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.
Two-ribbon C8.3 Flare in AR 12866 with Interesting Redshifts
Credit: IRIS, LMSAL/NASA, Wei Liu
On September 8, 2021, around 17:10 UT, IRIS captured an interesting C8.3-class flare in active region (AR) 12866. This flare exhibited typical double ribbons, as shown in the slit-jaw images on the right-hand side. The western ribbon (the one on the right) was captured by the IRIS spectrograph slit and produced brightenings in spectral lines and continuum shown in various spectral windows on the left. Another interesting feature was the redshifted emission at about 18:30 UT, more than 1 hour later after the flare onset, at the original location of the western flare ribbon across major spectral lines, including C II 1330 Angstrom, Si IV 1403 Angstrom, and Mg II 2796 Angstrom. One possibility to account for this redshift was falling material that was originally heated and lifted up to fill the flare loops via the so-called "chromospheric evaporation process", and had later cooled, condensed, and fell back to the chromosphere as coronal rain (in the "post-flare" loops). Another possibility was material falling back from earlier ejecta (such as a surge or prominence eruption associated with the flare). The latter possibility, however, is less favored by the observational evidence, as the expected blueshift was not detected earlier around the flare onset.