Solar Wind: Energetic Particles, DV Reames, P Murdin

Tags: abundances, Acceleration, flare, shock waves, energy spectra, solar wind, ions, Energetic Particles, intensities, Nature Publishing Group, event, UK, solar minimum, Institute of Physics Publishing, magnetic field, populations, energetic particle, solar atmosphere, magnetic fields, observed, ACRs, solar flares, Houndmills Basingstoke, WIND spacecraft, IOP Publishing Ltd, bow shock, Institute of Physics Publishing Bristol, electron temperature, heliosphere, ASTRONOMY AND ASTROPHYSICS, Particle acceleration, distant sources, particle interactions
Content: eaa.iop.org DOI: 10.1888/0333750888/2312 Solar Wind: Energetic Particles Donald V Reames From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2005 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Wed Jul 13 08:43:31 BST 2005 [127.0.0.1] Terms and Conditions
Solar Wind: Energetic Particles
ENCYCLOPEDIA OF Astronomy and Astrophysics
Solar Wind: Energetic Particles The energetic particle populations that we observe in the SOLAR WIND are accelerated in a variety of local and distant sources. We distinguish different populations by their temporal, spatial and angular distributions, energy spectra, abundances and ionization states. They tell us the physics of acceleration mechanisms and the properties of remote sites that are otherwise invisible or inaccessible. Sources The acceleration of particles to high energies is a remarkably common occurrence in the ionized plasmas of the heliosphere and presumably throughout the Galaxy. Often the presence of energetic particles in distant sources is inferred from observations of the photons they produce as they collide with matter. radio emission, hard x-rays and gamma rays are produced when energetic electrons and ions interact with magnetic fields or with material in the ambient plasma. However, at times it is also possible to observe directly a sample of the accelerated material that has propagated to us at high speed along the often-tangled magnetic fields in space. In the heliosphere these energetic particles have disclosed a variety of new acceleration sites where the matter is too tenuous for photon production. Particles are known to be accelerated to MeV and even GeV energies in solar FLARES, at the shock waves driven out from the Sun by SOLAR CORONAL MASS EJECTIONS (CMEs), in planetary MAGNETOSPHERES and at planetary bow shocks. They are also accelerated at corotating interaction regions (CIRs) between high- and low-speed streams in the solar wind (see SOLAR WIND: COROTATING INTERACTION REGIONS) and at the solar wind termination shock at the outer edge of the heliosphere (see SOLAR WIND SHOCK WAVES AND DISCONTINUITIES). In addition, we observe the galactic cosmic rays that have probably been accelerated by shock waves from supernovae. We distinguish these different populations of energetic particles and identify their sources by the particle arrival timing and associations with other phenomena, by their spatial distribution and arrival directions, by their energy spectra, and by the abundances of elements and the ionization states of the ions in these populations. As measurements have become more sensitive and complete, it has become possible to distinguish particle sources that were previously unclear. One important example has been the identification of different sources for the impulsive and gradual solar energetic particle (SEP) events as flares and CME-driven shocks, respectively. Once it was believed that all of these particles came from flares, the so-called `flare myth'. Another example has been the new evidence that particles upstream of the Earth's bow shock are actually
accelerated by that shock and are not merely escapees from the magnetosphere (see MAGNETOSPHERE OF EARTH: BOW SHOCK). Impulsive solar flares Impulsive solar flares are sources that provide us with considerable information on particle acceleration. We can observe the accelerated particles in space and the photons produced near the Sun. Most of the acceleration in flares takes place on closed magnetic loops in the CORONA. As these loops become tangled from circulation at their photospheric footpoints or as new magnetic flux emerges from the photosphere, energy can be suddenly released by magnetic reconnection. The associated wave turbulence is the probable source of energy for stochastic acceleration (see SOLAR FLARES: PARTICLE ACCELERATION MECHANISMS). As particles scatter into the loss cone, they plunge into increasingly dense material near the footpoints of the loops where they lose energy by Coulomb interactions. A small fraction of the electron's energy is lost to x-ray bremsstrahlung and a small fraction of the ions interact to produce -ray lines from excited nuclei in the beam or the ambient material. Electrons escaping along open field lines produce type III radio bursts as they stream out along the magnetic field through plasma of decreasing density. Finally, satellites in space can observe both the energetic electrons and ions directly. As the particles stream out along magnetic field lines they are scattered somewhat by Alfvйnic fluctuations of the magnetic field, although the scattering mean free path of ~1 AU is comparable with the distance to the Sun. This, together with different particle velocities and pitch angles with respect to the field, can spread the particle arrival times over a period of several hours. Typical profiles of particle intensity versus time for a series of impulsive events are shown in figure 1(a). Different particle speeds produce `velocity dispersion' in the earliest arrival times, a sophisticated way of saying that the fastest particles arrive first. The propagation time, t = L/µv, where L is the distance along the field line, µ is the average cosine of the pitch angle and v is the particle velocity. Photons and relativistic particles take at least 8 min to propagate 1 AU, 50 keV electrons or 100 MeV amu-1 ions take 18 min, and 1 MeV amu-1 ions take 2.9 h. This relationship can be used to associate all particle species and energies with a particular flare at the Sun, within measurement errors of a few minutes. Type III radio observations provide additional information that can track the 10­100 keV electron population in direction and distance as it moves out from the Sun along the field line. Particles from impulsive events are only seen over a solar-longitude interval of ~30° where field lines are fairly well connected to the flare (see SOLAR FLARES: IMPULSIVE PHASE). Although poorly measured, this longitude interval is of interest because it is controlled by the random walk of magnetic field lines.
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Solar Wind: Energetic Particles
ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS
Figure 1. Intensity­time profiles are shown for various sources using the same time and intensity scales for each. (a) Electron and proton data from ISEE 3 are shown for a series of flares at the times and solar longitudes indicated. (b) Proton intensities from the NOAA/GOES spacecraft are plotted for a series of gradual events at the energies shown. Times and longitudes of CME launch at the Sun and the shock arrival at Earth are indicated. (c) Intensities of He at various energies are shown during a CIR event as observed on the WIND spacecraft.
Particle abundances are one of the most distinctive features of impulsive-flare events. We often find the ratio of abundances of isotopes of He, 3He/4He~1 in these events while this ratio is ~5 Ч 10-4 in the solar atmosphere or the solar wind. For this reason, these impulsive-flare events are often called 3He-rich events. High electron abundances and more modest enhancements of heavy ions also occur. On average, Ne/O, Mg/O, and Si/O are enhanced by a factor of ~3 and Fe/O by a factor of ~7 in these events, relative to the corresponding coronal or solar wind abundances (see table 1 and SOLAR ABUNDANCES). Element abundances derived from the intensities of broad -ray lines emitted by energetic particles inside the flares are consistent with those measured for the energetic particles in space. These abundances are taken as evidence of resonant wave­ particle interactions during stochastic acceleration. The huge enhancement of 3He may occur because intense beams of streaming electrons excite electromagnetic ion
cyclotron waves between the gyrofrequencies of the dominant species, H and 4He, in the flare plasma. The rare isotope 3He, the only species whose gyrofrequency lies in this region, is preferentially accelerated as it absorbs energy from these waves. Heavy ions may be enhanced by other wave modes or by second-harmonic interaction with the same waves. Acceleration of particles by resonant interaction with waves also occurs in other regions of high magnetic field energy, such as the auroral region of the Earth where both the particles and resonant waves have been observed together in the phenomenon known as `ion conics'. Measurements of ionization states of the energetic ions show that all elements up to Si are fully ionized, while Fe has ~20 of its 26 electrons removed, corresponding to an electron temperature of ~15 MK (1.5 Ч 107 K). This is not surprising for a flare temperature; atomic spectral lines of highly ionized Fe are often seen from flares. However, if the elements C, N, O, Ne, Mg,
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and Si were fully ionized before acceleration, they would all have the same charge-to-mass ratio, Q/A = 0.5, hence the same magnetic gyrofrequency. In this case, it would be impossible to produce relative enhancements since all ions would resonate with the same waves. Actually, the pattern of enhancement is consistent with the values of Q/A that would exist in a 3­5 MK plasma, a typical active-region temperature. This suggests that the ions are drawn from the ambient plasma and accelerated early in flare; stripping then occurs later as the electron temperature rises. Gradual events and CME-driven shocks The most intense energetic-particle events we see near Earth are the `gradual' events produced by particles accelerated at shock waves driven out from the Sun by fast CMEs. Essentially all (>96%) large proton events are associate with CMEs. Since most CMEs have speeds that are near or slightly above that of the solar wind, they do not produce shocks, nor do they accelerate particles. Particle acceleration only occurs for the fastest ~1% of CMEs. Shocks with speeds >500 km s-1 usually accelerate particles while those with speeds >750 km s-1 nearly always do. The largest SEP events of a solar cycle are produced by shocks with speeds of 1500­ 2000 km s-1. Shock speeds can decrease by as much as a factor of 2 between the Sun and Earth. Acceleration of protons up to ~10 MeV usually continues to 1 AU and beyond in most moderate events. Acceleration of 100 MeV­1 GeV protons occurs primarily near the Sun in most events but continues out to 1 AU in the largest events. In the large 19 October 1989 event shown in figure 1(b), particle intensities peak at the time of shock passage (on 20 October) even at energies of ~1 GeV.
Acceleration occurs at shock waves as particles are scattered back and forth across the shock surface, gaining an increment in speed on each transit. The upstream and downstream scattering centers serve as approaching `walls' from which the particles are reflected. The distribution of particles streaming away from the shock is itself unstable to generation of Alfvйn waves that add to the scattering of particles that follow behind. This nonlinear process serves to trap particles near the shock and increase the efficiency of acceleration. However, in shock acceleration, the energy of the particles comes from the Kinetic Energy of the shock, not just from the energy in the waves. Ionization-state measurements have provided the clearest and most compelling evidence on the origin of the ions in the large gradual events. The ionization states are characteristic of a 1­2 MK plasma. Even C and O are not fully ionized and the mean ionization state of Fe, QFe ~ 14. These ions could not come from the hot plasma of a flare; they represent ambient unheated coronal plasma. Ionization states of Fe have now been measured over an energy interval of 0.3­600 MeV amu-1 by 6 different experiments on 4 spacecraft. The highest-energy measurements were made in some of the largest events of the last solar cycle. Not only do these ions place an upper limit on the source temperature, but also the high-energy ions would be stripped of electrons in seconds at densities of 1010 cm-3 found in the low corona where flares occur. The ions must have been accelerated from ambient coronal plasma at low density. This conclusion is in agreement with observations that the peak intensities of GeV protons are produced when the leading edge of the CME is at 5­10 solar radii.
Table 1. Element abundances in energetic particle sources.
Z H1 He 2 C6 N7 O8 F9 Ne 10 Na 11 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 Ar 18 K 19 Ca 20 Ti 22 Cr 24 Fe 26 Ni 28 Zn 30
FIP 13.53 24.46 11.22 14.48 13.55 17.34 21.47 5.12 7.61 5.96 8.12 10.9 10.3 12.95 15.68 4.32 6.09 6.81 6.74 7.83 7.61 9.36
Photosphere 1.35 Ч 106 132 000±11 000 479±55 126±20 1000±161 0.05±0.03 162±22 2.9±0.2 51±6 4.0±0.6 48±5 0.38±0.04 29±7 0.4±0.3 4.5±1.0 0.18±0.05 3.09±0.14 0.14±0.02 0.63±0.04 42.7±3.9 2.4±0.05 0.054±0.010
Gradual events (SEP corona) (1.57±0.22) Ч 106 57 000±3000 465±9 124±3 1000±10 <0.1 152±4 10.4±1.1 196±4 15.7±1.6 152±4 0.65±0.17 31.8±0.7 0.24±0.1 3.3±0.2 0.55±0.15 10.6±0.4 0.34±0.1 2.1±0.3 134±4 6.4±0.6 0.11±0.04
CIR events (Coronal hole) (1.81±0.24) Ч 106 159 000±1000 890±36 140±14 1000±37 170±16 140±14 100±12 50±8 97±11
Impulsive flares ~1 Ч 106 46 000±4000 434±30 157±18 1000±45 <2 400±28 34±8 408±29 68±12 352±27 4±3 117±15 <2 30±8 2±2 88±13 <2 12±5 1078±46 42±9 6±4
FIP: first ionization potential.
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Anomalous CR ~1000±500 5000±1000 <20 120±10 1000±10 <2 70±10 <0.2 1.2±0.3 <0.1 1.7±0.3 <0.1 0.6±0.2 <0.1 4.2±0.5 <0.1 <0.1 <0.1 <0.9 <0.05 <0.05
Galactic CR source (1.74±0.43) Ч 105 23 500±1600 843±67 61±24 1000±40 <5 125±16 11±7 207±12 20±8 196±12 <5 25±4 <3 5.9±1.4 <3.7 1.7±3.5 <4.7 <5.7 182±11 10±1.7 0.13±0.02 3
Solar Wind: Energetic Particles At energies of a few MeV amu-1, most element abundances in gradual SEP events do not vary significantly from event to event and the average abundances seem to directly reflect the abundances of the source plasma in the solar corona. In fact, SEP abundances provide the most complete information we have on coronal abundances on a total of 22 elements (see table 1). Element abundances in the corona differ from those in the photosphere in a way that depends upon their first ionization potential (FIP), or perhaps upon their first ionization time scale. Elements with FIP < 10 eV (e.g. K, Na, Al, Mg, Si and Fe) have abundances that are enhanced by a factor of ~4 relative to high-FIP elements (e.g. C, N, O, Ne, Ar). Low-FIP elements are ionized in the photosphere, while high-FIP elements are neutral. An ion­neutral fractionation occurs because ions and neutral atoms are transported differently from the photosphere into the corona. Once elements reach the 1­2 MK corona they all become highly ionized long before they are accelerated. A characteristic feature of gradual SEP events is their large and complex spatial structure that produces a wide variety of time profiles when viewed by spacecraft at different longitudes relative to the CME. Figure 2 shows time profiles of proton intensities seen by 3 spacecraft whose configuration relative to the CME is shown in the inset. As the spiral interplanetary field lines are swept over a spacecraft, the longitude of the magnetic connection point to the shock swings eastward with time. In this event, the 3 spacecraft become better connected to the intense acceleration region near the nose of the shock as time increases. HELIOS 1 is best connected. Here the intensity rises to a flat profile where the intensity is controlled by limits on the rate particles can stream away from the shock. A sharp peak in intensity is seen as the shock passes the spacecraft. The other two spacecraft see lower intensities around the flank of the shock; intensities increase as their connection point moves toward the nose of the shock. Behind the shock all 3 spacecraft enter an invariant-spectral region where particle intensities decrease slowly with time as the containment volume for the particles slowly expands. Energy spectra of ions from the 20 October 1995 event are shown in figure 3(b). These spectra were obtained during the invariant spectral period in this relatively small event by the WIND spacecraft. The proton spectrum from the plateau region in the large event of 19 October 1989 is also shown in the figure for comparison. This event, shown in figure 1(b), was the largest event that occurred during the last solar maximum.
ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS Figure 2. Intensity­time profiles of 3­6 MeV protons observed by 3 spacecraft are shown for a gradual event of modest extent. Spacecraft locations around the CME are shown in the inset. Lower panels show proton energy spectra taken early in the event (a) and during the `invariant' spectral period (b). Acceleration at the Earth's bow shock The bow shock produced when the solar wind encounters the Earth's magnetosphere is also a source of accelerated particles. These were first observed as intense bursts of particles (and resonant waves) seen by the ISEE spacecraft in the region sunward of Earth. Since magnetic flux tubes contact the Earth's bow shock for a short time, the energies of the accelerated ions are usually below 100 keV amu-1. Bursts were observed only during periods when the magnetic field connected the spacecraft to the bow shock. As the solar wind sweeps field lines past the Earth, they remain in contact with the bow shock for a limited period. For the nominal field direction, this contact point is usually swept across the sunward face of the shock from the dusk to the dawn side; the field direction becomes increasingly parallel to the shock normal. However, it was not possible in the early measurements to distinguish which elements comprised the `ions', or to clearly establish that the particles were accelerated at the shock rather than simply leaking out from the Earth's radiation belts.
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Figure 3. Energy spectra of various particle species indicated are shown for different particle populations. (a) Spectra for a small impulsive-flare event on 2 April 1995 observed on the WIND spacecraft are shown. (b) Spectra for various species in the small 20 October 1995 event observed on the WIND spacecraft are shown and a proton spectrum observed by NOAA/GOES during the large 19 October 1989 event (See Figure 1(b)) is shown for comparison. (c) Spectra of He during the May­June 1995 CIR event (see figure 1(c)) show variation with time. The low-energy GCR spectra of various species during solar minimum are shown, and the lowenergy rise in O from the presence of ACRs is noted on the figure.
The first measurements of element abundances in
The time evolution of He intensities from a large
these `upstream events' were made aboard the WIND CIR is shown in figure 1(c). Particles from the forward
spacecraft. It was found that the ions had the same shock are seen on 29 May. As the Sun rotates, the Earth
element abundances as the high-speed solar wind and crosses the stream interface from the slow to the fast
that events were much more common during high-speed solar wind on 30 May. At first, low-energy particles
solar-wind streams when the bow shock was strongest. In predominate from the newly formed shock. Later, as we
one case an upstream event even occurred during a small solar 3He-rich event so that the intense burst of ions flowing upstream from the shock was also 3He-rich.
become connected to the strengthening shock farther away from us, high-energy particle intensities increase while those at low energies are attenuated by the greater
Clearly the source of the accelerated ions was these transport distances. This behavior is the inverse of the
available particles from the upstream solar wind, not velocity dispersion seen in both impulsive and gradual
those from the magnetosphere.
SEP events. For the unusual event of May­June 1995,
particles were seen for 17 d while the sun rotated through
Co-rotating interaction regions (CIRs) CIRs are formed when high-speed solar-wind streams, emitted from coronal holes on the Sun, overtake the lowspeed solar wind emitted earlier in the solar rotation. This interaction produces a pair of shocks: a forward shock propagates outward into the slow solar wind and a reverse shock propagates sunward into the high-speed solar wind. The shocks usually begin to form outside 1 AU and they continue to strengthen out to several AU. Particles are accelerated at both shocks, although the reverse shock seems to produce greater intensities and harder energy spectra. Thus particles from the reverse shock are most easily observed near Earth flowing sunward through the high-speed stream.
an angle of ~225° and the reverse shock propagated across the high-speed stream and far into the following low-speed solar wind. The evolution of the energy spectra of He in this event is shown in figure 3(c). Averaged element abundances of energetic particles from CIRs are included in table 1. These abundances are most similar to those of the high-speed solar wind that serves as the source. The enhancement of low-FIP elements relative to high is only a factor of ~2, indicating that abundances of the material from coronal holes suffers less fractionation than that in the corona and in active regions. However, departures from solar-wind abundances are also seen. Elevated abundances of H and He, for example, probably result from the acceleration of interstellar pickup ions (see below) although O, the
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talisman of this source, is not enhanced relative to C, N or Ne, for example. Anomalous cosmic rays (ACRs) ACRs are yet another shock-accelerated particle population with unique element abundances. In the interstellar material outside the heliosphere, elements with FIP less than the ionization potential of H, 13.6 eV, are ionized while those of higher FIP are neutral. Interstellar H absorbs the photons of higher energy. As the solar system moves through this gas, neutral atoms easily penetrate, but ions are largely excluded by the magnetic fields. As these interstellar neutrals approach the Sun they are ionized by solar photons or, in the case of H, by charge exchange with the solar wind. These new ions are suddenly able to sense the presence of the magnetic fields of the solar wind. Thus they are `picked up' by the solar wind and they acquire a velocity distribution that extends to twice the solar-wind speed. The ions are eventually convected out to the solar-wind termination shock that is estimated to be formed 100 AU from the Sun. Here, the higher initial injection speed of the pickup ions leads to their preferential acceleration. The energetic ions are modulated as they propagate back to 1 AU where they are observed (see COSMIC RAYS). The ACRs were first observed by the anomalously high intensities of O (relative to C, for example) at ~10 MeV amu-1 near solar minimum (see figure 3(c)). O, as well as He, N and Ne, persisted during solar minimum but were then modulated with the 11 yr solar cycle in a manner similar to the galactic cosmic rays. The theory of the singly ionized interstellar pickup ions and of their acceleration at the termination shock was advanced 20 years before pickup ions were first observed in the solar wind. Recently, ionization states of ACRs have also been directly measured, confirming that most are singly ionized while a small fraction have become more highly ionized during acceleration and transport. Approximate abundances of ACRs at the termination shock are included in table 1; these abundances depend upon assumptions made to correct for solar modulation. Galactic cosmic rays (GCRs) Cosmic rays, probably accelerated at SUPERNOVA shock waves in the Galaxy, have a constant presence throughout the heliosphere (see COSMIC RAYS: PROPAGATION IN THE HELIOSPHERE). GCRs are easily distinguished from other particle populations by their high energies and their unusual element abundances. As observed, GCRs contain unusually high abundances of Li, Be and B that are produced in nuclear reactions with interstellar matter during the ~107 yr lifetime of the GCRs. It is possible to correct for this matter traversal to derive GCR source abundances that are shown in table 1. The first suggestion of the existence of an ion­neutral
fractionation process based upon FIP was made for the GCR source abundances by comparing them with the `universal' abundances of elements derived from meteorites (essentially equivalent to solar photospheric abundances). Similarities between the abundances of GCRs and those of large gradual SEP events were noted many years ago, suggesting that stellar coronae or stellar energetic particles provide the seed population for the GCRs. Competing theories explain the enhancement in low-FIP elements in GCRs by the preferential acceleration of INTERSTELLAR GRAINS that are rich in those elements. However, we have seen that the process of separating ions and neutral atoms across electromagnetic fields, seen in the SEP, CIR and ACR abundances, is quite common in the heliosphere. It would be surprising if this process were confined to the heliosphere while an entirely different mechanism governed the GCR abundances. The low-energy portion of the spectra of the dominant species in the GCRs is shown in figure 3(c). Above ~1 GeV amu-1 the differential intensity spectra attain an E-2.6 dependence that continues to ~1 TeV amu-1 where the spectrum steepens to form a `knee'. The low intensities of particles here and at higher energies can only be measured indirectly by large detector arrays at ground level. Hence, element abundances and their evolution as a function of energy have not been measured here. Solar cycle variations Different epochs in the 11 yr solar cycle bring an enormous change in the character of the energetic particles we see in the solar wind near Earth. The most favorable `season' for observing ACRs and low-energy GCRs is at solar minimum when their intensities are highest. These particles are suppressed as the solar cycle increases. In fact, ACRs have never been observed during the years around solar maximum. It is not surprising that the numbers of both impulsive and gradual SEP events increase as Solar Activity increases and more flares and CMEs occur. The number of impulsive-flare-related events varies from a few per year at solar minimum to a few hundred per year at solar maximum. The number of gradual events varies from about 5 yr-1 at minimum to 30 yr-1 at maximum. Of course these numbers depend upon the intensity threshold and the particle species and Energy Used to count events. However, the numbers are not the whole story. The events at solar minimum are also smaller and have softer energy spectra than those at solar maximum (see the proton spectra for the two gradual events in figure 3(b)). These are the products of less-energetic flares or of weaker shocks. For CIR events, the variation is more complicated. Energetic particles from CIRs can be seen at most phases of the solar cycle. However, they are difficult to observe
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Solar Wind: Energetic Particles (or to count) during solar maximum because they are often obscured by SEP events and the CIRs themselves are disrupted by CMEs. Thus the events are best studied near solar minimum. However, at times very near solar minimum, such as during 1996, the heliospheric current sheet becomes very flat and lies near the solar equatorial plane. Thus, the polar high-speed streams and the equatorial low-speed streams can flow out radially with no regions of interaction. During these times there are few CIRs formed and few particles accelerated. Bibliography Fisk L, Kozlovsky B and Ramaty R 1974 An interpretation of the observed oxygen and nitrogen enhancements in the low energy cosmic rays Astrophys. J. 190 L35 Gosling J T 1993 The solar flare myth J. Geophys. Res. 98 18 949 Reames D V 1990 Energetic particles from impulsive solar flares Astrophys. J. Suppl. 73 235 Reames D V 1995 Coronal abundances determined from energetic particles Adv. Space Res. 15 (7) 41 Reames D V 1999 Particle acceleration at the sun and in the heliosphere space science Reviews 90 413. See http://kapis1.wkap.nl/oasis.htm/231960 Donald V Reames
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