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.
Bibliographical noteFunding Information:
We thank the referee for the useful comments which helped improving the manuscript. V.P. is supported by the NASA grants NNX15AJ93G and NNX15AF50G, and by contract 8100002705 from Lockheed-Martin to SAO. G.G. is supported by the NSF-REU Solar Physics program at SAO, grant number AGS-1560313 and NASA grant NNX15AJ93G. K.R. is supported by the NASA grant NNX15AJ93G. The authors thank Dr. Lynsday Gleneser for insightful discussions. IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research center and major contributions to downlink communications funded by ESA and the Norwegian Space Center. AIA data are courtesy of NASA/SDO and the respective science teams. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway).
- Sun: UV radiation
- Sun: activity
- Sun: flares
- line: profiles
- techniques: spectroscopic