Dependence of very-low-frequency electric field antenna impedance on magnetospheric plasma density

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.