arXiv subjects

Donald V. Reames

Publications and source records attributed to Donald V. Reames.

At least 19 recordsLinked to original sources

arXiv2025

Solar Particle Acceleration

High-energy particles may be accelerated widely in stellar coronae; probably by the same processes we find in the Sun. Here, we have learned of two physical mechanisms that dominate the acceleration of solar energetic particles (SEPs). The highest energies and intensities are produced in "gradual" events at shock waves driven from the Sun by fast, wide coronal mass ejections (CMEs). Smaller, but more numerous, "impulsive" events with unusual particle composition are produced during magnetic reconnection in solar jets and flares. Jets provide open magnetic field lines where SEPs escape; closed magnetic loops contain this energy to produce bright, hot flares, perhaps even contributing to heating the low corona in profuse nanoflares. Streaming protons amplify Alfven waves upstream of the shocks. These waves scatter and trap SEPs and, in large events, modify the element abundances and flatten the low-energy spectra upstream. Shocks also reaccelerate residual ions from earlier impulsive events, when available, that characteristically dominate the energetic heavy-ion abundances. The large CME-driven shock waves develop an extremely wide longitude span, filling much of the inner heliosphere with energetic particles.

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arXiv2024

Seeds and Sequences of Element Abundances in Solar Energetic Particle Events

Solar energetic particles (SEPs) in the small "impulsive" events, primarily accelerated during magnetic reconnection in solar jets, have strong enhancements of the abundances of increasingly heavy elements. In contrast, the shock acceleration of ambient coronal plasma in most large "gradual" SEP events produces flat or decreasing abundances vs. element mass-to-charge ratios A/Q. However, heavy-ion enhancements in the largest gradual SEP events can occur in two ways: (1) strong streaming of protons away from the shock amplifies Alfven waves that preferentially scatter and retard protons near the shock while increasingly-heavy ions can leak out, and (2) strong shock waves re-accelerate SEPs fed from persistent impulsive SEP events streaming from some active regions, with their pre-enhanced heavy ions becoming dominant. Power-law fits of abundance enhancements vs. A/Q can distinguish the latter events by the presence of both impulsive and coronal-seed components, and the best-fit charges Q define characteristic source temperatures. Intense impulsively-seeded events can occur in sequences fed from a single persistent active-region as it rotates across the disk of the Sun. Three week-long event sequences, each producing two or three very large events, occur early in the strong solar-cycle 23. The weak solar-cycle 24, produces only one impulsively-seeded event sequence - perhaps a dearth of both impulsive seeds and sufficiently strong shocks. Solar cycle 25 has produced an unusual active period of short strong impulsive events In contrast, there are other active regions where large events alternate SEPs with and without impulsively-seeded sources. We also find that events with moderate Alfven-wave trapping near the shock can release ions slowly or rapidly as a function of A/Q. This A/Q-dependent trapping acts almost as a magnetic spectrometer that separates elements in space and time.

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arXiv2024

Element Abundances and the Physics of Solar Energetic Particles

Acceleration and transport of solar energetic particles (SEPs) causes their abundances, measured at constant velocity, to be enhanced or suppressed as a function of each ion's magnetic rigidity, and hence its atomic mass-to-charge ratio A/Q. Ion charges, in turn, depend upon source electron temperature. In small "impulsive" SEP events, arising from solar jets, acceleration during magnetic reconnection causes steep power-law abundance enhancements. These impulsive SEP events can have 1000-fold enhancements of heavy elements from sources at ~2.5 MK, and similar enhancements of 3He/4He and of streaming electrons that drive type-III radio bursts. Gamma-ray lines show that solar flares also accelerate 3He-rich ions, but their electrons and ions remain trapped on magnetic loops so they dissipate their energy as X-rays, gamma-rays, heat, and light. "Gradual" SEPs accelerated at shock waves, driven by fast coronal mass ejections (CMEs), can show power-law abundance enhancements or depressions, even with see ions from the ambient solar corona. In addition, shocks can reaccelerate seed particles from residual impulsive SEPs with their pre-existing signature heavy-ion enhancements. Different patterns of abundance often show that heavy elements are dominated by a different source from that of H and He. Nevertheless, the SEP abundances averaged over many large events define the abundances of the corona itself, which is found to differ from the solar photosphere as a function of the first ionization potential (FIP) since ions, with FIP < 10 eV, are driven upward by forces of electromagnetic waves which neutral atoms, with FIP > 10 eV, cannot feel. Thus, SEPs provide a measurement of element abundances in the solar corona, distinct from the solar wind, and may even better define the photosphere for some elements.

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arXiv2023

Review and Outlook of Solar-Energetic-Particle Measurements on Multispacecraft Missions

The earliest evidence on spatial distributions of solar energetic particles (SEPs) compared events from many different source longitudes on the Sun, but the early Pioneers provided the first evidence of the large areas of equal SEP intensities across the magnetically-confined "reservoirs" late in the events. More-detailed measurements of the importance of self-generated waves and trapping structures around the shock waves that accelerate SEPs were obtained from the Helios mission plus IMP 8, especially during the year when the two Voyager spacecraft also happened by. The extent of the dozen widest SEP events in a solar cycle, that effectively wrap around the Sun, was revealed by the widely separated STEREO spacecraft with three-point intensities fit to Gaussians. Element abundances of the broadest SEP events favor average coronal element abundances with little evidence of heavy-element-enhanced "impulsive suprathermal" ions that often dominate the seed population of the shocks, even in extremely energetic local events. However, it is hard to define a distribution with two or three points. Advancing the physics of SEPs may require a return to the closer spacing of the Helios era with coverage mapped by a half-dozen spacecraft to help disentangle the distribution of the SEPs from the underlying structure of the magnetic field and the accelerating shock.

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arXiv2023

How Do Shock Waves Define the Space-Time Structure of Gradual Solar Energetic Particle Events?

We revisit the full variety of observed temporal and spatial distributions of energetic solar protons in "gradual" solar energetic-particle (SEP) events resulting from the spatial variations in the shock waves that accelerate them. Differences in the shock strength at the solar longitude of a spacecraft and at the footpoint of its connecting magnetic field line, nominally 55 degrees to the west, drive much of that variation. The shock wave itself, together with energetic particles trapped near it by self-amplified Alfven waves, forms an underlying autonomous structure that can drive across magnetic field lines intact, spreading proton intensities in a widening SEP longitude distribution. During the formation of this fundamental structure, historically called an "energetic storm particle" (ESP) event, many SEPs leak away early, amplifying waves as they flow along well-connected field lines and broaden the distribution outward; behind this structure between the shock and the Sun a "reservoir" of quasi-trapped SEPs forms. Very large SEP events are complicated by additional extensive wave growth that can spread an extended ESP-like trapping region. The multiplicity of shock-related processes contributing to the observed SEP profiles causes correlations of the events to be poorly represented by the peak intensities commonly used. In fact, the extensive spatial distributions of SEPs are sometimes interwoven with the structures of the shocks that have accelerated them and sometimes free. We should consider new questions: Which extremes of the shock contribute most to the SEPs profile of an event, (1) the shock at the longitude of a spacecraft, (2) the shock ~55 degrees to the west at the footpoint of the field, or (3) SEPs that have collected in the reservoir? How does the space-time distribution of SEPs correspond with the underlying space-time distribution of shock strength?

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arXiv2022

Solar Energetic Particles: Spatial Extent and Implications of the H and He Abundances

One of the earliest indicators of the importance of shock acceleration of solar energetic particles (SEPs) was the broad spatial extent of the "gradual" SEP events produced as the shock waves, driven by wide, fast coronal mass ejections (CMEs), expand across the Sun with cross-field transport mediated by the shocks. Contrasting "impulsive" SEP events, with characteristic enhancements of 3He and of heavy elements, are now associated with magnetic reconnection on open field lines in solar jets. However, large shock waves can also traverse pools of residual impulsive suprathermal ions and jets can produce fast CMEs that drive shock waves; in both cases shocks reaccelerate ions with the "impulsive" abundance signatures as well as coronal plasma. These more-complex events produce "excess protons" that identify this process, and recently, differences in the distribution of 4He abundances have also been found to depend upon the combination of seed population and acceleration mode. Extreme differences in the 4He abundances may reflect underlying differences in the abundances of the coronal regions being sampled by solar jets and, surprisingly, SEP events where shock waves sample two seed-particle populations seem to have about twice the 4He/O ratio of those with a single source.

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arXiv2022

Energy Spectra vs. Element Abundances in Solar Energetic Particles and the Roles of Magnetic Reconnection and Shock Acceleration

We reexamine the relationship between energy spectral indices and element abundance enhancements in solar energetic particle (SEP) events at energies of a few MeV/amu. We find a correlated behavior only in the largest gradual SEP4 events when all ions are accelerated from the ambient coronal plasma by shock waves driven by fast, wide coronal mass ejections (CMEs). This correlated abundance behavior can track complex time variations in the spectral indices during an event. In other (SEP3) events, CME-driven shock waves, days apart, sample seed particles from a single pool of suprathermal impulsive ions contributed earlier. Of the smaller, Fe-rich, impulsive SEP events, previously related to magnetic reconnection in solar jets, over half are subsequently reaccelerated by CME-driven shock waves (SEP2) causing typical ion intensities to have a 64% correlation with shock speed. In these SEP2 events, onset of shock acceleration is signaled by a new component in the abundances, large proton excesses. The remaining SEP1 events lack evidence of shock acceleration. However, for all these events (SEP1 - SEP3) with abundances determined by magnetic reconnection, spectra and abundances are decoupled.

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arXiv2021

Fifty Years of 3He-rich Events

The early 1970s saw a new and surprising feature in the composition of solar energetic particles (SEPs), resonant enhancements up to 10,000-fold in the ratio 3He/4He that could even make 3He dominant over H in rare events. It was soon learned that these events also had enhancements in the abundances of heavier elements, such as a factor of ~10 enhancements in Fe/O, which was later seen to be part of a smooth increase in enhancements vs. mass-to-charge ratio A/Q from H to Pb, rising by a factor of ~1000. These events were also associated with streaming 10 - 100 keV electrons that produce type III radio bursts. In recent years we have found these "impulsive" SEP events to be accelerated in islands of magnetic reconnection from plasma temperatures of 2 - 3 MK on open field lines in solar jets. Similar reconnection on closed loops traps the energy of the particles to produce hot (>10 MK), bright flares. Sometimes impulsive SEP intensities are boosted by shock waves when the jets launch fast coronal mass ejections. No single theory yet explains both the sharp resonance in 3He and the smooth increase up to heavier elements; two processes seem to occur. Sometimes the efficient acceleration even exhausts the rare 3He in the source region, limiting its fluence.

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arXiv2021

Sixty Years of Element Abundance Measurements in Solar Energetic Particles

Sixty years ago the first observation was published showing solar energetic particles (SEPs) with a sampling of chemical elements. Thus began study of the direct products of dynamic physics in the solar corona. As we have progressed from 4-min sounding-rocket samples to continuous satellite coverage of SEP events, we have extended the observations to the unusual distribution of element abundances throughout the periodic table. Small "impulsive" SEP events from islands of magnetic reconnection on open magnetic-field lines in solar jets generate huge enhancements in abundances of 3He and of the heaviest elements. Solar flares involve the same physics but there the SEPs are trapped on closed loops, expending their energy as heat and light. The larger, energetic "gradual" SEP events are accelerated at shock waves driven by fast, wide coronal mass ejections (CMEs). However, these shocks can also reaccelerate ions from pools of residual suprathermal impulsive ions, and CMEs from jets can also drive fast shocks, complicating the picture. The underlying element abundances in SEP events represent the solar corona, which differs from corresponding abundances in the photosphere as a function of the first ionization potential (FIP) of the elements, distinguishing low-FIP (<10 eV) ions from high-FIP neutral atoms as they expand through the chromosphere. Dependence of SEP acceleration upon A/Q allows best-fit estimation of ion Q-values and hence of the source plasma temperature of ~1 - 3 MK, derived from abundances, which correlates with recent measures of temperatures using extreme ultraviolet emission from jets. New questions arise, however, about the theoretical basis of correlations of energy-spectral indices with power-laws of abundances, about the coexistence of mechanisms for enhancements of 3He and of heavy elements, and about the overall paucity of C in FIP comparisons.

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arXiv2021

On the Correlation between Energy Spectra and Element Abundances in Solar Energetic Particles

In solar energetic particle (SEP) events, the physical processes of both shock acceleration and scattering during transport can cause energy-spectral indices to be correlated with enhancement or suppression of element abundances versus mass-to-charge ratios A/Q. We observe correlations for those "gradual" SEP events where shock waves accelerate ions from the ambient coronal plasma, but there are no such correlations for "impulsive" SEP events produced by magnetic reconnection in solar jets, where abundance enhancement in different events vary from (A/Q)^+2 to (A/Q)^+8, nor are there correlations when shock waves reaccelerate these residual impulsive ions. In these latter events the abundances are determined separately, prior to the accelerated spectra. Events with correlated spectra and abundances show a wide variety of interesting behavior that has not been described previously. Small and moderate gradual SEP events, with relative abundances typically depending approximately upon (A/Q)^-1 and the spectra upon energy E^-2.5, vary little with time. Large SEP events show huge temporal variations skirting the correlation line; in one case O spectra vary with time from E^-1 to E^-5 while abundances vary from (A/Q)^+1 to (A/Q)^-2 during the event. In very large events, streaming-limited transport through proton-generated resonant Alfve'n waves flattens the spectra and enhances heavy ion abundances prior to local shock passage, then steepens the spectra and reduces enhancements afterward, recapturing the typical correlation. Systematic correlation of spectra and element abundances provide a new perspective on the "injection problem" of ion selection by shocks and on the physics of SEP acceleration and transport.

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arXiv2020

Solar Energetic Particles (Second Edition)

In a field overflowing with beautiful images of the Sun, solar energetic particle (SEP) events are a hidden asset, perhaps a secret weapon, that can sample the solar corona and carry away unique imprints of its most bizarre and violent physics. Only recently have we found that the abundances of the elements in SEPs carry a wealth of data, not only on their own acceleration and history, but on plasma temperatures at their source, and on aspects of the genesis of the corona itself. SEPs are the tangible product of differing energetic outbursts at the Sun. They come in extremes. Little "impulsive" SEP events from magnetic reconnection in solar jets (also in flares), have most unusual 1000-fold resonant enhancements of 3He and of heavy elements like Au or Pb, while large "gradual" SEP events accelerated at shock waves driven by coronal mass ejections (CMEs), sample the composition of the corona itself, but also accelerate GeV protons that threaten Mars-bound astronauts with hazardous radiation. Direct SEP measurements plus solar images provide complimentary, "multi-messenger" data on high-energy physics at the Sun.

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arXiv2020

Distinguishing the Rigidity Dependences of Acceleration and Transport in Solar Energetic Particles

In solar energetic particle (SEP) events, the power-law dependence of element abundance enhancements on their mass-to-charge ratios A/Q provides a new tool that measures the combined rigidity dependences from both acceleration and transport. Distinguishing these two processes can be more challenging. However, the effects of acceleration dominate when SEP events are small or when the ions even propagate scatter-free, and transport can dominate the time evolution of large events with streaming-limited intensities. Magnetic reconnection in solar jets produces positive powers of A/Q from +2 to +7 and shock acceleration produces mostly negative powers from -2 to +1 in small and moderate SEP events where transport effects are minimal. This variation in the rigidity dependence of shock acceleration may reflect the non-planer structure, complexity, and time variation of coronal shocks themselves. Wave amplification by streaming protons in the largest SEP events suppresses the escape of ions with low A/Q, creating observed powers of A/Q from +1 to +3 upstream of the accelerating shock, decreasing to small negative powers downstream. Of course, the powers of A/Q are correlated with the spectral indices of He, O, and Fe, yet unexplained departures exist.

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arXiv2020

Virtues of Including Hydrogen in the Patterns of Element Abundances in Solar Energetic Particles

We revisit a multi-spacecraft study of the element abundances of solar energetic particles (SEPs) in the 23 January 2012 event, where the power-law pattern of enhancements versus the mass-to-charge ratio A/Q for the elements C through Fe was partly disrupted by a break near Mg, which turned out to be an unfortunate distraction. In the current article we find that extending that least-squares fits for C - Fe down to H at A/Q = 1 lends much more credence to the power laws, even though H itself was not included in the fits. We also investigate the extent of an adiabatically invariant "reservoir" of magnetically-trapped particles behind the shock wave in this event.

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arXiv2020

Four Distinct Pathways to the Element Abundances in Solar Energetic Particles

Based upon recent evidence from abundance patterns of chemical elements in solar energetic particles (SEPs), and, ironically, the belated inclusion of H and He, we can distinguish four basic SEP populations: (1) SEP1 - pure "impulsive" SEPs are produced by magnetic reconnection in solar jets showing steep power-law enhancements of 1<= Z <= 56 ions versus charge-to-mass ratio A/Q from a ~3 MK plasma. (2) SEP2 - ambient ions, mostly protons, plus SEP1 ions reaccelerated by the shock wave driven by the narrow coronal mass ejection (CME) from the same jet. (3) SEP3 - a "gradual" SEP event is produced when a moderately fast, wide CME-driven shock wave barely accelerates ambient protons while preferentially accelerating accumulated remnant SEP1 ions from an active region fed by multiple jets. (4) SEP4 - a gradual SEP event is produced when a very fast, wide CME-driven shock wave is completely dominated by ambient coronal seed population of 0.8 - 1.8 MK plasma usually producing a full power law vs. A/Q for 1<= Z <= 56 ions. We begin with element abundances in the photosphere that are fractionated during transport up to the corona based upon their first ionization potential (FIP); this important "FIP effect" for SEPs provides our reference abundances and is different for SEPs from that for the solar wind. We then show evidence for each of the processes of acceleration, reacceleration, and transport that conspire to produce the four abundances patterns we distinguish.

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arXiv2019

Excess H, Suppressed He, and the Abundances of Elements in Solar Energetic Particles

Recent studies of the abundances of H and He relative to those of heavier ions in solar energetic particle (SEP) events suggest new features in the underlying physics. Impulsive SEP events, defined by uniquely large enhancements of Fe/O, emerge from magnetic reconnection in solar jets. In small, "pure," shock-free, impulsive SEP events, protons with mass-to-charge ratio A/Q = 1 fit the power-law dependence of element abundance enhancements versus A/Q extrapolated from the heavier elements 2 < Z < 57. Sometimes these events have order-of-magnitude suppressions of He, even though H fits with heavier elements, perhaps because of the slower ionization of He during a rapid rise of plasma from the chromosphere. In larger impulsive SEP events, He fits, but there are large proton excesses relative to the power-law fit of Z > 2 ions, probably because associated coronal mass ejections (CMEs) drive shock waves fast enough to reaccelerate the impulsive SEPs but also to sample protons from the ambient solar plasma. In contrast, gradual SEP events are accelerated by wide, fast CME-driven shock waves, but those with smaller, weaker shocks, perhaps quasi-perpendicular, favor impulsive suprathermal residue left by many previous jets, again supplemented with excess protons from ambient coronal plasma. In the larger, more common gradual SEP events, faster, stronger shock waves sample the ambient coronal plasma more deeply, overwhelming any impulsive-ion component, so that proton abundances again fit the same power-law distribution as all other elements. Thus, studies of the power-law behavior in A/Q of SEP element abundances give compelling new information on the varying physics of SEP acceleration and properties of the underlying corona.

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arXiv2019

Hydrogen and the Abundances of Elements in Gradual Solar Energetic-Particle Events

Despite its dominance, hydrogen has been largely ignored in studies of the abundance patterns of the chemical elements in gradual solar energetic-particle (SEP) events; those neglected abundances show a surprising new pattern of behavior. Abundance enhancements of elements with 2 <= Z <= 56, relative to coronal abundances, show a power-law dependence, versus their average mass-to-charge ratio A/Q, that varies from event to event and with time during events. The ion charge states Q depend upon the source plasma temperature T. For most gradual SEP events, shock waves have accelerated ambient coronal material with T < 2 MK with decreasing power-laws in A/Q. In this case, the proton abundances agree rather well with the power-law fits extrapolated from elements with Z >= 6 at A/Q > 2 down to hydrogen at A/Q = 1. Thus the abundances of the elements with Z >= 6 fairly accurately predict the observed abundance of H, at a similar velocity, in most SEP events. However, for those gradual SEP events where ion enhancements follow positive powers of A/Q, especially those with T > 2 MK where shock waves have reaccelerated residual suprathermal ions from previous impulsive SEP events, proton abundances commonly exceed the extrapolated expectation, usually by a factor of order ten. This is a new and unexpected pattern of behavior that is unique to the abundances of protons and may be related to the need for more streaming protons to produce sufficient waves for scattering and acceleration of more heavy ions at the shock.

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arXiv2019

Hydrogen and the Abundances of Elements in Impulsive Solar Energetic-Particle Events

Hydrogen has been almost completely ignored in studies of the abundance patterns of the chemical elements in solar energetic particles (SEPs). We seek to find impulsive events where H fits these abundance patterns and document the events that do not, suggesting possible reasons for the disparity. For 24 % of the smaller impulsive SEP events, the relative abundance of H fits within one standard deviation of the power-law fit of the abundances of elements 6 <= Z <= 56, relative to coronal abundances. In impulsive events with high intensities, H can be 10 to 100 times its expected value. In a few of these larger events, increased scattering at high wavenumber may preferentially detain H, perhaps with self-amplified waves; in some events pre-event proton background may con-tribute. In most large impulsive SEP events, however, associated shock waves must play a much greater role than previously thought; fast (>500km/s) coronal mass ejections con-tribute to 62 % of impulsive events. Shocks may sample protons from the ambient coronal plasma or residual background as well as reaccelerating heavier impulsive SEP ions injected from the region of magnetic reconnection in solar jets. Excess H may be a signature of shock acceleration.

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arXiv2019

Helium Suppression in Impulsive Solar Energetic-Particle Events

We have studied the element abundances and energy spectra of the small "He-poor" impulsive solar energetic-particle (SEP) events, comparing them with other impulsive SEP events with more-normal abundances of He. He-poor events can have abundances as low as He/O ~ 2, while both impulsive and gradual SEP events usually have source abundances of 30 < He/O < 100 with mean values of 50 - 60. He/C ratios are not only low, but often decrease with energy in He-poor events. Abundance enhancement patterns of other elements with atomic numbers 6 < Z < 56, and likely values of their mass-to-charge ratios A/Q, are generally unaltered in He-poor events, as are the probable source-plasma temperatures of 2.5 - 3.2 MK for all impulsive SEP events. One He-poor event is also an example of a rarer C-poor event with C/O = 0.08 +- 0.04, sup-pressed by a factor over 5 from the mean. We discuss suggestions of a possible A/Q threshold during acceleration and of the sluggish ionization of He entering the corona, because of its uniquely high first ionization potential (FIP), but the suppression of He and the decline of He/C with energy is difficult to explain if both He and C are fully ionized with A/Q = 2 as expected at 2.5 - 3.2 MK. Although less dramatic, a possible excess enhancement of Ne in some impulsive SEP events is also considered. Possible causes of the large ~30% spectral and abundance variations in impulsive events are also discussed. However, the physics of the He-poor events remains a mystery.

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