Spin modulation of spectrally broadened VLF signals

Abstract

The spectra of broadened VLF signals from ground transmitters received on the spinning ISIS spacecraft have a characteristic temporal dependence when the long receiving dipoles pass close to parallel to the earth's magnetic field direction. As the angle between the dipole and the field direction passes its minimum, the spectra evolve from having enhanced negative Doppler components to having enhanced positive Doppler components or vice versa. The relative orientation of the spin, spacecraft velocity, and magnetic field vectors determines whether the enhancement evolves from positive to negative Doppler or from negative to positive. Theoretical calculations of the Doppler spectra agree with observations from the ISIS spacecraft. These analyses show that wave vectors corresponding to broadened spectra are confined to a narrow range of azimuths about the magnetic field direction. The wave vectors are distributed through a range of polar angles with respect to the magnetic field direction, possibly as the result of scattering of the upcoming waves in the ionosphere below the spacecraft. The technique has been used to deduce the preferred azimuth in the case of one spectrum.