Characterizing Chromospheric Condensation with High-cadence Si iv Observations from IRIS

Abstract

Chromospheric flare ribbons are believed to result from coronal magnetic energy released by reconnection and propagating down to the cooler, lower atmosphere. They offer an opportunity to study flare reconnection using chromospheric observations at a spatial and temporal resolution higher than typically available by direct coronal observation. Transient downflows, known as chromospheric condensations (CCs), often occur at the leading edge of a ribbon as a prompt response to the energy input. These have properties that have recently been shown to relate to the coronal energy release process driving them. We use a set of IRIS observations of flare ribbons, made at high cadence in the sit-and-stare mode, to identify and analyze a sample of 134 CC episodes. Among this sample, 132 episodes have at least one time frame that exhibited a double-Gaussian profile. Clear CC appears to occur in only about one fifth of all ribbon pixels, indicating that flare energy release is necessary but not sufficient for producing the characteristic form of CC. We find that condensation pixels form clusters averaging 1 arcsec in extent, all undergoing similar and concurrent time evolution. This suggests that coronal reconnection and associated energy release occur in localized patches whose spatial extent maps to around 1 arcsec in the chromosphere.