  In this study, we investigate motions in the hot plasma above the flare loops
during the 2017 September 10 X8.2 flare event. We examine the region to the
south of the main flare arcade, where there is data from the Interface Region
Imaging Spectrograph (IRIS), and the Extreme ultraviolet Imaging Spectrometer
(EIS) on Hinode. We find that there are initial blue shifts of 20--60 km/s
observed in this region in the Fe XXI line in IRIS and the Fe XXIV line in EIS,
and that the locations of these blue shifts move southward along the arcade
over the course of about 10 min. The cadence of IRIS allows us to follow the
evolution of these flows, and we find that at each location where there is an
initial blue shift in the Fe XXIV line, there are damped oscillations in the
Doppler velocity with periods of ~400 s. We conclude that these periods are
independent of loop length, ruling out magnetoacoustic standing modes as a
possible mechanism. Microwave observations from the Expanded Owens Valley Solar
Array (EOVSA) indicate that there are non-thermal emissions in the region where
the Doppler shifts are observed, indicating that accelerated particles are
present. We suggest that the flows and oscillations are due to motions of the
magnetic field that are caused by reconnection outflows disturbing the loop-top
region.
