Transition Region and Chromospheric Signatures of Impulsive Heating Events. I. Observations

TL;DR

This study uses high-resolution IRIS observations to analyze how the transition region and chromosphere respond to impulsive heating during a small solar flare, revealing persistent red-shifts and small-scale bursts.

Contribution

It provides new insights into the small-scale, dynamic response of the solar atmosphere to impulsive heating, challenging existing hydrodynamic models.

Findings

Persistent red-shifts in Si IV, C II, Mg II during impulsive phase

Detection of small-scale bursts lasting less than 60 seconds

Evidence for energy release on many small-scale filaments with a power-law distribution

Abstract

We exploit the high spatial resolution and high cadence of the Interface Region Imaging Spectrograph (IRIS) to investigate the response of the transition region and chromosphere to energy deposition during a small flare. Simultaneous observations from RHESSI provide constraints on the energetic electrons precipitating into the flare footpoints while observations of XRT, AIA, and EIS allow us to measure the temperatures and emission measures from the resulting flare loops. We find clear evidence for heating over an extended period on the spatial scale of a single IRIS pixel. During the impulsive phase of this event the intensities in each pixel for the Si IV 1402.770, C II 1334.535, Mg II 2796.354 and O I 1355.598 emission lines are characterized by numerous, small-scale bursts typically lasting 60s or less. Red shifts are observed in Si IV, C II, and Mg II during the impulsive phase. Mg II shows red-shifts during the bursts and stationary emission at other times. The Si IV and C II profiles, in contrast, are observed to be red-shifted at all times during the impulsive phase. These persistent red-shifts are a challenge for one-dimensional hydrodynamic models, which predict only short-duration downflows in response to impulsive heating. We conjecture that energy is being released on many small-scale filaments with a power-law distribution of heating rates.