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

Abstract

Fast (V${}_{\mathrm{CME}}$>1000$\mathrm{km}{\mathrm{s}}^{-1}$) 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 <1day 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$\mathrm{km}{\mathrm{s}}^{-1}$) 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$\mathrm{km}{\mathrm{s}}^{-1}$), 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${}_{\mathrm{CME}}\lesssim$ 2000$\mathrm{km}{\mathrm{s}}^{-1}$ underwent CME-CME interactions within 10R${}_{\odot }$, while SGRE-associated CMEs faster than 3000$\mathrm{km}{\mathrm{s}}^{-1}$ did not undergo interactions.

References in corpus (10)

• Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares
• First detection of >100 MeV gamma rays associated with a behind-the-limb solar flare
• The First Fermi-LAT Solar Flare Catalog
• On the Statistical Relationship between CME Speed and Soft X-ray Flux and Fluence of the Associated Flare
• Comparing Long-Duration Gamma-Ray Flares and High-Energy Solar Energetic Particles
• Successive interacting coronal mass ejections: How to create a perfect storm?
• Source of Energetic Protons in the 2014 September 1 Sustained Gamma-ray Emission Event
• The coupling of an EUV coronal wave and ion acceleration in a Fermi-LAT behind-the-limb solar flare
• Clustering of fast Coronal Mass Ejections during the solar cycles 23 and 24 and implications for CME-CME interactions
• Novel scaling laws to derive spatially resolved flare and CME parameters from sun-as-a-star observables