Direct observation of a tangential electric field component at the magnetopause

The first direct, in situ measurements of the quasi‐static electric field at and near the magnetopause, in the magnetosheath, bow shock, and solar wind, have been made on the ISEE‐1 satellite. Both the large scale average electric field and the fine scale field observed during a magnetopause crossing show the existence of significant tangential electric field components on both sides of the magnetopause, in agreement with the requirements of reconnection theories. The data in the satellite frame also show significant normal electric field components that typically point towards the magnetopause on both sides, corresponding to plasma flows towards dawn, as expected for the pre‐noon magnetopause crossings. In combination with ISEE‐1 and ISEE‐2 magnetic field data, these measurements also suggest that a magnetopause passage over the spacecraft occured at a velocity of 20±10 km/s, that the magnetopause thickness was 300±150 km, and that the local Ī·Ē dissipation was 70±20 watts/km². This observed local level of power dissipation would correspond to a total power of 1012 watts, if it existed over the entire front of the magnetopause.