Ion acceleration dependence on magnetic shear angle in dayside magnetopause reconnection

Magnetic reconnection at the Earth's magnetopause plays an important role in magnetospheric dynamics. Understanding the dynamics requires theory and observations. Previous theoretical work suggests that for no guide field, ions in the exhaust region on the magnetosheath side of the boundary counterstream with a velocity separation that is twice the upstream Alfven speed (v(A)) and that the counterstreaming velocity decreases with increasing guide field. These theoretical predictions are tested for reconnection at the Earth's magnetopause using observations from the Cluster spacecraft. The difference between the incident and reflected ion velocities (v(sep)) in the magnetosheath boundary layer ion populations is used to determine the exhaust velocity. The ratio of v(sep) over twice the Alfven speed (R-V=v(sep)/2v(A,L)) is predicted to approach 1 for reconnection with shear angles near 180 degrees (no guide field) but is observed to reach a value of approximately 0.84 for the magnetopause crossings analyzed with shear angles near 180 degrees. This value is consistent with previous observations of ion velocities from reconnection at the magnetopause investigated using the Walen relation. While magnetic shear angle can contribute to the disagreement between observations and the Walen relation, it does not play a large role, given the reduced ratio for the events near 180 degrees in this study.