Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions

TL;DR

This study uses spectroscopic data to analyze hot plasma in solar active regions, providing evidence that impulsive nanoflare heating likely drives coronal heating, based on emission measure distributions.

Contribution

First comprehensive spectroscopic analysis of hot plasma in active regions supporting impulsive heating models.

Findings

EM distributions are flat or slowly increasing up to 3 MK

Distributions fall off rapidly at higher temperatures

Impulsive heating models reproduce observed EM distributions well

Abstract

EUV observations of warm coronal loops suggest that they are bundles of unresolved strands that are heated impulsively to high temperatures by nanoflares. The plasma would then have the observed properties (e.g., excess density compared to static equilibrium) when it cools into the 1-2 MK range. If this interpretation is correct, then very hot emission should be present outside of proper flares. It is predicted to be vey faint, however. A critical element for proving or refuting this hypothesis is the existence of hot, very faint plasmas which should be at amounts predicted by impulsive heating. We report on the first comprehensive spectroscopic study of hot plasmas in active regions. Data from the EIS spectrometer on Hinode were used to construct emission measure distributions in quiescent active regions in the 1-5 MK temperature range. The distributions are flat or slowly increasing up to approximately 3 MK and then fall off rapidly at higher temperatures. We show that active region models based on impulsive heating can reproduce the observed EM distributions relatively well. Our results provide strong new evidence that coronal heating is impulsive in nature.