Possible Signatures of a Termination Shock in the 2014 March 29 X-class Flare Observed by IRIS

The standard model of flares predicts the existence of a fast-mode magnetohydrodynamic shock above the looptops, also known as termination shock (TS), as the result of the downward-directed outflow reconnection jets colliding with the closed magnetic loops. A crucial spectral signature of a TS is the presence of large Doppler shifts in the spectra of high-temperature lines (≥10 MK), which has been rarely observed so far. Using high-resolution observations of the Fe XXI line with the Interface Region Imaging Spectrograph (IRIS), we detect large redshifts (≈200 km s^-1) at the top of the bright looptop arcade of the X1-class flare on 2014 March 29. In some cases, the redshifts are accompanied by faint simultaneous Fe XXI blueshifts of about -250 km s^-1. The values of red and blueshifts are in agreement with recent modeling of Fe XXI spectra downflow of the reconnection site and previous spectroscopic observations with higher temperature lines. The locations where we observe the Fe XXI shifts are co-spatial with 30-70 keV hard X-ray sources detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), indicating that nonthermal electrons are located above the flare loops. We speculate that our results are consistent with the presence of a TS in flare reconnection models.