We present a detailed investigation of the Doppler shift oscillations in a hot loop during an M7.1 flare on 2014 October 27 observed by the Interface Region Imaging Spectrograph .
The periodic oscillations are observed in the Doppler shift of Fe XXI 1354.09 Å ( log T \sim 7.05 ) , and the dominant period is about 3.1 minutes .
However , such 3.1-min oscillations are not found in the line-integrated intensity of Fe XXI 1354.09 Å , AIA EUV fluxes , or microwave emissions . SDO /AIA and Hinode /XRT imaging observations indicate that the Doppler shift oscillations locate at the hot loop-top region ( \geq 11 MK ) .
Moreover , the differential emission measure ( DEM ) results show that the temperature is increasing rapidly when the Doppler shift oscillates , but the number density does not exhibit the corresponding increases nor oscillations , implying that the flare loop is likely to oscillate in an incompressible mode .
All these facts suggest that the Doppler shift oscillations at the shorter period are most likely the standing kink oscillations in a flare loop .
Meanwhile , a longer period of about 10 minutes is identified in the time series of Doppler shift and line-integrated intensity , GOES SXR fluxes and AIA EUV light curves , indicating the periodic energy release in this flare , which may be caused by a slow mode wave .