Broadband whistler waves and differential electron fluxes in the equatorial region of the magnetosphere behind the plasmapause during substorm injections

This paper considers the whistler waves in the frequency band from 3 to 30 kHz observed on the Van Allen Probe-B satellite on March 17, 2019, when the satellite was on L-shells from 2.8 to 5.4. The upper frequency in the emission spectrum followed the course of the electron gyrofrequency fce and was lower than it by 1–5 kHz. The emission spectrum often had two spectral maxima (above and below fce/2); the maximum at frequencies above fce/2 could be either more or less intense. High-frequency whistler waves at frequencies > fce/2 were observed simultaneously with an increase in low-energy electron fluxes with energies more than 102 eV, which had transverse anisotropy. To explain the observed spectrum, we used simultaneous satellite measurements of the cold plasma density and differential fluxes of energetic electrons in the energy range from 0.015 to 250 keV in a wide range of pitch angles to determine the electron distribution function and calculate local linear growth rate as a function of frequency f and wave normal angle θ. The calculations were performed for three cyclotron resonances (n = 1, 0, –1) that make the largest contributions to the wave growth rate. The calculations showed the presence of a pronounced maximum at frequencies (0.8–0.9) fce. The energy range and pitch angles of electrons with a maximum contribution to wave excitation at these frequencies were estimated.