This talk presents work recently published in ApJL where we have argued that the ion temperature is several times the local electron temperature in the hot onset phase and in the above-the-loop region of solar flares. The talk considers:
(1) the evidence of spectral line Doppler widths (“nonthermal” broadening);
(2) evidence for “universal” ion and electron temperature increase scaling relations for magnetic reconnection in the solar wind, Earth’s magnetopause, Earth’s magnetotail, and numerical simulations; and
(3) thermal equilibration times for onset and above-the-loop densities, which are much longer than previous estimates based on soft X-ray flare loops.
We conclude that the ion temperature is likely to reach 60 MK or greater and that it may represent a substantial part of spectral line widths, significantly contributing to solving the long-standing issue of the excess nonthermal broadening in flare lines. The talk will also include an overview of new preliminary work addressing multi-temperature flare modelling.
Related paper: https://doi.org/10.3847/2041-8213/adf74a Russell et al. 2025, ApJ Letters, 990:L39. Co-authors: Vanessa Polito, Paola Testa, Bart De Pontieu and Sergey Belov