Ionospheric signatures of split reconnection X-lines during conditions of IMF Bz < 0 and |By| ∼ |Bz|: Evidence for the antiparallel merging hypothesis
 We present measurements of the convection electric field in the dayside ionosphere during extended intervals when the interplanetary magnetic field (IMF) has a negative Bz component and a significant By component (|By| ∼ |Bz|). Under these conditions, distinct differences in the ionospheric projection of the magnetopause reconnection X-line are predicted by the antiparallel and subsolar merging hypotheses. Close to solstice, in the winter hemisphere the antiparallel merging hypothesis predicts two ionospheric merging lines which are significantly separated either side of noon. This prediction is not expected from any subsolar merging hypothesis (e.g., component merging). Using the method of global convection mapping with line-of-sight velocity data from the SuperDARN HF radar network, we present evidence of two distinct merging regions on the dayside magnetopause under these IMF and seasonal conditions. The two merging regions manifest themselves in the ionosphere as two regions of convection flow with a strong poleward component separated by a region of convection flow with either a weak poleward or equatorward component corresponding to an adiaroic portion of the polar cap boundary. The spatiotemporal nature of the actual convection scenario can be complicated, but the predicted pattern represents a recurring feature of the convection electric field under these IMF and seasonal conditions. These observations lend support to the hypothesis that antiparallel merging is the dominant form of magnetopause reconnection during these steady state IMF intervals. This suggests that the magnetic shear between magnetosheath and magnetospheric magnetic field lines plays the major role in determining the location of reconnection on the dayside magnetopause.
Authors: Chisham, G. ORCID record for G. Chisham, Coleman, I.J., Freeman, M.P. ORCID record for M.P. Freeman, Pinnock, M., Lester, M.