Limb Event Brightenings and Fast Ejection Using IRIS Mission Observations

Abstract

The Interface Region Imaging Spectrograph (IRIS) of the recently commissioned NASA small explorer mission provides significantly more complete and higher resolution spectral coverage of the dynamical conditions inside the chromosphere and transition region (TR) than has been available ever before. High temporal, spatial ($0.3''$) and spectral resolution observations from the ultraviolet IRIS spectra near the solar limb reveal high-energy limb event brightenings (LEBs) at low chromospheric heights at about 1 Mm above the limb. They can be characterized as explosive events producing jets. We selected two events showing spectra of a confined eruption just off or near the quiet-Sun limb, the jet part showing obvious moving material with short-duration large Doppler shifts in three directions that were identified as macrospicules on slit-jaw (SJ) images in Si iv and He ii 304 Å. The events were analyzed from a sequence of very close rasters taken near the central meridian and the South Pole limb. We analyzed the processed SJ images and the simultaneously observed fast spectral sequences, which have large Doppler shifts, with a pair of redshifted elements together with a faster blueshifted element from almost the same position. Shifts correspond to velocities of up to $100~\mbox{km}\,\mbox{s}^{-1}$ in projection on the plane of the sky. Erupting spicules and macrospicules from these regions are visible in images taken before and after the spectra. The cool low first ionization potential (FIP) element simultaneous line emissions of the Mg ii h and k resonance lines do not clearly show a similar signature because of optical thickness effects, but the Si iv broadband SJ images do. The bidirectional plasma jets ejected from a small reconnection site are interpreted to be the result of coronal loop–loop interactions that lead to reconnection in nearby sites.