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tokens: of Fe xxi 1354.09 Å. On the other hand, the shorter period cannot be detected in these detrended fluxes, which further confirms that the 3.1 minute oscillations can only be observed in the Doppler shift of Fe xxi 1354.09 Å. Figure 9. Normalized Fourier power spectra of the detrended time series from IRIS Fe xxi 1354.09 Å (a, b), GOES SXR fluxes (c, d), and AIA EUV light curves (e, f). A horizontal purple line in each panel indicates the 99.99% confidence level. As mentioned above, the Doppler shift oscillations within a period of ∼3.1 minutes ( P 1 ) can be interpreted as the standing kink oscillations in the hot flare loop. On the other hand, the longer period ( P 2 ) is different from the shorter one ( P 1 ), because it can be observed simultaneously in SXR and EUV fluxes, as well as in the time series of Doppler shift and line-integrated intensity of Fe xxi 1354.09 Å. Those facts imply the process of periodic energy release in this flare (Tian et al. 2015 ; Li et al. 2017b ). The longer period is most likely to be linked to the quasi-periodic pulsations during the solar flare, which seems to be caused by an MHD wave in the slow mode (Fang et al. 2015 ; Van Doorsselaere et al. 2016 ). Based on the formation temperature ( 11 MK) of Fe xxi 1354.09 Å, the local sound speed ( v s ≈ ) in the flare loop is estimated to be ∼500 km s −1 (e.g., Nakariakov Ofman 2001 ; Kumar et al. 2013 , 2015 ). While the deprojected length ( L ) and longer period ( P 2 ) of this flare loop have been identified as ∼81 Mm and ∼10 ± 4 minutes, then the phase speed of this flare loop ( v h = 2L/ P 2 ) can be estimated as about 200−450 km s −1 . This phase speed is slower than the local sound speed, implying it might be a slow MHD wave (Mandal et al. 2016 ). In other words, the longer period ( P 2 ) might be explained as the MHD wave in a slow mode. To compare the Doppler shift oscillations, we also use the sine function with a damping amplitude (Equation ( 3 )) to fit the time series of Doppler shift during peaks “5” and “6” (e.g., Nakariakov et al. 1999 ; Wang et al. 2002 ; Anfinogentov et al. 2015 ), because the Doppler shift oscillations are most pronounced during this time interval. Figure 8 (upper) shows the fitting result, the overlaid turquoise profile gives the best fitting, and it appears to match well with the Doppler velocities (black).Here, v 0 is the initial Doppler velocity, v m is the initial oscillatory amplitude, and P 1 , ψ , and τ are the shorter dominant period, initial phase, and decay time of the oscillations, respectively. 5. Conclusions Using observations from multi-instruments, i.e., IRIS , SDO /AIA, SDO /HMI, Hinode /XRT, and NoRH, we investigate the Doppler shift oscillations in a hot loop during a GOES M7.1 flare on 2014 October 27. The main results are summarized as follows. (I) The Doppler shift oscillations are detected in a hot flare line, i.e., Fe xxi 1354.09 Å from IRIS spectroscopic observations. The time series of Doppler shift