A multi-wavelength analysis of small-scale brightenings observed by IRIS

Abstract

Small-scale brightenings in solar atmospheric observations are a manifestation of heating and/or energy transport events. We present statistical characteristics of brightenings from a new detection method applied to 1330, 1400, and 2796 \AA\ IRIS slitjaw image time series. 2377 events are recorded which coexist in all three channels, giving high confidence that they are real. $\approx$1800 of these are spatially coherent, equating to event densities of $\sim9.7\times10^{-5}$arcsec$^{-2}$s$^{-1}$ within a $90\arcsec\times100\arcsec$ FOV over 34.5 minutes. Power Law indices estimates are determined for total brightness ($2.78<\alpha<3.71$), maximum brightness ($3.84<\alpha<4.70$), and average area ($4.31<\alpha<5.70$) distributions. Duration and speed distributions do not obey a power law. A correlation is found between the events' spatial fragmentation, area, and duration, and a weak relationship with total brightness, showing that larger/longer-lasting events are more likely to fragment during their lifetime. Speed distributions show that all events are in motion, with an average speed of $\sim7$\kms. The events' spatial trajectories suggest that cooler 2796 \AA\ events tend to appear slightly later, and occupy a different position/trajectory to the hotter channel results. This suggests that either many of these are impulsive events caused by reconnection, with subsequent rapid cooling, or that the triggering event occurs near the TR, with a subsequent propagating disturbance to cooler atmospheric layers. The spatial distribution of events is not uniform, with broad regions devoid of events. A comparison of spatial distribution with properties of other atmospheric layers shows a tentative connection between high magnetic field strength, the corona's multithermality, and high IRIS brightening activity.