The 2024 July 16 solar event: a challenge to the coronal mass ejection origin of long-duration gamma-ray flares

Journal Article · Fri Dec 05 00:00:00 EST 2025 · Astronomy & Astrophysics

DOI:https://doi.org/10.1051/0004-6361/202556879· OSTI ID:3023523

Bruno, Alessandro ^[1]; Pesce-Rollins, Melissa ^[2]; Dalla, Silvia ^[3]; Omodei, Nicola ^[4]; Richardson, Ian G. ^[5]; Ryan, James M. ^[6]

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1. Heliophysics Division, NASA Goddard Space Flight Center; Department of Physics, Catholic University of America
2. Istituto Nazionale di Fisica Nucleare
3. Jeremiah Horrocks Institute, University of Lancashire
4. W.W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University
5. Heliophysics Division, NASA Goddard Space Flight Center; Department of Astronomy, University of Maryland
6. Space Science Center, University of New Hampshire

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 γ -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 energies well above the 300 MeV pion-production threshold to account for the observed γ -ray emission. Specifically, the CME was relatively slow (∼600 km/s) and the accompanying interplanetary type-II/III radio bursts were faint and short-lived, unlike those typically detected during large events. In particular, the type-II emission did not extend to kilohertz 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 γ -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 that persisted for the entire duration of this LDGRF.

• Research Organization: SLAC National Accelerator Laboratory (SLAC)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC02-76SF00515
• OSTI ID: 3023523
• Journal Information: Astronomy & Astrophysics, Journal Name: Astronomy & Astrophysics Vol. 704; ISSN 1432-0746; ISSN 0004-6361
• Publisher: EDP Sciences
• Country of Publication: United States
• Language: English