tokens: IRIS detected quasi-periodic fluctuations in the relative Doppler velocities of the C i , Si iv , and O iv lines from its pointing position about 40′′ east of the EIS slit's position, within the same ribbon. This may be because IRIS observed that portion of the ribbon to undergo quasi-periodic fluctuations before the onset of explosive chromospheric evaporation, when the amplitudes of velocity fluctuations may simply have been more readily detected. Other contributing factors may be that IRIS has higher spectral resolution, and spatial resolution nearly 10 times better than that of EIS. The IRIS observations revealed, however, that quasi-periodic fluctuations in the redshifted relative Doppler velocities were not as pronounced as those in the intensities (Brosius Daw 2015 ). The observed O iv , Mg vi , Mg vii , Si vii , Fe xiv , and Fe xvi fluctuations provide evidence for quasi-periodic energy injections into the chromosphere (at the location of the flare ribbon observed by EIS) by accelerated, nonthermal particles. This is similar to the conclusion drawn by Brosius Daw ( 2015 ) regarding fluctuations in C i , Si iv , and O iv at the location of the flare ribbon observed by IRIS , all of which occurred before the appearance of Fe xxi . In that work, each episode was thought of as a failed flare because no emission at temperatures characteristic of flares (Fe xxi emission) was produced. The quasi-periodic injections of nonthermal beam particles could be produced by MHD oscillations in a magnetic trap at the site of the initial flare energy release (by reconnection). In this scenario (Zaitsev Stepanov 1982 ), MHD oscillations in the trap are generated by the impulsive energy release itself, and modulate the periodic escape of particles therefrom. Alternatively, the injections could be driven by MHD oscillations in a nearby, nonflaring magnetic loop (Nakariakov et al. 2006 ). In this scenario, the oscillating loop perturbs the plasma external to the loop, which produces variations of the electric current density in the flaring region. These induce plasma micro-instabilities and, hence, anomalous resistivity in the flaring region, which can quasi-periodically trigger the magnetic reconnection events that accelerate nonthermal particles. Yet again, the injections could be supplied by quasi-periodic particle acceleration episodes during dynamic magnetic reconnection in a large-scale current sheet dominated by repeated formation of magnetic islands (Kliem et al. 2000 ). In this scenario, particles are presumably accelerated by DC electric fields near the reconnection X points. Recently, Guidoni et al. ( 2016 ) extended the approach of Drake et al. ( 2010 ; see also Karpen et al. 2012 ) to investigate electron acceleration in sunward-moving magnetic islands (plasmoids) formed by spatially and temporally variable reconnection in a flare current sheet. Their 2.5D simulations of an eruptive flare revealed that energy gains in a single island
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