Spatially Resolved Quasi-Periodic Fluctuations in C II, Si IV, and Fe XXI Emission During Explosive Chromospheric Evaporation in a Flare Ribbon Observed by IRIS and RHESSI on 2017 September 9

We investigate the onset of a GOES M3.7 flare in AR 12673 around 11:00 UT on 2017 September 9 with rapid cadence (9.4 s) UV stare spectra obtained with IRIS, and coordinated hard X-ray observations with RHESSI. We treat spatially averaged spectra from each of five 1-arcsec segments of the IRIS slit in and near a flare ribbon as if they occurred in separate, independent flares. Analyses of integrated line intensities and Doppler velocities for UV emission lines of C II at 1334.5 Angstrom, Si IV 1402.7, and Fe XXI 1354.1 reveal a variety of temporal behaviors. As an example, in the segment labeled 26, C II and Si IV exhibited a precursor phase during which their intensities underwent a series of four fluctuations characterized by quasi-periods of 237.9+/-61.1 and 258.8+/-89.6 s, respectively, prior to the appearance of Fe XXI emission. After Fe XXI was detected, C II, Si IV, and Fe XXI all underwent a series of four fluctuations characterized by quasi-periods of 117.4+/-35.6, 111.3+/-38.6, and 135.1+/-22.2 s; timing differences between successive peaks in Fe XXI upward velocities yield a quasi-period of 141.1+/-28.2 s. Time differences between the four C II intensity peaks and the four Fe XXI intensity peaks reveal that the C II peaks preceded their Fe XXI counterparts by 39, 44, 41, and 72 s, while the Fe XXI upward velocity peaks preceded their intensity counterparts by 71, 59, 75, and 51 s. The temporal behavior of the UV emission is consistent with chromospheric evaporation during a series of nonthermal particle beam injections into the chromosphere. RHESSI observations confirm the presence of nonthermal electrons, for which we estimate sufficient beam energy flux to drive explosive chromospheric evaporation.