Cluster and THEMIS Observations of Asymmetric Magnetic Reconnection at the Dayside Magnetopause

The goal of our project is to investigate characteristics of asymmetric magnetic reconnection at the dayside magnetopause. We present the observations from Cluster 3 which crossed the magnetopause at (X, Y, Z) = (8.3, 1.3, 4.5) RE GSM while travelling outbound in the northern hemisphere on March 5, 2007. Bursty high speed flows (Vz~250 km/s, Vx~-200 km/s) were observed from 18:55 UT to 19:10 UT. During this time interval, the IMF was southward with (Bx, By, Bz) = (-4, 0, -4) nT GSM and the solar wind speed was -400 km/s. The high speed flow satisfies the Walén relation indicating the spacecraft being in the reconnection exhaust region. We identify the separatries, flow boundaries, and central current sheet on the magnetospheric side (Msp) and magnetosheath side (Msh) by sharp boundaries in wave spectrogram, particle differential energy flux, flow, field, and density gradient. The separatrix, flow boundary angles, and J × B force in the Msh side are larger than these on the Msp side. Then the geometry of the asymmetric reconnection are deduced based on the different separatrix angles. The cold plasmaspheric ions are present on the Msp side and are accelerated by being tied to field lines which acquire large transverse motions once they become reconnected to the interplanetary magnetic field. THEMIS observations of a reconnection event near the equatorial plane ((X, Y, Z) = (9.9, -3.9, -1.5) RE GSM) on September 21, 2008 show similar results. Cold plasmaspheric ions was observed in the low-latitude boundary layer. The cold plasmaspheric ions and the magnetosheath ions behave differently in the reconnection process. The bulk velocity of transmitted magnetosheath ions has both parallel and perpendicular components. On the other hand, energy spectrum and velocity distributions show that the cold plasmaspheric ions are accelerated perpendicularly to the magnetic field by being tied to field lines after crossing the separatrix.