Spatial Distribution of Element Abundances and Ionization States in Solar Energetic-Particle Events

2018

Space Sci Rev

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The relative abundances of chemical elements and isotopes have been our most effective tool in identifying and understanding the physical processes that control populations of energetic particles. The early surprise in solar energetic particles (SEPs) was 1000-fold enhancements in 3 He/ 4 He from resonant wave-particle interactions in the small "impulsive" SEP events that emit electron beams that produce type III radio bursts. Further studies found enhancements in Fe/O, then extreme enhancements in element abundances that increase with mass-to-charge ratio A/Q, rising by a factor of 1000 from He to Au or Pb arising in magnetic reconnection regions on open field lines in solar jets. In contrast, in the largest SEP events, the "gradual" events, acceleration occurs at shock waves driven out from the Sun by fast, wide coronal mass ejections (CMEs). Averaging many events provides a measure of solar coronal abundances, but A/Q-dependent scattering during transport causes variations with time; thus if Fe scatters less than O, Fe/O is enhanced early and depleted later. To complicate matters, shock waves often reaccelerate impulsive suprathermal ions left over or trapped above active regions that have spawned many impulsive events. Direct measurements of ionization states Q show coronal temperatures of 1-2 MK for most gradual events, but impulsive events often show stripping by matter traversal after acceleration. Direct measurements of Q are difficult and often unavailable. Since both impulsive and gradual SEP events have abundance enhancements that vary as powers of A/Q, we can use abundances to deduce the probable Q-values and the source plasma temperatures during acceleration, ≈3 MK for impulsive SEPs. This new technique also allows multiple spacecraft to measure temperature variations across the face of a shock wave, measurements otherwise unavailable and provides a new understanding of abundance variations in the element He. Comparing coronal abundances from SEPs and from the slow solar wind as a function of the first ionization potential (FIP) of the elements, remaining differences are for the elements C, P, and S. The theory of the fractionation of ions by Alfvén waves shows that C, P, and S are suppressed because of wave resonances during chromospheric transport on closed magnetic loops but not on open magnetic fields that supply the solar wind. Shock waves can accelerate ions B D.V. Reames

Section: Spatial Distributions -Multi-spacecraft Observationsmentioning

confidence: 88%

Section: Spatial Distributions -Multi-spacecraft Observationsmentioning

confidence: 95%

Section: Spatial Distributions -Multi-spacecraft Observationsmentioning

confidence: 99%

See 1 more Smart Citation

Abundances, Ionization States, Temperatures, and FIP in Solar Energetic Particles

2018

Space Sci Rev

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“…To what extent can SEP abundances be transported laterally across the shock surface? Reames (2017b) found one event that showed the same unique non-power-law abundance pattern across 222 o of solar longitude. This event, on 23 January 2012, had flat or increasing abundance enhancements from He to Mg then decreasing abundances up to Fe.…”

Section: Discussionmentioning

confidence: 85%

The Abundance of Helium in the Source Plasma of Solar Energetic Particles

2017

Sol Phys

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Studies of patterns of abundance enhancements of elements, relative to solarcoronal abundances, in large solar energetic-particle (SEP) events, and of their power-law dependence on the mass-to-charge ratio A/Q of the ions, have been used to determine the effective source-plasma temperature T that defines the Q-values of the ions. We find that a single assumed value for the coronal reference He/O ratio in all SEP events is often inconsistent with the transport-induced power-law trend of the other elements. In fact, the coronal He/O actually varies rather widely from one SEP event to another. In the large Fe-rich SEP events with T ≈ 3 MK, where shock waves, driven out by coronal mass ejections (CMEs), have reaccelerated residual ions from impulsive suprathermal events that occur earlier in solar active regions, He/O ≈ 90, a ratio similar to that in the slow solar wind, which may also originate from active regions. Ions in the large SEP events with T < 2 MK may be accelerated outside active regions, and have values of 40 ≤ He/O ≤ 60.Mechanisms that determine coronal abundances, including variations of He/O, are likely to occur near the base of the corona (at ~ 1.1 R S ) and thus to affect both SEPs (at ~2 -3 R S ) and the solar wind. Other than He, reference coronal abundances for heavier elements show little temperature dependence or systematic difference between SEP events;He, the element with the highest first ionization potential, is unique. The CME-driven shock waves probe the same regions of space, at ~2 R S near active regions, which are also likely sources of the slow solar wind, providing complementary information on conditions in those regions.

“…In gradual or long-duration SEP events, particles are accelerated at shock waves driven out from the Sun by fast, wide CMEs (Kahler et al 1984). 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.…”

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.