The Origin of Element Abundance Variations in Solar Energetic Particles

2019

Sol Phys

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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 6 ≤ Z ≤ 56.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.

Section: Comparing Cmesmentioning

confidence: 99%

mentioning

confidence: 92%

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

2019

Sol Phys

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“…About 25% of gradual events have been found to have ≈3 MK source temperatures, relatively weak shocks, and abundances with enhancements that look like reaccelerated impulsive SEP events. These events are believed to involve preferential shock acceleration of residual impulsive suprathermal ions [28,[77][78][79]. Reacceleration of residual impulsive suprathermal ions might be especially favored when the magnetic field lies near the plane of the accelerating shock, so only fast particles can overtake the shock from downstream [77,78].…”

Section: Note In Panel (D) Ofmentioning

confidence: 99%

“…These gradual SEP events involving reaccelerated ≈3 MK ions have much smaller variations of source abundance than do single impulsive SEP events of the same temperature, especially in abundance ratios, like He/C, where both ions should be fully ionized at this temperature. Figure 7 shows a suggested explanation for this [75,79].…”

Section: Note In Panel (D) Ofmentioning

confidence: 99%

Element Abundances of Solar Energetic Particles and the Photosphere, the Corona, and the Solar Wind

2019

Atoms

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From a turbulent history, the study of the abundances of elements in solar energetic particles (SEPs) has grown into an extensive field that probes the solar corona and physical processes of SEP acceleration and transport. Underlying SEPs are the abundances of the solar corona, which differ from photospheric abundances as a function of the first ionization potentials (FIPs) of the elements. The FIP-dependence of SEPs also differs from that of the solar wind; each has a different magnetic environment, where low-FIP ions and high-FIP neutral atoms rise toward the corona. Two major sources generate SEPs: The small "impulsive" SEP events are associated with magnetic reconnection in solar jets that produce 1000-fold enhancements from H to Pb as a function of massto-charge ratio A/Q, and also 1000-fold enhancements in 3 He/ 4 He that are produced by resonant wave-particle interactions. In large "gradual" events, SEPs are accelerated at shock waves that are driven out from the Sun by wide, fast coronal mass ejections (CMEs). A/Q dependence of ion transport allows us to estimate Q and hence the source plasma temperature T. Weaker shock waves favor the reacceleration of suprathermal ions accumulated from earlier impulsive SEP events, along with protons from the ambient plasma. In strong shocks, the ambient plasma dominates. Ions from impulsive sources have T ≈ 3 MK; those from ambient coronal plasma have T = 1 -2 MK. These FIPand A/Q-dependences explore complex new interactions in the corona and in SEP sources.Forbush first observed the effects of SEPs [2], as increases in GeV particles that caused nuclear cascades through the atmosphere to produce what we now call ground-level events (GLEs). These SEPs were visually associated with solar flares and it was commonly assumed that the flares themselves caused SEP events in some way, but there were problems; for example, SEPs that were associated with these flares were eventually found to span more than 180° in solar longitude, which suggests a broad spatial distribution. How could the SEPs from a point-source flare cross magnetic field lines over such a distance? Newkirk and Wenzel [3] proposed the "birdcage" model where SEPs Atoms 2019, 7, 104 2 of 16 could follow coronal magnetic loops that then spread across the Sun like the wires of a birdcage. The birdcage model reigned many years, but Mason, et al. [4] argued that abundances of ions of different rigidity were unaffected by a trip through such a complex magnetic birdcage; they must arise from large-scale shock acceleration. At the same time, Kahler et al. [5] found a 96% correlation between large energetic SEP events and wide, fast coronal mass ejections (CMEs), which had just been discovered several years before. We now know that CMEs drive extensive expanding sun-spanning shock waves, where SEPs are accelerated to produce what we now call "long-duration" or "gradual" SEP events. Considerable controversy arose again with the publication of Gosling's [6] review article entitled "The Solar Flare Myth", which was alleged...

“…These shock waves accelerate SEPs (Lee, 1983(Lee, , 2005Zank, Rice, and Wu, 2000;Cliver, Kahler, and Reames, 2004;Ng and Reames, 2008;Gopalswamy et al 2012;Lee, Mewaldt, and Giacalone, 2012;Desai and Giacalone, 2016;Reames, 2017a), requiring shock speeds above 500 -700 km s -1 (Reames, Kahler, and Ng, 1997), and accelerating ions over an extremely broad region of the heliosphere (Reames, Barbier, and Ng, 1996;Rouillard et al, 2012;Cohen, Mason, Mewaldt, 2017;Reames, 2017b). Generally, the shock waves sample the 1 -2 MK coronal plasma (Reames, 2016a(Reames, , 2016b(Reames, , 2018b. Power-law dependence in gradual events can come from rigidity-dependent scattering of ions during transport (Parker, 1963;Ng, Reames, and Tylka, 1999Reames, 2016aReames, , 2016b, which results in dependence upon A/Q when ion abundances are compared at the same particle velocity.…”

Section: Introductionmentioning

confidence: 99%

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

2019

Sol Phys

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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.