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Particle Acceleration and Transport in the Heliosphere (PATH)
Objective: Determine the Mechanisms Responsible for Energetic Particle
Acceleration and Transport at the Sun and in the Interplanetary Medium
A white paper for the 2010 Solar and Space Physics Decadal Survey
Principal Author: M. I. Desai (SwRI)
Co-Authors (alphabetical): E. R. Christian (GSFC), C. M. S. Cohen (Caltech), J. F. Cooper
(NASA/GSFC), G. A. de Nolfo (GSFC), H. A. Elliott (SwRI), J. Giacalone (U. Arizona), S. Geier
(JPL), G. C. Ho (JHU/APL), B. Klecker (MPI/Garching), H. Kucharek (UNH), L. J. Lanzerotti
(NJIT), M. A. Lee (UNH), S. T. Lepri (U. Mich.), R. A. Leske (Caltech), R. Lin (UCBerkeley), J.
E. Mazur (Aerospace), D. J. McComas (SwRI), E. Möbius (UNH), E. C. Roelof (JHU/APL), J.
M. Ryan (UNH), N. A. Schwadron (UNH), T. T. von Rosenvinge (NASA/GSFC), R. F. WimmerSchweingruber (U. Kiel), P. Wurz (U. Bern)
Abstract
Solar Energetic Particles (SEP) above 10 MeV in energy with proton intensities nearly 5 orders of
magnitude more than the galactic cosmic rays pose a serious radiation hazard to both instrumentation and
humans in space. Identifying the sources and underlying physics that produce these SEPs is critical, if we
are to reliably forecast such events at 1 AU. However, the origin of this acceleration, whether it is in
flares or at CME-driven shocks, and the complex roles of the variations in the seed populations in such
shocks, is still not well understood. The Particle Acceleration and Transport in the Heliosphere (PATH)
mission consists of a suite of high sensitivity and high resolution in-situ and remote sensing instruments
designed to determine the mechanisms responsible for the acceleration and propagation of SEPs through
the inner heliosphere. PATH is a small-class mission consisting of a spinning spacecraft pointed near the
Sun at the L-1 Lagrange point. This configuration and instrument suite also enables PATH to provide the
critical real-time solar wind, magnetic field, and energetic particle parameters that are required for Space
Weather monitoring and forecasting. In terms of science objectives, estimated cost, and schedule, the
PATH mission is fully compatible with (1) the goals of NASA’s Heliophysics Division, (2) the future
plans described in the 2009 Heliophysics Roadmap and the 2002 NRC decadal survey, and (3) plans and
activities of other space agencies.
1. Introduction
Massive explosions on the Sun routinely accelerate energetic ions up to ~GeV energies and electrons
up to ~100s of MeV and generate intense electromagnetic (EM) radiation in the form of x-rays and γ-rays.
Understanding the mechanisms that are responsible for accelerating these solar energetic particles (SEPs)
and influencing their propagation to Earth are of critical importance for developing physics-based
capabilities that predict the hazardous effects of solar radiation on technological systems and humans on
Earth and in interplanetary space.
Recent observations from many spacecraft in the Heliophysics System Observatory such as Wind,
SoHO, RHESSI, ACE, and STEREO have contributed dramatically to our understanding of SEPs.
However, it is now abundantly clear that a number of different physical effects that vary with time and
location contribute to the properties of SEP events seen at 1 AU. These effects include: the magnetic
topology and connection of solar eruptions such as flares and coronal mass ejections (CMEs), the
relationship between interacting and escaping particles, the origin, structure, and obliquity of CME
shocks, the nature of wave-particle interactions and the type of turbulence that is present near the shocks,
the availability, distribution and composition of the suprathermal seed populations, the type of injection
and acceleration processes involved, and the possible modification of SEP properties (e.g., time-profiles
of Fe/O, intensities etc.) due to transport-related effects.
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By making new measurements in the inner heliosphere from ~10 Rs up to ~0.8 AU, future
Heliophysics missions such as ESA’s Solar Orbiter (SO) and NASA’s Solar Probe Plus (SPP) will
provide critical breakthroughs regarding SEP acceleration and transport. However, to fully distinguish
between the effects of the above processes and achieve closure with models that seek to predict key SEP
properties near Earth and beyond, we require coordinated measurements of CMEs and their shocks, the
suprathermal seeds and the accelerated SEPs in conjunction with observations of the EM radiation
signatures from the flares and CMEs on a single spacecraft located much closer to Earth.
To make these critical measurements, the Particle Acceleration and Transport in the Heliosphere
(PATH) mission will be located at the L1 Lagrange point near 1 AU and designed to address three main
science topics: A. the origin of the suprathermal seed population; B. the origin and acceleration of SEPs;
and C. the role of transport in modifying SEP signatures. PATH is intended to provide comprehensive,
stand-alone measurements that bring closure to these science objectives. However, we note that if PATH
is operational during the rise-through-maximum of solar cycle 25 (~2019–2023), then together with SO
and SPP, PATH will provide unprecedented, radially distributed measurements of the same physical
phenomena and address the critical questions of how propagating structures, suprathermal tails and SEP
properties evolve en route to Earth. PATH will focus on:
1. Observations of ion charge states across a wide range of energies;
2. Observations of the suprathermal seed population with improved time cadence and sensitivity over
previous missions;
3. Observations of neutral (x-rays, γ-rays, neutrons, and energetic neutral atoms) signatures in
flare/CME events with improved sensitivity and energy resolution;
4. SEP observations up to several hundred MeV/n; and
5. Vital contextual data (plasma, electric, and magnetic field observations).
The charge state measurements (1) are absolutely necessary because they provide one of the best
remote diagnostics of the plasma conditions at the acceleration site (e.g. Geiss et al., 1995 and Klecker,
et al., 2007). The SEPs are presumed to be accelerated from the suprathermal tail, rather than the bulk
plasma, but these seed particles (2) have never been measured with a cadence that allows understanding
of how changes in the tails correlate with the high energy data. Neutral particles (3), which include very
energetic neutral atoms, (as observed by Mewaldt et al. 2009), neutrons, and x-rays and -rays, give us a
direct channel into the acceleration region. SEP observations (4) provide the end result of the
acceleration. Since these observations are being made at 1 AU, whereas most of the acceleration happens
much closer to the Sun, the transport of the SEPs is impossible to understand without contextual data (5)
on the solar wind plasma and magnetic field.
In addition to the prime science goal, the PATH instrument complement will also provide new data to
understand the origin of particles accelerated elsewhere in the heliosphere (CIRs, ESPs), as well as
provide Space Weather monitoring that is vital to the nation’s infrastructure.
The PATH mission comprises sophisticated, heritage-based instruments that measure (1) the
distribution functions and composition of the solar wind plasma and energetic particles with comparable
or better sensitivity and resolution compared with existing instruments, (2) charge-state composition of
suprathermal and energetic particles with significantly improved sensitivity compared with current
instruments, (3) magnetic and electric fields at sufficiently high cadence and sensitivity to characterize the
plasma and magnetic field waves and turbulence in the interplanetary medium and near shocks, (4) high
cadence, high sensitivity measurements of x-rays, -rays, and neutrons to provide the energy spectra and
timing of solar flares. Finally, the PATH mission concept includes a real-time downlink capability that
provides observations of the interplanetary magnetic field, solar wind, and energetic particles to the Space
Weather Community.
A reasonable implementation of PATH is a spinning spacecraft pointed toward the Sun at the L-1
Lagrange point, similar to ACE. This configuration also makes communication easy, utilizing a large
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antenna on the earthward deck, and also allows the possibility of a Real Time data stream similar to ACE
that can be used as a Space Weather station.
2. Science Measurements and Implementation
Summarized below are strawman instruments for PATH that collectively provide measurements of a)
the energy spectra, the elemental, isotopic, & charge state composition, and the anisotropy of ions and
electrons covering an extended energy range from ~few eV up to 100s of MeV/nucleon; b) the very
energetic neutral atoms with temporal, spectral and spatial resolution; c) the energy spectra and timing of
neutrons, X-rays and -rays from solar flares, d) the solar wind ions and electrons, and e) the DC and AC
electric and magnetic fields. These measurement capabilities also provide real-time observations of the
solar wind, interplanetary magnetic field (IMF), and energetic particles to the Space Weather community.
2.1 Charge State and Composition of Solar Wind, Suprathermal, and Energetic Ions
High sensitivity, high time resolution measurements of the elemental, isotopic, and charge-state
composition of solar wind, pickup, and suprathermal H-Fe ions from ~10s eV/e up to ~50 keV/e.
Instruments that employ E/q selection techniques, followed by a time-of-flight versus residual energy
analysis in solid-state detectors like Ulysses/SWICS and ACE/SWICS [Gloeckler et al., 1992; 1997] are
required to make these measurements.
High sensitivity measurements of the differential fluxes of H – Ni ions covering the energy range
from ~few keV/q up to ~100s of keV/q to determine the elemental and isotopic (e.g., 3He/4He ratio)
composition and the ionic charge states of suprathermal Fe in small and large individual solar energetic
particle events. Instruments like Wind/STICS (Gloeckler et al., 1995) and ACE/SEPICA (Möbius et al.,
1997) can provide these measurements.
2.2 Very Energetic Neutral Atoms
The fact that there is only one flare during which very energetic neutral atoms (VENAs) are observed
(Mewaldt et al., 2009) indicates that only rarely does a flare produce VENAs at a location from where
they can escape without being ionized and/or they are narrowly beamed. Yet they provide a very
intriguing and unexpected insight into SEP acceleration processes. An instrument specifically designed to
observe VENAs could consist of a large area particle detector with a narrow field of view aimed at the
sun. This could possibly be combined with either the -ray instrument, neutron detector, and/or the
energetic particle instrument.
2.3. Neutrons, X rays, & γ-Rays
High sensitivity, good spectral and imaging resolution of neutrons and γ rays to probe the acceleration
processes at play directly at the Sun. Neutrons would be measured on an event-by-event basis between
15-150 MeV (the spectral range appropriate for 1 AU) with angular resolution adequate to identify
neutrons of solar origin. We envision an imaging neutron spectrometer based on the SOlar Neutron
TRACking telescope (SONTRAC) that double (n,p) scatters in densely packed scintillating plastic fibers
arranged in alternating orthogonal planes (Ryan et al. 1999). Because it is compact, it would have several
times the sensitive area of COMPTEL, the most sensitive such detector to have flown. The imaging
property would greatly reduce the local neutron background effects. For γ rays >10 MeV, we require an
effective area of order 50 cm2 and 20% energy resolution. To measure γ-rays from 0.2 to 10 MeV one or
two ganged LaBr3 scintillators would suffice. By adding a thin window to one of the two detectors, we
can incorporate x-ray measurements down to 20 keV (just above most thermal emission).
2.4. Suprathermal-through-Energetic Ions and Electrons
Differential fluxes of H – up to Ultra-Heavy nuclei covering the energy range from ~few 10s
keV/nucleon up to ~100s of MeV/nucleon to determine the elemental and isotopic composition (e.g.,
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He/4He, 22Ne/20Ne ratios) in small and large individual solar energetic particle events. Such
measurements can be provided by instruments that employ two types of techniques, namely, (a) time-offlight versus residual energy technique as in ACE/ULEIS (Mason et al., 1997) and
STEREO/IMPACT/SIT (Mason et al., 2006), and (b) dE/dx versus residual energy technique in
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instruments like ACE/SIS (Stone et al., 1997b) and STEREO/IMPACT/LET & HET (Mewaldt et al.,
2006).
Differential fluxes and anisotropy of ions and electrons between ~2 keV–10 MeV energy at a time
resolution of ~1 minute or less to distinguish between particles arriving from the Sunward and antisunward directions. These measurements can be made with instruments like ACE/EPAM [Gold et al.,
1998] and STEREO/IMPACT/LET & HET (Mewaldt et al., 2006) and will also be used to provide realtime energetic particle (RTEP) observations.
2.5. Contextual Solar Wind Plasma, Magnetic & Electric Field Measurements
Velocity distribution functions (VDFs) of solar wind protons and alphas in the ~0.1-20 keV/q energy
range, and 3D VDFs of solar wind electrons in ~0.005-10 keV energy range at a time resolution of <1
second. Measurement of the interplanetary magnetic field is required for the primary science on PATH,
but an ACE/MAG equivalent (dual fluxgate magnetometer with 24 vectors per second) is acceptable. A
higher cadence and/or the inclusion of a search-coil magnetometer would improve the return on the
secondary science goals. Antennae that give electric field/plasma wave measurements would also greatly
enhance all five secondary science goals and the radio measurements are also important for remote
sensing of acceleration regions and will improve our understanding of the origin of SEPs.
These measurements allow complete determination of the properties of waves, compression regions,
CME shocks, and magnetic topology and are critical for understanding in-situ particle acceleration and
wave-particle interactions. These data also provide real-time solar wind and IMF data to the community
and can be made with instruments like Ulysses/SWOOPS [Bame et al., 1992], ACE/SWEPAM
[McComas et al., 1998], and ACE/MAG [Smith et al., 1998].
3. Advances in Solar and Space Physics Science
The PATH Mission will yield significant advances in the following critical research areas.
Primary 1: Origin, Acceleration, and Transport of Solar Energetic Particles. PATH will provide
comprehensive measurements of the differential fluxes, anisotropies, and composition (elemental,
isotopic, and charge state) of ions, electrons and EM radiation over a continuous and extended energy
range during a large number of CME and flare-related SEP events. Also, considerable numbers of
protons, ions and electrons are accelerated in flares and are measurable through the neutrons, γ rays and x
rays they radiate. The relationship between these particles and those measured in space is often unclear.
This neutral radiation is not affected by transport effects and, it carries with it vital information about
particle acceleration at the flare site. For example, γ rays and neutron measurements allow us to study the
time evolution of different processes throughout the solar event and provide a bulk measure of the
metallicity of the accelerated ions. Understanding the clear differences between particle populations
accelerated during solar flares and by CME-driven shocks will provide the necessary ground-truth for
SEP acceleration models that seek to specify and reliably predict the radiation environment near Earth
and its impact on our space and ground-based assets.
Primary 2: Origin and Acceleration of Suprathermal Tails. Suprathermal ions between ~2 keV and
100s of keV are observed continuously at various locations throughout the heliosphere, but their origin
and acceleration processes remain poorly understood. Observing and characterizing exactly how their
key properties (e.g., intensities, distribution functions, composition etc.) vary on short-timescales ranging
from hours to minutes, will provide much needed breakthrough for understanding the origin of this
population and how it feeds into the mechanisms responsible for injecting and accelerating particles to
even higher energies.
Secondary 1: Self-excited waves, Energy Partition, and Shock Dissipation Mechanisms. CMEdriven interplanetary shocks dissipate their energy by heating the bulk plasma and accelerating
suprathermal and energetic particles. The exact fraction of energy that is dumped into these different
populations varies with radial distance and depends on several factors such as the availability and
injection of suprathermal ions, the local structure of interplanetary shocks, the nature and influence of
pre-existing turbulence, and the generation and subsequent dissipation of self-excited Alfvén waves.
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Comparing properties of shock-associated particle events with CME and shock properties as a function of
radial distance will differentiate between the above processes and ascertain the importance of their roles.
Secondary 2: Kinetic, Microphysical, and Turbulence-related Processes in the Solar Wind.
Mechanisms responsible for heating and accelerating the solar wind include wave-particle interactions,
turbulence-driven dissipation, shocks and compression regions etc. Studying the spatial evolution of the
the solar wind at different radial distances in the inner heliosphere is necessary to pinpoint the responsible
processes and distinguish between the influence of local effects such as plasma instabilities and the
generation and dissipation of MHD turbulence from the remnants of coronal heating processes.
Secondary 3: Origin and Evolution of fast and slow solar wind streams, CIRs, ICMEs, and other
solar wind structures. Determining the locations and properties of the source regions of the different
types of solar wind and its embedded structures and understanding how they get modified during transit
through the interplanetary medium remains a key objective of most Heliophysics missions. Comparing
high-resolution solar wind and magnetic field measurements near Earth with properties of the remotely
observed coronal structures will provide the elusive missing links between solar and heliospheric
observations of the same phenomena.
Secondary 4: Particle Acceleration and Transport of CIR-associated Particles. Corotating
Interaction Regions are quasi-stationary, large-scale heliospheric structures that occur primarily around
the declining phase of solar activity cycle when fast solar wind streams overtake slower solar wind
streams that were ejected earlier. Beyond Earth-orbit, such CIRs are typically bounded by MHD shocks
that accelerate ions up to ~20 MeV/nucleon and electrons up to ~few MeV. Studying the properties of
CIRs and their associated particles at 1 AU provides deep insights into the origin of the source
populations, their efficient acceleration to ~10s of MeV in energy, the mechanisms that influence their
subsequent transport back into the inner heliosphere, and the effects of time-dependent magnetic
connection between 1 AU and the CIR shocks beyond Earth orbit.
Secondary 5: Origin of Upstream Particle Events near Earth. The origin and acceleration of
energetic ions with energies ranging from a few keV up to 1–2 MeV and electrons up to ~100s of keV
that escape from the vicinity of the Earth is highly controversial. Using sophisticated instruments that can
measure the elemental abundances and ionic charge state composition along with the energy spectra and
anisotropy will help unravel the source material and the physical mechanisms that are responsible for
accelerating the upstream ions and electrons.
4. Cost Estimate
PATH can be implemented on a spinning spacecraft pointed toward the Sun at the L-1 Lagrange
point, similar to ACE. This configuration also makes communication easy, utilizing a large antenna on
the earthward deck, and also allows the possibility of a Real Time data stream similar to ACE that can be
used as a Space Weather station. The payload is more extensive than ACE, and is more like STEREO in
size and complexity. However, by extension from STEREO which was ~$520M (NASA and European)
for two spacecraft (and assuming one spacecraft would cost 70 to 75 % of the two spacecraft cost), even
with inflation the PATH spacecraft, payload, launch vehicle and mission operations and data analysis
should be achievable within the budget of a small class mission (< $500M).
With the loss of Delta-II launch capability there is an uncertainty in launch vehicle availability for
missions of this size. Clearly a Pegasus is too small, but this mission should be achievable without using
a large rocket such as the Delta IV or Atlas V. Possibilities include the Minotaur, Falcon, or Taurus.
5. Evaluation Criteria for Heliophysics Decadal Survey
5.1 Identification of PATH measurements as a high priority or requirement in previous NRC studies and
Roadmaps.
PATH addresses several fundamental questions of critical importance for Heliophysics. For instance,
PATH directly addresses the goals and related science questions of two missions recommended in the
Future Priority Science Queue in 2009 Heliophysics Roadmap. As stated, the central objective of Solar
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Terrestrial Probes (STP #6) SEPAT – Solar Energetic Particle Acceleration and Transport mission is to
“Understand how and where solar eruptions accelerate energetic particles that reach Earth”, while the
goal of the next Living With a Star mission (LWS #7) – Climate Impacts of Space Radiation – CISR after
Solar Probe Plus is to “Understand our atmosphere’s response to auroral, radiation belt, and solar
energetic particles, and the associated effects on nitric oxide and ozone.”
PATH also directly addresses four of the five challenges in the previous NRC decadal survey, entitled
“The Sun to the Earth – and Beyond: A Decadal Research Strategy in Solar and Space Physics (2002)”
Challenge 2: Understanding heliospheric structure, the distribution of magnetic field and matter
throughout the solar system, and the interaction of the solar atmosphere with the local interstellar
medium.
Challenge 3: Understanding the space environments of Earth and other solar system bodies and their
dynamical response to external and internal influences.
Challenge 4: Understanding the basic physical principles manifest in processes observed in solar and
space plasmas.
Challenge 5: Developing a near real-time predictive capability for understanding and quantifying the
impact on human activities of dynamical processes at the Sun, in the interplanetary medium, and in
Earth’s magnetosphere and ionosphere.
5.2 Contributions to more than one of the Panel themes.
Stationed at L1, PATH measures the solar wind, suprathermal populations, energetic particles, x-rays
and γ rays, magnetic and electric fields, and cosmic rays to revolutionize our understanding of the
dynamic and often violent interactions between the particle & plasma populations and the field
environments in the heliosphere. PATH also provides real-time high cadence measurements of the IMF
and solar wind data that are required for monitoring and predicting Earth’s response to solar and
interplanetary disturbances.
5.3 Contributions to important scientific questions facing solar and space physics today.
PATH contributes to two of the three critical science investigations associated with the three
Research Focus Areas defined in the 2009 NRC Roadmap:
1. What is the composition of matter fundamental to the formation of habitable planets and life?
3. What is the magnetic structure of the Sun-heliosphere system?
5.4 Contributions to applications and/or policy making
PATH monitors the arrival and properties of all major causes (CMEs, shock, CIRs etc.,) of Space
Weather at L1 and helps us understand the evolution and impact of radiation hazards from SEPs and
GCRs to astronauts and technological assets. Similar measurements from ACE have proved to be of
tremendous benefit, with the real-time ACE data being used in essentially all space weather forecasting
models and centers. Further, the real-time energetic particle (RTEP) data are particularly relevant during
large SEP and Ground Level Enhancement (GLE) events when solar particle populations not only
precipitate in polar regions but can also reach lower latitudes, causing severe delays along commercial
airline flights flying polar routes (e.g., New York to Tokyo).
5.5 Complement to other observational systems or programs available.
PATH makes the necessary near-Earth measurements to fully study the spatial origin, acceleration
and evolution of solar wind and energetic particles as they propagate through the inner heliosphere and
reach 1 AU. If operational during the rise-through-maximum of solar cycle 25 (~2019-2023), PATH
observations of solar wind structures and SEPs at 1 AU will clearly augment and complement the
corresponding SO and SPP measurements of the same phenomena in the inner heliosphere. PATH also
measures the composition of different samples of matter, building on the successes of the ACE, Wind,
and Ulysses. In addition, PATH fills in a critical between gap the current low-energy space-based
observations and the high-energy neutron monitor observations of GLEs, making it possible to identify
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the composition content of GLEs. Finally, PATH provides real-time solar wind monitoring and serves
the needs of the Space Weather Community.
5.6 Cost-to-Benefit Ratio
PATH is a small-class ( < $500M) mission stationed at L1 and has a highly favorable cost-to-benefit
ratio since the new measurements will provide major breakthroughs in our understanding of how charged
particles are accelerated to energies that produce harmful radiation, and will simultaneously provide realtime Space Weather monitoring capability.
5.7 Degree of technical, resource, and expertise readiness.
All the technology and expertise needed for PATH, including spacecraft systems, science payload,
and mission operations at the L1 orbit either already exists or requires simple modifications to heritage
instruments and existing sub-systems, thereby resulting in a low-risk, high return mission concept.
5.8 Fitting with other national and international plans and activities.
The major goal of the PATH mission is to determine the mechanisms responsible for particle
acceleration and transport in the heliosphere, which concurs with a major objective of two of the most
exciting Heliophysics Missions currently in development – SO and SPP – as well as that of the SEPAT
mission (STP #6) described in the 2009 Heliophysics Roadmap. The 2009 Roadmap also recommended a
launch date of 2021 for SEPAT, which is compatible with the amount of time (<10 years) required for the
design, development, and launch of the PATH mission. Thus in terms of science objectives, estimated
cost, and schedule, the PATH mission easily fits in with the future plans and ongoing activities of national
and international space agencies.
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