IRIS Movie of the Day
At least once a week a movie of the Sun taken by NASA's Interface Region Imaging Spectrograph (IRIS) is posted by one of the scientists operating the instrument.
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20 Feb 2018
A Hot Solar Flare, Rapidly Cooling
Credit: IRIS, LMSAL/NASA, Jean-Pierre Wuelser
By design, these IRIS images are sensitive to very hot, 10 million degree material and relatively cool, 100,000 degree material, but not to temperatures in between. This leads to some interesting effects when observing solar flares. This flare first shows a set of fuzzy loops above the limb. IRIS spectra reveal that these fuzzy loops are very hot, while all the sharply defined features are quite cool. Later, more fuzzy loops light up further to the right, but start to fade around the middle of the movie sequence. At that time, sharply defined, cool, post-flare loops become visible which later give way to cool material that drains down from the region where the fuzzy loops used to be. This demonstrates that hot gas cools very quickly, once the flare heating stops. Note that the brief fading of the whole image about one third into the movie has nothing to do with the Sun. The movie was taken at a time of the year when IRIS periodically looks through the upper part of the Earth's atmosphere, which strongly absorbs ultraviolet light.