Erlangen 2026 – scientific programme

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EP: Fachverband Extraterrestrische Physik

EP 8: Sun and Heliosphere II

EP 8.5: Talk

Thursday, March 19, 2026, 12:15–12:30, KH 01.019

Constraining ion acceleration in behind-the-limb gamma-ray flares with Fermi-LAT, SolO/STIX, and ground-based radio observations — •Alexander Warmuth — Leibniz-Institut für Astrophysik Potsdam (AIP), Potsdam, Germany

Compared to energetic electrons in solar flares, which can be readily observed in hard X-rays and radio, our understanding of energetic ions is severely deficient. Our main diagnostics for ions are gamma-ray observations, which remain challenging. A particularly intriguing case are behind-the-limb (BTL) gamma-ray flares, where the flare is occulted as seen from Earth, but nevertheless gamma-ray emission is detected by near-Earth spacecraft. Here, we investigate the relationship between the gamma-ray emission measured with Fermi-LAT, hard X-ray observations from STIX on Solar Orbiter, and ground-based radio observations, for small sample of BTL gamma-ray flares. In all events, type II radio bursts were present that were synchronized in time with the gamma-ray emission. Conversely, we find a significant delay between the impulsive phase of the flare as recorded by STIX and the gamma-ray emission. These findings support the notion that the highly relativistic ions that produce the gamma-rays in BTL flares are accelerated at CME-driven propagating coronal shock waves rather than in large-scale coronal loops.

Keywords: solar flares; coronal mass ejections; energetic particles; solar radio bursts