Magnetic field and electron observations near the dawn magnetopause

Observations of electron fluxes between 25 ev and several kev by the triaxial electron spectrometer and of the magnetic field by the fluxgate magnetometer on the satellite Ogo 5 are used to study the structure and movement of the magnetopause. The observations were made at both quiet and fairly disturbed times between 6.4 and 7.6 hours local time and at geomagnetic latitudes between 16° and 40°. The boundary layer electron fluxes for V<8000 km/sec (energy <200 ev) are typical of the magnetosheath and that the non-Maxwellian tails of the spectra for V>8000 km resemble those observed in the magnetosphere. The magnetic field in the boundary layer has either magnetospheric or transitional characteristics. These observations suggest the possibility of mixing of the magnetosheath plasma with the magnetospheric plasma, which would be important in determining the structure of the boundary layer. It is inferred from pressure balance across the magnetopause, in agreement with previous work, that β (=plasma energy density/magnetic field energy density) is close to or exceeds unity in the region of the magnetosphere immediately adjacent to the dawn magnetopause. Values of βc calculated from the observed electron spectra indicate substantial contributions to β from other particle populations.