The emission measure distribution of impulsive phase flare footpoints

TL;DR

This study uses Hinode/EIS data to determine the emission measure distributions of impulsive phase flare footpoints, revealing a consistent peak temperature around 8 MK and an EM gradient supporting a specific heating scenario.

Contribution

First to obtain detailed emission measure distributions of impulsive phase flare footpoints using high-resolution spectral data.

Findings

EMDs peak at ~8 MK indicating high-temperature plasma presence

EM(T) ~ T gradient consistent with direct heating of top chromosphere layers

Results support conduction-based heating models in flare footpoints

Abstract

The temperature distribution of the emitting plasma is a crucial constraint when studying the heating of solar flare footpoints. However, determining this for impulsive phase footpoints has been difficult in the past due to insufficient spatial resolution to resolve the footpoints from the loop structures, and a lack of spectral and temporal coverage. We use the capabilities of Hinode/EIS to obtain the first emission measure distributions (EMDs) from impulsive phase footpoints in six flares. Observations with good spectral coverage were analysed using a regularized inversion method to recover the EMDs. We find that the EMDs all share a peak temperature of around 8 MK, with lines formed around this temperature having emission measures peaking between 10^28 and 10^29 cm^-5, indicating a substantial presence of plasma at very high temperatures within the footpoints. An EMD gradient of EM(T) ~ T is found in all events. Previous theoretical work on emission measure gradients shows this to be consistent with a scenario in which the deposited flare energy directly heats only the top layer of the flare chromosphere, while deeper layers are heated by conduction.