Abstract
Statistical studies of the intensity of very low frequency (VLF) electromagnetic waves in the Earth's inner magnetosphere are usually performed in terms of their frequency, L-shell, magnetic local time (MLT), and geomagnetic activity dependencies. We use the measurements performed by the DEMETER (2004-2010) and Van Allen Probes (2012-2016, still operating) spacecraft to perform an analysis with respect to the geomagnetic longitude, i.e., a parameter which is usually not considered.
The longitudinal dependence is observed only inside the plasmasphere and varies with the wave frequency. We distinguish four frequency ranges: (i) up to 500 Hz, which exhibits no longitudinal dependence at all, (ii) from 0.5 to 1 kHz, (iii) from 1 to 2 kHz, in which the longitudinal dependence is most pronounced, and (iv) larger than 2 kHz, where the overall wave intensity significantly falls.
The obtained results, especially during the MLT night, are in good agreement with the normalized flash rate calculated from the OTD/LIS data. This strongly suggests that the waves from the lightning activity significantly contribute to the overall wave intensity in the night-side plasmasphere at frequencies from 0.5 to 2 kHz.