Magnetopause electric field inferred from energetic particle measurements on ATS 5

Energetic (>30 kev) proton and electron flux measurements were made at 6.6 RE on ATS 5 during magnetopause crossings on September 29, 1969, and March 8, 1970. In each event an order of magnitude decrease in the electron flux was observed more than 200 electron gyroradii before the boundary was reached, while the proton flux remained unchanged until after the boundary crossing. To explain these data, an outward-directed electric field is assumed to have existed in a region 800 to 1000 km wide on the earthward side of the magnetopause, creating an E×B drift oppositely directed to and greater than the electron ∇B drift, thereby excluding electrons but not protons from this region. The required electric field is about 65 mv/meter, producing a potential of about 50 kv across the boundary layer. These data are consistent with models of a closed magnetosphere and are inconsistent with a completely open magnetosphere, but they apply only during periods of extreme magnetic activity.

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