Driving major solar flares and eruptions: a review

TL;DR

This review discusses the mechanisms behind major solar flares and eruptions, emphasizing the role of flux-rope emergence, magnetic reconnection, and the challenges in predictive modeling due to observational limitations.

Contribution

It synthesizes current understanding of solar flare triggers, highlighting the importance of flux-rope emergence and interaction with magnetic fields, and outlines a comprehensive scenario for eruptions.

Findings

Flux-rope emergence is critical in flare initiation.

The internal twist of flux ropes influences eruption type.

Current forecasting methods are unreliable due to observational uncertainties.

Abstract

This review focuses on the processes that energize and trigger major solar flares and flux-rope destabilizations. Numerical modeling of specific solar regions is hampered by uncertain coronal-field reconstructions and by poorly understood magnetic re- connection; these limitations result in uncertain estimates of field topology, energy, and helicity. The primary advances in understanding field destabilizations therefore come from the combination of generic numerical experiments with interpretation of sets of observations. These suggest a critical role for the emergence of twisted flux ropes into pre-existing strong field for many, if not all, of the active regions that pro- duce M- or X-class flares. The flux and internal twist of the emerging ropes appear to play as important a role in determining whether an eruption will develop predom- inantly as flare, confined eruption, or CME, as do the properties of the embedding field. Based on reviewed literature, I outline a scenario for major flares and erup- tions that combines flux-rope emergence, mass draining, near-surface reconnection, and the interaction with the surrounding field. Whether deterministic forecasting is in principle possible remains to be seen: to date no reliable such forecasts can be made. Large-sample studies based on long-duration, comprehensive observations of active regions from their emergence through their flaring phase are needed to help us better understand these complex phenomena.