Asymmetry in the bipolar signatures of flux transfer events

[1] Several conceptual models have been proposed for the formation of flux transfer events (FTEs), including models based on reconnection at a single reconnection line (X line) and at multiple X lines. Two-dimensional magnetohydrodynamic models have previously been used to simulate both scenarios and have found a tendency for FTEs generated by single X line reconnection to exhibit an asymmetry in the bipolar BN signature that is the major in situ signature of FTE structures, with the leading peak being substantially smaller than the trailing peak. On the other hand, simulated FTEs generated by multiple X line reconnection led to more symmetric signatures. We present a comparison of these simulation results with observations made at the Earth's magnetopause by the Cluster spacecraft, using a data set of 213 FTEs which were observed by all four spacecraft in 2002/2003 at the high-latitude magnetopause near local noon and at low latitudes on the flanks, and 36 FTEs which were observed by one or more Cluster spacecraft near the subsolar point in 2007 and 2008. A tendency is found for the BN signatures to be asymmetric but with the leading peak larger in amplitude than the trailing peak, opposite to the prediction made by the 2-D single X line simulations. This tendency is weaker in the subsolar FTEs. Therefore, the observations are not consistent with 2-D MHD simulations of single X line reconnection. The signatures observed near the subsolar point are more consistent with those predicted by 2-D simulations of multiple X line reconnection, although the multiple X line simulation studies did not report any net asymmetry. We propose that the observed asymmetry can be explained by a compression of magnetic flux ahead of the propagating FTE structure and a rarefaction behind it. The weaker tendency nearer the subsolar point is consistent with a weaker compression and rarefaction due to lower FTE velocities.