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

Abstract

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.

Four Distinct Pathways to the Element Abundances in Solar Energetic Particles

“…Unfortunately, these CME speeds are not usually determined for the same magnetic flux tube as the SEP abundances. (Reames 2019d) While the proton excess or suppression may be dependent on the time evolution of the wave spectrum, and thus may occasionally increase or decrease in large gradual events, these SEP4 events are clearly distinguished from the SEP3 events by their acceleration of ambient plasma with T < 2 MK.…”

“…In (b) the shock from a wide, fast CME is poised to sweep up residue from many precious SEP1 or SEP2 jets, generating averaged abundances with much smaller variations in the corresponding distribution to the right. At T ≈ 3 MK, He and C are fully ionized and should not participate in A/Q-dependent enhancements, only event-to-event source fluctuations (Reames 2019d).…”

“…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.…”

“…For as many as 25% of gradual events, reacceleration of the 3-MK impulsive suprathermal ions actually dominates the abundances of the elements with Z > 2. These events seem to involve weaker shocks and perhaps quasi-perpendicular shocks, where the angle between the field B and the shock normal θ Bn > 60°, so that thermal ions downstream would have difficulty in overtaking the shock in order to scatter back and forth to gain energy (Tylka et al 2005;Tylka and Lee 2006;Reames 2019d).…”

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

“…Presumably, these shock waves in larger impulsive events sample protons from the ambient plasma in addition to favoring the high intensities of preaccelerated SEPs that are already enhanced, as shown in Figure 5. ions, plus additional protons from the ambient coronal plasma to produce the observed proton excesses [74,75].…”