Abstract
Magnetospheric line radiation (MLR) events are electromagnetic waves in the frequency range of about 1-8 kHz observed in the inner magnetosphere that, when presented in a form of frequency-time spectrograms, consist of several nearly parallel and almost equidistant intense lines. Although many observations of these events have been reported using ground-based instruments and a survey of a large data set based on low-altitude satellite data has been published recently, their origin remains unclear.
We use low-altitude satellite observations of MLR events to study their detailed properties, namely, the frequency spacing of individual lines and their frequency drift. Since the satellite, unlike ground observatories, is moving, it allows us to analyze the properties of the events as a function of the position, especially L-shell.
We show that neither the frequency spacing of the events nor their frequency drift varies significantly with the L-shell where the event is observed. Moreover, the frequency drift is generally positive.
The individual lines forming the events cannot be explained as harmonics of the base frequency equal to the frequency spacing. We suggest that a possible generation mechanism might be an interaction between a wave of a carrier frequency and an additional wave with the frequency equal to the observed frequency spacing.
We cannot exclude that it comes from human activity (power lines), but a magnetospheric origin is more likely. We suggest that the emissions might be guided by the plasmasphere inner boundary before they deviate to lower L-shells at altitudes of a few thousands of kilometers.