QUASI-PERIODIC FLUCTUATIONS AND CHROMOSPHERIC EVAPORATION IN A SOLAR FLARE RIBBON OBSERVED BY IRIS

ABSTRACT The Interface Region Imaging Spectrograph (IRIS) satellite obtained rapid cadence (9.4 s) stare spectra of an M7 flare ribbon in AR 12036 on 2014 April 18. Chromospheric and transition region line emission exhibited quasi-periodic intensity and velocity fluctuations in the ribbon prior to the appearance of Fe xxi emission. Seven intensity peaks were observed in light curves from small ( ) tracked spatial locations in the ribbon, the first four of which show variable time separations around 3 minutes, and the last four of which show variable time separations about half that value, i.e., the frequency appears to have doubled. The Fe xxi intensity increased rapidly and impulsively after the quasi-periodic fluctuations in chromospheric and transition region lines. The entire Fe xxi line profile was blueshifted when the line first appeared, corresponding to an upward velocity around −100 km s−1. This upward velocity increased to a maximum of about −150 km s−1 before diminishing to zero around the time of maximum intensity. Simultaneous, cospatial velocities observed with Si iv line emission were directed downward, consistent with explosive chromospheric evaporation. During this flare the Fe xxi line's profile is well fit with only one Gaussian component that is either wholly blueshifted or wholly at rest; no significant secondary blueshifted or redshifted components are observed. This suggests that IRIS may have sufficient spatial resolution to resolve loop strands in these flare observations. Under the assumption that the Fe xxi line is at rest when its width is thermal, we derive a rest wavelength of 1354.0714 0.0108 for this forbidden line.