Geotail observation of the dissipation region in magnetotail reconnection

Magnetic reconnection is an important fundamental process in and around the Earth's magnetosphere. Since the reconnection process is controlled by a compact region surrounding the X-point, it is important to understand the structure and local physics there. Recent large-scale kinetic simulations have revealed a complex structure near the X-point such as a central dissipation region, quadrupole Hall fields, and bi-directional electron jets. In this work, we verify the above picture in the Earth's magnetotail. We examine Geotail data of the 2003-05-15 reconnection event in the pre-midnight magnetotail. This is the best ever reconnection event and electron-scale signatures are well resolved. Introducing an approximate dissipation measure D_e* and evaluating it from Geotail data, we successfully detect a compact dissipation region, for the first time in a planetary magnetotail. The results are consistent with the theory: The region is associated with the electron flow reversal, its spatial length is 2400--3600 [km] or 1--2 local ion inertial length, and the dissipation rate is 45 [pWm**-3]. We also introduce Lorentz work W, the work rate by Lorentz force to plasmas. It is positive over the reconnection region and it has a peak around the pileup region away from the X-point. These new measures D_e* and W provide useful information to understand the reconnection structure.Geotail crossing of the reconnection site on 15 May 2003. Plasma velocities and the energy dissipation D_e* are shown.