Two-step flux rope evolution and microwave emission in a compound solar flare

Bernhard Kliem, University of Potsdam, Institute of Physics and Astronomy

We present an analysis of a compound solar flare on 2015 June 21, which consisted of three confined precursor flares (C to M2.0 class), followed by an eruptive M2.6 flare and a coronal mass ejection (CME). EUV data from the Solar Dynamics Observatory reveal the formation of a sigmoidal hot channel during the confined flares, which can only be interpreted as a magnetic flux rope. The structure erupts in the eruptive flare and evolves into the CME. Overall, the sequence demonstrates that the erupting flux rope was formed prior to (not during) the CME. The microwave emission showed a stationary source between the similarly located main ribbons of all four flares, which perfectly corresponds to the location of the vertical (flare) current sheet in this event near disk center. A second, remote microwave source formed during the eruptive flare above a parasitic polarity, where also a closed, roughly circular ribbon developed. We suggest that these structures signify reconnection and particle acceleration in the separatrix layer associated with a coronal magnetic null above the parasitic polarity. Different from the standard picture of separatrix perturbation from below (often by an erupting filament), the energy release in the separatrix was here triggered by a lifting of overlying flux when the extended pre-formed sigmoidal flux rope erupted.