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

Abstract

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.

Four Distinct Pathways to the Element Abundances in Solar Energetic Particles

“…It is only when we include the abundance of the element H, which significantly extends the span of A/Q, that we find revealing upward breaks in the power-law dependence that could be direct evidence of two-component seed population for shock acceleration, at least for weak shocks, and is therefore a signature of shock-acceleration itself (Reames 2019b(Reames , 2019c(Reames , 2019d. The intermediate energies we study allow us to exploit the simple power-law behavior in A/Q, avoiding high-energy spectral breaks (e.g.…”

“…Measurements have progressed for individual elements up through Fe (Teegarden et al 1973;McGuire et al 1979;Cook et al 1984;Meyer 1985;Reames 1995aReames , 2014Reames , 2017aReames , 2018aMewaldt et al 2002) and groups of elements throughout the rest of the periodic table (Reames 2000;Mason et al 2004;Reames and Ng 2004;Reames et al 2014a). Yet, surprising new patterns have begun to emerge only recently as we extended abundances down to H (Reames 2019b(Reames , 2019c(Reames , 2019d.…”

“…Panel (a) shows selected particle intensities and (b) shows abundance enhancement ratios for the two SEP4 events of 26 and 28 October 2003. Panel (c) shows color-coded best-fit temperatures vs. time while panel (d) shows χ 2 /m vs. T for each time interval and panel (e) shows best-fit enhancements vs. A/Q for each time interval displaced ×0.1, with numbers indicating Z of the ions(Reames 2019c). Proton enhancements at A/Q = 1 fall close to the fit lines from Z > 2 in all periods in (e).…”

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

“…Recent studies of H enhancements in gradual SEP events (Reames, 2019b) show clearly that those gradual SEP events with positive powers of A/Q, like impulsive SEP events, tend to have H abundances a factor of ten or more above expectation, while most gradual SEP events, with negative powers of A/Q, tend to have H enhancements nearer expectation. For gradual SEP events, these two distributions can also be distinguished based upon source plasma temperature.…”

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

“…For larger impulsive events, and especially those associated with faster, but characteristically narrow, CMEs, the enhancement of H can exceed that fit line by an order of magnitude or more -a significant proton excess. Gradual SEP events also fall into two groups (Reames, 2019c). For those associated with ambient coronal temperatures of 0.8 -1.5 MK, H generally falls close to the fit line from the elements with Z ≥ 6 extrapolated down to A/Q = 1.…”