Why do only some fast CMEs produce sustained gamma-ray emissions?
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| Number: | 535 |
| 1st Author: | Atul MOHAN |
| 2nd Author: | |
| Published: | September 14, 2026 |
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| Previous Nugget: | The Variance of Solar X-ray Flux |
Introduction
Certain strong solar flares may accelerate particles producing intense gamma-ray emissions during their impulsive phase. In some flares, however, gamma rays with energies >100 MeV continue to arrive for several hours well beyond the impulsive phase. These events are called sustained gamma-ray emission events (SGREs), and require >300 MeV protons to interact in the dense photosphere. Fast (>1000 km/s) coronal mass ejections [https://www.spaceweather.gov/phenomena/coronal-mass-ejections CMEs] and their shocks are likely accelerators of the high-energy protons that are transported sunward to generate SGREs lasting over 3h (>3h-SGREs) after the impulsive phase. Nevertheless, many fast CMEs do not cause SGREs. Some ARs can produce several SGRE-associated CMEs during their lifetime within a gap of a few days. Such periods of high CME-occurrence rates often cause CME-CME interactions, resulting in particle acceleration (Ref. [1]) These observations raise two central questions which our work (Ref. [2]) addresses:
1. What makes a major CME-productive AR SGRE-producing? a. Is there a role for temporal clustering of CME events? 2. Why only some of the fast CMEs from these ARs associated with SGRE? a. Is there a role for CME-CME interactions?
Definitions and the major AR catalog
We studied CMEs associated with flares stronger than GOES C-class during 2011-2019, covering the period in solar cycle 24, for which Fermi gamma-ray data is available. The following definitions were adopted to identify the relevant active regions and periods of clustered CME-activity in our catalog:
1. Major AR: An AR that produced >1 CME-associated-flare of class M or stronger. 2. SGRE-producing AR: A major AR that produced at least one >3h-SGRE. 3. SGRE-lacking AR: A major AR that did not produce any >3h-SGRE. 4. CME cluster: A sequence of CMEs, each associated with a flare stronger than C class, from the same AR with a waiting time <1 day. 5. Fast CME epoch: A 2-day period centered around the fast CME onset time.
Figure 1 shows an example of CME event occurrence plots and identified clusters in an SGRE-producing and an SGRE-lacking major AR.
We identified 76 major ARs during the study period, of which 12 produced all of the 22 >3h-long SGREs reported in Cycle 24. Overall, SGRE-associated CMEs generally tend to occur in CME clusters during epochs with high chances of interaciton with the preceeding CME.
What makes some fast CMEs in clusters SGRE-associated?
A likely candidate here could be the CME-CME interaction, which is known to often cause enhanced proton flux in >10MeV channels, observed near Earth. We investigated this possibility in detail for two especially major ARs that produced multiple long-duration SGREs. See Ref. [2] for full detail, and refer to Figure 2 for an artist's conception of the process.
Conclusions
Our detailed case studies on two very active major ARs lead us to the hypothesis that an extremely fast (>3000 km/s) CME may produce an SGRE through the strength of its shock alone, whereas moderately fast ones (<2000 km/s) require a CME/CME interaction below 10 RSun, as evidenced by the CME height-time plots and coronagraph images for the selected events. Testing the hypothesis against the Fermi LAT SGRE catalog gave a high success rate, with just two outliers, in which cases other factors of heliospheric preconditiong and local AR conditions may explain the anomaly making them unique cases.
References
[1] "Radio Signatures of Coronal Mass Ejection Interaction: Coronal Mass Ejection Cannibalism?"
[2] "Role of CME Clusters and CME-CME Interactions in Producing Sustained γ-Ray Emission Eents"