Coupled Intensity Variations in Hot Coronal Loops Observed with SDO/AIA
Data from the AIA on board SDO allow us to study EUV intensity variations of coronal loops. Here we study strong EUV brightenings of coronal loops in NOAA Active Region 11250 on 13 July 2011 between UT 13:32 and 14:32. The brightenings are seen only in the 94� band of Fe XVIII, implying an effective temperature ~ 8 MK. The relevant loops run directly between the AR opposite polarities and are low and short. One example is an apparently single loop north of the AR midline. At UT 13:50 (18 min into the observational sequence - see the image) it began a strong brightening in the 94 � band. This occurred at the same moment all along 15 Mm of its length to ~ 12 s accuracy. This suggests the presence of stored free energy along the loop that was released by an instability that must have propagated along it at 1500 km/s or faster. The maximum rate of increase of the 94 � intensity occurred at 20 min into the sequence, and the maximum was reached at 23 min. It then decayed, reaching its former level at time 40 min. Another example lay south of the AR midline (see the image). On its trailing end this loop had two feet, suggesting that it is a superposition of two loops seen in projection. This structure showed two strong 94 � brightenings peaking at 32 min and 37 min into the data sequence. The first brightening was associated with an intensity increase in the northern trailing foot, while the second was associated with an intensity increase in the southern foot. Again, this points to the presence of two superimposed loops. Brightenings in the loop feet indicate upward motion of intensity features toward the loop center at various speeds from 50 - 200 km/s. The intensity in the central loop section shows a sequence of weaker increases at 4 min intervals preceding the strong brightenings and a still weaker sequence at 5 min intervals afterward. These indicate releases of stored energy along a 10 Mm loop segment by a periodically repeating instability. The onset of the periodic energy releases in Example 2 and also the start of the loop brightening in Example 1 both occur at time 18 min into the data sequence. This coincides with the time of disturbances in other features of AR 11250 and suggests a complex coupling among its various structures.; SDO/AIA images of AR11250 made in the EUV 94 � band at UT 13:53 and 14:03 on 13 July 2011. The images are 94 Mm wide. The grayscale is reversed.