Escape of Flare-accelerated Particles in Solar Eruptive Events

TL;DR

This study uses 3D magnetohydrodynamic simulations to demonstrate how magnetic reconnection enables flare-accelerated particles to escape from CMEs into interplanetary space, resolving a longstanding paradox.

Contribution

It extends previous axisymmetric models to fully 3D geometries, revealing how interchange reconnection facilitates particle escape from deep within CME flux ropes.

Findings

Reconnection occurs before and during CME eruption.

Particles gain access to open interplanetary field via reconnection.

Results align well with observational data.

Abstract

Impulsive solar energetic particle events are widely believed to be due to the prompt escape into the interplanetary medium of flare-accelerated particles produced by solar eruptive events. According to the standard model for such events, however, particles accelerated by the flare reconnection should remain trapped in the flux rope comprising the coronal mass ejection. The particles should reach the Earth only much later, along with the bulk ejecta. To resolve this paradox, we have extended our previous axisymmetric model for the escape of flare-accelerated particles to fully three-dimensional (3D) geometries. We report the results of magnetohydrodynamic simulations of a coronal system that consists of a bipolar active region embedded in a background global dipole field structured by solar wind. Our simulations show that multiple magnetic reconnection episodes occur prior to and during the CME eruption and its interplanetary propagation. In addition to the episodes that build up the flux rope, reconnection between the open field and the CME couples the closed corona to the open interplanetary field. Flare-accelerated particles initially trapped in the CME thereby gain access to the open interplanetary field along a trail blazed by magnetic reconnection. A key difference between these 3D results and our previous calculations is that the interchange reconnection allows accelerated particles to escape from deep within the CME flux-rope. We estimate the spatial extent of the particle-escape channels. The relative timings between flare acceleration and release of the energetic particles through CME/open-field coupling are also determined. All our results compare favourably with observations.