Criteria for and statistics of electron diffusion regions associated with subsolar magnetic field reconnection

The definition of “electron diffusion regions” and criteria for identifying them in magnetic field reconnection events are given. By employing these criteria and further constraints on the measured parallel electric field, 117 electron diffusion regions have been found in searching through 3 years of Polar satellite subsolar data. They exist in filamentary currents in which parallel electric fields and depressed plasma densities are found and where the electron beta is generally less than 1. The average parallel electric field in these events is about 30% of the average 38 mV/m perpendicular field. The size of these regions is the order of the electron skin depth or less. These electron diffusion regions are topological boundaries in the electron and magnetic field line flows because the components of E × B /B 2 on their opposite sides are frequently different. These regions are found throughout the magnetopause but mainly at the magnetospheric separatrix. The divergence of the pressure tensor in the Generalized Ohm's Law may be the leading term that balances the parallel electric field if the observed large plasma density variations (and hence electron pressure variations) were spatial and not temporal. The picture resulting from this data is of a magnetopause that is highly structured and filamentary and very different from a linear, laminar, symmetric structure sometimes considered in theories or simulations. However, it is emphasized that events such as those described have been found in fewer than 20% of the magnetopauses examined, so the conventional picture may be more prevalent.