SM13A-4155:
Ion acceleration dependence on shear angle in dayside magnetopause reconnection
Monday, 15 December 2014
Sarah K. Vines1,2, Stephen Fuselier1, Karlheinz J Trattner3, Steven M Petrinec4 and James Frederick Drake5, (1)Southwest Research Institute San Antonio, San Antonio, TX, United States, (2)University of Texas at San Antonio, San Antonio, TX, United States, (3)Laboratory for Atmospheric and Space Physics, Boulder, CO, United States, (4)Lockheed Martin Space Systems, Cupertino, CA, United States, (5)University of Maryland College Park, College Park, MD, United States
Abstract:
Reconnection at the magnetopause of Earth plays an important role in magnetospheric dynamics. Previous theoretical work suggests that ion exhaust velocities are equal to the Alfvén speed for no guide field and exhaust velocities decrease with increasing guide field. These theoretical predictions are tested at the Earth’s magnetopause using observations from the Cluster spacecraft. Cluster magnetopause crossings, from 2002 – 2009, cover the dayside reconnection region (9 – 15 LT with |MLAT| < 60°) for a wide range of IMF clock angles (90° – 270°) and magnetic shear angles (88° – 180°). The difference between the incident and reflected ion velocities (vsep) in the magnetosheath boundary layer (MSBL) ion populations is used to determine the exhaust velocity. The ratio of vsep over twice the Alfvén speed (vsep/2vA) is predicted to approach one for reconnection with shear angles near 180° (no guide field), but is observed to reach a maximum value of approximately 0.8 for the magnetopause crossings analyzed. This value is consistent with previous observations of ion velocities from reconnection at the magnetopause investigated with the Walén relation. While magnetic shear angle can contribute to the disagreement between observations and the Walén relation, it does not play a large role, given the reduced ratio for the events near 180° in this study.
