The 2024 July 16 Solar Event: A Challenge To The Coronal Mass Ejection Origin Of Long-Duration Gamma-Ray Flares

TL;DR

This study analyzes the 2024 July 16 solar event, showing that long-duration gamma-ray flares can occur without strong CME-driven shocks or high-energy SEP events, suggesting alternative local acceleration mechanisms.

Contribution

It provides evidence that high-energy gamma-ray emission can originate from local coronal processes rather than CME-driven shocks, challenging previous assumptions.

Findings

CME was slow and weak, unlikely to produce observed gamma rays.

High-energy gamma-ray emission persisted without strong SEP events.

Local coronal loop acceleration is a plausible source for the gamma-ray flare.

Abstract

We present a multi-spacecraft analysis of the 2024 July 16 Long-Duration Gamma-Ray Flare (LDGRF) detected by the Large Area Telescope on the Fermi satellite. The measured >100 MeV $\gamma$-ray emission persisted for over seven hours after the flare impulsive phase, and was characterized by photon energies exceeding 1 GeV and a remarkably-hard parent-proton spectrum. In contrast, the phenomena related to the coronal mass ejection (CME)-driven shock linked to this eruption were modest, suggesting an inefficient proton acceleration unlikely to achieve the energies well-above the 300 MeV pion-production threshold to account for the observed $\gamma$-ray emission. Specifically, the CME was relatively slow (~600 km/s) and the accompanying interplanetary type-II/III radio bursts were faint and short-duration, unlike those typically detected during large events. In particular, the type-II emission did not extend to kHz frequencies and disappeared ~5.5 hours prior to the LDGRF end time. Furthermore, the associated solar energetic particle (SEP) event was very weak, short-duration, and limited to a few tens of MeV, even at magnetically well-connected spacecraft. These findings demonstrate that a very-fast CME resulting in a high-energy SEP event is not a necessary condition for the occurrence of LDGRFs, challenging the idea that the high-energy $\gamma$-ray emission is produced by the back-precipitation of shock-accelerated ions into the solar surface. The alternative origin scenario based on local particle trapping and acceleration in large-scale coronal loops is instead favored by the observation of giant arch-like structures of hot plasma over the source region persisting for the entire duration of this LDGRF.