Are All Flare Ribbons Simply Connected to the Corona?

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Friday, July 21, 2017

We consider the observational basis for the belief that flare ribbons in the chromosphere result from energy transport from the overlying corona. We study ribbons of modest flares using magnetic as well as intensity data from the Hinode, SDO and IRIS missions.

Magnetic data image
Magnetic data are shown for active region 11890, before, during, and after the two flares at 12:07 (ribbon extent traced with a white line) and 12:15 (black line) shown in Figure 1. Each row shows longitudinal, tangential fields, field azimuth (with the 180◦ ambiguity unresolved) and the continuum intensity. The upper panels show the Hinode scan obtained before the flares, the middle panels show HMI data closest to the flaring events, and the lower panels show Hinode data from the next complete scan some six hours later. Six hours is a considerable fraction of the lifetimes of supergranules, so surface field evolution has occurred between the two HINODE scans. High cadence HMI magnetic field evolution animation for the time-range in between the two Hinode scans is available as electronic supplementary material.

While most ribbons appear connected to the corona, we find one ribbon with no evidence for such a connection. Evolving horizontal magnetic fields seen with Hinode suggest that reconnection with pre-existing fields can explain the data. We conclude that at least two mechanisms are responsible for the heating that leads to flare ribbon emission.