Download Magnetic coupling in the solar system

Davis et al.: Magnetic coupling
1: An image from the Heliospheric Imager
on the STEREO-A spacecraft, taken on 5
November 2007, showing a coronal mass
ejection entering the frame from the right,
the Milky Way and Jupiter to the left. (NASA)
Magnetic coupling
in the solar system
Abstract
On 10 October 2008, UK solar, solar–
terrestrial and planetary scientists hosted
a meeting at the RAS on the theme of
“Magnetic coupling in the solar system
– from the Sun into the heliosphere”.
This meeting was proposed in order
to stimulate discussion on how the
variations of the solar magnetic field
influence other solar system bodies such
as comets, planets (including our own)
and their moons. Energy from within the
convection zone is transferred through
magnetic coupling into the corona
and the heliosphere beyond on a wide
range of spatial and temporal scales
A&G • April 2009 • Vol. 50 that are related to the different types of
magnetic field emergence at the solar
surface. Thus in order to understand
the global heliospheric magnetic field
and its influence throughout the solar
system, we have to understand the role
that flux emergence on all scales plays
in structuring in the global field and the
associated solar wind. With the successful
launch of two major new space-based
missions, Hinode and STEREO (Solar
Terrestrial Relations Observatory),
scientists are well placed to study the
emergence of the Sun’s magnetic field and
track it into the heliosphere.
Sarah Matthews, Danielle
Bewsher and Chris Davis discuss
the complexities of the solar
magnetic field, informed by data
from Hinode and STEREO.
T
he Sun’s magnetic field is carried far
away from the solar surface by the solar
wind, out to the edge of the heliosphere
and the interface with the galactic neighbourhood. The environment between the Sun and the
outer planets is thus dominated by the processes
occurring on the Sun that shape the heliospheric
magnetic field, and our understanding of the
evolution of planetary magnetospheres and
atmospheres within our own and other solar
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Davis et al.: Magnetic coupling
systems relies heavily on our ability to understand the whole coupled system. At the outer
edges of the heliosphere the galactic neighbourhood exerts the controlling influence. Studies of
solar system physics thus have the potential to
extend our knowledge of the universe far beyond
the sum of its constituent parts, provided that
we can make the connections between the physics occurring in different parts of the system.
The ultimate source of all solar activity and
its subsequent impacts within the solar system
is of course the Sun’s magnetic field. There are
still many areas in which our knowledge of the
origin and evolution of the solar magnetic field
throughout the solar cycle is incomplete, and
the meeting began with a summary of observations of solar activity deduced from sunspot
activity (figure 2) in cycles 21–23 by Valentina
Zharkova (University of Bradford).
The evolution of sunspots through the solar
activity cycle is well known, but recent work has
focused on the asymmetries between the two
hemispheres. Observations show that the background magnetic field changes through the solar
cycle and there appears to be a “royal zone” – a
restricted latitude region where magnetic flux
can emerge as sunspots. The combined area of
these sunspots differs between the northern and
southern hemispheres and the same is true for
the combined area of active regions. By defining
an asymmetry index (S – N) / (N + S), this difference can be quantified. Over a given cycle, the
asymmetry decreases but (particularly for active
regions) lasts throughout the cycle. The hemisphere in which the magnetic field dominates
oscillates throughout the cycle with a period
of around 2.5 years. The significance of this
asymmetry manifests itself in a variety of ways
such as in the correlation with the occurrence
of flares (seen in Hα emissions).
Lidia van Driel-Gesztelyi (Mullard Space Science Laboratory) then talked about the emergence, evolution and eruption of magnetic flux
from the Sun. Such emergence is ubiquitous,
even when the Sun is at the minimum of its activity cycle, when the emergence is on a relatively
small scale. In her presentation, Lidia concentrated on large-scale events. Observations show
loops emerging from the surface of the Sun and
quickly reconnecting, followed by flares. For
such energy releases, free energy is required in
the magnetic field and it has been proposed that
the source of this energy results from twists in
the emerging flux. Modelling shows that, if there
is no twist in the emerging field, the rising flux
tubes quickly fragment. Because the flux rises
through a complex turbulent convective zone on
its way to the surface, twist in the field is vital
if it is to escape being torn apart by vortices in
its wake. If such flux is twisted sufficiently, it
goes through the helical kink instability and
produces complex patterns in the magnetic
fields of emerging flux tubes. This brings great
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2: A typical sunspot region in G-band at 430 nm, from Hinode, shows the solar surface in the
photosphere with unprecedented clarity, including the typical granulation. (Hinode)
complexity to sunspot regions. But most emerging loops are not strongly twisted. It is common
to see “nested” flux emergence: one loop after
another in the same region. Large active regions
can be formed from many dipoles that emerge
separately. Flux emerges over many days to form
an active region, gradually dispersing over several months. Many flares are associated with
the formation of such a region, with numbers
decreasing steeply as the active region decays,
eventually vanishing. Associated coronal mass
ejections (CMEs) also occur most frequently at
the formation of an active region, but the rate
declines to a constant level. This is linked to the
local complexity of magnetic field structures.
The flare index and CME acceleration correlate with the complexity of an active region.
Twisted magnetic fields are clearly important
in the emergence and intensity of active regions
as well as in the occurrence of CMEs.
Impulsive magnetic energy release
While the majority of the solar physics community believe that reconnection is responsible
for impulsive energy release, there are still issues
with understanding how this process works,
particularly in terms of how fast the energy is
released in solar flares. David Tsiklauri (University of Salford) spoke about a new, fast reconnection model in the collisionless regime that can
address some of these issues. A growing number
of numerical simulations have shown that in the
collisionless regime, the off-diagonal electron
tensor plays an important role in regions where
the magnetic field is weak and the plasma is not
completely “frozen in” to the magnetic field.
In the Sweet–Parker (collisional) model, where
advection is balanced by diffusion, any plasma
outflow occurs at the Alfvén speed. By making
assumptions about the nature of the electron
motion and magnetic pressure, David balanced
advection with the pressure tensor. The resulting
collisionless model fits the laboratory data well,
with the width of the electron diffusion region
comparable with the skin depth. The implications of this model for the production of flares
are that it predicts reconnection rates around
100 times larger and a heating flux 10 times that
predicted by the Sweet–Parker model.
Critical for understanding energy release in
the solar atmosphere on all scales, is a knowledge of the coronal field because, in many cases,
this is where reconnection is believed to take
place. The coronal field is notoriously difficult
to measure, so many indirect, but very effective
techniques have been developed and these were
discussed in several posters. Tom van Doorsalaere (University of Warwick) and Youra Taroyan (University of Sheffield) both reported the
spectroscopic detection of MHD oscillations
of solar coronal structure by Hinode’s EUV
Imaging Spectrometer (EIS) that allowed them
to estimate the coronal magnetic field strength
in several cases. The oscillation of the example
presented by Tom (figure 3) had an amplitude
of 1 km s –1 and a period of 296 s, demonstrating
the impressive sensitivity of the EIS instrument
for these kind of measurements.
Stephane Regnier (University of St Andrews)
described a study of coronal Alfvén speeds in
an isothermal solar atmosphere, important for
understanding energy release in the corona, and
the initiation and propagation of CMEs. The
global properties of the magnetic field strength
above four active regions associated with different eruptive events were determined and most of
the magnetic flux found to be localized within
50 Mm of the photosphere; most of the energy to
be stored below 150 Mm, and the magnetic field
strength was seen to decay with height more
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Davis et al.: Magnetic coupling
2007–07–09T13:46:32.566
0
CME initiation and evolution
y (arcsec)
–100
–200
–300
–400
–950
Institute for Solar System Research and Armagh
Observatory) both examined the response of
the atmosphere to impulsive energy release by
studying flaring plasma and the dynamics of a
jet, showing the enormous range of effects seen
in all wavelengths. Observations of this kind are
extremely important for constraining models,
and the increasing quality of the data in terms
of spatial, spectral and temporal resolution continuously presents new challenges.
–900
–850 –800
x (arcsec)
–750
3: The active region observed by EIS. Later on
in the observation run, we see coronal loop
oscillations, which we interpret as fast kink
mode oscillations. (Van Doorsselaere T et al.
2008 A&A 487 L17)
slowly for a nonlinear force-free field than for a
potential field. The derived Alfvén speeds varied
by up to two orders of magnitude and showed
sensitivity to the magnetic configuration, with
average speeds in flaring regions departing
strongly from potential field values.
William Simpson’s (St Andrews) poster discussed an investigation into the magnetic field
topology of coronal null points and solar flares.
Mathematical models have been proposed for
both phenomena that require the presence of
a magnetic null point in the solar atmosphere.
William used magnetic observations and 3-D
modelling to investigate these null points as they
are very difficult to observe directly. In this way,
he could extrapolate the coronal magnetic field
from the field associated with the photosphere
of an active region. By understanding the 3-D
topology of the magnetic field, it becomes possible to determine the positions of the null points.
The example presented was the magnetic field in
the vicinity of active region AR0486 during an
X-class flare. The analysis confirmed the presence of a stable null point near the flare site.
Sotiris Adamakis (University of Central Lancashire) and Maria Madjarska (Max-Planck
A&G • April 2009 • Vol. 50 The question of whether or not a reliable indicator or precursor for CME onset can be found is
a question that has occupied researchers in the
field for some time. Deb Baker (MSSL) addressed
this question with observations of up-flows in
an active region/coronal hole complex. Were
these up-flows an enigma or a CME precursor?
Hinode EIS was used to make multiwavelength
observations of an active region in an equatorial
coronal hole from 11 to 18 October 2008. A
highly sheared sigmoidal structure containing a
filament erupted to form a CME. The dimming
and eruption were observed by STEREO EUVI
(EUV Imager) and in the SOHO/LASCO coronagraph. The Hinode EIS instrument observed the
active region break up and expand over several
days. By over­laying SOHO MDI magnetogram
data over EIS velocity maps it is possible to see
that closed loops were associated with downflows while there were upflows at the edges of
the active region where the magnetic field was
strong. The velocity maps also revealed a strong
temperature dependence of such outflows, suggesting it is the hotter coronal material that is
flowing outwards. Simulations of this event
reveal three potential types of plasma up-flow:
hot reconnection jets; slower (cooler) plasma
along new open field lines resulting from interchange reconnection between the active region
and coronal hole; and plasma upflows driven by
the active region expanding along compressed
neighbouring open field lines of the coronal hole.
The intensification of velocities towards the end
of the observation period required greater compressive forces. Lots of minor eruptions were
observed before the CME. Intensified upflows
were seen where the active region was adjacent
to open coronal hole field lines, which may be a
pre-CME signature.
In terms of CME initiation, Danielle Bewsher
(Rutherford Appleton Laboratory/Aberystwyth
University) discussed the relationship between
EUV dimming and coronal mass ejections.
While there have been many studies of EUV
dimming in association with CME onsets, there
has never been a thorough statistical study of
this association, covering appropriate temperature ranges. Danielle used a large campaign
database from SOHO/CDS and SOHO/LASCO
to associate dimming events detected at 1 and
2 MK with CME activity. The results confirm
the CME–EUV dimming association using
statistical analysis for the first time and stress
that one emission line may not be sufficient for
associating dimming regions with CMEs.
Richard Harrison (RAL) picked up this topic
from a more global viewpoint with interesting observations of outflows in STEREO
Heliospheric Imager (HI) data. In a sample of
two months’ worth of data, 15 CMEs occurred
of which seven were preceded by outflows of
material in a stream along a narrow range of
solar latitudes. Prior to the CME, these outflows appeared to intensify with a series of dense
blobs travelling away from the Sun’s surface. If
these streamers are associated with the CME,
this could provide another tantalizing possibility for predicting the occurrence of CMEs. After
several examples, Richard homed in on one particular event, from 19 April 2007. By plotting
the outflow of material crossing a surface of
constant elongation, it is possible to map the
latitude of the streamers prior to the CME and
measure the speed and intensity of these outflows. Detailed analysis showed that there were
indeed enhanced outflows, forming eight or so
transient blobs in the two days before the CME.
These all appeared to have similar velocities,
around 175 km s –1. This is much less than the
expected Alfvén speed, which may indicate that
there is a geometric factor involved, for example, if the flows are not in the plane of the sky.
Such activity is consistent with reconnection
occurring at one foot-point of a magnetic loop
structure, releasing previously contained mat­
erial. Further analysis is continuing in order to
determine whether these “fuses” are physically
associated with the subsequent CMEs, what
they represent and how often they occur.
One of the keys to differentiating between
competing CME models is determining when
the flux rope forms relative to the eruption – is
it pre-existing or formed as a consequence of
the eruption? Lucie Green (MSSL) presented
work that used soft X-ray observations of sigmoids from Hinode to answer specifically this
question. Lucie discussed sigmoids (“S” shaped
structures in the solar atmosphere), which are
seen in both active and quiet regions. In active
regions, they are found in the same locations
as filaments and the “S” shape is related to the
twist in the local magnetic field. Sigmoids occur
in both positive and negative forms (mirror
images of each other), with the “S”-shaped sigmoids generated by a positive (right-hand) twist
in the magnetic field, and mirrored “S” sigmoids
are generated by a left-hand twist. They are of
interest as their occurrence represents a high
probability of an eruption occurring, and Lucie
showed several examples related to eruptions
where it seemed clear that there was stronger
evidence for an arcade to flux rope transition,
rather than a pre-existing flux rope.
Bernhard Kliem (MSSL) then broadened the
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discussion towards what drives the eruption
of coronal flux out into the heliosphere. These
eruptions are defined by an instantaneous onset
and rapid acceleration indicative of the release
of stored energy followed by a huge expansion
at a rate that exceeds the local Alfvén speed.
The CME is thus characterized by three main
phases: the final two are acceleration and propagation into the solar wind, but they are preceded
by a long-term evolution of magnetic topology.
Bernhard stressed the point that CME models
are not exclusive and all probably play a part in
CME onset to some degree. All invoke a flux
rope and reconnection and modelling work is
able to reproduce the observations well. In those
that contain twisted magnetic fields, the degree
of twist drives the growth phase of such features
and can lead to a CME. Kink instabilities cannot explain everything, however, because some
events are seen as simple expanding loops. The
torus instability – well known in fusion research
– requires an external field to retain the field in
a loop structure. If the external field decreases
rapidly, the contained material is released. It
seems that we might gradually be converging
towards the point where we have a CME equivalent of the canonical solar flare model.
Towards the Earth and beyond
In terms of determining the impact that a CME
might have on Earth or the terrestrial planets,
being able to track accurately an ejection and
its evolution en route is of key importance.
STEREO is designed to do precisely this and
Jaz Pearson (UCLan) demonstrated how it is
possible to track an event from the solar corona
(with STEREO/EUVI data) into the heliosphere
at around 1 AU with the STEREO/HI. For an
event on 25 March 2008, Jaz used two methods
to determine the direction of the mass ejection.
First, he used the stereo views of the event in
the STEREO EUVI cameras to reconstruct the
solar longitude of the expanding magnetic field
structures. Secondly, he tracked the event into
the HI images on STEREO-A. By making the
assumption that a CME will be travelling at
a constant speed within the HI field of view,
it is possible to use the apparent acceleration
across the wide-field optics to estimate the
angle of propagation with respect to the spacecraft. The two estimates were consistent with
each other for this event. Thomson scattering
of photospheric light results in events that are
most prominent when they are propagating at
right-angles to the Sun–spacecraft line. As the
STEREO spacecraft diverge, this will favour the
observations of CMEs propagating along the
Sun–Earth line and such techniques will provide a powerful method for investigating Earthimpacting CMEs in future observations.
Alexis Rouillard (University of Southampton)
also discussed a CME observed by STEREO on
the east limb of the Sun with the HI instrument
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on the ahead spacecraft. It was possible to track
this CME all the way from the Sun to Venus.
A “V”-shaped structure was seen accelerating
out in the STEREO coronagraph COR2. This
structure appeared to hit others ahead of it, and
two features looked to be the outer loop and
inner “V”-shaped structure of the same event.
Fitting model parameters to this structure gave
a best-fit for a flux rope some 50–60° off the
Sun–spacecraft line. Tracking these structures
through the HI images using the techniques
employed by Jaz Pearson, showed that the direction of propagation was only a few degrees in
longitude from Venus and the Messenger spacecraft. In situ observations from Venus Express
and Messenger indicated the passage of a flux
rope. Its structure and orientation correlated
well with the HI observations and indicated that
the structure had expanded at between 30 and
40 km s –1 in the radial direction.
Of course, our ability to have confidence in
the results that we obtain from these analyses
relies heavily on the quality of the data and the
calibration. Danielle Bewsher showed a new
method for deriving the instrument pointing
for the HI along with other optical parameters,
by comparing the locations of stars identified
in each HI image with the known star positions
predicted from a star catalogue. The pointing
and optical parameters are varied in an autonomous manner to minimize the discrepancy
between the predicted and observed positions
of the stars. With this method, the HI images
can be considered as self-calibrating.
The Earth is, of course, not alone in being
affected by all that the Sun has to throw at us,
and the effects of varying solar activity on the
evolution of planetary atmospheres may well
have a key role to play in determining conditions
for life. Focusing on impacts of the solar wind
and solar transients on the terrestrial planets
within our solar system, Janet Luhmann (University of California, Berkeley) presented the theories and observations of the ways in which the
solar wind affects Earth, Mars and Venus. This
is a particularly interesting time for progressing
such studies, as not only is there a comprehensive suite of solar observatories in operation,
but there are also missions operating at each
of the terrestrial planets. Janet highlighted the
difference in conditions and subsequent effects
at Venus and Mars compared to the Earth.
Although they both have atmo­spheres, Venus
and Mars do not have active magnetic fields
and so solar storms interact with their atmo­
spheres directly. The ultraviolet aurora on Venus
responds to conditions in the heliosphere, while
aurorae have been observed on Mars where the
solar wind is concentrated in the martian atmosphere by pockets of crustal magnetic field. Janet
then emphasized that the main drivers of solar
activity are those events that enhance the solar
wind speed and dynamic pressure and BZ , viz.
co-rotating interaction regions (CIRs) caused
by a region of fast wind compressing a region
of slower wind ahead of it and CMEs.
In his talk, Alexis Rouillard also showed
details of how CIRs evolve as they move out
into the heliosphere using STEREO HI and in
situ data. HI observations show blobs of mat­
erial travelling out into the CIR structure that
change the nature of the CIR as it rotates from
the location of the STEREO-B spacecraft to the
STEREO-A spacecraft, and the effect that is
seen in the in situ STEREO data indicates how
important it is to our understanding of these
events to have multiple perspectives (figure 4).
CMEs are, of course, comparatively large and
are the most studied of all transients in the interplanetary medium. They exhibit strong solarcycle dependence because they are associated
with active regions, with differences between
each cycle determined by the dominant polarity of the interplanetary magnetic field. Consecutive cycles have an alternating preference
that governs the behaviour of the shock. If the
interplanetary magnetic field is southward after
a CME shock, reconnection occurs with the
Earth’s magnetic field initiating a sudden onset
of a geomagnetic storm. A major challenge in
CME research is to connect the behaviour of a
CME with the nature of the associated active
region. While the polarity of a CME is of great
importance in predicting how it will interact
with the Earth’s magnetic field, the relationship
between the polarity of the CME and the source
region is not simple. A southward component
in the interplanetary magnetic field engages the
magnetosphere like a clutch in a vehicle, while
more subtle changes are seen when the field is
northward. On this topic, Laura Bone’s (MSSL)
poster discussed the interaction of active region
and quiescent filaments associated with a flare
and CME. The CME erupted on 19 May 2007,
accompanied by a flare, and resulted in a magnetic cloud near Earth that was registered by the
STEREO-B and WIND spacecraft on 22 May.
Two independent filaments, one in an active
region and the other quiescent, were observed
in the days before the flare and eruption and
seen to merge. Heating cycles preceded the final
filament disappearance, heating and eruption
observed by Hinode, STEREO and TRACE.
The effect of the interplanetary medium at
Earth is not just confined to CMEs and CIRs,
however. The occurrence of solar energetic
protons (SEPs) also varies with the solar cycle;
the emergence of magnetic flux at times of peak
solar activity shields the Earth from cosmic
rays and so the cosmic ray flux at Earth is anticorrelated with the solar cycle. SEPs have been
associated with atmospheric changes such as a
modulation of stratospheric ozone while cosmic
rays have been associated with the seeding of
high-altitude clouds, modulating the Earth’s
albedo, making their study important for longA&G • April 2009 • Vol. 50
Davis et al.: Magnetic coupling
4: STEREO’s observations from three distinct vantage points in space help scientists to unravel
complex regions in the solar atmosphere. (NASA)
term climate-change research.
Janet Luhmann also highlighted the fact that
ionized particles in planetary atmospheres can
be lost to space as the solar wind impacts on a
planetary atmosphere. At the Earth, the magnetic field largely contains these within the magnetosphere, but there is evidence from both the
Mars Express and Venus Express missions that
this mechanism is occurring at these planets
and, in the case of Mars, this may be responsible
for the loss of the martian atmosphere. Current
loss estimates do not take solar activity into
account, but at times of increased solar activity
the flux of O+ ions lost in this way can increase
by a factor of 100. Do these interactions dictate
the fate of atmospheres in our solar system and
in other planetary systems?
Global field and the solar cycle
Having touched on the effects on other planets,
the discussion focused back on understanding
the global evolution of the heliospheric field and
its effects. Mat Owens (Imperial College London) discussed the possible role of CMEs in the
evolution of the Sun’s global magnetic field during the solar cycle. Observations of photo­spheric
flux have long been known to demonstrate a
strong solar-cycle variation, but this emission
is associated with magnetic field structures that
close in the corona and do not extend into the
heliosphere. Although we only have single point
measurements of the heliospheric field from
spacecraft, it is apparent that the variation of
total magnetic flux in the heliosphere is also
closely associated with the solar cycle. Observations of supra-thermal electrons (STEs) allow us
to infer whether the field lines that contain them
are open or closed. Open field lines contain a
single beam, while counter-streaming electrons
are observed on closed field lines. The latter case
is nearly always associated with CMEs that add
closed flux to the heliosphere. Averages of the
intensity of the interplanetary magnetic field
A&G • April 2009 • Vol. 50 and the CME rate are well correlated. Matt left
the interesting question of whether this demonstrated that CMEs were the cause of the solar
cycle variation for another day, but did address
how the flux was removed to return the flux levels back to their minimum value. It appears that
the most likely mechanism is interchange reconnection, where a CME closed loop is opened.
This has been confirmed by observation where
counter-streaming electrons are replaced by a
single beam. Observations also show that this
reconnection tends to favour one polarity footpoint over another, thereby preserving the polarity of the open field lines. CME foot-points tend
to follow the same rules as sunspots, thereby
providing an effective way of transporting flux
and therefore generating the polarity cycle. During the rising phase of the cycle, open flux fills
the polar regions, while CMEs transport closed
flux into the heliosphere around the equator.
Interchange reconnection takes place at the
trailing edge of these foot-points, transporting
open flux from the pole to the equator. At solar
maximum, the dominant polarities flip and in
the declining phase equatorial flux is transported
to the pole. There is sufficient flux to explain the
field reversals so long as the CME foot-points are
separated by more than 5°. It may be that CMEs
are not facilitating all flux transport with such
a mechanism, but it is certainly an important
process. The solar dynamo drives the system,
but CMEs are the response to this driver.
The current solar cycle has been generating
substantial interest because of the unusually
high number of spotless days observed during
the current minimum, the reduced value of the
photospheric magnetic field strength and the
heliospheric magnetic field observed at 1 AU,
raising questions about whether we may be
seeing a long-term change in solar activity, or
whether the current cycle is anomalous. Bob
Forsyth (Imperial College London) presented a
comparison of the solar heliospheric magnetic
field measured over two solar minima by the
Ulysses spacecraft. Ulysses has been making
observations over two solar cycles and is now on
its third orbit of the Sun. From the first orbit, the
solar magnetic field was clearly symmetric, with
fast solar wind over the two polar coronal holes,
approximating to a tilted dipolar field. In addition to the high-latitude polar coronal holes,
these observations revealed the ubiquitous presence of large-amplitude Alfvén waves while the
composition of the plasmas from the fast wind
regions indicated that they originated from
regions of lower coronal temperatures. Three
orbits on, the solar magnetic field structure has
become more complex. The polar coronal holes
extend over far greater latitudes, while the tilt of
the current sheet has increased by 10°. There has
also been a reduction in flux density by around
34% since the minima of first orbit. According
to observations of the Wilcox solar observatory, the photospheric field has also reduced by
a similar amount between the two cycles and
this may be, at least in part, the cause of the
decrease observed by Ulysses. This reduction
will have led to a reduction in the magnetic field
wave power and many other solar wind parameters, as discussed by Tim Horbury (Imperial).
He showed that the amplitude of waves and turbulence in the high-speed wind is about 40%
lower than the previous minimum. The “global”
observations of Ulysses compare well with point
measurements made by the Advanced Composition Explorer spacecraft at the first Lagrangian
point whenever Ulysses is within 30° in longitude, even if Ulysses is at high latitudes. Hence
the observed changes affect the entire heliosphere and may have important consequences
at Earth, in areas such as increased cosmic-ray
flux. Recent modelling has shown that the magnetic flux powers the solar wind. Lower coronal
temperatures lead to a reduction in the size of
the heliosphere and this could explain why the
Voyager spacecraft crossed the heliospheric termination shock sooner than expected.
As the meeting concluded, it was clear that
while our understanding of complex solar phen­
omena and their influence on the heliosphere
was being greatly enhanced by both theory and
observations, there are still many unresolved
questions and many new and exciting challenges
in understanding the interactions between the
many different components that comprise our
solar system. The organizers would like to
thanks all the participants. ●
Sarah Matthews is Head of Solar and Stellar
Physics at UCL’s Mullard Space Science
Laboratory, [email protected]. Danielle Bewsher
is a solar physicist working with SOHO,
STEREO and Hinode at the Rutherford Appleton
Laboratory. Chris Davis is the Project Scientist
for the STEREO Heliospheric Imagers at the
Rutherford Appleton Laboratory.
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