http://arxiv.org/abs/1709.00741
The relative abundances of chemical elements and isotopes have been our most effective tool in understanding the physical processes that control populations of energetic particles. The early surprise in solar energetic particles (SEPs) was 1000-fold enhancements in 3He/4He from resonant wave-particle interactions in the small “impulsive” SEP events with electron beams that produce type III radio bursts. Further studies found enhancements in Fe/O, and abundances increasing with mass-to-charge ratio A/Q, rising by a factor of 1000 from He to Pb arising in magnetic reconnection 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 events measures solar coronal abundances, but A/Q-dependent scattering during transport causes variations with time. To complicate matters, shock waves often reaccelerate impulsive suprathermal ions left over or trapped above active regions that have spawned many impulsive events. 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. Comparing coronal abundances from SEPs and from the slow solar wind, remaining differences are for the elements C, P, and S. We propose that these elements, with intermediate values of first ionization potential (FIP), act like high-FIP neutral atoms in cool sunspots beneath active regions where SEPs are accelerated, but behave like low-FIP ions in the warmer photosphere that contributes to the slow solar wind, ions being more readily swept into the corona than neutral atoms.
D. Reames
Tue, 5 Sep 17
3/76
Comments: 32 pages, 17 figures, submitted to Space Sci. Rev
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