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

atoms Review Element Abundances of Solar Energetic Particles and the Photosphere, the Corona, and the Solar Wind Donald V. Reames Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742–2431, USA; Received: 24 September 2019; Accepted: 18 November 2019; Published: 20 November 2019



 Abstract: From a turbulent history, the study of the abundances of elements in solar energetic particles (SEPs) has grown into an extensive field that probes the solar corona and physical processes of SEP acceleration and transport. Underlying SEPs are the abundances of the solar corona, which differ from photospheric abundances as a function of the first ionization potentials (FIPs) of the elements. The FIP-dependence of SEPs also differs from that of the solar wind; each has a different magnetic environment, where low-FIP ions and high-FIP neutral atoms rise toward the corona. Two major sources generate SEPs: The small “impulsive” SEP events are associated with magnetic reconnection in solar jets that produce 1000-fold enhancements from H to Pb as a function of mass-to-charge ratio A/Q, and also 1000-fold enhancements in 3 He/4 He that are produced by resonant wave-particle interactions. In large “gradual” events, SEPs are accelerated at shock waves that are driven out from the Sun by wide, fast coronal mass ejections (CMEs). A/Q dependence of ion transport allows us to estimate Q and hence the source plasma temperature T. Weaker shock waves favor the reacceleration of suprathermal ions accumulated from earlier impulsive SEP events, along with protons from the ambient plasma. In strong shocks, the ambient plasma dominates. Ions from impulsive sources have T ≈ 3 MK; those from ambient coronal plasma have T = 1 – 2 MK. These FIP- and A/Q-dependences explore complex new interactions in the corona and in SEP sources. Keywords: solar energetic particles; element abundances; magnetic reconnection; shock acceleration; solar wind 1. Introduction Solar energetic particles (SEPs) carry an invisible, yet direct, record of high-energy physics at the Sun that we have slowly learned to read in a field that is dominated by beautiful images of the Sun. Improving measurements of the relative abundances of the chemical elements in SEPs have been shown to carry an imprint of corresponding abundances in the solar corona, of the source conditions, and of characteristic modifications that show unique histories of SEP acceleration and transport. This is becoming a rich field. However, the path to the identification of the sources of acceleration has not been tranquil [1]. 1.1. A Turbulent History: Gradual and Impulsive SEP Events Forbush first observed the effects of SEPs [2], as increases in GeV particles that caused nuclear cascades through the atmosphere to produce what we now call ground-level events (GLEs). These SEPs were visually associated with solar flares and it was commonly assumed that the flares themselves caused SEP events in some way, but there were problems; for example, SEPs that were associated with these flares were eventually found to span more than 180◦ in solar longitude, which suggests a broad spatial distribution. How could the SEPs from a point-source flare cross magnetic field lines over such a distance? Newkirk and Wenzel [3] proposed the “birdcage” model where SEPs could follow coronal Atoms 2019, 7, 104; doi:10.3390/atoms7040104 www.mdpi.com/journal/atoms Atoms 2019, 7, 104 2 of 16 magnetic loops that then spread across the Sun like the wires of a birdcage. The birdcage model reigned many years, but Mason, et al. [4] argued that abundances of ions of different rigidity were unaffected by a trip through such a complex magnetic birdcage; they must arise from large-scale shock acceleration. At the same time, Kahler et al. [5] found a 96% correlation between large energetic SEP events and wide, fast coronal mass ejections (CMEs), which had just been discovered several years before. We now know that CMEs drive extensive expanding sun-spanning shock waves, where SEPs are accelerated to produce what we now call “long-duration” or “gradual” SEP events. Considerable controversy arose again with the publication of Gosling’s [6] review article entitled “The Solar Flare Myth”, which was alleged to “wage an assault on the last 30 years of solar-flare research" [7], apparently based upon a fear that flare research would no longer be funded unless flares produced the radiation that was hazardous to astronauts, which they actually do not. Of course, flare science remains quite healthy 26 years later. However, improving measurements made the shock acceleration of gradual SEPs increasingly difficult to ignore [1,8–13]. These included onset timing of SEPs [14,15], multi-spacecraft studies [16] with improved shock imaging [17–22], temperature and ionization-state measurements [13,23–28], shock modeling [24–35], and correlations of SEP intensities with CME speed [22,36], which continued to strengthen the evidence [1,34]. Meanwhile, there was early evidence for a completely different kind of SEP event being first noted by the abundance of the isotope 3 He, which was enhanced by orders of magnitude. The ratio 3 He/4 He ≈ 5 × 10−4 in the solar wind, but Serlemitsos and Balasubrahmanyan [37] found 3 He/4 He = 1.52 ± 0.10 in a small SEP event, completely unaccompanied by any 2 H, 3 H, Li, Be, or B (Be/O and B/O < 4 × 10−4 ), failing to suggest any evidence of nuclear fragmentation at all. These 3 He-rich events also had enhancements of Fe/O and they were associated with streaming electrons [38] and the type III radio bursts [39] that they produce. Current theory [40] suggests that 3 He resonates with electromagnetic ion cyclotron waves that were generated by the streaming electrons. More recently, the enhancements of heavy elements have been found to extend across the periodic table from H and He to Pb with a 1000-fold enhancement in the elements (76 ≤ Z ≤ 82)/O, relative to that in the gradual SEP events or the solar corona [41–44]. These heavy-element enhancements are found to originate in magnetic reconnection [45] on open field lines in solar jets [46–48]. Energetic ions from similar acceleration in flares are magnetically trapped on loops and they deposit their energy in the footpoints producing γ-ray lines [49–51] and hot (10–40 MK), bright flares, but the charged ions do not escape. The time scale of associated X-rays, which now seem less clear and considerably less relevant, originally distinguished impulsive and gradual SEP events. It is true that most impulsive events last for hours, while most gradual events last days, but these times are not well resolved. It now seems that the cleanest separation of the event types comes from bimodal enhancements in Fe/O. Impulsive SEP events, which are associated with solar jets, have over four times the average Fe/O of gradual events [13,44]. 1.2. SEP Abundance Measuements To evaluate element ab (...truncated)