Role of CME clusters and CME-CME interactions in producing sustained $\gamma$-ray emission

TL;DR

This study investigates how fast CME clusters and CME-CME interactions contribute to sustained gamma-ray emission, highlighting the significance of CME speed, flare class, and interactions in producing prolonged high-energy solar emissions.

Contribution

It provides new insights into the role of CME interactions in SGRE production, especially during active periods with multiple CMEs from major active regions.

Findings

SGRE events are more common during CME clusters with interactions.

Faster CMEs and higher flare classes are associated with SGRE production.

CME-CME interactions are prevalent in SGRE-associated CMEs below 2000 km/s.

Abstract

Fast (V$_{\rm CME}$>1000${\rm \,km\,s^{-1}}$) coronal mass ejections (CMEs) capable of accelerating protons beyond 300MeV are thought to trigger hours-long sustained $\gamma$-ray emission (SGRE) after the impulsive flare phase. Meanwhile, CME-CME interactions can cause enhanced proton acceleration, increasing the fluxes of solar energetic particles. This study explores the role of fast CME interactions in SGRE production during CME clusters, which we define as a series of CMEs linked to >C-class flares with waiting times <$\,$1$\,$day from the same active region (AR). We focus on clusters in major CME-productive ARs (major ARs), by defining a major AR as one that produced >$\,$1 CME-associated major (>M-class) flare. The study identified 76 major ARs between 2011 and 2019, of which 12 produced all SGRE events. SGRE-producing ARs exhibit higher median values for the speed of their fastest CMEs (2013 vs. 775${\rm \,km\,s^{-1}}$) and the class of their strongest flares (X1.8 vs. M5.8), compared to SGRE-lacking ARs. They also produced relatively faster CMEs (median speed: 1418 vs. 1206.5${\rm \,km\,s^{-1}}$), with the SGRE-associated CMEs occurring during periods of higher CME rates than typical fast CME epochs. Twelve of 22 (54.5%) SGRE events and 5 of 7 (71.4%) long-duration (>$10\,$h) SGRE events occurred during CME clusters, with high chances of CME-CME interactions. A case study on very active major ARs showed that all SGRE-associated CMEs with V$_{\rm CME}\lesssim$ 2000${\rm \,km\,s^{-1}}$ underwent CME-CME interactions within 10$\,$R$_\odot$, while SGRE-associated CMEs faster than 3000${\rm \,km\,s^{-1}}$ did not undergo interactions.