The magnetopause and boundary layer for small magnetic shear: Convection electric fields and reconnection

Convection electric fields, E c = −V p ×B, measured during a magnetopause/boundary layer encounter at low latitude by the AMPTE/IRM spacecraft, are shown to yield an accurate determination of the magnetopause normal vector, N, even though the magnetic shear across the magnetopause was small. The average plasma velocity along N is used to deduce a boundary layer thickness of about 1.3 R E . Analysis of E c in the magnetopause itself reveals the existence of a deHoffmann‐Teller frame in which the flow is nearly field aligned and approximately Alfvénic, indicating that the magnetopause was a rotational discontinuity; this is interpreted as a signature that magnetic‐field reconnection was occurring somewhere on the magnetopause.