Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem

TL;DR

This paper proposes that large-scale Alfven waves transport energy during solar flares and can accelerate electrons, offering a new perspective on the impulsive phase energy transfer and electron acceleration mechanisms.

Contribution

It introduces a novel scenario where Alfven waves facilitate rapid energy transport and electron acceleration, bridging MHD and particle perspectives in solar flare physics.

Findings

Alfven waves can generate field-aligned electric fields for electron acceleration.

Reflections and mode conversions of waves can lead to turbulence and further electron acceleration.

The model explains rapid magnetic field variations and addresses the electron supply problem in flares.

Abstract

The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the pre-flare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario in which products of the reconfiguration - large-scale Alfven wave pulses - transport the energy and magnetic-field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. Firstly, in a coronal plasma with beta < m_e/m_p, the waves propagate as inertial Alfven waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wavefronts. Secondly, when they reflect and mode-convert in the chromosphere, a cascade to high wavenumbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle-based views of a flare, provides an interpretation of the recently-observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare "number problem" of finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission.