A Model for the Escape of Solar-Flare Accelerated Particles

TL;DR

This paper presents a model explaining how solar flare-accelerated particles can escape promptly into the heliosphere through magnetic reconnection, supported by 2.5D MHD simulations of CME events near coronal-hole boundaries.

Contribution

It introduces a novel explanation for rapid particle escape via interchange reconnection in the breakout CME model, supported by detailed simulations.

Findings

Interchange reconnection enables particles to access open magnetic flux lines quickly.

Prompt escape of particles is feasible when eruptions occur near coronal-hole boundaries.

The model aligns with observations of impulsive solar energetic particles.

Abstract

We address the problem of how particles that are accelerated by solar flares can escape promptly into the heliosphere, on time scales of an hour or less. Impulsive solar energetic particles (SEP) bursts are generally observed in association with so-called eruptive flares consisting of a coronal mass ejection (CME) and a flare. These highly prompt SEPs are believed to be accelerated directly by the flare, rather than by the CME shock, although the precise mechanism by which the particles are accelerated remains controversial. Whatever their origin, within the magnetic geometry of the standard eruptive-flare model, the accelerated particles should remain trapped in the closed magnetic fields of the coronal flare loops and the ejected flux rope. In this case the particles would reach the Earth only after a delay of many hours to a few days, when the bulk ejecta arrive at Earth. We propose that the external magnetic reconnection intrinsic to the breakout model for CME initiation can naturally account for the prompt escape of flare-accelerated energetic particles onto open interplanetary magnetic flux tubes. We present detailed 2.5D MHD simulations of a breakout CME/flare event with a background isothermal solar wind. Our calculations demonstrate that if the event occurs sufficiently near a coronal-hole boundary, interchange reconnection between open and closed field can occur, which allows particles from deep inside the ejected flux rope to access solar wind field lines soon after eruption. We compare these results with the standard observations of impulsive SEPs and discuss the implications of the model for further observations and calculations.