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U‐Shaped Spectrograms Registered by the DEMETER Satellite: Observational Features and Formation Mechanism
Author(s) -
Shklyar D. R.,
Parrot M.,
Titova E. E.
Publication year - 2018
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2018ja025656
Subject(s) - ionosphere , physics , attenuation , trajectory , cutoff frequency , satellite , cutoff , spectrogram , computational physics , spectral line , wave propagation , latitude , geophysics , optics , quantum mechanics , astronomy , computer science , computer vision
U‐shaped spectrograms observed by the DEMETER satellite in the equatorial region of the upper ionosphere are presented and explained for the first time. These spectra are characterized by the latitude‐dependent upper cutoff frequency that increases with the latitude during nighttime. The observations suggest that this very low frequency wave phenomenon is closely connected with the anomalous increase in cold plasma density at and below the satellite. We show that restricting the analysis to propagation effects is insufficient to explain U‐shaped spectra, and other effects, in particular, the collisional wave damping should be taken into consideration. To calculate the wave damping, we use the method of successive approximations. In the first approximation, we calculate the ray trajectory and the wave normal vector along it, using the equations of geometrical optics neglecting collisions. In the second approximation we calculate the collisional wave damping along the precalculated trajectory. We show that the effects of very low frequency wave propagation and attenuation in the equatorial region of the upper ionosphere explain the main features of the phenomenon under discussion.