Preferential Acceleration of Heavy Ions in a Spontaneously Fragmenting Flare Current Sheet

• Published in: The Astrophysical journal Vol. 927; no. 2; pp. 177 - 194
• Main Authors: Kramoliš, David, Bárta, Miroslav, Varady, Michal, Bučík, Radoslav
• Format: Journal Article
• Language: English
• Published: 2230 Support The American Astronomical Society 01.03.2022; American Astronomical Society; IOP Publishing
• Subjects: Abundance; Active solar corona; Astrophysics; Current sheets; Electromagnetic fields; Energetic particles; Energy spectra; Heavy ions; Ion acceleration; Ion trajectories; Ions; Light ions; Mathematical analysis; Pitch (inclination); Plasma temperature; Relativistic particles; Slopes; Solar abundances; Solar energetic particles; Solar flares; Solar magnetic reconnection
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac4fc9

We study the ion acceleration in a mesoscale, spontaneously fragmenting flare current sheet (SFCS) characterized by the presence of a plasmoid cascade. The main subject of our investigation is to determine whether and how plasmoid cascades at intermediate scales in a fragmented current sheet of a solar flare can impact the (preferential) acceleration of specific ions. The time evolution of the SFCS is obtained from high-resolution 2.5D MHD simulations. The ion trajectories (in the background fields resulting from the MHD model), energies, and pitch angles are calculated using a relativistic test-particle code based on the half-acceleration–rotation–half-acceleration method. For light ions, the main acceleration effects of electromagnetic fields within the SFCS are analyzed using the guiding center approximation. We identify regions with the most-efficient ion acceleration within the SFCS, the accelerator efficiency, and spectra of the accelerated ions. The influence of the charge-to-mass ratio on ion behavior is also studied and resulting ion abundances are compared with observational data. The main ion acceleration takes place in the regions with a strong polarization term, which is part of the first-order Fermi acceleration. Because the term is mass dependent, heavier ions undergo preferential acceleration. The ion energy spectra, abundance-enhancement factors, and differential fluxes, obtained from the model, exhibit power-law profiles, in agreement with observed solar energetic particle events. Nonetheless, the obtained slopes for the abundance-enhancement factor do not exactly match the observed data. The computed slopes and profiles are not sensitive to changes in the initial plasma temperature.