University of Cambridge > Talks.cam > DAMTP Astro Mondays > Plasma heating and particle acceleration by magnetic reconnection in solar and stellar flares

Plasma heating and particle acceleration by magnetic reconnection in solar and stellar flares

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Solar flares are dramatic releases of stored magnetic energy in the solar corona, with signatures across the electromagnetic spectrum due to heating of the plasma and the generation of high-energy (non-thermal) electron and ion beams. It is widely accepted that energy is released through the process of magnetic reconnection, but many questions remain. Understanding flares may also shed light on the long-standing question of how the solar corona itself is heated to temperatures of over a million degrees K, as it is likely this results from the combined effect of many small “nanoflares”.

Relaxation theory in which the magnetic field relaxes to a state of minimum energy subject to conserved helicity provides a powerful framework to interpret energy dissipation in flares. I will describe models of confined solar flares in twisted magnetic flux ropes, showing how free magnetic energy may be converted into heat and non-thermal particle kinetic energy through magnetic reconnection. First, energy release in individual flux ropes will be discussed, with relaxation triggered by the ideal kink instability. Using 3D magnetohydrodynamic simulations coupled with a test-particle code, it will be shown how multiple magnetic reconnections can both heat plasma and accelerate charged particles. Forward modelling of the observational signatures in EUV , hard X-rays and microwaves will be described, and the potential for observational identification of twisted magnetic fields discussed. Then, interacting magnetic flux ropes will be considered, showing how instability in a single unstable loop may trigger reconnection with stable neighbours, leading to an “avalanche” of heating events, with important implications for solar coronal heating.

Many other stars exhibit flares, and I will discuss recent work on modelling flares in T-Tauri stars. In particular, the enhanced radio luminosity of these stars relative to scaling laws for the Sun and other Main Sequence stars will be considered, based on a model in which the flare fills a magnetic flux rope with non-thermal electrons.

This talk is part of the DAMTP Astro Mondays series.

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