It is important to know the electric field in the tail current sheet in order to understand how particles behave and how much energy is being dissipated. The electric field is also a measurement of the reconnection rate during substorms. For the CDAW-6 substorm period of March 22, 1979, We use the ion data from the medium energy particles experiment (MEPE) on the ISEE-1 satellite, and studied nine measurements of the 3D distribution function centered on the center of the current sheet. The measured distribution function was then integrated to obtain the average of bulk flow velocity in the geocentric solar ecliptic (GSE) frame. This bulk flow velocity was then broken up into its components perpendicular and parallel to the magnetic field for the nine cases. It was further assumed that the perpendicular component was due, in part, to an energy dependent drift and to an energy independent electric field drift. Using the bulk flow velocities from any two energy channels we can separate out the electric and energy dependent drifts and thus obtain electric field and energy dependent components. The two lowest energy channels (34.3 keV and 54.9 keV) give the main results, and the 80.4 keV and 118.8 keV channels are used as a cross check.

We find that Ex fluctuates approximately ¿5 mV/m, and Ey¿10 mV/m, in ressonable agreement with measurements by the electric field instrument [Pedersen et al., 1985>, with most of the fluctuation presumably due to the motion of the current sheet. Using current sheet oscillation theory and the central current sheet data points, we can estimate Ey in the frame of the current sheet and find a positive average Ey with a magnitude of ≈0.1 mV/m, which is also consistent with that expected for reconnection in this substorm time period. The Ez component has a remarkable linear correlation with Bx, with a correlation coefficient of 0.91. Assuming Bx varies linearly with z, a positive ion density of 3.14¿10-20/d(Re) Coulombs/m3 is implied. Such a positive space charge near the current sheet center is expected theoretically.