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

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.

• The paper challenges the conventional CME-shock origin for long-duration gamma-ray flares using detailed multi-spacecraft analysis.
• It demonstrates a hard parent-proton spectrum with >1 GeV photons and weak SEP signals, favoring local particle trapping in giant coronal loops.
• The study advocates for revised theoretical models and improved observational strategies to capture the complex dynamics of solar high-energy events.

The 2024 July 16 Solar Event: Re-evaluating the Coronal Mass Ejection Paradigm for Long-Duration Gamma-Ray Flares

Introduction

This paper presents a comprehensive multi-spacecraft analysis of the 2024 July 16 Long-Duration Gamma-Ray Flare (LDGRF), challenging the prevailing hypothesis that such events are causally linked to coronal mass ejection (CME)-driven shock acceleration. The authors leverage remote-sensing and in-situ data from Fermi-LAT, GOES, SOHO, STEREO-A, Parker Solar Probe (PSP), and Solar Orbiter (SolO) to dissect the temporal, spectral, and spatial characteristics of the flare, associated CME, radio emissions, and solar energetic particle (SEP) profiles. The event is notable for its $>$7 hour duration, photon energies exceeding 1 GeV, and a parent-proton spectrum that is among the hardest ever inferred for solar LDGRFs. The analysis reveals a set of contradictory observations with respect to the CME-shock paradigm and instead supports a scenario involving local particle trapping and stochastic acceleration in large-scale coronal loops.

Multi-Spacecraft Observational Context

The spatial configuration of observing spacecraft is critical for interpreting SEP propagation and magnetic connectivity to the flare site. The paper provides a detailed mapping of spacecraft positions in Stonyhurst heliographic coordinates at the time of the event.

Figure 1: Spacecraft radial and longitudinal configuration at 13:30 UT on 2024 July 16.

The event originated from Active Region 13738 near the western limb (S05W83), with favorable magnetic connectivity to Earth and STEREO-A, enabling robust SEP detection and contextual analysis.

Remote-Sensing and In-Situ Diagnostics

X-ray and Gamma-ray Emission

The impulsive phase of the flare was characterized by an X1.9-class soft X-ray peak at 13:26 UT, followed by hard X-ray emission observed by SolO/STIX. Fermi-LAT detected $>$100 MeV gamma-ray emission persisting for over seven hours, with four photons above 1 GeV and a maximum energy of 1.68 GeV. Time-resolved spectral analysis using power-law with exponential cutoff (PLEXP) and pion-decay templates yielded a minimum parent-proton index $\alpha_p^{min} = 3.31 \pm 0.21$, indicating a remarkably hard spectrum. This value is second only to the 2017 September 10 LDGRF and significantly harder than the average for delayed-emission events.

Figure 2: Multi-wavelength remote-sensing observations, including X-ray, gamma-ray, and radio fluxes, with temporal alignment to the LDGRF and associated radio bursts.

CME and Radio Emission Properties

The associated CME was a partial-halo event with a projected speed of $\sim$600 km/s, decelerating through the LASCO field of view. The CME lacked the impulsive acceleration and high velocity typical of events producing strong SEP and LDGRF signatures. The interplanetary type-II radio burst was weak, confined to metric-DH wavelengths, and terminated 5.5 hours before the LDGRF ended, with no extension to kilometric frequencies. Type-III and type-IV radio emissions were similarly faint and short-lived.

Extreme Ultraviolet Imaging and Coronal Loop Structures

EUV imaging at 94 Å from SDO/AIA and GOES/SUVI revealed the formation of giant, hot, quasi-static coronal loops immediately following the eruption. These structures persisted for the entire duration of the LDGRF and exhibited sustained emission at temperatures of $\sim$6–10 MK, with no evidence of post-flare cooling or propagating shock waves.

Figure 3: Extreme-ultraviolet images at 94 Å showing the evolution and persistence of giant coronal loops over the source region during the LDGRF.

SEP Profiles and Magnetic Connectivity

Despite optimal magnetic connectivity, SEP intensities at Earth and STEREO-A were weak, short-lived, and limited to energies below a few tens of MeV. PSP and SolO, at less favorable longitudes, detected no significant SEP enhancements. The SEP event lacked the gradual, high-energy profile expected from efficient CME-driven shock acceleration.

Figure 4: Temporal profiles of SEP intensities from multiple spacecraft, highlighting the weak and low-energy nature of the event despite favorable connectivity.

Critical Evaluation of the CME-Shock Paradigm

The paper systematically refutes the CME-shock origin for the 2024 July 16 LDGRF based on several lines of evidence:

• CME Kinematics: The modest speed and width of the CME are inconsistent with the high-energy requirements for LDGRF production. Previous LDGRFs have also been associated with slow or absent CMEs, but none with such extended duration and hard spectra.
• SEP Observations: The lack of a significant SEP population, even at energies an order of magnitude below the pion-production threshold, contradicts the hypothesis of a common shock-accelerated parent population for both SEPs and LDGRFs.
• Radio Burst Correlation: The absence of temporal and spectral correlation between the LDGRF and interplanetary type-II radio emission undermines claims of a direct causal relationship.
• Magnetic Mirroring and Transport: Modeling studies indicate that magnetic mirroring severely limits the back-precipitation of shock-accelerated ions, with precipitation fractions $\lesssim$2%, far below the $>$20%–40% required to explain the observed gamma-ray fluxes.

Coronal Loop Acceleration Scenario

The alternative scenario posits that seed ions are injected and trapped within large-scale coronal loops, where they undergo 2nd-order Fermi acceleration driven by coronal turbulence and wave activity. The observed loop lengths ($\sim$1.1–1.4 $R_s$) and sustained EUV emission are consistent with theoretical requirements for prolonged trapping and efficient stochastic acceleration. The loop model naturally explains the smooth exponential decay of the gamma-ray emission, the lack of SEP correlation, and the spatially extended nature of behind-the-limb LDGRFs.

Implications and Future Directions

The findings have significant implications for the understanding of high-energy solar phenomena:

• Reassessment of LDGRF Origins: The event demonstrates that fast, wide CMEs and strong SEP events are not necessary conditions for LDGRF production, challenging the universality of the CME-shock paradigm.
• Role of Coronal Loops: The visibility and persistence of giant hot coronal loops suggest that local acceleration and trapping mechanisms may be dominant in a subset of LDGRFs, especially those with extended durations and hard spectra.
• Observational Strategies: Limb events and wide-field EUV imaging are critical for identifying large-scale loop structures. Future missions should prioritize multi-wavelength, multi-point observations to disentangle the contributions of different acceleration mechanisms.
• Theoretical Modeling: Further work is needed to quantify turbulence levels, wave energy input, and particle injection pathways in coronal loops, as well as to model the interplay between flare-associated and CME-driven processes.

Conclusion

The 2024 July 16 LDGRF provides a compelling counterexample to the CME-shock origin hypothesis for long-duration gamma-ray flares. The event's hard parent-proton spectrum, extended gamma-ray emission, weak CME and SEP signatures, and the presence of persistent giant coronal loops collectively support a scenario of local particle trapping and stochastic acceleration within closed magnetic structures. This analysis underscores the necessity of revisiting theoretical models of solar high-energy phenomena and highlights the importance of comprehensive, multi-spacecraft observational campaigns for advancing the field.