Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

Alessandro Bruno, Melissa Pesce-Rollins, Silvia Dalla, Nicola Omodei, Ian G. Richardson, James M. Ryan

TL;DR

This study analyzes the 2024 July 16 LDGRF using multi-spacecraft observations, revealing gamma-ray emission exceeding $>100$ MeV for $7.25$ hours with photons reaching $1.68$ GeV and a notably hard inferred parent-proton spectrum with $α_p^{min} \\approx 3.31$–$3.46$. The associated CME/shock was slow and weak, with a DH type-II burst not extending to kHz frequencies and only modest SEP activity at 1 AU, challenging the idea that shock-accelerated ions back-precipitate into the solar atmosphere to produce the LDGRF. Instead, the authors favor a loop-trapping scenario where seed ions are confined in giant coronal arches and stochastic acceleration via second-order Fermi processes sustains high-energy particle populations, consistent with persistent $94 \\AA$ EUV emission and smooth decay of the gamma-ray flux. The event thus provides a rigorous counterexample to the CME-shock origin for LDGRFs and suggests a broader, possibly hybrid, framework in which large-scale coronal loops play a dominant role in at least some long-duration gamma-ray events.

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 $γ$-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 the 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-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 $γ$-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 persisting for the entire duration of this LDGRF.

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 LDGRF using multi-spacecraft observations, revealing gamma-ray emission exceeding MeV for hours with photons reaching GeV and a notably hard inferred parent-proton spectrum with –. The associated CME/shock was slow and weak, with a DH type-II burst not extending to kHz frequencies and only modest SEP activity at 1 AU, challenging the idea that shock-accelerated ions back-precipitate into the solar atmosphere to produce the LDGRF. Instead, the authors favor a loop-trapping scenario where seed ions are confined in giant coronal arches and stochastic acceleration via second-order Fermi processes sustains high-energy particle populations, consistent with persistent EUV emission and smooth decay of the gamma-ray flux. The event thus provides a rigorous counterexample to the CME-shock origin for LDGRFs and suggests a broader, possibly hybrid, framework in which large-scale coronal loops play a dominant role in at least some long-duration gamma-ray events.

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 -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 -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 -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.

Paper Structure

This paper contains 10 sections, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and Analysis
  3. X-Ray Emission
  4. CME Properties
  5. Gamma-Ray Emission
  6. Radio Emission
  7. Extreme Ultraviolet Emission
  8. Solar Energetic Particles
  9. Discussion
  10. Summary and Conclusions

Figures (4)

  • Figure 1: Spacecraft radial and longitudinal configuration at 13:30 UT on 2024 July 16.
  • Figure 2: Remote-sensing observations between 12--23 UT on 2024 July 16. From top to bottom: the soft X-ray flux measured by GOES-18 (a); the hard X-ray counts measured by SolO (b); the $>$100 MeV $\gamma$-ray flux measured by Fermi-LAT (c), with the blue dashed line marking the quiet-Sun background and the shaded areas indicating the intervals where the Sun was outside the detector FoV; and the radio emission measured by PSP (d), SolO (e), STEREO-A (f) and Wind (g). The red arrow in panel f) marks the end of the interplanetary type-II emission at 15:47 UT (1.4 MHz) according to the CDAW DH type-II burst catalog.
  • Figure 3: Extreme-ultraviolet images at 94 Å taken by SDO/AIA (left) and GOES/SUVI (right) on 2024 July 16 at $\sim$14 UT (top panels), $\sim$17 UT (middle panels) and $\sim$21 UT (bottom panels), approximately corresponding to the start, middle and end times of the $>$100 MeV $\gamma$-ray emission (see Figure \ref{['fig:2']}c). The red arrows indicate the system of giant loops persisting over the source region for the entire LDGRF duration.
  • Figure 4: Temporal profiles of SEP intensities measured between 2024 July 10--23 by GOES and SOHO (a), STEREO-A (b), PSP (c) and SolO (d). The vertical dotted line marks the peak time of the X1.9-class flare associated with the LDGRF event.