New insights into the proton precipitation sites in solar flares

¹ Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of informatics, Università della Svizzera italiana, Locarno, Switzerland
² University of Applied Sciences and Arts Northwestern Switzerland (FHNW), 5210 Windisch, Switzerland
³ Space Sciences Laboratory, University of California, 94720 Berkeley, USA
⁴ ISEE, Nagoya University, Nagoya, Japan

^⋆ Corresponding author; andrea.francesco.battaglia@irsol.usi.ch

Received: 25 November 2024
Accepted: 16 December 2024

Abstract

Aims. We revisit the Reuven Ramaty High Energy Solar Spectrocopy Imager (RHESSI) γ-ray observations of the extraordinary GOES X25 flare SOL2003-10-28T11:10 to investigate previously reported conclusions that flare-accelerated electrons and protons precipitate along spatially separated flare loops.

Methods. In contrast to previous works that reconstructed 2.223 MeV images over extended time periods (∼20 minutes), we selected shorter integration times on the order of 2 to 3 minutes. Using simulations of the 2.223 MeV profile in combination with observations of the prompt γ-ray lines from the INTEGRAL mission, we obtained two separated integration time ranges representing the peak of the flare and the start of the decay, respectively. The resulting γ-ray images were then compared to GONG white-light (WL) observations to identify where along the flaring ribbons electron and proton precipitation occurs.

Results. We point out that previously reported results comparing RHESSI hard X-ray (HXR) and γ-ray images only hold if the relative time evolution in the two energy ranges is the same. As the decay time for the October 28, 2003, flare is different at the two energy ranges considered (200–300 keV and around 2.223 MeV), the previously published conclusion that electrons and protons precipitate at different locations is an overstatement. Using shorter integration times reveals that the γ-ray and HXR sources spatially coincide with the WL flare ribbons.

Conclusions. Our key conclusion is that electron and proton precipitation sites coincide with the flare ribbons, suggesting that the electron and proton precipitation sites are the same, at least within RHESSI’s imaging capabilities. This result solves the 20-year-long mystery surrounding the different previously reported electron and proton precipitation sites.