Dominance of Bursty over Steady Heating of the 4–8 MK Coronal Plasma in a Solar Active Region: Quantification Using Maps of Minimum, Maximum, and Average Brightness

Abstract

A challenge in characterizing active region (AR) coronal heating is in separating transient (bursty) loop heating from the diffuse background (steady) heating. We present a method of quantifying coronal heating’s bursty and steady components in ARs, applying it to Fe xviii (hot 94) emission of an AR observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The maximum-, minimum-, and average-brightness values for each pixel, over a 24 hr period, yield a maximum-brightness map, a minimum-brightness map, and an average-brightness map of the AR. Running sets of such three maps come from repeating this process for each time step of running windows of 20, 16, 12, 8, 5, 3, 1, and 0.5 hr. From each running window’s set of three maps, we obtain the AR’s three corresponding luminosity light curves. We find (1) the time-averaged ratio of minimum-brightness-map luminosity to average-brightness-map luminosity increases as the time window decreases, and the time-averaged ratio of maximum-brightness-map luminosity to average-brightness-map luminosity decreases as the window decreases; (2) for the 24 hr window, the minimum-brightness map’s luminosity is 5% of the average-brightness map’s luminosity, indicating that at most 5% of the AR’s hot 94 luminosity is from heating that is steady for 24 hr; (3) this upper limit on the fraction of the hot 94 luminosity from steady heating increases to 33% for the 30 minute running window. This requires that the heating of the 4–8 MK plasma in this AR is mostly in bursts lasting less than 30 minutes: at most a third of the heating is steady for 30 minutes.