Branches of Resonant Ultra-Low-Frequency Absorption in the Earth’s Magnetosphere

Doppler observations of the auroral electrojet monitored by the Norwegian STARE radar during one solar rotation preceding a strong magnetic storm are analyzed. Minimum frequencies of resonant ultra-low-frequency (ULF) absorption are defined as frequencies of stepwise drops in the power spectral density (PSD) of the detected signal. In turn, the frequency of the stepwise PSD drop is determined by solving the minimum variational problem, which makes the best fitting (by least squares) of the stepwise model of the mean spectral power into the PSD profile of real observations. Successive reductions of the spectral window, in which the variational problem is solved, enable us to construct diagrams of minimum problem solutions on the frequency–power plane for the resonant absorption. Using these diagrams, the higher-frequency and lower-frequency branches of resonant ULF absorption have been revealed. It is found for weakly disturbed conditions that variability of the resonant ULF absorption is first attributed to variations occurring in the absorption power of both branches.