Photospheric response to a flare

TL;DR

This paper models how large-amplitude shear Alfvén waves from the corona cause permanent magnetic and velocity shear changes in the photosphere during solar flares, potentially explaining particle acceleration.

Contribution

It introduces a wave-based model explaining photospheric magnetic and velocity shear changes observed during flares, linking coronal dynamics to photospheric responses.

Findings

Photospheric magnetic shear increases along the neutral line.

Photospheric velocity shear exhibits opposite signs on either side of the neutral line.

Model suggests a mechanism for particle acceleration in flares.

Abstract

Flares produce sudden and permanent changes in the horizontal photospheric magnetic field. In particular flares generally produce increased magnetic shear in the photospheric field along the neutral line. Recent observations show also that flares can produce sudden photospheric motion. We present a model for the observed changes as the response of the photosphere to a large-amplitude shear Alfv\'{e}n wave propagating down from the corona on either side of the neutral line. The Alfv\'{e}nic front is assumed to impact the photosphere close to the neutral line first, and then successively further away with time, such that the line of impact coincides with the flare ribbon. The wave introduces magnetic shear and velocity shear. The magnetic shear introduced at the photosphere has the same sign on either side of the neutral line, while the velocity shear has the opposite sign. We discuss the possibility that this process is responsible for particle acceleration in flares.