Electrons re-acceleration at the footpoints of Solar Flares

TL;DR

This paper proposes a new model where electrons are re-accelerated at the footpoints of solar flares, challenging the traditional view that they only lose energy there, and offers a potential solution to existing modeling problems.

Contribution

It introduces the Local Re-acceleration Thick Target Model (LRTTM), providing an alternative to the standard collisional thick target model for solar flare hard X-ray sources.

Findings

Re-acceleration at footpoints can explain observed X-ray emissions.

LRTTM requires fewer electrons than traditional models.

Numerical results support the feasibility of re-acceleration scenarios.

Abstract

Hinode's observations revealed a very dynamic and complex chromosphere. This require revisiting the assumption that the chromospheric footpoints of solar flares are areas where accelerated particles only lose energy due to collisions. Traditionally electrons are thought to be accelerated in the coronal part of the loop, then travel to the footpoints where they lose their energy and radiate the observed Hard X-ray. Increasing observational evidence challenges this assumption. We review the evidence against this assumption and present the new Local Re-acceleration Thick Target Model (LRTTM) where at the footpoints electrons receive a boost of re-acceleration in addition to the usual collisional loses. Such model may offer an alternative to the 'standard' collisional thick target injection model (TTM) (Brown 1971) of solar HXR burst sources, requiring far fewer electrons and solving some recent problems with the TTM interpretation. We look at the different scenarios which could lead to such re-acceleration and present numerical results from one of them.