Tests of impedance theories for a transmitting dipole in an ionospheric plasma

The voltage at the output of the sounder transmitter in both of the International Satellites for Ionospheric Studies (ISIS) ionospheric spacecraft is routinely recorded over the entire frequency range of 0.1-20 MHz. The observed dependence of the output voltage on the sounder frequency is determined by the values of the local plasma parameters, of which there is a considerable range in the ISIS data. Impedance theories for an ionospheric dipole have been tested by comparing observed curves of voltage versus sounder frequency with computed curves based on those theories. In cases where the plasma frequency is low, around 0.1 MHz, a realistic curve is obtained using standard vacuum dipole theory. For higher plasma frequencies, the observed response departs significantly from the low-density case at frequencies close to the electron characteristic frequencies. For fixed working frequency and plasma parameters, the voltage exhibits a slight dependence on antenna orientation. Attempts to explain these results for higher plasma frequencies using a transmission-line theory have qualified success when a vacuum sheath of 1-m radius is assumed. The need for a large sheath is just one kind of evidence that the sounder fields profoundly change the nature of the plasma near the dipole.