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

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 2<Z<57. 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.


Introduction
The Wind spacecraft has enabled studies of hundreds of solar energetic-particle (SEP) events in its highly productive 25 years of operation. All of these events are different.
One most profound difference lies in the relative abundances of the elements in SEPs compared with corresponding abundances in the solar photosphere or corona. New abundances of H and He add information that is sensitive to the physics of particle selection and acceleration. Some elements in SEP events are preferentially selected for acceleration while others are preferentially scattered as they propagate away from the source. 1975) contrasted with 5 × 10 -4 in the solar wind. Since the 3 He was completely unaccompanied by 2 H or the elements Li, Be, and B, the 3 He had nothing to do with nuclear fragmentation, rather with a resonance process (e.g. Temerin and Roth, 1992) that was associated with streaming electrons and radio type III bursts (Reames, von Rosenvinge, and Lin, 1985;Reames and Stone, 1986). Element enhancements up to Fe (e.g. Reames, Meyer, and von Rosenvinge, 1994;Mason, 2007) and subsequently to heavier elements (Reames, 2000;Mason et al., 2004;Reames and Ng, 2004) were apparently accelerated directly in islands of magnetic reconnection (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) Reames, Cliver, and Kahler (2014a) and Reames (2016a) are indicated.
Events defined by Fe/O in this way distinguish meaningful physical processes such as those of associated CMEs. Figure 2 shows properties of two impulsive SEP events that help to illustrate the concepts of "proton excess" and "He suppression." Throughout this article, He without a superscript always means 4 He. Protons in Event 79 barely rise above the pre-event background, yet they do not exceed the power-law fit from the ions with Z ≥ 6. Of the 111 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, At a temperature of 3 MK, He and C are fully ionized with Q = Z and O is nearly so. Relative abundances of these ions with A/Q ≈ 2 should remain unaltered during acceleration and transport. Figure 3 shows panels of O/C versus He/C with <20% errors, with various quantities represented in the color and size of the points. The scatter in the points is quite large, especially in the He/C direction. Normalization based upon reference values of He/O = 57 and 91 are shown as dashed lines. Various panels show the 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
It is important to emphasize that most gradual SEP events do not show proton excesses. These are events with T < 2MK. A new example is shown in Figure 4    The slope of the power law fitting the enhancements of Z ≥ 6 ions tends to decrease with time. For the event in Figure 5 this power decreases from 1.9±0.2 to 0.44±0.35 during the five eight-hour periods. During the final period it becomes so flat that χ 2 also becomes flat and the temperature becomes uncertain.
The lower panel of Figure 6 shows O/C versus He/C for gradual SEP events at the same scale as it was shown for impulsive SEP events in Figure 3. In this case the scale leaves considerable white space because of the tighter grouping of gradual SEP events in O/C versus He/C. The upper panels in Figure 6 show the tighter grouping of orange and red events with T ≈ 3 MK in the He-rich, Fe-rich corner of the temperature panels. Big intense events, as measured by 20-MeV protons and associated with the fastest CMEs (e.g. Kahler, 2001;Kouloumvakos et al., 2019), lie along the He-poor side and drift down in Fe/O with time. Thus acceleration in the largest gradual SEP events is dominated by ambient coronal plasma with T < 2 MK, not by recycled impulsive material. The gradual SEP events characterized by reaccelerated T ≈ 3 MK ions from prior impulsive events are modest in size in terms of their energetic proton intensities. Perhaps slower shocks are more strongly biased in favor of pre-accelerated suprathermal ions.
The shock geometry is also a factor in selecting ions that are accelerated from the seed population (Tylka et al., 2005;Tylka and Lee, 2006). When θ Bn , the angle between the magnetic field and the shock normal, approaches 90°, only the fastest ions can overtake the shock from downstream, so the faster pre-accelerated impulsive ions are favored.
It was only possible to assign temperatures to about 70% of gradual SEP events since at least four reasonably-consistent eight-hour periods were required (Reames, 2016a). Most events that failed this test did so because the dependence on A/Q was too flat, so that the values of A/Q, and hence the value of T, were indeterminate, i.e. there was no "best" value. Since this occurs when the measured abundances are similar to the reference abundances, these events may be less likely to involve many reaccelerated impulsive suprathermal ions with their strong A/Q enhancements, and may be biased in favor of sampling ambient coronal material.

Comparing CMEs
We compare some properties of the CMEs associated with gradual and impulsive SEP events in Figure 7. Relevant CME data were found in the SOHO/LASCO CME catalog (Gopalswamy et al., 2009; https://cdaw.gsfc.nasa.gov/CME_list/). The gradual SEP events on our list have wide, mostly halo, fast CMEs with a mean speed of 1815 km s -1 .
The impulsive event list has narrow, mostly < 120°, slow CMEs with a mean speed for these little CMEs of 687 km s -1 . For many impulsive events, CME are too small to be recorded (e.g. Nitta et al., 2006).  Reames (2016b). Both populations involve T ≈ 3MK impulsive SEP ions from magnetic reconnection sites in solar jets which may be reaccelerated by a shock wave; proton excesses are produced when those shocks also include protons from the ambient coronal material. In the impulsive case, the shock, if any, is produced by the CME from the same event; ions from a single jet reflect local variations in element abundances. In the second case, a large shock from a wide CME sweeps up residual impulsive suprathermal ions from many (N) individual jets that have occurred recently in the vicinity of an entire active region, reducing the abundance fluctuations by a factor of √N.
The residue from many jets collects in large regions for substantial periods so that 3 Herich, Fe-rich background levels are often seen (Desai et al., 2003;Bučík et al., 2014Bučík et al., , 2015Bučík et al., , 2018aBučík et al., , 2018bChen et al., 2015). Having distinguished differences in the CMEs associated with impulsive and gradual SEP events, we now turn to differences in CMEs that favor different ion seed populations for shock acceleration in gradual SEP events. Figure 6 suggests that the most intense gradual events are associated with cooler plasma while events with the hotter reaccelerated impulsive ions are not so large and intense. Figure 9 shows the timing and CME-speed distributions of ions with differing source-plasma temperatures. Events with the hotter reaccelerated impulsive-SEP source plasma involve somewhat slower, weaker CMEs, probably with quasi-perpendicular shock waves, that tend to arrive early in solar cycle 23 and to dominate the weak solar cycle 24. Faster CMEs that occur later in cycle 23 tend to be dominated by cooler ambient coronal plasma in which they sample deeplythese more powerful events have less need for preaccelerated ions. Like SEP events, all solar cycles are not the same.

Discussion
Can the situation described by Figure  What happens when chromospheric material is suddenly thrust up into the corona in a jet as suggested by the emerging-flux model in the lower-left panel of Figure 8? Would it be surprising for more rapid and dynamic FIP processing to lead to a much greater suppression of He? Much less He + may be processed during the rapid fractionation in some jets.
However, while 3 He and 4 He fractionation should be similar, 3 He/ 4 He tends to be highest when 4 He/C is suppressed.
He suppression is the only FIP-related event-to-event variation we can identify in SEP events. The theory of He suppression (Laming 2009) also suggests the possible suppression of Ne, but Ne is nearly always enhanced in impulsive SEP events (see Figure   1), mainly because of the general increase in enhancement with A/Q. It is possible that the variations in O/C for impulsive events in Figure 8 could occur because O is not quite fully ionized at 2.5 MK so its value of A/Q may be a bit higher than that of C. Naïve researchers occasionally mistake the enhancement in Fe/O in impulsive SEP events as a possible FIP effect, but it is actually part of the 1000-fold power law that stretches from H to U (see Reames, Cliver, and Kahler, 2014a;Reames, 2018b). This power law also For the smaller "pure" (magnetic-reconnection only) impulsive SEP events with no shock reacceleration, and for the largest gradual SEP events which accelerate ambient coronal plasma with T < 2 MK, H/O enhancements are generally consistent with powerlaw fits to the A/Q dependence of abundances of elements with Z ≥ 6. Any events dominated by shock reacceleration of impulsive suprathermal ions show substantial proton excesses. The only suggested explanation of these proton excesses is shown in Figure 10 where the shock wave primarily selects the enhanced impulsive suprathermal ions which dominate the SEPs with Z > 2, but protons from the ambient coronal plasma dominate the SEP protons. Abundances of ions with Z > 2 rise sharply with A/Q because the impulsive suprathermal source ion abundances already increase sharply with A/Q, but the ambient ion abundances and, in fact, all accelerated ion abundances, usually decrease modestly with A/Q because of rigidity-dependent transport from the shock. Note that the cooler ambient ions will have lower Q and higher A/Q than the impulsive ions with T ≈ 3 MK. Figure 10. 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.
While Figure 10 shows data from an impulsive SEP event, the concept can apply to any event, impulsive or gradual, where a weak or quasi-perpendicular shock wave preferentially reaccelerates residual impulsive suprathermal ions which dominate high Z, while protons are mainly sampled from the ambient coronal plasma. However, any 4 He accelerated from the ambient plasma in impulsive SEP events will certainly tend to reduce 3 He/ 4 He, so impulsive events with proton excesses may not remain 3 He-rich.
Regarding the general issue of multiple components and the seed population, shock acceleration at corotating interaction regions (CIRs) sometimes shares this complexity (Reames, 2018c). CIRs are formed when high-speed solar-wind streams overtake and collide with slow solar wind emitted earlier in the solar rotation (e.g. Richardson, 2004). This collision leads to two shock waves, a forward shock propagating outward into the slow wind and a stronger reverse shock propagating sunward into the fast wind.
Usually these shocks form outside 1 AU and strengthen out to 5 AU (Van Hollebeke, McDonald, and von Rosenvinge, 1976). Low-rigidity ions tend to be retained near the shock while high-rigidity ions spread widely (e.g. Reames et al., 1997). To first order, the only source of particles available for acceleration at CIR shocks is the solar wind, and the energetic particles at CIRs typically show solar-wind abundances near the shocks and abundances that decrease as power-laws in A/Q more and more steeply with distance from the shock (Reames, 2018c). However, seed particles, or simply background, from SEP events, especially small impulsive SEP events, can inject Fe-rich material that easily dominates the otherwise-suppressed abundance of Fe from the solar wind, but contributes much less at lower Z (Reames, 2018c). This is reminiscent of the pattern shown in Figure   10. These multi-component events tended to hide the A/Q dependence and complicated the source composition in early measurements of CIR events, especially for the weaker shocks associated with slower solar-wind streams.
In SEP events, the rigidity dependence that leads to the A/Q dependence can come from several factors. The acceleration of ions in islands of magnetic reconnection leads directly to power laws in A/Q when the ions undergo Fermi acceleration as they are reflected back and forth from the ends of collapsing magnetic islands (Drake et al., 2009).
Scattering during transit along magnetic fields also depends upon A/Q as noted above (Parker, 1963;Ng, Reames, and Tylka, 2003;Reames, 2016aReames, , 2016bReames, , 2019b. In shocks, the dominant protons play a special role, not only are they spectators, resonantly scattering from existing waves like all other ions, but they can generate or amplify resonant waves as they stream away from the shock, increasing the scattering and acceleration of ions that follow (Bell, 1978;Lee, 1983Lee, , 2005Ng and Reames, 2008;Reames and Ng, 2010). The wave number of resonant Alfvén waves is k ≈ B/µP, where B is the magnetic field strength, µ is the cosine of the ion's pitch angle with the field, and P is the ion's rigidity. Thus, if we assume µ ≈ 1 for simplicity, 2.5 MeV protons are scattered by selfgenerated waves at 2.5 MeV, for example, but 2.5 MeV amu -1 He, C, or O with A/Q = 2 (at T ≈ 3 MK) resonate with waves generated by 10-MeV protons, and 2.5 MeV amu -1 Fe at A/Q = 4 resonates with waves generated by streaming 39-MeV protons. A pure powerlaw dependence of enhancements upon A/Q may require a power-law proton rigidity spectrum. Differences in resonance can produce a proton excess early in a gradual event when, for example, 2.5 MeV protons begin to arrive, as yet unscattered, while 2.5 MeV He at the same time has been scattered and retarded by traversing waves generated by 10-MeV protons that arrived much earlier, increasing H/He .
The understanding of SEP events could be greatly enhanced by improved theory and modeling of time dependent generation of resonant waves by protons and the coupling of those waves to other ions. While the full time-dependent evolution of proton and wave spectra at shock waves have been modeled in some detail (e.g. Ng and Reames, 2010), the co-evolution of the spectra of other ions has not yet been included.

Summary
The Plasma may also be ejected from the event, producing a narrow CME that is too slow to drive a shock wave.
ii) An impulsive + shock event occurs when the narrow CME from a jet, that of an otherwise pure impulsive event, is fast enough to drive a shock wave. The shock wave samples all available ions, those from the ambient plasma and residual energetic ions from the pure event. The abundant ambient protons form the wave structure at the shock and dominate at Z = 1, but the pre-enhanced, preaccelerated ions are selected by the weak shock and dominate the Z > 2 region.
This appears as a large proton excess.
iii) A fast, wide CME from an eruptive event drives a fast shock wave that expands broadly, producing an energetic gradual SEP event that lasts many days. If the shock is quasi-parallel or samples deeply into the tail of the thermal distribution of the ambient plasma with T < 2 MK, it will produce a "pure" gradual event with essentially coronal ion abundances modified by a power-law dependence on A/Q that may be enhanced or suppressed during ion transport. Protons generally fit with other ions although some regions of unusual transport may produce modest local excesses or depletions of protons. Any impulsive suprathermal ions present are also accelerated by the shock, but their contribution is overwhelmed but the accelerated ambient coronal ions; these events do not need pre-accelerated ions.
iv) A fast, wide CME from an eruptive event drives a moderately fast shock wave producing a gradual SEP event. The shock may be quasi-perpendicular (or just weak) so its final contribution from sampling of ambient plasma is limited mainly to protons, while preferring residual impulsive suprathermal ions surviving from a dozen or so earlier impulsive events that combine to produce well-defined average impulsive-SEP abundances for Z >2.
Thus, we still think impulsive SEP events are produced in solar jets, initially from magnetic reconnection, and ions in gradual SEP events are still swept up and accelerated by shock waves driven by wide, fast CMEs. However, weak or quasi-perpendicular shock waves in either event class, manage to accelerate only significant protons from the ambient coronal plasma, but preferentially recycle pre-accelerated impulsive-SEP ions, with their built-in abundance bias, at higher Z.
The study of power-law patterns of element abundance enhancements has given us new parameters to better distinguish and organize physical processes in the large sample of SEP events we have seen. Soon the Wind spacecraft will enter its third solar cycle; will the abundances of the new SEP events return to the pattern of solar cycle 23, as we would predict, or will they differ entirely?
Acknowledgements CME data were taken from the CDAW LASCO catalog. This CME catalog is generated and maintained at the CDAW Data Center by NASA and The Catholic University of America in cooperation with the Naval Research Laboratory.

Disclosure of Potential Conflicts of Interest
The author declares he has no conflicts of interest.

Appendix: Reference Abundances of Elements
The average element abundances in gradual SEP events are a measure of the coronal abundances sampled by SEP events (Reference SEPs in Table 1). They differ from photospheric abundances in Table 1 by a factor which depends upon FIP (e.g. Reames, 2018a;2018b;Laming et al., 2019). More complete tables of SEP abundances of 21 elements that include rarer species are given by Reames (2017a;2018a). Ion "enhancements" are defined as the observed abundance of a species, relative to O, divided by the reference abundance of that species, relative to O.