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Magnetic reconnection is thought to drive a wide variety of dynamic phenomenain the solar atmosphere. Yet the detailed physical mechanisms drivingreconnection are difficult to discern in the remote sensing observations thatare used to study the solar atmosphere. In this paper we exploit thehigh-resolution instruments Interface Region Imaging Spectrograph (IRIS) andthe new CHROMIS Fabry-Perot instrument at the Swedish 1-m Solar Telescope (SST)to identify the intermittency of magnetic reconnection and its association withthe formation of plasmoids in so-called UV bursts in the low solar atmosphere.The Si IV 1403A UV burst spectra from the transition region show evidence ofhighly broadened line profiles with often non-Gaussian and triangular shapes,in addition to signatures of bidirectional flows. Such profiles had previouslybeen linked, in idealized numerical simulations, to magnetic reconnectiondriven by the plasmoid instability. Simultaneous CHROMIS images in thechromospheric Ca II K 3934A line now provide compelling evidence for thepresence of plasmoids, by revealing highly dynamic and rapidly movingbrightenings that are smaller than 0.2 arcsec and that evolve on timescales oforder seconds. Our interpretation of the observations is supported by detailedcomparisons with synthetic observables from advanced numerical simulations ofmagnetic reconnection and associated plasmoids in the chromosphere. Our resultshighlight how subarcsecond imaging spectroscopy sensitive to a wide range oftemperatures combined with advanced numerical simulations that are realisticenough to compare with observations can directly reveal the small-scalephysical processes that drive the wide range of phenomena in the solaratmosphere.

Intermittent reconnection and plasmoids in UV bursts in the low solar atmosphere