Plasma and magnetic field observations during the magnetosphere passage of Pioneer 7

Data are presented from simultaneous measurements of magnetic field and the electron component of the plasma in the near geomagnetic tail (out to 40 RE) during the outbound passage of Pioneer 7 on August 17 and 18, 1966. A quasi-static model of the geomagnetic tail based on a balance of particle and field pressures is found to be in general agreement with the data. Pressure balance was maintained even through the single well-defined traversal of the neutral sheet. During this traversal the electron concentration remained approximately constant, but the thermal energy increased to compensate for the reduced field pressure. Typical electron number densities and thermal speeds in the plasma sheath were 0.5/cm³ and 107 m/sec. The proton pressure in the plasma sheath inferred from the pressure balance model was 1–3 times greater than the electron pressure. A possible second crossing of the neutral sheet was observed near the magnetopause. A period of low particle flux is found to correlate well with the observation of a geomagnetic bay event.