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Abstract
Solar energetic particles, or SEPs, from suprathermal (few keV) up to relativistic ( few GeV) energies are accelerated near the Sun in at least two ways: (1) by magnetic reconnection-driven processes during solar flares resulting in impulsive SEPs, and (2) at fast coronal-mass-ejection-driven shock waves that produce large gradual SEP events. Large gradual SEP events are of particular interest because the accompanying high-energy ( s MeV) protons pose serious radiation threats to human explorers living and working beyond low-Earth orbit and to technological assets such as communications and scientific satellites in space. However, a complete understanding of these large SEP events has eluded us primarily because their properties, as observed in Earth orbit, are smeared due to mixing and contributions from many important physical effects. This paper provides a comprehensive review of the current state of knowledge of these important phenomena, and summarizes some of the key questions that will be addressed by two upcoming missions-NASA's Solar Probe Plus and ESA's Solar Orbiter. Both of these missions are designed to directly and repeatedly sample the near-Sun environments where interplanetary scattering and transport effects are significantly reduced, allowing us to discriminate between different acceleration sites and mechanisms and to isolate the contributions of numerous physical processes occurring during large SEP events.
Keywords: Coronal mass ejections; Particle radiation; Shocks; Solar activity; Solar energetic particles; Space weather.
© The Author(s) 2016.
Figures
Carrington’s (1859) drawing of sunspot group 520 on September 1, 1859: the first visual record of a solar flare. The initial (A, B) and final (C, D) positions of the white-light emission are shown. Solar east is to the right
Neutron monitor observations during the 1956 solar flare event. Image reproduced with permission from Meyer et al. (1956), copyright by APS
The two-class picture for SEP events where a the gradual event is produced by a large-scale CME-driven shock wave that accelerates the SEPs and populates interplanetary magnetic field (IMF) lines over a large longitudinal area, and b the impulsive event is produced by a solar flare that populates only those IMF lines well-connected to the flare site. Intensity-time profiles of electrons and protons in c a large gradual SEP event, and d a small impulsive SEP event (adapted from Reames 1999)
Intensity-time profiles of 1–30 MeV protons during gradual SEP events observed at three different solar longitudes relative to the flare or CME lift-off location (see text; after Cane et al. ; Reames ; Cane and Lario 2006)
Longitudinal distributions of the solar sources associated with a gradual and b impulsive SEP events. Image reproduced with permission from Reames (1999), copyright by Springer
a Dynamic spectrum from the Culgoora radio observatory showing a type II burst with fundamental (F) and harmonic (H) structure. The fundamental component starts around 150 MHz. b A section of the nearest STEREO A EUVI-A image showing the CME. The CME height can be directly measured from this frame as . Image reproduced with permission from Gopalswamy et al. (2013), copyright by COSPAR
Peak proton intensity in SEP events at two energies versus CME speed. Pink circles represent data from wind/energetic particles—acceleration, composition, and transport/low energy matrix telescope (EPACT/LEMT) and SoHO/LASCO; green triangles show data from Helios and Solwind, P78-1; blue lines are linear least-squares fits, r are the corresponding correlation coefficients. Image reproduced with permission from Kahler (2001), copyright by AGU
Scatter-plot of CME kinetic energy versus SEP kinetic energy for 23 large SEP events from solar cycle 23. Image adapted from Mewaldt et al. (2008)
Left Comparison of the mass distribution of all CMEs observed from 1996–2003 (Gopalswamy 2006) to the masses of CMEs associated with 23 of the 50 largest SEP events of solar cycle 23 (scaled up by 20). Right Comparison between the distributions of the kinetic energy of CMEs associated with 23 large SEP events from solar cycle 23 and all CMEs observed from 1996–2003. Images reproduced with permission from Mewaldt et al. (2008), copyright by AIP
Left Peak proton intensity versus CME speed for SEP events with a preceding frontside CME (P; red diamonds) and for no preceding CME (NP; plus symbols). Solid lines are regression lines for the P and NP groups. The dashed regression line is for all data points. Right The “twin-CME” scenario for a large SEP event. Two CMEs erupt from the same or nearby source active regions. Interchange reconnection between open magnetic field lines and those draping the first CME can release seed particles accelerated by the first CME shock into the disturbed downstream region, which has enhanced turbulence levels. This material can then be subsequently accelerated by the second CME shock. Images reproduced with permission from (left) Gopalswamy et al. (2004), copyright by AGU, and (right) Li et al. (2012), copyright by Springer
Left Event-integrated fluence spectra of C, O, and Fe. Right C/O and Fe/O ratios in two large SEP events measured by the Ultra Low Energy Isotope Spectrometer (ULEIS) and the Solar Isotope Spectrometer (SIS) on board ACE (adapted from, Tylka et al. ; Desai et al. 2006a)
Fe and O intensity-time profiles at during three large gradual SEP events measured by ACE/SIS. Image reproduced with permission from Cane et al. (2003), copyright by AGU
a Temporal profiles of He and He ions in a large CME-related SEP event. b 0.5–2.0 MeV/nucleon He mass histogram obtained during several large SEP events. The right scale corresponds to the open histogram. Image reproduced with permission from Mason et al. (1999), copyright by AAS
a Average heavy ion abundances at – in 64 large SEP events events relative to those measured in the fast and slow solar wind, normalized to oxygen and plotted versus M/Q (adapted from Desai et al. 2006a). b Average abundances measured in the slow solar wind divided by those measured in 40 large CME-related SEP events above , plotted versus the FIP of each element (adapted from Mewaldt et al. 2002)
Hourly averaged intensity of suprathermal Fe (red) and number density of solar wind Fe (blue) during a 100-day period in 2004. Image reproduced with permission from Mason et al. (2005), copyright by AIP
Left Fluences of 12–80 MeV/nucleon Fe in large SEP events from solar cycle 23 versus the suprathermal Fe density averaged over the day before the SEP event. The dashed black line is 3333 times the number-density scale. Right Histogram of daily averaged suprathermal Fe densities for all days from March 1998 to December 2005 (left scale) compared to a histogram of suprathermal Fe densities, measured one day before the associated SEP events (right scale). Image reproduced with permission from Mewaldt et al. (2012b), copyright by AIP
Left Schematic of a CME-driven shock as seen at azimuthally-separated 1 AU spacecraft illustrating the variation in shock obliquity and the corresponding regions of variable injection threshold speeds (adapted from Zank et al. 2006). Right According to the Tylka and Lee (2006) model, the suprathermal seed population for shock-accelerated ESPs and SEPs comprises both coronal (or solar wind) and flare-accelerated ions. Flare suprathermals are more likely to be accelerated at quasi-perpendicular shocks with higher injection thresholds. The inset shows the energy-dependence of Fe/O ratio in the accelerated population (adapted from Tylka et al. 2005)
Model calculations for the Fe/O ratio versus energy. The Fe/O ratio is normalized to 0.134, which is taken as typical of the coronal population. The bottom curve in both panels shows the quasi-parallel case in which the spectra are averaged over , while the rest of the curves represent quasi-perpendicular shocks where the spectra are averaged over the full range of . The calculations are performed by assuming different fractions of the flare component in the seed population, as specified by the parameter R (see text for more details). Other energetic particle parameters are fixed: spectral index and . a Injection of ions from the coronal component is suppressed at quasi-perpendicular shocks. b Same calculations without coronal seed suppression. Image reproduced with permission from Tylka and Lee (2006), copyright by AAS
Left Average ionic charge of Fe in the energy range 0.18–0.24 MeV/nucleon in impulsive and gradual SEP events; see text for details. Right Mean ionic charge Q at 0.18–0.25 MeV/nucleon versus that at 0.36–0.43 and 28–65 MeV/nucleon. Image reproduced with permission from Klecker et al. (2007), copyright by Springer
Left Event-integrated fluences of O, Ne, Mg, Si, S, Ca, and Fe plotted versus energy during the large SEP event on October 26, 2003. All the spectra except O and Mg have been scaled to better compare the spectral shapes. The solid lines are the oxygen spectra scaled appropriately in energy (see text). Right Abundance ratios relative to oxygen, calculated from the spectra shown on the left, plotted versus scaled energy. Image reproduced with permission from Cohen et al. (2005b), copyright by AGU
One-hour averaged elemental composition normalized to coronal values (Reames 1995a) measured by Wind/EPACT/LEMT during the April 20, 1998 SEP event. Image reproduced with permission from Tylka et al. (1999), copyright by AGU
a Fe and O intensity-time profiles during the November 4, 2001, (DOY 308) large SEP event at . b Time-intensity profiles for the event in (a), with O at twice the kinetic energy of Fe (adapted from Mason et al. 2006, 2012)
Columns show low energy ion data for three SEP events observed on: a May 1 (DOY 122), 2000; b April 21 (DOY 105), 2001; and c January 20 (DOY 20), 2005. a–c Spectrograms for 6–80 AMU ion arrivals plotted as 1/v versus time; red diagonal lines show arrival pattern for pure velocity dispersion along a 1.2 AU IMF line for particles injected at the time of the associated X-ray flare; red dashed vertical lines marked S show times of shock passage. d–f 386 keV/nucleon O and Fe intensity profiles during the events. g–i Fe intensities at 386 keV/nucleon from the (d), (e), and (f) compared with O at 773 keV/ nucleon in (g) and (i) and at 546 keV/nucleon in (h). Image reproduced with permission from Mason et al. (2012), copyright by AAS
Left Turn-overs in the energy spectra of H and O in 5 large GLEs. Right Proton spectra in 2 GLEs with large differences in proton intensities at . Image reproduced with permission from Reames and Ng (2010), copyright by AAS
Left Measured angle with respect to the Sun-s/c line for individual 1.6–12 MeV protons observed on December 5, 2006 by the low energy telescopes (LET) on STEREO. Red STEREO A and blue STEREO B. A small group of events arrived from within of the Sun-s/c line between 1130 UT to 1350 UT, i.e., well before the SEP onset at 1445 UT. The range of magnetic field orientations connecting to the Sun between 1130 and 1350 UT is shown as a bar. Right Comparison between the timing of associated solar events and ENA arrival at STEREO (adapted from Mewaldt et al. 2009)
Examples of the variety of time-intensity profiles of –68 keV and –4.8 MeV ions and –53 keV electrons observed in IP shock-associated ESP events at ACE. Image reproduced with permission from Lario et al. (2003), copyright by AIP
A histogram of the classification of time-intensity profiles of energetic protons and electrons associated with 168 interplanetary shocks observed by ACE from 1997–2001. Image reproduced with permission from Lario et al. (2003), copyright by AIP
Left Distribution of IP shocks with measurable intensities of He ions versus shock speed (green) within the total distribution of 258 shock waves versus shock speed (yellow and green) observed by Wind. Right Scatter-plot of the background-corrected peak intensity of –
He nuclei versus shock speed. Image reproduced with permission from Reames (2013), copyright by Springer
Hourly averages of a
–0.23 and – C, O, and Fe intensities; b C/O ratios; and c Fe/O ratios from June 22–29, 1999, measured by ACE/ULEIS. The IP shock is identified using 5 min averages of d the magnetic field magnitude B, and e the SW speed V. Blue vertical lines marked S1 and S2 denote the IP shock arrival times at ACE. Dashed black vertical lines represent the time interval for measuring shock-associated energetic ions. Solid black vertical lines represent the time interval for measuring ambient energetic ions in the interplanetary medium. Image reproduced with permission from Desai et al. (2004), copyright by AAS
Left Energy spectra of C, O, and Fe during three ESP events. The solid curves show fits with the Jones and Ellison expression, where the differential intensity is given by ). Right C/O and Fe/O ratios versus energy for the three IP shock events. Image reproduced with permission from Desai et al. (2004), copyright by AAS
Left Heavy ion energy spectra from ACE and GOES following an IP shock on October 29, 2003, are fitted with the Ellison and Ramaty (1985) spectral form for differential intensity, ). All elements are fitted with power-laws of the same spectral index: (Mewaldt et al. 2005c). Spectra of different elements are scaled for clarity. Right Values of the e-folding energies versus the ion’s Q/M ratio. Fits to the values of for give a (Q/M) dependence, which is somewhat weaker than that predicted by Li and Zank (2005)
Left Shock compression ratio versus spectral index of 30–50 keV energy ions in 50 ESP events (adapted from van Nes et al. 1984). The symbols denote ESP events with four different types of time-intensity profiles (see text for details). Right Scatter plot of the 0.1–0.5 MeV/nucleon O spectral indices versus for 60 ESP events; here M is the magnetic compression ratio (adapted from Desai et al. 2004). The solid curve represents , as expected from diffusive shock acceleration theory; the dashed curves arise from uncertainty in the compression ratio (left) and M (right). N is the number of events, r is the correlation coefficient, and p is the probability that the value of the correlation coefficient can be exceeded by a pair of uncorrelated parameters
O spectral index at IP shocks versus that for the ambient suprathermals. Image reproduced with permission from Desai et al. (2004), copyright by AAS
The 0.5–2.0 MeV/nucleon mass histograms during four He-enriched IP shock events at ACE. Image reproduced with permission from Desai et al. (2001), copyright by AAS
a Hourly averaged intensity profiles of 0.25–0.8 MeV/nucleon He, He, O, and Fe (solid lines) measured by ACE/ULEIS and the /He ratio (filled circles) measured by ACE/SEPICA during an IP shock event. The yellow-shaded region identifies the ambient interval, the purple-shaded region shows the time interval for sampling the shock-associated energetic particles, the brown line denotes the shock arrival time at ACE, and arrows at the top indicate the estimated and actual CME lift-off times near the Sun. b He mass histogram from ULEIS, and c He charge state histogram from SEPICA showing well-resolved He and peaks during the ESP event (adapted from Allegrini et al. 2008)
Mean abundances in 72 IP shocks normalized to: a slow SW values, and b mean abundances measured upstream of the shocks, plotted versus the ion’s M/Q ratio (adapted from Desai et al. 2003)
Wind/STEP C N O observations during the August 6, 1998 (left) and the February 18, 1999 (right) IP shock events. Each figure shows 10-min averages of a, b
– CNO intensities; c, d first-order ; and e, f second-order , anisotropy components for – CNO ions in the solar wind frame. Vertical lines marked S indicate the IP shock arrival times at 1 AU. The yellow bar in (f) is the ICME interval. Image reproduced with permission from Desai et al. (2012), copyright by AIP
Temporal evolution of the energy spectra of CNO from Wind/STEP and O from ACE/ULEIS measured during three separate intervals identified by the color-coded horizontal bars shown in the two panels of Fig. 37, normalized to the intensity at
Upper panels ACE measurements of the magnetic power versus time in three different frequency ranges. Lower panels PSDs at four time intervals identified by the vertical bars in the upper panels. The local proton cyclotron frequency is shown in each of the four lower panels. The two shocks arrived at ACE at 0644 UT on August 6, 1998, and 0211 UT on February 18, 1999, i.e., min earlier than at Wind. ACE was located at the L1 point and separated from Wind by and , respectively. Image reproduced with permission from Desai et al. (2012), copyright by AIP
Left Arrival of He ions of different energies at the Wind s/c during the May 6, 1998 GLE. Right Ion intensity onset time in each energy interval versus 1/v. Parameters of the linear fit (solid line) are: slope gives the path-length, and the intercept yields the SPR time at the Sun. Image reproduced with permission from Reames (2009b), copyright by AAS
Onset times for two impulsive SEP events (left) and two GLEs (right). Blue curves show the intensities of hard X-rays (left panels) and of 4–7 MeV -rays (right panels). The red curves are the GOES 1.5–12 keV soft X-ray intensities. The red vertical lines represent the SPR times inferred from measuring the arrival times of the GLE particles. Also shown are onset times of type II and type III radio bursts and CME lift-off times (after Tylka et al. 2003)
Top Radial height of CME shocks at the time of the first SPR versus flare longitude. Bottom SPR time—the type II onset time at the Sun versus source longitude during several GLEs. Image reproduced with permission from Reames (2009b), copyright by AAS
Proton fluence spectra during two GLEs observed during the October–November (Halloween) 2003 SEP events. Both spectra are fitted with the double power-law function of Band et al. (1993), with different spectral slopes above and below the break energy. Green curves show the GCR fluence levels. Image reproduced with permission from Mewaldt et al. (2012a), copyright by Springer
Proton energy spectra during the SEP events of April 1998 (green Tylka et al. 2000) and September 1989 (blue Lovell et al. 1998). Yellow hazardous radiation portion of the spectrum during the April 1998 event; red additional hazardous radiation from the September 1989 event. Image reproduced with permission from Reames (2013), copyright by Springer
Left Dose rates (16-min averages) recorded by MSL-RAD in a silicon detector (black circles) and in a plastic scintillator (red circles) during MSL’s journey to Mars. Five SEP events were observed during the cruise phase. For a given incident flux, the dose rate in silicon is generally less than the dose rate in plastic because of the comparatively large ionization potential of silicon. Right Radiation exposure compared with that measured by MSL-Rad on its way to Mars. Image reproduced with permission from Zeitlin et al. (2013) and Kerr (2013), copyright by AAAS
Top 10-min averaged energetic proton intensities measured on STEREO A in three different energy ranges: SEPT from 0.4–0.6 MeV, LET from 4 to 6 MeV, and HET from 40 to 60 MeV. Bottom The energetic proton pressure in three energy ranges and their total during the event. Magnetic pressure is included for comparison. Image reproduced with permission from Russell et al. (2013), copyright by AAS
Oxygen fluences measured by several instruments on board ACE during a 3-year period. The black data points show the fast and slow solar wind components along with a suprathermal tail that extends well above . Also shown are representative particle spectra obtained for gradual and impulsive SEPs, corotating interaction regions (CIRs), anomalous cosmic rays (ACRs), and GCRs. Image adapted from Mewaldt et al. (2001)
Phase space density during the extreme quiet SW conditions in 2009. a Highest and lowest observed tail densities during the first 82 days of 2009. b Four spectra selected according to their tail densities. Image reproduced with permission from Fisk and Gloeckler (2012), copyright by Springer
Solar wind parameters, the density of the suprathermal proton tail, and the spectral index of the ST tail for the extreme quiet conditions in the solar wind during 2009. Possible shocks are marked with thin vertical lines; the compression ratio across the shock is also shown. The shaded regions indicate time periods during which spectra shown in Fig. 48 were taken. Image reproduced with permission from Fisk and Gloeckler (2012), copyright by Springer
Yearly averages of the quiet-time suprathermal a Fe and CNO spectral indices, , given by fitting a power-law of the form to the measured differential intensities j at energy E; E is in MeV/nucleon, b C/O ratio, c Fe/CNO ratio, and d
He/He ratio measured by the Wind/STEP and ACE/ULEIS from 1995–2009 (adapted from Desai et al. ; Dayeh et al. 2009)
Fraction of time that energetic He above MeV/nucleon is present in the interplanetary medium, compared with the sunspot number, and current sheet tilt (adapted from Wiedenbeck and Mason 2013)
Top left Proton intensity-time profiles during the March 1, 1979 event at 3 s/c; ‘S’ represents the the time of shock passage at each s/c. Top right Energy spectra in the reservoir or the spectral invariant region behind the shock at time ‘R’. Lower panel Spacecraft trajectories through the the CME. Image reproduced with permission from Reames (2013), copyright by Springer
Left Proton and electron intensities in several energy ranges as measured by 4 s/c whose relative locations are shown at right during the January 1978 SEP event. Vertical lines show the time and longitude of the flare (E6) and the times of shock passage at each s/c. Blue asterisks represent data from Helios 1, green circles represent Helios 2 data, red squares show data from IMP 8, and violet triangles are Voyager 2 data. Image reproduced with permission from Reames et al. (2013), copyright by Springer
Left Contour plots of the simulated plasma radial speed showing the solar ecliptic plane from the solar north pole at 1700 UT on April 3, 2010 (top), at 2100 UT on April 03, 2010 (middle), and at 1600 UT on April 04, 2010 (bottom). Black/white lines indicate the magnetic field lines passing through the L1 s/c. Right Shock speed (), compression ratio (N1/N0), and the angle between the magnetic field lines threading the shock and the shock normal () simulated along the magnetic field connected to L1. Image reproduced with permission from Rouillard et al. (2011), copyright by AAS
Observations of a single He-rich SEP event seen by three s/c widely separated in heliolongitude. A large solar active region, AR11045, was seen on the Sun at about the center of the solar disk when the He-rich SEP event was observed by all three s/c; arrows depict the delayed onset times of the event. Image reproduced with permission from Wiedenbeck et al. (2013), copyright by AAS
Three possible causes of the large longitudinal spread of SEP events as observed by the STEREO s/c. Image reproduced with permission from Dresing et al. (2014), copyright by ESO
Left 10–30 MeV proton intensities for the April 11, 2013 event from STEREO A, ACE, and STEREO B. Inset shows s/c locations relative to the flare. Center 12–33 MeV/nucleon event-integrated fluences of He, O, and Fe versus s/c longitude; STEREO B is at and ACE is at . Right ACE/SIS He mass histograms compared with those measured during two Fe-rich SEP events in solar cycle 23. Image from ACE News #170, adapted from Cohen et al. (2014)
Two SEP events displayed such that each dot in the top panels represents the detection of an ion by ACE/ULEIS. SEP velocity dispersion, and the presence (left) or absence (right) of flux dropouts are easily identified. Image reproduced with permission from Mazur et al. (2000), copyright by ESO
Analysis of ACE/ULEIS observations of the edges of impulsive SEP dropout events. Left Superposition of dropout edges plotted in units of diffusion length, L (km), which take into account the convection of flux tubes that passed the s/c with the observed solar wind speed. Right Superposition of six dropouts observed during the impulsive SEP events that occurred on DOY 225 of 2000. Each panel shows three different energies. Image reproduced with permission from Chollet and Giacalone (2011), copyright by AAS
Interplanetary magnetic lines of force showing field-line meandering due to solar supergranulation, as discussed in the text. The coordinate system chosen has Z pointing normal to the heliographic equatorial plane. is the time scale associated with supergranulation that was chosen for each case. The right columns show the case of no transverse fluctuations at the source surface, and the result is the usual Parker spiral. Image reproduced with permission from Giacalone (2001), copyright by AGU
Numerical simulations of trajectories of individual charged particles moving through kinematically prescribed, turbulent electric and magnetic fields in the vicinity of two different shocks. The trajectories are displayed with kinetic energy along the vertical axis and position, relative to the position of the shock, which is fixed along the horizontal axis. In this frame, plasma flows from the upstream region at left to the downstream region at right. The average upstream magnetic field for the shock on the left makes an angle of relative to the shock normal. This is a nearly parallel shock. The shock on the right has an average upstream shock-normal angle of , making it a quasi-perpendicular shock. The particle on the left was followed for 270 gyro-periods (using the average upstream magnetic field) and the one on the right for 65 gyro-periods. See the text for more details. Image reproduced with permission from Decker (1988), copyright by Kluwer, who adapted it from Decker and Vlahos (1986)
Distributions of charged particles averaged over the downstream region of three shocks that differ only in the the angle between the average upstream magnetic field and the unit shock normal direction. The distributions were obtained from test-particle orbit integrations of particles moving through kinematically prescribed electric and magnetic fields associated with strong collisionless shocks. Image reproduced with permission from Giacalone (2005a), copyright by AAS
Cartoon illustration of a CME-driven shock expanding in the solar corona (courtesy of Allan Tylka)
The maximum momentum from diffusive shock acceleration at two different CME-driven shocks, at the shock location, as a function of time as the shock moves outwards in the heliosphere. The curves are for protons (solid), CNO ions (dashed), and Fe ions (dot-dashed). Image reproduced with permission from Li et al. (2005), copyright by AGU
Elemental abundance ratios of various heavy ion nuclei associated with a large, gradual SEP event as a function of time seen by an observer located 1.125 AU from the source of the event. Time is measured relative to the start of the event. The shock arrival time is indicated by a vertical arrow. Image reproduced with permission from Ng et al. (1999), copyright by AGU
Solutions to the Parker equation for particles accelerated at a shock moving through a spatially dependent magnetic field whose lines of force connect to the shock in two places. a Geometry of the magnetic field lines in the upstream and downstream regions. Color-coded representations of the distribution function of particles of b low energy (3–5 times the injection momentum), and c high-energy particles (15–30 times the injection momentum), where red is the most intense and black is the least intense. Image reproduced with permission from Guo et al. (2010), copyright by AAS
The distribution of thermal protons in the shock-heated plasma downstream of two collisionless shocks with similar Mach number and shock-normal angle, and initial thermal proton temperature. For the distribution shown in red, the upstream plasma consisted only of the thermal protons and electrons. The distribution in black includes thermal protons and electrons, as well as suprathermal pickup ions with density that of the thermal protons. These distributions are discussed in greater detail in Giacalone (2005a) and Giacalone and Decker (2010)
Electron (e) and He () time profiles from Helios 1 (0.3 AU) and IMP 8 (1 AU) during five SEP events in 1980. Magnetic connections to the flare site are indicated at upper right. Helios 1 observed five separate injections, while IMP 8 observed only one. Future missions, SPP and SolO, will enable us to separate the effects of transport by making key near-Sun measurements where SEP acceleration takes place. Image adapted from Wibberenz and Cane (2006)
Simulations of proton-amplified Alfvén-wave spectra during a large SEP event. Accelerated ions resonate at lower wave numbers (from right to left), and once they pass the peak, the acceleration efficiency and slope of the spectrum decrease. Solar Probe Plus and Solar Orbiter will enable measurements of such wave spectra inside 0.5 AU in multiple CME shock-associated events and determine how they affect SEP properties such as temporal evolution of heavy ion composition and spectral breaks over a broad energy interval. Image reproduced with permission from Ng et al. (2003), copyright by AAS
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