U‐Shaped Spectrograms Registered by the DEMETER Satellite: Observational Features and Formation Mechanism

• Published in: Journal of geophysical research. Space physics 2018-08, Vol.123 (8), p.7077-7088
• Main Authors: Shklyar, D. R., Parrot, M., Titova, E. E.
• Format: Article
• Language: English
• Subjects: Approximation; attenuation; Cold plasmas; cutoff frequency; Damping; DEMETER; Earth Sciences; Equatorial regions; Geometrical optics; Geophysics; Ionosphere; Ionospheric propagation; Latitude; Low frequencies; Mathematical analysis; Optics; Physics; Plasma density; propagation; Satellite observation; Satellites; Sciences of the Universe; Space Physics; spectrogram; Spectrograms; Trajectories; Upper ionosphere; Wave attenuation; Wave propagation; whistler mode waves
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2018JA025656

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. Key Points U‐shaped spectrum registered in the equatorial region of the upper ionosphere during nighttime is formed by lightning‐induced whistler waves Latitudinal dependence of the upper cutoff frequency is determined by the peculiarities of oblique wave propagation and attenuation The wave attenuation can be calculated along the trajectories found from the equations of geometrical optics that disregard collisions