New insights into the proton precipitation sites in solar flares

TL;DR

This study revisits RHESSI gamma-ray data of a major solar flare, revealing that electron and proton precipitation sites coincide with flare ribbons, challenging previous claims of spatial separation due to analysis time differences.

Contribution

The paper demonstrates that shorter integration times and combined observations show electron and proton precipitation sites are the same, resolving a long-standing debate.

Findings

Electron and proton precipitation sites coincide with flare ribbons.

Previous conclusions of spatial separation were due to longer integration times.

Shorter time analysis aligns gamma-ray and hard X-ray sources with flare ribbons.

Abstract

The Reuven Ramaty High Energy Solar Spectrocopy Imager (RHESSI) $\gamma$-ray observations of the extraordinary GOES X25 flare SOL2003-10-28T11:10 are revisited to investigate previously reported conclusions that flare-accelerated electrons and protons precipitate along spatially separated flare loops. In contrast to previous works which reconstructed 2.223 MeV images over extended time periods ($\sim$20 minutes), we selected shorter integration times of the order of 2 to 3 minutes. Using simulations of the 2.223 MeV profile in combination with observations of the prompt $\gamma$-ray lines from the INTEGRAL mission, we obtain two separated integration time ranges representing the peak of the flare and the start of the decay, respectively. The resulting $\gamma$-ray images are then compared to GONG white-light (WL) observations to identify where along the flaring ribbons electrons and protons precipitation occurs. We point out that previously reported results comparing RHESSI hard X-ray (HXR) and $\gamma$-ray images only hold if the relative time evolution in the two energy ranges is the same. As the decay times for the 28 October 2003 is different at the considered two energy ranges (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 $\gamma$-ray and HXR sources spatially coincide with the WL flare ribbons. 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 twenty-years-long mystery around the previously reported different electron and proton precipitation sites.