The Astrophysical Journal (Jan 2023)

Heavy-ion Acceleration in 3He-rich Solar Energetic Particle Events: New Insights from Solar Orbiter

  • G. M. Mason,
  • I. Roth,
  • N. V. Nitta,
  • R. Bučík,
  • D. Lario,
  • G. C. Ho,
  • R. C. Allen,
  • A. Kouloumvakos,
  • R. F. Wimmer-Schweingruber,
  • J. Rodriguez-Pacheco

DOI
https://doi.org/10.3847/1538-4357/acf31b
Journal volume & issue
Vol. 957, no. 2
p. 112

Abstract

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We present Solar Orbiter energetic particle observations of two ^3 He-rich events with features more clearly observed than in prior studies. The event of 2022 November 9 observed from 0.59 au contained hundreds of ultraheavy (UH; mass >78 amu) ions whereas previous observations at 1 au have shown only an occasional count or two. The event of 2023 April 8 observed from 0.29 au fortuitously had very low ambient activity, making it possible to observe spectra from the ^3 He acceleration mechanism without contamination, revealing extremely low H and ^4 He intensities arriving simultaneously with other ions observed in typical ^3 He-rich events. Taken together with previous studies, we believe these data show that ^3 He-rich events have a single acceleration mechanism that is responsible for the unique abundance features of ^3 He, heavy ions, and UH ions. Considering the acceleration model of Roth & Temerin that heats the ions over a broad range of gyrofrequencies away from those damped by H and ^4 He, we calculate reasonable fits to the observed abundances O–Fe. A key result is that high values of, e.g., Fe/O typical of such events is not due to preferential Fe heating, but on the contrary is due mainly to the depletion of O, which at elevated temperatures has a charge-to-mass ratio in the region of the waves damped by ^4 He. The model also naturally incorporates features of high-ionization states and neutron-rich isotope enhancements that have been long-standing puzzles in observations of this type of flare.

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