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We report on observations of highly-varying Si IV 1402.77 line profilesobserved with the Interface Region Imaging Spectrograph (IRIS) during theM-class flare from 2022 January 18 at an unprecedented 0.8 s cadence. Momentanalysis of this line observed in flare ribbon kernels showed that theintensity, Doppler velocity, and non-thermal broadening exhibited variationswith periods below 10 s. These variations were found to be correlated withproperties of the Gaussian fit to a well-resolved secondary component of theline redshifted by up to 70 km s$^{-1}$, while the primary component wasconsistently observed near the rest wavelength of the line. A particularly highcorrelation was found between the non-thermal broadening of the line resultingfrom the moment analysis and the redshift of the secondary component. Thismeans that the oscillatory enhancements in the line broadening were due toplasma flows (away from the observer) with varying properties. A simplede-projection of the Doppler velocities of the secondary component based on athree-dimensional reconstruction of flare loops rooted in the kernel suggeststhat the observed flows were caused by downflows and compatible with strongcondensation flows recently predicted by numerical simulations. Furthermore,peaks of the intensity and the trends of Doppler velocity of the Gaussian fitto the secondary component (averaged in the ribbon) were found to correspond toone of the quasi-periodic pulsations (QPPs) detected during the event in thesoft X-ray flux (as measured by the Geostationary Operational EnvironmentalSatellite, GOES) and the microwave radio flux (as measured by the ExpandedOwens Valley Solar Array, EOVSA). This result supports a scenario in which theQPPs were driven by repeated magnetic reconnection.

Rapid variations of Si IV spectra in a flare observed by IRIS at a sub-second cadence