Observations and simulations of asymmetric magnetic field reconnection

Comparisons of asymmetric magnetic field reconnection at the subsolar magnetopause are made between satellite data and computer simulations for the ratio of the antiparallel components of the magnetic fields at the boundaries being about three and the density ratio being 10 to 30. The bipolar electric field and quadrupolar magnetic field of symmetric reconnection are not found in either the simulations or the experimental data because the Hall term in the generalized Ohm's Law ( the leading term on its right side on the ion scale) is a factor up to 100 smaller on the magnetosheath side of the current layer than on the magnetospheric side. Average plasma densities and fields at six THEMIS magnetopause crossings agree with open simulations provided that the experimental data is in the joint variance, not minimum variance, frame. This is because the magnetic field component normal to the assumed planar current sheet is not constant for asymmetric reconnection while it is forced to be approximately constant by the mathematics of minimum variance analyses. A north-south asymmetry of the fields and flows in asymmetric reconnection is found in both the simulations and the space measurements. The simulations and the satellite data both have regions the order of the electron skin depth in size that contain significant electric fields. In such structures, parallel electric fields exist in the simulation throughout the current layer. Such parallel fields are similar to those reported earlier from Polar satellite data.