We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnection separatrix region. This region separates two plasmas with significantly different temperatures and densities, at a large distance from the $X$ line. We identify which terms in the generalized Ohm’s law balance the observed electric field throughout the separatrix region. The electric field inside a thin $\sim c/{\omega }_{pi}$ Hall layer is balanced by the $\mathbf{j}\times \mathbf{B}/\mathrm{ne}$ term while other terms dominate elsewhere. On the low density side of the region we observe a density cavity which forms due to the escape of magnetospheric electrons along the newly opened field lines. The perpendicular electric field inside the cavity constitutes a potential jump of several kV. The observed potential jump and field aligned currents can be responsible for strong aurora.

• Received 4 April 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.205003