The apparent spectral broadening of VLF transmitter signals during transionospheric propagation

Abstract

VLF/ELF electric field wave data acquired on the ISIS 1, ISIS 2, and ISEE 1 satellites demonstrate the existence of a new phenomenon in which initially narrowband (∼1 Hz) upgoing signals from ground‐based VLF transmitters undergo a significant spectral broadening as they propagate through the ionosphere and protonosphere up to altitudes in the range 600–3800 km. For transmitter signals in the range 10–20 kHz, the spectral broadening can be as high as 10% of the nominal frequency of the input signal. Spectral broadening occurs only in the presence of impulsive VLF hiss and/or a lower hybrid resonance (LHR) noise band with an irregular lower cutoff frequency, and only for signals whose frequency exceeds the LHR frequency at the satellite location. It is often observed in association with a band of impulsive ELF hiss below 700 Hz. In many cases, the bandwidth of the spectrally broadened signals is a strong function of the electric dipole antenna orientation with respect to the local direction of the earth's magnetic field. Unusual dispersion in the components of the spectrally broadened pulses suggests that the spectral broadening may be due to a doppler shift effect in which the initial signals scatter from irregularities in the F region and couple into quasi‐electrostatic modes of short wave length. The large doppler shift associated with these short wavelength modes produces a significant increase in the bandwidth of the signal, as observed on a moving satellite. Since impulsive VLF hiss and irregular LHR noise bands have been linked to energetic (<1 keV) electron precipitation in the past, it is conjectured that the spectral broadening effect may be driven by precipitating electrons.