VLF Transmitter Signals Observed by the Cluster Spacecraft Over a Wide Range of Latitudes

Abstract

Very low frequency transmitter signals can penetrate through the ionosphere and propagate in the inner magnetosphere, being observed by spacecraft instruments with sufficient frequency resolution and range. We use 23 years of measurements performed by the Waves of High frequency and Sounder for Probing of Electron density by Relaxation instrument on board the four Cluster spacecraft at frequencies up to 80 kHz to investigate the observed wave intensities. The transmitter signals are about an order of magnitude more intense during the night than during the day, and they are apparently confined within the plasmasphere. The extensive latitudinal coverage of the measurements allows us to investigate frequency cut-offs observed in the transmitter spectra. These are mostly in line with nonducted propagation, but occasional partial ducting seems to be necessary to explain signals spanning otherwise inaccessible regions. Finally, we compare the observed intensity patterns with the calculations of Starks et al. (2020), https://doi.org/10.1029/2019ja027029. Although overall agreement is obtained, the observed wave intensities tend to be a factor of about 2–3 lower than the calculated intensities.