Transition to Petschek-type Reconnection in Solar Wind Reconnection Exhausts

Abstract:

The Petschek reconnection model predicts that for antiparallel symmetric conditions, slow-mode shocks should form along reconnection exhaust boundaries, and that the reconnection current sheet should bifurcate. However, closer to the X-line it is expected that Hall physics effects should play a more significant role in controlling the reconnection dynamics. Whilst in-situ observations at the magnetopause and magnetotail have provided a detailed insight into reconnection physics, imprecise knowledge of the spacecraft location both within the reconnection exhaust and relative to the X-line limits the extent to which the spatial structure of reconnection exhausts can be probed. In the solar wind, however, the rapid transit of a spacecraft across the solar wind exhausts allow us to make detailed observations with precise knowledge of the spacecraft location within the jet, such that the magnetic structure of the reconnecting boundary can be directly deduced. If two spacecraft measure the reconnection jets either side of the X-line, this enables a much more precise reconstruction of the reconnection geometry, but this is a rare occurrence.

Here we present three solar wind reconnection events where different spacecraft (ACE, Cluster and Wind) sampled both of the oppositely directed reconnection exhausts from a common reconnection X-line, which allows us to estimate each spacecraft’s distance from the X-line. We find that in all three cases spacecraft furthest from the reconnection site observed bifurcated current sheets, consistent with Petschek reconnection, whereas spacecraft closer to the reconnection site did not. This suggests that bifurcations of reconnection current sheets develop with increasing distance from the X-line, and that Petschek-type signatures are less developed close to the reconnection site. We discuss these results and consider what may control the point at which these signatures appear, and implications for other reconnection environments.