Two-Ribbon Flares Spreading in the Third Dimension

Abstract:

Two-ribbon flares, often associated with eruptive filaments or CMEs, are
textbook demonstration of the standard flare model that describes
simultaneous reconnection of an arcade of anti-parallel magnetic field lines
crossing at the 2-dimensional macroscopic current sheet in the corona.
However, flare ribbons are often observed to brighten sequentially along
their length with an apparent speed ranging from 10 to 200 km/s, indicative
of a slow (sub-Alfvenic) and organized pattern of reconnection spreading
along the assumed current sheet. This can be hardly explained by the
mechanism of driven reconnection due to ideal MHD instability that
explosively rushes open coronal field lines leading to subsequent
reconnection. We present observations of flare ribbon spreading
either uni-directionally or bi-directionally along the magnetic polarity
inversion line, or the assumed direction of the overlying current sheet and
explore diagnostics of 3-dimensional (for example with a guide
field) physics that may govern the observed spread of
reconnection (e.g. Shepherd and Cassak, 2012).