Helium Suppression in Impulsive Solar Energetic-Particle Events

Abstract

We have studied the element abundances and energy spectra of the small "Hepoor" 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, suppressed 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.

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

“…distribution of temperature, proton excess, peak proton intensity, and CME speed. It is clear that the proton excess is associated with events with higher He/C, with high proton intensities, and with fast CMEs, as noted by Reames (2019a). CMEs with speeds above 500 km s -1 are quite capable of ion acceleration.…”

“…Element enhancements, labeled by Z, versus A/Q for sample impulsive Event 54, with two possible seed-particle sources for shock acceleration from pre-accelerated impulsive ions (blue) and from ambient coronal ions or preevent plasma (red). Helium is assumed to receive comparable contributions from both sources for this event (Reames, 2019a). Any weak or quasi-perpendicular shock waves may prefer impulsive suprathermal ions at high Z from a two-component seed-particle source.…”

“…Drake et al, 2009). However, since 3 He/ 4 He varies widely with energy in single events or from event to event (Mason, 2007), while Fe/O exhibits a bimodal distribution with impulsive events enhanced (Reames, 1988), Reames, Cliver, and Kahler (2014a) impulsive SEP events studied (Reames, Cliver, and Kahler, 2014a), 70 have proton intensities above background and 17 (24%) of these have proton intensities within one standard deviation of the value predicted by the best-fit line of the ions with Z ≥ 6 (Reames, 2019a). Of these 17 events, 6 have order-of-magnitude He suppression.…”

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

“…Mason, Mazur, and Dwyer (2002) noted the presence of modest enhancements of 3 He in large SEP events that would otherwise be called gradual and the complexity of the seed population was soon realized Tylka et al, 2005;Tylka and Lee, 2006;Bučík et al, 2014Bučík et al, , 2015Bučík et al, , 2018Chen et al, 2015;Reames 2016aReames , 2016b. Thus gradual SEP events sometimes exhibit the abundance enhancement patterns of impulsive SEP events, which we can recognize from power-law enhancements increasing with A/Q and source plasma temperatures of ≈3 MK (Reames, Cliver, and Kahler, 2014a, 2014bReames, 2019aReames, , 2019b which actually dominate 24% of gradual events (Reames 2016a(Reames , 2016b. The 3 He/ 4 He ratio is also enhanced in these events (Reames, Cliver, and Kahler, 2014b) but its extreme variation with energy and from event to event (Mason 2007) makes Fe/O a more stable and reliable indicator of impulsive material.…”

“…In contrast to H , He shows substantial source abundance variations 30 ≤ He/O ≤ 100 in both impulsive and gradual events (Reames, 2017c(Reames, , 2019a) with a few impulsive SEP events showing extreme suppression of He/O ≈ 2 (Reames, 2019a). These variations in He may result from FIP-related inefficient ionization of He during transport up into the corona (Laming, 2009).…”