Download Letter of Intent for submission of a Mission Proposal for a Flexi

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Mission Proposal in Response to the ESA Call for Mission Proposals for Two Flexi-Missions (F2 and F3)
Submitted January 27, 2000
Solar Orbiter
High-Resolution Mission to the Sun and Inner Heliosphere
Proposal coordinators: Eckart Marsch, Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau,
Germany, Tel: 49-5556-979-292, Fax: 49-5556-979-240, Email: [email protected]; Rainer Schwenn,
Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany.
Proposal team members:
E. Antonucci, Osservatorio Astronomico di
Torino, Italy
P. Bochsler, University of Bern, Switzerland
V. Bothmer, University of Kiel, Germany
R. Bruno, Istituto Fisica Spazio Interplanetario del
CNR, Roma, Italy
C. Chiuderi, University of Florence, Italy
A. Cacciani, University of Rome, Italy
L. Damé, Service d' Aéronomie du CNRS,
Verrières-le-Buisson, France
R. Harrison, Rutherford Appleton Laboratory,
Chilton, UK
P. Hoyng, SRON, Utrecht, The Netherlands
O. Kjeldseth-Moe, University of Oslo, Norway
O. von der Lühe, Kieperheuer Institut für
Sonnenphysik, Freiburg, Germany
G. Mann, Astrophysikalisches Institut Potsdam,
Potsdam, Germany
R. Marsden, ESA/ESTEC, Noordwijk, The
Netherlands
V. Martinez Pillet, Instituto de Astrofisica de
Canarias, Spain
E. Priest, University of St. Andrews, UK
W. Schmutz, Physikalisch-Metorologisches
Observatorium, Davos, Switzerland
J. Stenflo, Eidgenössische Technische
Hochschule Zürich, Switzerland
J.-C. Vial, University of Paris XI, Orsay, France
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Executive Summary
The scientific rationale of the Solar Orbiter (SO) is to provide, at high spatial and temporal resolution,
observations of the solar atmosphere and unexplored inner heliosphere. The most interesting and novel
observations will be made in the almost heliosynchronous segments of the orbits at heliocentric distances
near 45 Rs and out-of-ecliptic at heliographic latitudes of up to 38°. During most of the time of its journey the
SO will be out of the Earth-Sun line, thus carrying remote sensing instruments to unexplored territory in the
heliosphere and new vantage points for solar studies. The first near-Sun interplanetary measurements together
with concurrent remote observations of the Sun will permit us to disentangle spatial and temporal variations and
to understand, through correlative studies, the characteristics of the solar wind and energetic particles in close
linkage with the plasma and radiation conditions in their source regions on the Sun. By going to within 45 R s the
SO will allow remote sensing of the solar surface and atmosphere with unprecedented spatial resolution. Over
extended periods the SO will stay behind the Sun and deliver images and data from the side of the Sun invisible
from the Earth. It will also provide the first optical solar observations from outside the ecliptic. The SO will
achieve its many and wide-ranging aims with relatively small and simple, but state-of-the-art instruments
through a clever choice of orbits. The many original features of the SO will lead to new insights into how the
Sun works. In addition to helping resolve major open questions in solar physics the SO possesses a potential
for unexpected discoveries.
In particular, the plasmas and magnetic fields in those regions of the heliosphere, which are associated either
with magnetically active regions on the Sun or the streamer belt as well as quiet coronal holes extending to
near-ecliptic latitudes, will be investigated in situ in unprecedented detail. The high-resolution imagers of SO
will be able to resolve much smaller features of the solar atmosphere than in past missions. These capabilities
in concert will enable us to analyse the time variability, evolution and fine-scale structure of the dynamic
chromosphere, transition region and corona, to study the Sun's magnetic activity on multiple scales, to
investigate thermal and energetic particles, their origin, confinement, acceleration and release, and to reveal
plasma and radiation processes underlying the heating of the chromosphere and corona. The SO will ideally
complement studies (by STEREO near 1 AU) of the transient slow wind and coronal mass ejections (and the
consequences for space weather), and of the quiet fast wind (by the Solar Probe at a few solar radii; Feldman
et al., 1989).
The payload encompasses two instrument packages, in keeping with the solar and heliospheric science
objectives: Heliospheric instruments - Solar wind analyser for electrons and ions, magnetometer for DC and
AC fields, particle detectors for solar energetic ions and electrons, interplanetary dust detector, plasma wave
analyser, radio science experiment, neutral particle detector. Solar instruments - X-ray/EUV full-Sun imager;
high-resolution (<100 km) EUV imaging spectrometer, covering selected emission lines from the chromosphere
to the corona (10 kK-2 MK); high-resolution visible telescope and magnetograph; EUV and visible-light
coronagraphs; solar neutron and -ray detectors, radiometer.
The orbital design follows the Mercury Orbiter trajectory design. The mission requires solar electric
propulsion (SEP). Using SEP in conjunction with multiple planet swing-by manoeuvres, it will take SO only two
years to reach a perihelion of 45 Rs at an orbital period of 149 days, with an inclination ranging from 6.7° to
23.4° w.r.t. the ecliptic. During an extended mission phase of about two years the inclination to the ecliptic
increases up to 31.7°. A maximum heliographic latitude of 38.3° will thus be achieved. The maximum orbital
rate at perihelion is 13.1°/day, enabling near-synchronous observations (siderial rotation rate of the Sun:
14.4°/day). The orbital scheme offers seven perihelion passages at low differential rotation between SO and
Sun for about 10 days, and seven maximum northern and southern latitude viewing-periods of 10 days each. A
Soyuz-Fregat launcher can lift 1560 kg, sufficient for the SO lift-off mass to be launched from Kourou.
The spacecraft must be 3-axis stabilised and always Sun-pointing, except during SEP firing. Power is supplied
by a set of large cruise solar arrays (SAs), which are jettisoned after the last firing of the SEP thrusters and
have edges that are thermally protected. The two orbiter SA wings are orientable at 90° - 165° with respect to
the Sun. The SO strongly relies on and benefits from technology developed for the Mercury Cornerstone
mission. In particular, SEP in connection with Venus gravity-assisted manoeuvres is used and powered by large
cruise SAs. The thermal shield protecting the S/C bus is based on standard multi-layer insulation. Radiation
protection of sensitive electronic components is provided by Al shielding. Telemetry will be handled via X-band
LGAs, and for data dump by a 2-axis steerable Ka-band HGA. Transmission durations and distances from
Earth are dependent on the orbits: Near perihelion there will be high-rate data acquisition and on-board
storage. The data will be dumped to the ground when the distance from the Sun is larger than 0.5 AU.
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1. Introduction
The Solar Orbiter (SO) mission addresses a large variety of scientific questions related to
the Sun and the heliosphere. By virtue of its unique orbit it not only can study in situ the
unexplored inner heliosphere in detail, but can provide extremely high-resolution optical
observations of the Sun. The first regular out-of-ecliptic observations of the Sun's polar
regions will be made by the SO. Moreover, it will go sufficiently close to the Sun so that the
spacecraft temporarily reaches a quasi-corotational orbit. The SO satisfies a variety of
important demands for long-term near-Sun observations of the interplanetary medium, very
high-resolution solar observations and stereoscopic viewing of the corona.
The SO mission provides unique possibilities to do optical and remote sensing observations
of the Sun from close distances (perihelion of 45 Rs). Unprecedented spatial resolution at
scales below 100 km will be achieved in the images obtained in various wavelength bands.
Any full-Sun or disk-segment images will hardly be influenced by solar rotation during the
heliosynchronous orbital phase of the spacecraft. Therefore, time variability of the magnetic
field and its optical and interplanetary-particles manifestations can be studied extensively for
many days within a given range of heliolongitudes. The favourable vantage points along a
heliosynchronous trajectory are unattainable by any other means and will naturally enable
corona-tomography measurements to be carried out, covering a large volume of the outer
corona and solar wind. The remote sensing instruments will bridge the gap between the
phenomena in the source region of the wind and interplanetary space.
The SO achieves its many aims with a suite of relatively small but state-of-the-art
instruments through a clever choice of orbits, making use of low-thrust (LT) electric
propulsion (EP) technology, and an appropriate observational strategy. It is envisaged to
group these instruments into two packages, addressing in-situ and remote-sensing
objectives, respectively. Based on currently-available technology, it is estimated that the
complete payload would have a mass approaching 137 kg. Clearly, a major task in the next
phase of the mission study will be to refine the candidate instrument designs in order to bring
this figure close to 100 kg. This will be done through the application of, for example, lowmass, integrated structures and common electronics.
2. Key Scientific Objectives
2.1 Scientific rationale
The scientific rationale of the SO mission is to provide at high spatial and temporal resolution
observations of the solar atmosphere and unexplored inner heliosphere. The most interesting
and novel observations will be made in the almost heliosynchronous segments of the orbits
at heliocentric distances near 45 Rs and out-of-ecliptic at heliographic latitudes up to 38°.
The near-Sun interplanetary measurements together with concurrent remote observations of
the Sun will permit us to determine and understand, through correlative studies, the
characteristics of the solar wind and energetic particles in close linkage with the plasma and
radiation conditions in their source regions on the Sun. The Solar Orbiter mission will permit
us to correlate in detail the in-situ observations with magnetic activity occurring near the
solar surface. The quasi-heliosynchronous phase of the orbit near perihelion will allow us
to observe active regions continuously, in particular their dynamics and evolution at the
surface and in the atmosphere of the Sun, and to measure the interplanetary consequences
at distances beyond 45 Rs.
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The high-resolution imaging of the solar atmosphere will be better than in past missions by
an order of magnitude in spatial resolution. The SO instruments in concert will enable us to
analyse thoroughly the time-variability, evolution and fine-scale structure of the dynamic
chromosphere, transition region and corona, to study the Sun's magnetic activity on
multiple scales, to investigate energetic particle acceleration, confinement and release, and
to reveal plasma and radiation processes underlying the heating of the chromosphere and
corona. The Sun is the only star that can be resolved at the level at which the physical
processes responsible for magnetic activity take place.
The SO will ideally complement studies (by STEREO near 1 AU) of the transient slow wind
and coronal mass ejections (and the consequences for space weather), and of the quiet fast
wind (by the Solar Probe at a few solar radii; Feldman et al., 1989). In contrast to the Solar
Probe the SO will be able to study the inner heliosphere for a number of weeks over each of
its many orbits. Over most of its orbits the SO will lie outside the Sun-Earth-line and for
extended periods stay behind the Sun, delivering images and data from the side of the Sun
invisible from the Earth. The planned evolution of the orbit of SO allows us to distinguish
between two main phases of the mission with partly complementary scientific goals: a nearSun phase and an out-of-ecliptic phase.
2.2 Key goals for the near-Sun phase
The in-situ and remote-sensing measurements from a near-Sun perspective should permit us
to determine the:
 properties of plasma and electromagnetic fields within the coronal streamer belt and
coronal holes extending to the near-ecliptic regions (slow- and high-speed solar wind)
 properties of energetic particles close to the Sun to understand their origin, acceleration
and transport processes in the corona and inner heliosphere
 characteristics of the near-Sun dust and its origin and spatial distribution
 features of coronal radio emission in connection with particles originating from the same
source
 structure and evolution of the Sun's magnetic field at the fundamental scale of solar
magnetic elements, providing new insights into magnetoconvection
 nature of the coronal heating processes in the transition region and at the coronal base in
the source regions of the solar wind, at a much finer scale than previously possible
 nature of coronal and interplanetary disturbances associated with magnetic flux tubes,
magnetic activity, flares (shocks), eruptive prominences and coronal mass ejections
 evolution of solar active regions, such as sunspots, loops, and prominences.
2.3 Key goals for the out-of-ecliptic phase
Building on our knowledge and experience gained with Ulysses (which carries no
imaging/spectroscopic instruments), and SOHO (which remains in the ecliptic plane at 1 AU),
we formulate a number of key objectives for the out-of-ecliptic phase of a Solar Orbiter
mission. These include the investigations of the:
 nature and evolution of the solar polar coronal holes, as well as their boundaries
 origin of solar wind streams at intermediate and high latitudes
 coronal mass ejections: their global distribution, longitudinal extent, onset and
propagation
 magnetic field structure and evolution, in particular over the poles where it is poorly
known
 working of the solar dynamo, including the reversals of the polar field
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dynamics and rotation of the solar corona near the poles
magnitude and nature of the solar luminosity variations
global coronal waves and their effects over the poles
acceleration of the solar wind over the poles by Doppler velocity measurements.
3. Specific Observations and Science Objectives
3.1 Global solar corona and solar wind
There are two characteristic types of solar corona and wind, prevailing at different
heliographic latitude regions of the Sun and heliosphere. The high-speed flow is the basic
equilibrium state of the solar wind. It is most conspicuous near solar minimum, when it
emanates steadily from the magnetically open coronal holes around the poles, whereas the
slow wind originates in an unsteady fashion from the equatorial streamers. These are for
most of the time magnetically closed, but seem to open intermittently to release abruptly
mass ejections (CMEs) (see Figure 3.1) and more continuously the slow wind embedding
the heliospheric current sheet. The transient nature is also evident in the high variability of its
abundances in helium and other heavier elements found in association with large-scale
magnetic activity on the Sun. The solar magnetic field in the equatorial regions reveals a rich
morphology and many fine-scale structures, such as the low-latitude rays resembling the
polar plumes, which are clearly evident in visible light coronagraph pictures and in the EIT
(Moses et al., 1997) images. Remnants of these features are even found in the meso-scale
stream variations of the solar wind, as observed in situ by Helios and Ulysses and through
interplanetary scintillations.
Figure 3.1: Giant coronal mass ejection as seen by LASCO on SOHO in 1998 (adapted from
Srivastava et al., 1999). The filament material of the ejected prominence exhibits twisted
helix-shaped structures. The SO at its perihelion of 45 Rs would be able to measure the
ejecta in situ and out of the ecliptic in much more detail than possible from the Earth's orbit.
High inclinations will presumably be reached by the SO (depending on the launch year)
during the declining phase of the next solar cycle, when the streamer belt will be warped but
well-defined at mid-low latitudes and be completely observable through the polar regions.
The SO observations will allow us
 to determine for the first time the longitudinal extent of CMEs
 to provide in conjunction with a S/C in Earth orbit and ground-based observations a 360°longitude observation of the entire Sun
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to monitor global effects and to determine the extension of coronal waves associated with
CMEs
to find out whether or not the Sun quakes associated with flares, discovered by SOHO,
can propagate across the interior of the Sun and reach the opposite side.
The slow and transient solar wind can only be resolved and understood properly by a direct
probing close to its sources at a fixed heliographic longitude. It is a key advantage of the SO
that from the corotational vantage point the temporal and spatial variations of the solar wind
can unambiguously be disentangled. The observations to be made by SO will concentrate on
the slow streams and on solar disturbances associated with magnetic solar activity, flares,
loops, erupting prominences, and CMEs, when SO is close to the ecliptic, and on the fast
streams, when SO is close to maximum inclination. The temporal evolution and spatial
structure of such phenomena at the coronal base will for the first time be measured at very
high spatial (< 100 km) and temporal (< 1 s) resolution.
SO will for the first time image directly the polar regions, where the fast solar wind is formed,
from an out-of-ecliptic position (with inclinations ranging from 6.7° to 23.4° during the nominal
mission and up to 38.3° at the end of the extended mission). The almost radial expansion of
the fast solar wind has not yet been determined, since the expansion over the poles is
essentially perpendicular to the line of sight (LOS) of spectroscopic instruments observing
from the ecliptic, and therefore the Doppler effect is negligible. With increasing inclination
angle of its orbit, the SO will perform out-of-ecliptic measurements and
 render possible velocity measurements due to a sizable flow component along the LOS
 image the border of polar coronal holes and their deformation in response to the coronal
waves (discovered with SOHO) that develop during the CMEs
 observe globally the entire streamer belt in the extended corona (beyond 2.6 Rs at the
end of the nominal mission and beyond 1.3 Rs at the end of the extended mission)
 find the magnetic signal associated with coronal plumes and clarify their nature.
3.2 Structure of the magnetic network and origin of the solar wind
The supergranulation network, which dominates the chromospheric plasma dynamics, is
apparent in the EUV emission pattern as seen by the SUMER instrument on SOHO (see
Figure 3.2). Magnetograms from SOHO (see, e.g., Fleck et al., 1997) have revealed the
ubiquitous appearance of small magnetic bipoles at the solar surface. After emergence, the
polarities separate and are carried to the network boundaries by the supergranular flow,
where they merge with the pre-existing network flux. This leads to flux cancellation,
submergence and reconnection events. The magnetograms also show that the magnetic
field exists in the network in two components side-by-side, i.e. in uncanceled unipolar fields
together with a carpet of closed loops and flux tubes. The small loops will either emerge or
contract downwards and collide, and thus constitute a permanent source of energy, which
can be tapped by the particles through magnetic field dissipation. Numerical simulations
suggest that many of the bipolar structures have scales smaller than 100 km.
As a consequence of these observations, theoretical ideas about the origin of the solar wind
have been put forward (see the review of Axford and McKenzie, 1997) according to which
the wind originates in the chromospheric network, and draws its energy from high-frequency
waves generated by magnetic reconnections of the dynamic and complex fields prevailing
there. Above mid-chromospheric altitudes the open field expands in the form of coronal
funnels, fills the overlying corona and guides the solar wind mass flux, which emanates from
the open chromosphere where the plasma is created by photo ionisation. Plasma outflow has
indeed been detected by SUMER on SOHO and is illustrated in Figure 3.2. The SO with its
unprecedented spatial resolution will for the first time
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reveal the fine structure of the network and provide definitive answers to the question of
where and how the solar wind originates
allow, while corotating with and thus remaining magnetically connected with the wind
source regions, to reliably disentangle spatial and temporal structures in the solar wind
observe the source signatures by remote sensing together with in-situ analysis.
Figure 3.2: SUMER observations of the solar-wind source regions and magnetic structure of
the chromospheric network. The insert shows the measured Doppler shifts of Neon ions,
indicating blue-shifts, i.e. outflow, at the network cell boundaries and lane junctions below the
polar coronal hole, and red-shifts (downflow) in the network regions underlying the globally
closed corona (adapted from Hassler et al., 1999).
3.3 Waves in the corona
Magnetohydrodynamic waves generated in the photosphere by convective motion are
primarily of low frequency. Small-scale magnetic activity is expected to continually produce
energetic particles, rapidly-moving plasma and waves. Their dissipation could involve
cyclotron damping, which is observed to operate in the distant solar wind (Marsch, 1991). SO
will make the first observations of such key plasma processes from a near-Sun perspective
and thus address the extended heating of the outer corona. In the small-scale magnetic
structures of the strongly inhomogeneous network fields higher-frequency waves could be
excited up to the kilo-Hertz range. Such waves would transfer their energy, e.g. into the
transverse kinetic degrees of freedom of the protons, and particularly the heavy ions, and
thereby heat the particles very effectively to high kinetic temperatures, a process for which
the UVCS and SUMER instruments on SOHO have recently found evidence in the strong
Doppler broadenings of emission lines (Kohl et al., 1997; Antonucci et al., 1997; Wilhelm et
al., 1998; Tu et al., 1998). Due to its proximity to the heating sites in the corona and the high
sensitivity and spatial resolution of its instrumentation, the SO will for the first time be able to
 see very dim emissions, which are not visible from 1 AU due to the low density of the gas
and the high contrast in emissivity, when compared with plasma confined in small loops
 resolve and diagnose dilute plasma on open fields and the outflow in coronal funnels
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search for high-frequency waves in the corona (with reasonable chances of success due
to the high resolution).
3.4 Photospheric magnetic flux elements
The major part of the magnetic flux permeating the solar photosphere outside sunspots is
concentrated in small (scales of 100 km and below) flux tubes of kilo-gauss field strength.
The structure and dynamics of these fundamental elements of the near-surface magnetic
field has profound implications for a number of basic questions:
 How do magnetic foot-point motion, wave excitation, flux cancellation and reconnection
contribute to the flux of mechanical energy into the corona?
 In which way do the emergence, evolution and removal of magnetic flux elements
determine the magnetic flux budget of the Sun? Is there a local dynamo operating on the
scale of granulation?
 What is the origin of the facular contribution to the variability of the solar constant?
 What is the physics of the interaction between convection and magnetism?
Answers to these questions require the study of magnetic flux elements on their intrinsic
spatial scale (<100 km). SO will allow us to achieve this resolution with rather modest
instruments of an 25-cm aperture. The SO high-resolution visible-light imager and
magnetograph is intended to monitor the emergence, dynamics, twist, shearing, mutual
interactions and possible coalescence and subduction below the surface in order to follow
the evolution and scrutinise the life cycles of magnetic flux elements.
The different viewing angles of the Sun from SO is of great importance for better recordings
of the high-latitude and polar magnetic fields than is possible from Earth. SO be able to
overcome a major obstacle in the derivation of vector magnetic fields from the measured
Stokes vector. The longitudinal and transverse Zeeman effects respond in fundamentally
different ways to these spatially unresolved fields, when averaging over the angular
resolution element. One important novel aspect offered by the SO is, when combining
magnetograms obtained with different viewing angles from the SO and Earth, that a
transversal Zeeman effect as seen from Earth may become a longitudinal Zeeman effect as
seen from SO, and vice versa. This is of great importance for the disentangling of various
effects in the diagnostics of spatially unresolved vector magnetic fields.
3.5 Fine morphology and fast dynamics of coronal magnetic fields
A key scientific objective of SO is to study the emergence and the cancellation of
photospheric magnetic flux (the latter is the disappearance of opposite polarity regions in
close contact), and to investigate the consequences of such processes for the overlying
global coronal magnetic loops and for the chromospheric and transition region magnetic
network. Flux cancellations are known to be associated with or at the origin of various active
phenomena, such as filament formation and eruption, evolution of small points of emission
bright in radio or X rays, or the occurrence of flares. Magnetograms combined with EUV and
soft-X-ray images as well as EUV spectra are the key data necessary to understand the
bearing that small-scale magnetic activity has on the corona.
The Yohkoh, SOHO and TRACE extreme ultraviolet and soft-X-ray telescopes (see, for
example, Strong et al., 1994; Schrijver et al., 1999) illustrate the existence of fine-scale
structures in the corona, such as polar plumes and thin post-flare loops, and reveal
continuous dynamics occurring on all resolved scales, in particular the finest. There is strong
evidence that the size of the actual brightness structures lies well below the best current
spatial resolution. This points to the need for still higher spatial resolution, which, as in the
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visible, can be obtained by modest means with the SO. The very active Sun has still not
been imaged with sufficient resolution, a task to be performed by SO from its unique vantage
point. The triggering phase of solar flares and small activity regions as well as the evolution
of point-like events or bright X-ray spots will be monitored, with simultaneous observations in
other relevant wavelengths being made and foot-point magnetic field measurements being
carried out. The wide coverage of coronal temperatures by the telescopes on the SO will
enable complete images to be obtained in fast cadence. From these the density and
temperature distributions can reliably be derived, such that the traits of coronal heating
processes in current sheets, shock fronts, or acceleration in small explosive events and rapid
plasma jets might become clearly visible and be resolved in time and space.
3.6 The Sun's polar magnetic field and the dynamo
Knowledge of the nature and evolution of the magnetic field near the solar poles is a key for
understanding the solar dynamo mechanism. The amplitude of geomagnetic disturbances
during solar minimum, which are caused by recurrent fast solar wind streams originating from
the polar coronal holes, is well correlated with the height of the following activity maximum
(Legrand and Simon, 1981). This suggests that the polar field is directly related to the
dynamo process, presumably as a source of poloidal field to be wound up by the differential
rotation at the shear layer at the base of the convection zone. Theoretical models (Choudhuri
et al., 1995) indicate that the meridional circulation pattern found by SOHO could be crucial
for the solar dynamo, by introducing the equatorward drift of the activity belts and
transporting surface magnetic flux toward the poles and downward to the shear layer. Such
models predict a substantial magnetic flux concentration near the poles. SO will allow us
 to measure reliably the strength of the polar field for the first time and thus
 to provide a new and crucial observational constraint for dynamo models.
Studies of the evolution of solar features such as active regions, loops, prominences or
sunspots are greatly complicated by the fact that their evolution time scales are comparable
to the solar rotation period. Thus the evolution is entangled with other effects such as centreto-limb variation, foreshortening and projection effects. In order to disentangle these effects it
is necessary to co-rotate with the Sun. SO will for the first time provide such an opportunity
and will thus help resolve old and otherwise intractable problems related to the solar dynamo
and the diffusion of the magnetic field across the solar surface.
3.7 Solar neutrons
A large fraction of the particles accelerated by flares (and microflares) interact in closed
magnetic field lines of the solar atmosphere (at heights much less than 3 R s) and do not
escape into interplanetary space. They are detected through the X-ray, -ray, microwave and
decimetric-metric emissions they produce, and through secondary particles, such as
neutrons produced in interactions with the dense layers of the solar atmosphere. The neutron
flux varies dramatically with the distance because of the beta decay. It indeed decreases
exponentially with the product of the neutron speed and mean lifetime (being eight minutes
only). Direct detection at the Earth orbit is only possible for neutrons with energies greater
than 100 MeV. So far, very few neutron events have been reported as compared with several
hundreds of -ray events. During the perihelion passages of SO, it will be possible to directly
measure also the less energetic neutrons not detectable otherwise. Also, the neutron fluxes
at energies above 100 MeV will increase by orders of magnitude.
The  rays and neutrons are produced by interactions between the primary accelerated ions
and the constituents of the solar atmosphere. Such events contain important information
about the composition and charge states of energetic particles, and about the existence of
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distinct phases of proton acceleration, being either prompt and occurring within seconds of
the release of flare energy, or delayed and associated with the shock wave possibly
generated by the flare ejecta. SO will provide new information about nuclear reactions in the
close vicinity of the Sun and thus open up a new window: solar neutron astronomy.
3.8 Microstate of the interplanetary solar wind
The ultimate causes of interplanetary kinetic phenomena are to be found in the dynamic
corona itself. The closer a S/C comes to the Sun the more likely it is to detect remnants of
coronal heating and related plasma processes, occurring on a broad range of scales in
space and time. The radial evolution of the internal state of the expanding wind resembles a
complicated relaxation process, in which free energy stored in the form of stream structure
and shear as well as in non-Maxwellian particle velocity distributions is converted into wave
and turbulence energy. Plasma waves in the collisionless solar corona and wind play a role
analogous to collisions in ordinary fluids. These wave modes can theoretically be excited by
a variety of free energy sources, including drifts, currents, temperature anisotropies and
beams, which must be resolved in detail. All the wave modes of primary importance together
with the ions and electrons will be measured by SO at high time resolution, in order to
provide the comprehensive wave and particles diagnostics necessary to study the waveparticle interactions and kinetic processes.
SO, while approaching the Sun to about 0.2 AU with its plasma and wave analysers, will
enable high-resolution measurements of kinetic processes be made, which holds much
promise of unexpected revelations. It will address fundamental solar wind science and key
plasma-physics questions such as:
 How do particle distributions develop velocity-space gradients and deviations from
Maxwellians?
 What processes drive plasma instabilities and cause wave growth and damping?
 What regulates transport and ensures the observed fluid behaviour in the collisionless
solar wind?
 What are the radial, latitudinal and longitudinal gradients of plasma parameters in the
inner heliosphere?
 What is the microstructure of stream interfaces and boundaries near the Sun?
 How does the chemical and charge-state composition of the plasma vary spatially?
3.9 Solar wind ions as tracers of coronal structures
The solar wind carries along information on its coronal source regions, e.g., through its
elemental and isotope composition, and the ionisation states of the various atoms. At the
coronal base, a compositional bias is introduced according to the first ionisation potential
(FIP) of the elements. This FIP effect appears to be significantly different for slow and fast
solar wind flows. Further, the ionisation state of various species indicates that slow wind
must have undergone substantially more heating than fast wind. These signatures will be
used for the first time to resolve fine structures and boundaries in the solar wind. This
diagnostic technique works most reliably close to the Sun where the flow has not yet been
processed (compressed, deflected, etc.) by interactions between streams of different speed.
With Helios it was found that at least in high-speed wind a basic flow-tube structure was still
recognisable in situ at 0.3 AU (Thieme et al., 1990). The scale size observed matches that of
supergranules fairly well, if an appropriate flux-tube expansion is taken into account. The SO,
using modern ion-composition instruments and being closer to the Sun, will
 reveal through compositional variations the fine structures in all types of solar wind
 link the flow tubes directly with the underlying chromospheric network observed remotely
 identify pick-up ions stemming from dust and interplanetary sources.
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3.10 Magnetohydrodynamic turbulence
The solar wind plasma is in a highly turbulent state composed of various components (Tu
and Marsch, 1995). The energetically dominant component consists of largely
incompressible Alfvénic fluctuations. The minor component has a much lower amplitude than
the Alfvénic fluctuations, is compressible and clearly enhanced in the mixed low-speed flows.
SO will scan a belt ranging roughly from -40° to 40° in heliographic latitude, while being
within 0.3 AU of the Sun. From these vantage points SO will be able to answer important
questions such as:
 How does MHD turbulence evolve spatially at higher latitudes near the Sun?
 What are the crucial conditions for in-situ turbulence generation?
 How does the turbulence pattern vary with stream structure closer to the Sun?
The overall radial trends as seen by Ulysses and Helios suggests strong variations of the
local production rate of the Alfvénic fluctuations in the region just inside 0.3 AU. The
reduction of the perihelion distance from 0.3 AU (Helios) to 0.2 AU (Solar Orbiter) will offer
unique opportunities to study the local generation, nonlinear coupling and spatial evolution of
MHD waves near the Sun. Measuring Alfvénic fluctuations and MHD turbulence in situ
represents also a means of diagnosing their coronal sources. SO will address these issues
from its unique vantage point and help to answer basic questions such as:
 How and where are Alfvénic fluctuations generated in the solar corona?
 How does MHD turbulence evolve radially and dissipate in the inner heliosphere?
 Do the spectra contain indications or relics of high-frequency wave heating of the corona?
The solar wind is the only available plasma “laboratory” where detailed studies on MHD
turbulence can be carried out free from interference with spatial boundaries, and in the
important domain of very large magnetic Reynolds numbers. Detailed comparison between
experimental in situ data and theoretical concepts will allow to put MHD turbulence theory on
more solid physical ground, which will be of paradigmatic importance to understand the solar
(stellar) coronal heating mechanism and the role or turbulence in the solar (a stellar) wind.
3.11 Coronal transients and their impact on the heliosphere
A subject that has recently attained much attention is “space weather”. It is concerned with
transient events such as flares, coronal mass ejections (CMEs), eruptive prominences, and
shock waves and their impacts upon the Earth's magnetosphere and atmosphere.
Fundamental questions in this context remain to be solved, e.g.:
 What are the indicators for imminent violent eruptions? Can they be predicted?
 Why are there so extremely different types of solar transients?
 What determines their propagation properties?
 How far around the Sun do the resulting shock waves reach?
SO will be ideally located, being closer to the sources of transients in the solar atmosphere,
to measure the input into the heliosphere and determine the boundary conditions near the
Sun. These dramatic events literally shatter the whole heliosphere, and their effects can be
felt at all planets. SO will be a key link in a chain of solar terrestrial observatories to be
stationed in Earth's orbit and at the libration points in that it provides near-real-time event
alerts from its unique orbit close to the Sun. SO will image the solar atmosphere and
determine the photospheric fields at the footpoints of coronal loops and thus reveal the field
evolution leading to solar eruptions. The associated effects on the interplanetary medium can
be studied without significant delays or transport-related changes of the particles and fields.
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3.12 Acceleration and transport of solar energetic particles
A continuing source of difficulty has been our inability to predict the intensity of solar
energetic particles at the Earth from observed transient activity on the Sun. An important part
of the problem is that we do not know the suprathermal population that feeds the
acceleration processes near the Sun. The efficiency for transferring energy from flares to
energetic particles cannot be inferred from remote observations, because an unknown
fraction of the accelerated ions remains trapped by strong magnetic fields near the Sun for a
significant time after acceleration. Subsequent -ray and neutron emissions resulting from
their eventual loss to the atmosphere are often too low in flux to be observable. Present
observations indicate that small transients occur sufficiently often to allow a determination of
the efficiency, e.g., by neutrons as proxies for the magnetically bound component. Our ability
to solve this issue will be greatly enhanced by SO, because during its multiple perihelion
passages we will
 gain a better knowledge of the source spectrum,
 obtain new observations on particle motion in the hypothetical storage region,
 measure changes in the spectrum as the ions and electrons propagate from the Sun to
the spacecraft after escape from the trapping region.
The SO will for the first time investigate the particles' environment in close proximity to the
different source regions on the Sun, such as coronal holes, streamers, CMEs and associated
shocks, active regions, and flare locations. Concerning CMEs in particular, the SO will
 determine the solar source conditions for different particle species (e.g., e, p, 3He, heavy
ions, p/He ratios) from composition measurements, energy spectra and time evolution,
 distinguish clearly between gradual (shock-associated CME events) and impulsive flaretype events related with magnetic reconnection,
 study the effects of particle acceleration and turbulence-moderated propagation at
different locations with respect to the CME centre,
 find the differences between the particle signatures associated with parallel and
perpendicular shocks at the east and west flanks of CMEs,
 probe the effects of magnetic reconfigurations in the aftermath of CME launches, at times
when the acceleration processes still occur in the corona,
 detect perhaps for the first time energetic particle populations from microflares, a
measurement which is not possible further away from Sun due to background problems.
In addition, we can study with SO important global aspects of the Sun and heliosphere by
 utilising the energetic particles as probes for the coronal and heliospheric magnetic field,
 analysing the propagation of solar particles and modulation of galactic cosmic rays.
Generally, the processes which accelerate particles to very high energies are of great
interest in astrophysics. Observations of energetic particles close to their sources on the Sun
will allow us to study our nearest star, the Sun, as a particle accelerator.
3.13 Solar radio emission
The Sun is an intense source of thermal as well as nonthermal radio emission. The
nonthermal radiation is generated by suprathermal and/or highly energetic electrons
produced by sudden releases of magnetic energy in the active corona. Nonthermal radiation
at frequencies below 1 GHz is generally assumed to be plasma radiation emitted near the
local electron plasma frequency which depends on the electron density. Consequently, the
higher and lower frequencies correspond to the lower and higher corona, respectively, and
remote sensing of the radial electron density profile is possible.
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In the corona the nonthermal radio radiation is generated by energetic electrons. The
measurement of this radiation will allow us to study the plasma processes associated with
such electron populations and to address such questions as:
 What is the elementary process of magnetic energy release?
 How are ions and electrons accelerated up to high energies within a few seconds?
 What are the relationships between flares, coronal mass ejections, radio bursts,
interplanetary shocks, and energetic particle events?
 How does the quiet Sun produce suprathermal particles?
The radio spectrometer data are particularly valuable since they will be obtained in
connection with complementary data from the optical telescopes imaging the activity site,
and data of the particles and fields instruments onboard. Detailed correlative studies
between the coronal/interplanetary electrons and the radio waves they generate will be
possible for the first time from a near-Sun and out-of-ecliptic vantage point on SO.
3.14 Neutral particles from the Sun
The SO provides the opportunity of observing for the first time neutral particles emitted from
the Sun, the neutral solar wind. Thus we obtain an independent means of determining the
solar wind velocity profile in the outer corona and the electron temperature close to the Sun.
In addition, Energetic Neutral Atoms (ENAs) can be produced by charge-exchange
processes in interplanetary space. They might be detected by SO for the first time near the
Sun. ENAs have been used for imaging planetary magnetospheres and can be used similarly
for imaging the outer solar corona. The ENAs originating from a seed-ion population or
background neutral gas will allow one to derive properties of the background gas as well as
to remotely sense the energy distribution, spatial distribution and temporal evolution of the
seed populations. The neutral gas could be either of interstellar origin or a neutral component
stemming from the solar wind itself, being produced, e.g., by neutralisation of solar wind ions
on interplanetary dust particles close to the Sun. Possible seed ion populations are the solar
wind protons, energetic solar particles and anomalous cosmic rays. Measuring the neutrals
means probing and inferring the physical properties of the local interstellar gas and
circumsolar dust. Such measurements possess a potential for unexpected discoveries.
3.15 Circumsolar and interplanetary dust
The observed distribution of interplanetary dust particles (IDPs), micrometeorites of
submicron to millimetre size, forms a flat spheroid centred on the ecliptic plane. Its spatial
density decreases with increasing heliocentric distance and increasing latitude. Different
forces and effects are acting on IDPs: gravitational attraction, radiation pressure, corpuscular
pressure, magnetic forces, planetary perturbations, erosion processes (sputtering,
sublimation), and mutual catastrophic collisions. IDPs originating from comets and asteroids,
or even from interstellar space approach the Sun on time scales of 10000 to 100000 years
due to the deceleration by the Poynting-Robertson effect. In the vicinity of the Sun, IDPs
suffer, as their temperature becomes high, a complicated evolution caused by their
progressive sublimation, which depends on the particle properties and chemical composition.
On the other hand these processes are the source of an additional ion population in the solar
wind: outgassing, sublimation and sputtering produce neutrals which are then ionised and
picked up by the solar wind. Therefore, they contribute to the suprathermal particle
population. The study of these particles will contribute significantly to the understanding of
the evolution of interplanetary dust in the vicinity of the Sun. The interaction between the
plasma and the solar magnetic field with IDPs makes this picture even more complex: far13
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reaching, dense coronal structures as well as the changing pattern of the magnetic field can
perturb the IDPs dynamics.
The present understanding of IDPs results from Zodiacal Light observations, in-situ impact
detectors and laboratory analysis of collected particles. The processes taking place in the
circumsolar region remain poorly understood. Their description is, to a large extent, based on
theoretical studies. The possibilities of remote sensing experiments are limited due to
instrumental problems. Especially the chemical composition and the dynamics of submicron
grains can only be investigated by in-situ methods. The dust observations on SO will
 render possible the first in-situ detection of grains which have suffered extreme radiation
and corpuscular impacts, processes relevant for star formation,
 help in determining the extent of the dust-free zone near the Sun,
 has the potential for discovery of a dust disc fed partly by Sun-grazing comets,
 deliver data relevant for an understanding the physics of protoplanetary discs.
3.19 Solar luminosity variations
The irradiance of the Sun (i.e. its brightness as measured above the Earth’s atmosphere) is
known to vary by 0.1% over the solar cycle. There is also some evidence for a longer term,
secular variation. In spite of its small magnitude, the irradiance variation is a potential cause
for climate change. Two basic questions need to be answered before we can reach an
understanding of the causes of this variability:
 How does the solar luminosity (i.e. the radiation escaping in all directions) vary? Does it
change or is a brightening at the equator compensated by a darkening over the poles?
 Why is the irradiance variability of the Sun a factor of three smaller than that of Sun-like
stars?
By carrying out irradiance studies from out of the ecliptic and on the side away from the
Earth, the SO will provide a unique opportunity to answer these questions. For example, one
major difference between the Sun and Sun-like stars is that whereas we observe the Sun
nearly equator on, stars are on average seen from a latitude of approximately 30 o. SO will
reach this latitude in the course of the extended mission and test whether this inclination
effect really is responsible for the Sun's anomalous behaviour.
4. Scientific Payload
4.1 Measurement requirements
The Helios, Ulysses, Yohkoh, SOHO and TRACE missions may be used as a baseline for
designing the payload of SO. It must be state-of-the-art, withstand the considerable thermal
load at 45 Rs, comply with the general requirements of a low-mass, compact and integrated
design, make use of on-board data compression/storage and require a modest data
transmission rate. The payload meeting the solar and heliospheric science objectives
encompasses two instrument packages:
In-Situ Heliospheric Instruments (see Table 4.1 below) - Solar wind plasma analyser
measuring electrons and ions, magnetometer for DC and AC fields, particle detectors for
solar energetic ions and electrons, interplanetary dust detector, neutral particle detector,
radio instruments. Heritage for such instruments is from Helios, SOHO, Ulysses, Wind, ACE.
Remote-Sensing Solar Instruments (see Table 4.4 below) - X-ray/EUV full-Sun imager;
high-resolution EUV spectrometer covering selected emission lines from the chromosphere
to corona; high-resolution visible light telescope and magnetograph; EUV and visible-light
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coronagraph; solar neutron and -ray detector and a radiometer. Heritage for such
instruments is from Yohkoh, SOHO, TRACE.
4.2 Interplanetary particles and fields instrumentation
4.2.1 Solar Wind Plasma Analyser (SWA)
The SWA will measure separately the three-dimensional distribution functions of the major
solar wind constituents: protons, alpha particles and electrons. The basic moments of the
distributions, such as density, velocity, temperature tensor, and heat flux vector will be
obtained under all solar wind conditions and be sampled fast enough to characterise fully the
fluid and kinetic state of the wind. In this way we will be able to determine possible nongyrotropic features of the distributions, ion beams, temperature anisotropies and particle
signatures of wave excitation and dissipation.
The SWA will measure protons, alpha particles and electrons. In addition, measurements of
representative high-FIP elements (the C, O, N group) and of low-FIP elements (such as Fe,
Si or Mg) will be carried out, to obtain their abundances, velocities, temperature anisotropies
and charge states, and in order to probe the wave-particle couplings (heavy-ion wave
surfing) and determine the freeze-in temperatures (as a proxy for the coronal electron
temperature). Also, pick-up ions of various origins, such as singly-ionised ions stemming
from interplanetary dust, will be measured.
The SWA will unambiguously separate ion species and provide M/Q determination with an
energy resolution of at least 5 % in (E/Q) /(E/Q) and range between 10 eV/Q and 30 keV/Q.
The angular resolution will be better than 5° in azimuth and elevation angle. The electron
analyser will be capable of resolving the strahl (heat flux) and cover a large fraction of 4 in
solid angle, with a pitch-angle resolution better than 22.5° and an energy range between a
few eV up to 10 keV. To cope with fast wave-particle effects to be expected in the velocity
distributions, the sensors will have time resolutions of about 1s or even shorter. The
specifications can be met by existing state-of-the-art plasma instruments, using time-of-flight
techniques with post-acceleration for particle discrimination.
4.2.2 Plasma Waves Analyser (PWA)
The PWA will identify the various plasma waves and kinetic modes comprising the highfrequency part of the fluctuation and turbulence spectra. The instrument will cover a broad
band in frequencies, extending from some Hz into the MHz range. The expected field
strength may range between a few V/m and mV/m, and perhaps up to a V/m for the
convection electric field. The magnetic field strength is expected to vary between a few nT
and T, with large differences between the longitudinal and transverse components with
respect to the mean magnetic field and the solar wind flow direction (substantial Doppler
shifts are to be expected).
The electromagnetic fluctuations are measured with search coil magnetometers and electric
field dipole antennas, minimised in size and arranged in a compact configuration. The PWA
will perform on-board processing of the data and deliver selectable wave forms, to identify
nonlinear coherent fluctuations. To resolve the vector components of the electric and
magnetic fields, which is scientifically highly desirable and required to determine wave
modes unambiguously, the search-coils are to be mounted on short booms required for
magnetic cleanliness reasons. More than one electric field antenna is needed, which will be
short enough to adjust to the variable Debye length of about one meter.
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4.2.3 Magnetometers (MAG)
The magnetometers will measure the interplanetary magnetic field components over a broad
dynamic range from a few nT up to the T range, and for frequencies well below the local
heavy-ion or proton gyrofrequencies. Conventionally, flux gate sensors are used for weak
fields and magnetoresistive or similar devices for strong fields. The magnetometer may
consist of a triaxial identical sensor system. A compromise has to be found between the
requirements of the spacecraft and payload systems and the demand of the magnetometer
for magnetically-clean conditions.
4.2.4 Energetic Particle Detector (EPD)
The EPD measurements on SO will allow to study the sources, acceleration and propagation
of solar energetic particles in association with coronal and interplanetary shocks. Energetic
pick-up particles originating from outgassing or sputtering of near-Sun dust should be
measured as well in conjunction with the SWA measurements. The EPD will determine
chemical and charge composition of ions in a wide energy range, from about the typical solar
wind energies of a few keV to several 100 MeV/nucleon for protons and heavy ions.
Electrons should be measured from 10 keV to 10 MeV.
The combination of electrostatic E/Q-analysis with time-of-flight E/M-determination and
subsequent direct energy measurement in a solid state detector has been employed in many
EPDs in the past. Ongoing research in solid state detector technology is promising and will
certainly lead to further improvements in energy thresholds, and noise level as well as to size
reduction. It is essential that all measurements are done fast, typically at 1 s, with complete
angular coverage to resolve the pitch-angle distributions. These requirements can readily be
met by a low-mass and compact sensor assembly with different look directions and modest
aperture sizes. We propose to place several detectors at different positions onboard the
spacecraft, in order to obtain a sufficient field of view and angle-of-incidence coverage
without using a scanning platform.
4.2.5 Dust Detector (DUD)
The DUD will analyse interplanetary dust particles with respect to their composition and
orbital characteristics. The spatial distribution of particles with masses ranging between 10-16
g and 10-6 g should be determined. These objectives can be reached with an instrument of
Ulysses, Galileo or Cassini heritage. The minimal dust experiment may consist of a multiple
sensor assembly, with each sensor looking in a different direction to resolve incidence
angles. A low-mass dust-detecting element could consist of a thin polarised polyvinylidene
fluoride foil with conducting electrodes on opposite sides. The particle impact irreversibly
depolarises this foil in the penetration crater volume and thus creates a fast (ns) current
pulse as input to the electronics. The calibrated detector signal carries information about
mass and velocity of the impact particle. Its mass can be determined by the foil thickness
setting a threshold value. The differential dust particle fluxes are directly obtained from the
differences of the integral fluxes measured with foils of various thickness. Full chemical
analysis and mass resolution of the important elements H, C, N, O of carbonaceous material
and the metals like Na, Mg, Al, Si, Ca and Fe contained in chondritic silicates is desirable.
4.2.6 Radio Instruments
Radio Spectrometer (RAS) The RAS will measure the solar and interplanetary radio waves
in the frequency range from 100 kHz to 1 GHz, with a sweep period of 1 s and a high
spectral resolution (f/f  0.01). The RAS will observe plasma processes associated with
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energetic electrons from the low corona up to 50 Rs. It will probe the plasma at distances
ranging from the solar surface to the spacecraft location, thereby connecting the low-altitude
coronal regions observed by the optical instruments with the near-Sun heliospheric
conditions specified by the in-situ measurements. The instrument could be divided into four
sub-spectrometers according to a frequency ratio of 1:10. Each sub-spectrometer may
consist of 128 channels, leading to a total data rate of 4 kb/s.
Coronal Radio Sounding (CRS) A passive radio science experiment can be carried out using
the available radio links. The S/C will pass behind the Sun (superior solar conjunction) on
many occasions, making it possible to investigate the solar corona by radio sounding at solar
distances much less than the perihelion distance down to at least 2 Rs. Integrated line-ofsight parameters such as electron content (densities), Faraday rotation, scintillations and
angular broadening can be recorded. The CRS experiment with a two-way radio link via the
S/C high-gain antenna (ranging capability) would require a radio subsystem with dualfrequency phase coherent downlinks at X-band and Ka-band. Linear polarisation of the
downlink signals would enable Faraday rotation measurements as an option. The RF power
for both radio links (X and Ka) planned for this mission is sufficient for this investigation. A
two-way dual-frequency coherent radio link (X-band uplink; X-band and Ka-band
simultaneous coherent downlinks) is considered as the optimal configuration for a sufficiently
stable link and for detecting signatures of CME events traversing the radio ray path. If only a
one-way radio link (S/C to ground) is feasible due to operational constraints during solar
conjunction, the transmitted radio signal must be stabilised by an onboard ultra-stable
oscillator (frequency stability 10-11 to 10-12 at 3 s integration time, 200g , 3 W).
4.2.7 Neutral Particle Detector (NPD)
Neutral H is used in spectroscopic observations on SOHO to determine the solar wind
velocity profile in the corona up to several solar radii. The NPD on SO will measure neutral
hydrogen and thus allow us to detect the interplanetary "neutral solar wind". The NPD will
also measure energetic neutral atoms emitted from various coronal sources, and thus enable
images of these coronal emission region to be constructed from rays of atoms with different
masses and energies. As a baseline, the instrument parameters could be: Energy range: 0.1
- 10 keV/nucleon (100 keV/nucleon as a secondary objective); moderate (~0.8) energy
resolution; mass resolution such that H, He, O (Na, K) will be discriminated. Imaging
capability of the NPD is required to produce novel results.
Table 4.1: Heliospheric Instrumentation (*depth x width x length)
Name
Abbreviation
Solar
Wind
Plasma
Analyser
SWA
Plasma
Wave Analyser
PWA
Magnetometer
MAG
Energetic
Particle
Detector
EPD
Neutral
Particle
Detector
NPD
Measurement
Thermal
ions
and electrons
AC Electric
and
magnetic
fields
DC
magnetic
field
Solar
and
cosmic-ray
particles
Neutral
hydrogen and
atoms
Specifications
Mass
kg
Size
cm x cm x cm
Power
W
Telemetry
kb/s
6
20 x 20 x 20
5
5
3
20 x 20 x 20
3
4
1
10 x 10 x10
1
0.5
Ions
and
electrons
0.01-10 MeV
4
20x 20 x20
3
2.5
1-100 keV
2
10 x 10 x 10
2
0.5
0-30 keV/Q;
0-10 keV
V/m - V/m
0.1nT - T
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Dust Detector
DUD
Radio
Spectrometer
RAS
Coronal Radio
Sounding
CRS
Interplanetary
dust particles
Coronal radio
waves
Wind density
and velocity
Mass (g):
10-16 - 10-6
100 kHz 1 GHz
X-band
Ka-band
2
10 x 10 x 10
1
0.5
4
10 x 10 x 10
antennae TBD
5x5x5
10
1
3
0
(USO
200g)
4.3 Solar instrumentation
4.3.1 General considerations
An integrated ensemble of optical instruments is suggested for SO which are combined in a
package involving several teams of scientists. Emphasis is put here on the scientific
requirements and on a demonstration of feasible designs. To save cost and simplify the
instrumentation, the telescopes could perhaps share parts of the mechanical low-mass
structure, thus enforcing a coordinated fine pointing by construction. All the imagers must
aim at high spatial (100 km or better) and temporal (1 s) resolution, to resolve the small-scale
dynamical processes in the solar atmosphere and to study the rapid changes in morphology
associated with coronal magnetic activity.
To understand what SO's proximity to the Sun implies for high-resolution solar imaging, just
consider a 25-cm diffraction-limited telescope (at about 5000 Å), which has an angular
resolution element of about 0.25 arcsec, corresponding to 180 km on the solar disk when
observed from 1 AU. From 45 Rs the same telescope would yield a resolution that is
enhanced by a factor of 4.8, corresponding to only 37 km on the Sun. Therefore, a significant
order-of-magnitude improvement of the spatial resolution is to be expected from the
telescopes on SO, in comparison with large Earth-based solar telescopes at best seeing
conditions. Telescopes with mirrors of different sizes (to achieve the same resolution at the
various wavelengths) could be arranged in a common structure such that the smaller EUV
and X-ray mirrors surround the larger visible-light mirrors. Critical issues like heat ingress into
the telescopes, aperture locations, mechanical structures of the instruments, and their
alignment have to be evaluated in detail in a future assessment study. Given the existing
analysis of the thermal load problem (Pre-Assessment Study Report, 1999) for the SO, we
do not expect serious problems for the instruments.
4.3.2 Visible-light imager and magnetograph (VIM)
The purpose of the VIM is to give the photospheric or lower-chromospheric context for the
coronal imagers. It will observe the morphology, dynamics, and strength of the magnetic
elements and flux tubes at the photospheric level with a resolution that is consistent with the
resolution of the EUV telescopes. It will also provide images, Dopplergrams and
magnetograms (e.g., Figure 4.1) from the side of the Sun which is invisible from Earth at a
low cadence throughout the distant orbit phases. During the perihelion phases, the VIM will
take data at high cadence (15 s to 30 s time resolution) for limited time in co-ordination with
the other on-disk remote sensing instruments to study fast phenomena with high resolution.
The instrument will be capable of automatically detecting targets of opportunity (e.g., onsets
of flares or other events of interest). During other phases of the orbit, the large Sun-S/C
relative velocity will be used in connection with the narrow-band filter to obtain precise
absolute calibration of the magnetic and Doppler signals as well as maps of the linear
polarizations in different parts of the Na D1 and D2 line profiles.
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A Magneto-Optical Filter (MOF) is used here as the baseline for the VIM, although other
narrow-band filter devices may also be considered. The MOF is a narrow double-band filter,
which is made of a sealed glass cell containing a metal vapour and transmits the red and/or
the blue side of a given photospheric/chromospheric line with intrinsic wavelength stability. In
addition to the high stability, its second most important characteristic is that the two bands
have orthogonal polarizations, so that they can be selected at will. Even if, in principle, any
Zeeman-sensitive line can be adopted, in practice only the Sodium and Potassium
resonance lines have been used so far. We propose to use the MOF with the Na D lines. The
MOF is capable of acquiring simultaneously intensity, Doppler, and magnetic field maps at
video rates. Two modulators, one in front and one behind the MOF, produce the desired
Doppler and magnetic information in the video signal. The signal is sent in parallel to three
image processors, which produce simultaneously the Doppler, magnetic and line-intensity
images. Using a suitable detector and integration time, the noise level of the global Doppler
signal integrated over the whole solar disk, is 1 cm/s in less then 30 s.
Figure 4.1: Global images of the Sun taken with an MOF-type instrument. From left to right:
Sodium Doppler image, line intensity image and Stokes-V image (U and Q images are also
attainable; courtesy, A. Cacciani, 1999).
The MOF does not require mechanisms for operation, has a mass as low as 2 kg, and
dimensions as small as 15 x 15 x 25 cm 3. The wavelength stability is assured by an intrinsic
wavelength reference. Temperature drifts can hardly change the wavelength reference. A
temperature variation of 1 K simulates a velocity signal of 10 cm/s, therefore the MOF is fairly
insensitive to temperature. The MOF life time depends on the glass cell that contains the
sodium vapour. The MOF has not been tested in space, but on the ground such an
instrument has been operating since 1995 without problems. Spare glass cells may be flown
as part of the payload.
Figure 4.2: Sketch of the VIM telescope optics. Left: High-resolution telescope, right: full-disk
zoom refractor. The filtergraph is located between the collimator/camera lenses in the lower
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part of the figures and not shown. This part is the same for both telescopes. The detector is
located at the image plane in the lower left corner in both cases.
The MOV is fed by two telescopes which are used alternatively. A 25-cm-diameter Gregorian
reflector provides the high-resolution image, with a spatial resolution as high as 37 km on the
solar surface during the near-Sun orbital phase. A 6-cm-singlet refractor with a variable focal
length provides a full-disk image during all orbital phases. This telescope uses either a zoom
lens or a revolver for the objective to adjust its focal length according to the Sun-S/C
distance. A mechanism with a flat mirror selects either of the two telescopes for the
filtergraph. Both telescopes are equipped with a broad-band interference filter at the entrance
to limit the incoming light flux to the desired spectral regime. The optics of the reflector will
be made from SiC. The structure will be either a CF or a SiC structure. The basic design
parameters of the instrument are given in the subsequent Table 4.2.
Table 4.2 Parameters for the Visible Imager / Magnetograph (VIM)
Telescope
Spatial resolution element (per pixel)
Raster Mechanism
Detector
Operational Wavelength
Mechanisms
Telemetry
Mass
Power
Size
Thermal
I: Gregorian type; 250 mm dia. feff 4400 mm
II: Singlet refractor; 60 mm dia. feff variable
I: 0.25 arcsec (37 km on Sun at 0.2 AU)
II: variable, to fit solar disk on detector
I: Through motion of the secondary
II: none
Back thinned CCD; 9 micron pixels; 2048 x 2048 array.
Na D1 / D2 589.0 and 589.3 nm
1: beam switching mirror
2: telescope II focal length
20 kbit/s
35 kg
30 W
Instrument = 120 cm x 40 cm x 30 cm
Operating temperature 20oC with passively cooled (-80oC,
radiator to space) CCD.
4.3.3 EUV spectrometer (EUS)
Observations of the EUV spectral range allow plasma diagnostics in the solar atmosphere
across the broad temperature range from tens of thousands to several million Kelvin.
Analysis of the emission lines from trace elements in the Sun’s atmosphere can provide
information on plasma density, temperature, element/ion abundances, flow speeds and the
structure and evolution of atmospheric phenomena. Current spacecraft instrumentation
provides EUV spatial and spectral resolution elements of order 1-2 arcsec and 0.05 Å,
respectively.
The basic design parameters of the EUS are given in the subsequent Table 4.3. The
heritage of this instrument comes from the SOHO/CDS and Solar-B/EIS projects. We
envisage a semi-synoptic operation - i.e. a basic set of operation modes which are run in a
predefined sequence or over long periods. Day-to-day planning will be minimised. The
instrument requires a spacecraft pointing stability of the order of 1 arcsec/15min, with
absolute pointing to better than 2 arcmin.
The proximity of SO to the Sun means that we can size the instrument to an observing
distance of, say, 0.2 AU. A spatial resolving element of 0.1 arcsec at 1 AU would represent
75 km on the Sun; the same resolution can be achieved by 0.5 arcsec at 0.2 AU. With regard
to spectral resolution, our basic aim is to return a sufficient number of spectral lines to allow
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thorough spectroscopic studies, whilst recognising the limitations of the telemetry. We
anticipate using a CCD-based detector system with 9 micron pixels in an array of 4k x 4k.
Thus, we would expect to return a spectral range of, say, 40 Å at 0.01 Å/pixel. The same
array will give a spatial extent (vertical distance on the detector = slit length) of 0.5 arcsec x
4096 = 2048 arcsec = 34 arcmin. The solar diameter at 0.2 AU is 160 arcmin, i.e. we have a
slit length of 0.2 of the solar diameter. Thus, a pointing mechanism is required. For a given
pointing location, rastered imaging will be made up from movement in only one direction. The
telemetry limitation requires that most observations will demand selection of data from the
given wavelength range and from the full slit length prior to being returned to Earth.
Table 4.3 Parameters for the EUV spectrometer (EUS)
Telescope
Spatial resolution element
Spectral resolution element
Raster Mechanism
Detector
Initial Wavelength Selection
Slit Length/Width
Field of View/Pointing
Telemetry
Mass
Power
Size
Thermal
Ritchey-Chretien type; 120 mm diameter
0.5 arcsec (75 km on Sun at 0.2 AU)
0.01 Å/pixel (5 km/s)
Through motion of the secondary
Back thinned EUV sensitised CCD; 9 micron pixels; 4096
x 4096 array.
580-620 Å (e.g., 1st and 2nd order Fe XII 291, Si X 293,
He I 584, Si IX 296, O III 599, He II 304, Mg X 610).
34 arcmin length; width TBD
Raster over 34 arcmin; pointing to anywhere on the Sun
20 kbit/s
35 kg
30 W
Instrument = 140 cm x 15 cm x 30 cm
Operating temperature 20oC with passively cooled (-80oC,
radiator to space) CCD.
The instrument structure could be made of carbon fibre, with silicon carbide optical
components. Multi-layers will be considered if the final wavelength selection requires it. The
Ritchey-Chretien telescope design was chosen here to minimise the size of the instrument,
yet alternative designs could be considered. Assuming a spectrograph magnification of 1.5
and given the resolving elements, band width and field of view stated above, the telescope
has an effective focal length of 2.48 m. We assume a telescope magnification of about 3,
giving a physical focal length of 0.83 m. Including the spectrometer will make the total
instrument envelope approximately 140 cm x 15 cm x 30 cm. The associated electronics box
is 30 cm x 20 cm x 20 cm.
4.3.4 EUV imager and X-ray imager (EXI)
The EXI should combine virtues of the SOHO, NIXT and Yohkoh, or the TRACE imaging
instruments. The images in various selected EUV and X-ray emission lines, corresponding to
temperatures between 40000 K in the lower transition region of the Sun and a few million
Kelvin in the corona, will allow studies of the two-dimensional morphology of magnetic
structures on the disk and off the limb. A bundle of telescopes with small appropriately
coated primary mirrors, suitably arranged in accord with their different sizes, is envisioned for
the SO mission. To keep the instrumentation small, a Cassegrain design is suggested for the
individual telescopes. There will be at least one telescope capable of delivering full-Sun
images routinely, whereas the others will concentrate on selectable target areas on the solar
disk at high-resolution and with limited fields of view.
At a wavelength of 300 Å, a mirror of only 3 cm diameter would theoretically give an angular
resolution of 0.2 arcsec at the diffraction limit. Thus, even with comparatively tiny mirrors one
could easily achieve a spatial resolution of 30 km at perihelion. The ideal mirror sizes for the
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EXI still need to be determined. Multilayer coatings and dimensions of the mirrors and
spectroscopic capabilities and spectral resolution of the instruments will depend on the lines
selected. Within a broad pass band, a narrow wavelength window can for example be
selected for spectroscopy by a pair of identically-coated movable flat mirrors like in a doublecrystal monochromator, yielding a tunable imager without image motion in the focal plane
(Golub et al., 1997). The scientifically optimal selection is left open and should be made in
connection with future detailed designs. The detectors employed may be coated
microchannel plates currently in use, providing photon detection and signal amplification,
combined with a multilayer cross-delay-line anode or a CCD (with glass-fibre optics for image
adaptation) accomplishing electronic readout. Detector technology in this field (in particular
for soft X-rays) is advancing rapidly and low-mass detectors dedicated to the specific
bandpasses will be available soon.
4.3.5 Ultraviolet and visible-light coronagraph (UVC)
The main objective of the UVC is to image and diagnose the structures of the dynamic
corona in EUV and white light on the relevant scales. Full-corona images are required,
because the Sun's side seen by the spacecraft during the perihelion passage will not be
visible from Earth. Furthermore, context images of the corona are necessary for the
simultaneous analysis with the EXI of features observed on the disc, such as sunspots,
filaments and active regions.
For UVC we propose an innovative design: a Hydrogen-Helium-Coronagraph instrument
for imaging the corona in the light of its most abundant elements. After hydrogen, helium is
the second largest contributor to the density of coronal plasma, making it also important for
the dynamics of solar wind acceleration. Particle flux measurements at 1 AU show variable
helium abundances ranging from 1% to 30% (in CMEs), with typical values of 4% (quite
constant) in high-speed flows and of 2% (highly variable) in the slow solar wind. The overall
low abundance of helium is most easily attributed to its high First Ionization Potential (FIP),
suggesting fractionation in the chromosphere, but the abundance variability is an open
question.
The UVC will allow the determination of the H and He abundances in the corona and of their
flow speeds in the solar wind as well as the measurement of the electron density in the
extended corona between 1.1 and 2.6 solar radii. The coronagraph will also be able to
measure the helium outflow velocity and abundance in polar coronal holes. It will allow the
first determination of the absolute abundance (i.e., relative to hydrogen) of helium in solar
corona. The UVC encompasses an extreme ultraviolet channel for the observation of the Ly
 emission lines H I 1216 Å and He II 304 Å, and a broad-band visible-light channel. For the
first time, the UVC will image, using narrow-band transmission filters, the entire corona in
these two lines simultaneously. It thus extends the UVCS/SOHO capabilities both
quantitatively and qualitatively.
UVC is an externally occulted telescope designed for narrow-band imaging of the EUV
corona (the He II 304 Å line with an Al-polyimide filter, and the H I 1216 Å line with an
interference filter), and for broad-band polarisation imaging of the visible K-corona
(polarimeter channel from 4500 Å - 6000 Å). The visible-light channel includes an achromatic
polarimeter, based on electro-optically modulated liquid crystals, with no moving parts. The
telescope optical configuration could be a two-reflection Gregorian, giving real images of the
occulter and the edge of the telescope's primary mirror (blocked with stops). The external
occulter ensures good stray-light rejection at shorter wavelengths and simplifies the thermal
design of the UVC. The field of view during the perihelion passage may range from about 1.1
to 2.6 solar radii, with a spatial resolution of 20-30 arcsec. The mirrors with coatings
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optimised for 304 Å (e.g., Mo/Si multilayer coating) still have good reflectivity at 1216 Å and
in the visible. Two detectors are foreseen: an ICCD with an EUV photo-cathode and a CCD
for the visible band.
4.3.6 Neutron and -ray detector (NGD)
The SO provides a unique opportunity to measure many, and also low-energy (few MeV),
neutrons in the perihelion passages. Although the simultaneously produced  rays have
been measured frequently on other spacecraft, co-ordinated measurements are possible in
scintillation detectors via pulse-height and shape analysis of the signals. Proton-recoil
spectrometers or inorganic scintillators allow neutrons to be detected in the energy range
between 0.5 and 100 MeV and  rays between 0.1 and 10 MeV, with a large dynamic range,
energy resolution of a few percent, and time resolution in the s range. A serious problem is
the large mass of several kg of conventional detectors. Mass reduction to less than about 1
kg is technologically feasible (e.g. with a fission detector as used on Pioneers 10/11). For
example, McKibben et al. (1997) have suggested a low-mass design based on detection of
neutron-induced fission of heavy nuclei. Their instrument will allow measurements down to
very low neutron fluxes for a mass of 1-2 kg and power less than 2 W.
4.3.7 Radiometer (RAD)
On SO the mass will be a crucial limitation, which implies a focus of the RAD on the main
science goal: measuring the solar constant only. This task has to be performed with high
precision and accuracy, otherwise the scientific objective cannot be met. Three different
versions with increasing instrument capability are conceived:
1) Basic equipment includes only one type of radiometer (e.g. a next generation active cavity
radiometer of SOHO heritage). For the time the S/C is closer than 0.6 AU to the Sun, the
observations need either be interrupted or continued with lower accuracy.
2) Enhanced equipment includes, in addition to 1), a second radiometer of different type to
guarantee absolute accuracy.
3) Sufficient equipment for full-duration solar observations requires an additional "high
irradiance" radiometer that can measure up to 25 times the solar irradiance at 1 AU. No
experience exists as to how accurate such instruments can be. But we anticipate, that when
having an in-flight calibration against standard radiometers, one might achieve a value <10 -4
in relative precision. A flight instrument for SO will be a compromise between these three
options, and substantial efforts have to be made to reduce mass where possible.
4.3.8 Other potential solar instruments
A valid option to be considered in future studies of the SO is the addition of three coronarelated instruments:

A wide-angle white-light instrument with a FOV from 1.5 to 6 Rs, equivalent to the
LASCO-C2 on SOHO, would allow one to see extended coronal structures and evolving
CMEs. It would cover the cusp regions of helmet streamers, where the slow solar wind is
released and begins to accelerate. The highly successful LASCO-C3 instrument (FOV
from 3 to 32 Rs at 1 AU) could serve as a design baseline. The required mass should not
exceed 4 kg (assuming common electronic systems with the other imagers).

A heliospheric imager might extend the view of the heliosphere to almost a full
hemisphere, from about 3 Rs (at 0.2 AU) to as much as the anti-solar direction. Thus, it
would have almost the full Earth-Sun line within its FOV for major parts of the observation
time, depending on the spacecraft attitude. This instrument would allow for the first time
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24
to observe the propagation of transient disturbances towards the Earth from various
distances and aspect angles, in particular from outside the ecliptic plane. Preliminary
design studies have demonstrated that such an instrument can probably be built for less
than 2 kg. A crucial requirement would be that the hemisphere to be watched must be
kept free of Sun-illuminated S/C appendages.

An X-ray instrument dedicated to the high-temperature portion of the coronal emission
could excellently complement the SO payload. A novel configuration for an imaging
spectroscopic instrument would operate in the wavelength range from 40 Å to 6 Å and
detect emissions from plasma at temperatures between a few 10 6 K and a few 107 K.
Such an instrument working in extremely-grazing incidence will obtain very high spectral
and spatial resolution. It would also be able to measure densities equal to or greater than
109 cm-3 (through density-sensitive lines in this high-temperature range).
Table 4.4: Solar Instrumentation (*depth x width x length)
Name
Visible-light
Imager and
Magnetograph
EUV Imager
and
Spectrometer
X-ray / EUV
Imager
Ultraviolet and
Visible Light
Coronagraph
Neutron and
-ray Detector
Radiometer
Abbreviation
VIM
EUS
EXI
UVC
NGD
RAD
Measurement
Specifications
High-res. disk
imaging and
polarimetry
Imaging
and
diagnostics of
TR and corona
Na D1
Na D2
EUV
emission
lines
Corona imaging
He and Fe
Ion lines
Imaging
and Coated mirror
diagnostics
of coronagraph
the corona
CCD detector
Solar neutrons
0.5-100 MeV
gamma rays
0.1-10 MeV
Solar constant
Visible light
Mass
kg
Size
cm x cm x cm
Power
W
Telemetry
kb/s
35
30 x 40 x 120
30
20
35
30 x 15 x 140
30
20
15
10 x 10 x 30
5
5
25
20 x 20 x 50
10
10
1
10 x 10 x 10
2
0.5
4
15 x 15 x 25
5
0.5
5. Spacecraft Design Concepts
5.1 Payload complement effect on mission and spacecraft system
The spectrometer and imager package requires a three-axis stabilised platform. These
instruments have to point to the Sun with a very high pointing stability (1 arcsec/15 min)
during science data acquisition. Some of the instruments may have to provide their highpointing stability by independent internal mechanisms, such as by piezoelectric devices. The
absolute pointing accuracy requirement satisfying the needs of all instruments is better than
2 arcmin. The instruments have to be protected from Sun illumination (except for the
telescope apertures) and the instrument thermal system has to ensure moderate
temperatures. Due to the high data rate during scientific operation, the spacecraft must
provide sufficient storage memory to ensure three 10-day periods of operation per orbit for
the high-data-rate experiments (at perihelion, northern maximum latitude and southern
maximum latitude). The low-data-rate instruments shall be operable over the full orbit
(pending occasional data storage and downlink limitations). The communication system has
to be able to ensure the downlinking of these data to Earth.
All deployment mechanisms and deployment monitoring devices (pyros, deployment
mechanisms/sensors and motors) are part of the instruments. The achievement of a
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magnetic cleanliness compatible with the magnetometer sensitivity requirement has to be
studied in detail in the next phase. The thermal design of the instruments is part of the
respective designs. The spacecraft system will ensure that the instruments are protected
from Sun illumination. The power interface consists of a single 28 V regulated bus provided
to the experiments. Power conditioning (including the generation of additional voltages) is
under the responsibility of the experiments. The commanding, the control and the acquisition
of the scientific data is performed via direct digital I/O lines, analogue I/O lines and by a serial
data bus, being all connected to the spacecraft data handling system.
5.2 Science accommodation
The optical science payload will be accommodated behind the front heat shield. The present
scientific instrument accommodation is illustrated in the Pre-Assessment Study Report. The
magnetometer will need to have short booms. The plasma wave electric antennae should be
composed of a refractory material that will permit permanent exposure during the perihelion
phase. The spacecraft will be 3-axis stabilised and always Sun-pointing, except during SEP
firing, with one axis directed towards the Sun. A star sensor system is required. Gyro
references will minimise the risk of using star trackers in a possibly dusty environment near
perihelion. Hydrazine thrusters will be used to control the attitude. We have considered
several DPUs for the instrument packages, also for the reason of avoiding loosing the whole
payload if a single DPU would fail, and because the data of the various instruments will be
very different in character and amount. This DPU splitting seems to be a necessary and
efficient option. Constraints on view angles of the particle instruments and their location on
the bus are still to be discussed and will be optimised in future studies.
5.3 Power supply
Power is supplied by two sets of solar arrays (SAs). The cruise array consists of two wings
(2 panels each) that are based on standard telecom satellite technology. These wings have
thermal shields to protect the edges when rotated edge-on to the Sun, and are jettisoned
along with their array drives after last firing of the SEP. The two wings (with 2 panels each)
for the orbiter array are essentially a new development. Both SAs are based on standard
GaAs cell technology.
6. Mission Requirements and Orbit Design
6.1 Mission requirements
The main scientific requirements for the orbit are: a low perihelion allowing corotation or
heliosynchronous phases; possibly high inclination with respect to the solar equator, and an
aphelion not higher than the Venus heliocentric distance. A ballistic trajectory design
satisfying these requirements leads to a high v and/or long transfer times. The requirements
can only be met if low-thrust propulsion and gravity-assisted encounters with planets are
used. This was the option chosen for the SO, and therefore the orbital design closely follows
the Mercury Orbiter trajectory design. The mission requires solar electric propulsion (SEP)
with plasma thrusters. With 0.3 N thrust by SEP (specific impulse: 2100 s) one obtains: Total
v = 4.77 km/s (nominal mission) with a total thrust time of 6033 h. The orbit is resonant (2:3)
with Venus so that each swing-by increases the inclination of the orbit. With this baseline
orbit design, it needs only 1.86 y to reach an acceptable science orbit (0.89 x 0.21 AU), with
perihelia near 45 Rs, an orbit period of 149 days, and an inclination ranging from 6.7° to
23.4° on the ecliptic (13.2° to 30.0° highest latitude with respect to Sun). During the extended
mission the inclination to the ecliptic increases up to 31.7°. A maximum heliographic latitude
of 38.3° will be achieved.
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6.2 Orbit design
The trajectory is composed of three phases: Cruise phase, starting at spacecraft separation
from the launcher and ending at start of scientific operations (some science will also be
performed during the cruise phase). Nominal phase, during which the scientific mission is
performed. Extended phase, in which the mission is prolonged and further gravity-assisted
manoeuvres allow the S/C to reach a high orbital inclination. During the cruise phase (0 1.86 y) there will be five thrust phases, with duration ranging from 6 to 105 days. Perihelion
passages are at 0.33 AU (thrust phase) and 0.25 AU. Venus swing-bys are needed for
changing the semi-major axis and inclination increase of the orbit.
During the nominal mission (1.86 - 4.74 y, duration: 2.88 y), there are 2 Venus swing-bys
for inclination increase during 7 orbits. During the extended mission (4.74 - 7.01 y, duration:
2.28 y), there are 2 Venus swing-bys for inclination increase during 6 orbits. The ecliptic
projection of the Solar Orbiter trajectory is shown in Figure 6.1. The maximum orbital rate at
perihelion is 13.1 °/day, enabling near-synchronous observations (siderial rotation rate of the
Sun: 14.4 °/day). The orbital design offers seven perihelion passages at low differential
rotation between SO and Sun for about 10 days, and seven maximum northern and southern
latitude viewing-periods of 10 days each. The perihelion radius as a function of flight time is
shown in Figure 6.2a, and the solar latitude (in degrees) with respect to the solar equator as
a function of flight time in days after launch is shown in Figure 6.2b.
Figure 6.1 The ecliptic projection of the Solar Orbiter trajectory, indicating the
important phases and milestones of the mission.
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Figure 6.2a Perihelion radius as a function of flight time in days after launch.
Figure 6.2b Solar latitude with respect to the solar equator as a function of flight time.
Table 6.2: Extended Mission Orbit Characteristics
Orbit
Maximum solar latitude
Perihelion (AU)
Orbital rate at perihelion
8, 9
30°
0.27
8.5 (°/day)
10, 11, 12
35°
0.32
6.5
13
38°
0.36
5.3
Table 6.1: Nominal Mission Orbit Characteristics
Orbit
Maximum solar latitude
Perihelion (AU)
Orbital rate at perihelion
1, 2, 3
13°
0.21
13.1 (°/day)
27
4, 5, 6
22°
0.23
10.9
7
30°
0.27
8.5
28
7. Telecommunications, Science Operations and Archiving
7.1 Telecommunication
Telemetry (TM) and telecommunications are provided by X-band LGAs (omni coverage, 20
W), and for data dump by a 2-axis steerable Ka-band HGA, 1.5 m diameter and 20 W
(Mercury Orbiter heritage). There exists a well defined data transmission strategy to Earth.
The transmission duration and S/C distance from Earth are variable with the SO orbits. The
high data-acquisition rate is 75 kb/s, the low one 11.5 kb/s when the optical instruments are
in stand-by mode. Near perihelion a high-rate data acquisition is foreseen with an on-board
data storage by a 240 Gb memory. Data are dumped to ground when the distance from
Sun is >0.5 AU. The observation strategy is illustrated in Figure 7.1, showing the three 10day long observation periods.
Figure 7.1 Modes of operation during nominal and extended phases of the mission.
7.2 Spacecraft and payload operations
The operational scenario for the in-situ measurements and vantage points of solar
observations is illustrated in Figure 7.1. The cruise phase will be used for in-situ particles
and fields measurements as much as possible, given that there are frequent phases when
the SEP system is used and cruise science is rendered impossible. Functional tests of the
optical part of the payload have to be performed during the cruise phase to ensure that these
instruments are fully operational during the primary perihelion passage. The data rates and
volume of the data are given in the Tables 4.1 and 4.4. The instruments are expected to
operate autonomously, but largely according to predefined observational strategies and not
in an interactive way, which is made impossible anyway when the S/C is in solar conjunction.
This type of operations demands capable and independent on-board processing units for the
imagers and spectrometer. Concerning the issues of
 instrument control and operational modes,
 command strategy and rate,
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 typical time between commanding and reconfiguration of instruments,
the wide experience gained from the SOHO operations is alive in the community and can be
exploited for SO. These aspects will be discussed in more detail during the Phase-A study.
7.3 Instrument ground segments and data processing
Concerning data processing aspects, ample experience exists in the community from
missions such as Yohkoh, SOHO and TRACE, and therefore we are sure that these points
will be adequately handled by the scientists involved. The SOHO EOF at GSFC may serve
as a paradigm for the requirements and solutions for quick look data evaluation and real-time
operations, the main difference being that fast interactive-type operations of the optical
instruments or rapid reactions to on-orbit events are excluded. Instrument ground support
equipment and software will be provided, and utilised throughout the mission by the
individual PI-teams. The hardware, software and manpower for mission support and data
analysis will be funded by the PI-teams and financed by the national agencies.
7.4 Science operations and archiving
The science operations are expected to be performed by ESA. We anticipate an operation
facility and full data archive in Europe. It is proposed that the science programme should
support the archive, which could be located, e.g., at ESTEC in Noordwijk or at ESOC in
Darmstadt. Also, the MEDOC for SOHO at Orsay near Paris in France could be continued
and used for the Solar Orbiter.
The SO mission must give European solar and heliospheric science clear visibility, a political
reason that demands for the operations centre to be located in an ESA member state. We
therefore highly recommend that use of the existing facilities and technologies in Europe is
made. The expected volume and format of the data coming from the various instruments on
SO will be comparable to what was received from, e.g., SOHO and Ulysses, and thus ample
experience exists in the community involved on how to handle and archive such data.
Preference is given to a full mission data centre rather than separate PI-data centres.
The mission exploitation must not be restricted to the instrument PI teams, but the data
should be made public and accessible to a larger scientific community, a procedure which
has been shown by the SOHO community to be largely successful and would ensure that the
best possible use of the data is made. Other agencies if contributing to the mission should
also be involved.
The science operation centre will archive all science data together with any instrument data
and software necessary for data analysis. For SO, we anticipate the downlink capability of
more than 70 kb/s, corresponding to a maximum of compressed data per day of about 10
Gb. The data would be stored on remotely retrievable systems accessible via the web to the
user community. This kind of approach is already in use, e.g. for the SOHO archives. The
data should be accessible as images or timelines in standard data formats such as FITS
files. The data would be accessible to anyone, the only requirement being to register on entry
to the user web site.
8. Technological Development Requirement
No critical technological developments are required. The SO strongly relies on and benefits
from technology developed for the Mercury Cornerstone mission. In particular, SEP in
connection with Venus gravity-assisted manoeuvres is used and powered by large cruise
SAs. The thermal shield protecting the S/C bus is based on standard MLI. Radiation
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protection of sensitive electronic components is provided by Al shielding. More technical
details can be found in the SO Pre-Assessment Study (ESA, 1999)
9. Management and Funding
The payload provision and funding would rest with the PI and Co-I institutes being supported
by their national funding agencies. There is no doubt that sufficient scientific interest, special
capabilities and hardware experience has been built up in Europe. From past missions many
successful collaborative arrangements exist, which could be strengthened and new ones
established.
10. Communication and Outreach
The SO will explore unknown territories of the inner solar system in situ and deliver
unparalleled images of the Sun from a close vantage point. As we have learnt from SOHO
experience, the fascination of such material to the general public should not be
underestimated. Therefore, images and other science results will be made public as much as
possible and meaningful. We propose that all data be processed as they are received and be
distributed over public data networks. Thus, through the SO, the Sun our star, and such
issues as space weather will receive a wide publicity. SO will raise the public’s understanding
of the role of our Sun and appreciating the role of space technology in modern civilisation.
11. International Partners
The Solar Orbiter mission is a further development of the InterHelios concept (Marsch et al.,
1997; Kogan, 1997), which was presented at the ESA Conference “A Crossroads for
European Solar and Heliospheric Physics” in March 1998 in Tenerife and unanimously
supported by the participants. Additional mission scenarios, including polar orbits and closer
approaches to the Sun, were then also discussed but had to be discarded for technical
reasons. Because of the potential risks and technology developments required, the ESA
Solar Physics Planning Group (SPPG) asked ESA to conduct a pre-assessment study. The
results were presented at ESTEC on September 29, 1999, and are very encouraging. This
Solar Orbiter study is published as an ESA Pre-Assessment Study (1999) and forms the
basis of the present proposal, which has been discussed by the ESA Solar Physics
Planning Group and is one of the two proposals endorsed by this group.
In the spirit of the ongoing international solar-terrestrial physics programme (Ulysses,
Yohkoh, SOHO, TRACE) it is highly desirable from scientific and monetary points of view
that SO is at the outset made an international effort involving partners, such as NASA,
ISAS, RSA or national European agencies. Given the present cost estimates of the SO, such
partners are indeed needed. Interest of international partners has been indicated in a
preliminary way and by individuals and institutes only. Official negotiations between agencies
are encouraged by the proposing team.
12. Cost Estimate
The ESA F-mission cost cap is 176 M Euro, at 1999 economic conditions. This must include
costs of ESA manpower and services, the spacecraft development and build, the launch and
science operations, but does not include the cost of any instrumentation, which is supplied by
ESA member states (or other agencies). Some of the cost estimates are based on the Mars
Express figures presented by ESA at the F-mission briefing meeting at ESTEC. It is difficult
to modify these costs for the SO mission without further information from ESA. The
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spacecraft operations costs are also difficult to estimate without input from ESA (i.e., costs
for ground stations, deep space network antennae, etc.). The science operations costs for
Mars Express are 1.5 M Euro. A solar mission involves more active planning and uplink
activities throughout the mission and, thus we expect higher cost for the SO science
operations.
Cost item
ESA project management
Technical support
Spacecraft
Launch
Spacecraft operations
Science operations
ESA overheads
TOTAL
M Euro
16
4
80
30
30
6
4
170
Comment
Based on Mars express
Based on Mars express
Estimate
Estimate
S/C to operate for 7 years
Nominal plus extended mission
Publicity, education etc.
13. References
Axford, W.I., McKenzie, J.F. 1997, in “Cosmic Winds and the Heliosphere”, J.R. Jokipii, C.P.
Sonett, and M. S. Giampapa (Eds.), The University of Arizona Press, Tucson, USA, 31
Antonucci, E., et al. 1997, ASP Conference Series 118, 273
ESA, Cesar Team 1999, ESA Pre-Assessment Study Solar Orbiter, Report of the Final
Presentation, ESTEC, The Netherlands, 29-30 September, Report-CDF-02(A)
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14. Mission and Spacecraft Summary
Scientific
Objectives
View of the Sun from an out-of-ecliptic, near-Sun orbit and do
· Spectroscopy and imaging at high spatial and temporal resolution
· In-situ sampling of particles and fields from a quasi-corotational perspective
· Remote-sensing of the polar regions of the Sun
Payload
Instruments totalling 137 kg, consuming 110 W, requiring 70 kb/s telemetry rate
Heliospheric instrumentation - Particles and fields experiments:
– Solar Wind Analyser
– Plasma Wave Analyser
– Magnetometer
– Energetic Particles Detector
– Neutral Particle Detector
– Dust Detector
– Radio Spectrometer
– Coronal Radio Sounding
Solar instrumentation - Imagers and spectrometers:
– Visible light Imager and Magnetograph
– EUV Imager and Spectrometer
– EUV/X-Ray Imager
– Ultraviolet and Visible Light Coronagraph
– Neutron and -ray Detector
– Radiometer
Launcher
Dedicated launch with Soyuz-LV Fregat (launcher payload 1560 kg). Preferred
launch site is Kourou.
Spacecraft
· Design lifetime = 4.74 y, consumables sized for 7.01 y (extended mission)
· Total mass = 1510kg
· Main S/C bus: 3000 mm x 1200 mm x 1600 mm.
· 3-axis stabilised
· Solar electric propulsion system: 4 x 0.15 N stationary plasma thrusters.
· Pointing stability better than 3 arcsec/15min
· Deployable and rotatable cruise solar arrays, total 28 m²; GaAs cells jettisoned
after last SEP thrusting
· Deployable and tiltable orbit solar arrays, total 10 m2 , 16 % GaAs cells, 84 % OSR.
· 5.8m magnetometer boom shielded by the spacecraft body
· X-band LGAs, omni coverage, for TT&C.
· Ka-band HGA, 1.5m diameter, for telemetry after cruise.
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