Simultaneous observations of field‐aligned currents and plasma drift velocities by Atmosphere Explorer C

Abstract

A three‐axis flux gate magnetometer was included in the Atmosphere Explorer C (AE‐C) payload for engineering functions. The instrument was activated during the last half of AE‐C's life, and its output appears to be useful only during approximately one‐half of the data gathering time when the vehicle was spinning. Still, the addition of even a partial magnetic field data set to the already broad spectrum of AE‐C measurements is an exciting prospect. The high telemetry rate (128 bits/s per axis) could produce a spatial resolution in B of ∼½ km, but the 8‐bit word size yields an amplitude resolution of only 562 nT. A large modulation (15,000 nT) of the output from the spin axis (Z) magnetometer occurs during sunlit orbit segments. Application of a spin‐averaging technique to the modulated output signal has enabled us to improve the amplitude resolution of the signal to ∼40 nT (rms), although a corresponding loss in spatial resolution from ∼½ to ∼15 km results from the averaging procedure. By taking the difference between the spin‐averaged signal and a model magnetic field in a manner similar to that used by McDiarmid et al. (1978) we deduce a resultant magnetic perturbation ΔBZ. Attributing ΔBZ to field‐aligned current (FAC) sheets, we arrive at typical values of ∼1 µA/m² for sheet current densities. The amplitudes and distributions of these FAC’s are observed to be consistent with those measured by Triad (Iijima and Potemra, 1976a, b). Simultaneously measured Z components of the plasma drift velocity (Vz) show correlations with magnetometer measurements indicating that the positions of FAC's correspond to the gradients in the component of the electric field along the satellite track (X component). By using the current continuity equation for the ionosphere (▽ · I⊥=J∥), values of height‐integrated Pedersen conductivity Σp from 2.5 to 17.5 mhos are calculated.