Simultaneous measurements of energetic electrons and electrostatic waves in the midlatitude trough region indicate that events characterized by strong pitch‐angle diffusion in the altitude range 2000‐4500 km occur in the presence of ELF electrostatic waves attributed to a current‐driven ion cyclotron instability. Scattering by the electrostatic waves is sufficient to account for the strong pitch‐angle scattering commonly observed by low‐altitude satellites. The resulting precipitation creates a sharply defined, high‐latitude boundary for energetic electrons as observed by low‐altitude satellites. This boundary is similar to the feature generally referred to as the high‐latitude trapping boundary. However, the precipitation boundary occasionally occurs on closed field lines at a significantly lower latitude than the trapping boundary. When present, the precipitation boundary is most sharply defined in the nighttime hemisphere.
Data from a VLF electric field antenna on board the spacecraft OV3‐3 show that the antenna response depends upon the local plasma density. Antenna impedances, calculated on the basis of a cold, magnetized plasma model and a warm, nonmagnetized, collisionless plasma sheath model, are compared with the data, showing that the sheath model is more appropriate. The response of the electric field antenna to constant onboard sources of electromagnetic interference is used with the antenna sheath impedance model to calculate plasma densities. The resulting densities are similar to those obtained from OGO 2 plasma density data scaled to the OV3‐3 orbital parameters. The impedance of the TRW Systems Group VLF electric field antenna, also on board OV3‐3, is calculated and compared with the data. It is shown that the sheath antenna impedance model applies equally well to both antennas even though they were quite dissimilar. Much of the signal variation observed by the Aerospace and TRW electric field antennas is caused by antenna impedance changes as a function of local plasma density.
The propagation of nonaxisymmetric Alfvén waves in a linear discharge tube filled with a nonuniform plasma under a 30-kG axial field has been investigated theoretically and experimentally in the frequency range 1–5 Mc/sec. Typical plasma densities were 3–4 × 1015 ions/cm3 near the center of the tube and fell to low values at the wall. It was found that the density gradient introduces a frequency dependent coupling between the torsional and compressional waves of the uniform plasma. As a result, an axial component of the magnetic field perturbation, which is normally associated only with the compressional wave, is now carried by the torsional wave as well, and was in fact detected at frequencies well below the lowest waveguide cutoff of the compressional wave. At still lower frequencies, this component disappeared, and the remaining transverse field components were found to propagate at a radially varying speed which agreed well with the local Alfvén speed deduced from the measured ion density profile. A theoretical analysis is presented, and its predictions are compared with experimental measurements of signal amplitudes, wave speeds, attenuation rates, and density profiles.
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