Occurrence and statistics of IRIS bursts

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 using a k-means analysis that entails classifying and selecting Si IV spectra with superimposed blend lines on top of bursts, which indicate low atmospheric heating. We find that ∼8% of all observations show IBs with about 0.01% of all recorded IRIS spectra being IB spectra. We find varying blend absorption levels, which may indicate different depths of the reconnection event and heating. IRIS bursts 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 light curves, 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 time-frame of ±6 min 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 light curves simultaneously.