Occurrence and Statistics of IRIS Bursts

TL;DR

This study statistically analyzes IRIS observations to characterize IRIS bursts, revealing their occurrence rate, spectral properties, and limited atmospheric heating influence, suggesting confinement of heating to lower atmospheric layers.

Contribution

Introduces a k-means spectral classification method for IRIS bursts and provides the first large-scale statistical analysis of their properties and atmospheric impact.

Findings

IRIS bursts occur in about 8% of observations, comprising 0.01% of spectra.

IBs show similar properties across Mg II, C II, and Si IV lines, but not in Fe XXI.

Heating from IBs appears confined to lower atmospheric layers, with limited impact on hotter coronal lines.

Abstract

Small reconnection events in the lower solar atmosphere can lead to its heating, but whether such heating can propagate into higher atmospheric layers and potentially contribute to coronal heating is an open question. We carry out a large statistical analysis of all IRIS observations from 2013 and 2014. We identified "IRIS burst" (IB) spectra via a k-means analysis by classifying and selecting Si IV spectra with superimposed blend lines on top of bursts, which indicate low atmospheric heating. We found that ~8% of all observations show IBs with about 0.01% of all recorded IRIS spectra being IB spectra. We found varying blend absorption levels, which may indicate different depths of the reconnection event and heating. IBs are statistically visible with similar properties and timings in the spectral lines Mg II, C II, and Si IV, but invisible in Fe XXI. By statistically analyzing co-spatial AIA lightcurves, we found systematic enhancements in AIA 1600 and AIA 1700, but no clear response to bursts in all other AIA wavelengths (94, 131, 171, 193, 211, 304, 335) in a timeframe of $\pm 6$ minutes around the burst. This may indicate that heating due to IBs is confined within the lower atmosphere and dissipates before reaching temperatures or formation heights covered by the hotter AIA lines. Our developed methods are applicable for statistical analyses of any co-observed data sets and allow us to efficiently analyze millions of spectra and lightcurves simultaneously.