Observation and Generation of Two-Band Noise Emissions above and below Half the Electron Gyrofrequency near the Geomagnetic Equator

The spectral characteristics of very low-frequency (VLF) waves observed on board the Van Allen Probe B satellite on November 30, 2015, are investigated. The feature of the observed spectrum is the presence of two spectral maxima below and above half the electron gyrofrequency. This feature, which is typical of the spectra of chorus emissions, has been observed for noise-like emissions. Along with observing VLF emissions, the satellite also measured differential fluxes of energetic electrons, the magnetic field, and the concentration of cold plasma. Wave growth rates in the whistler band are calculated on the basis of the obtained data. Calculations are made in two different ways that yield similar results. A comparison of the observed noise emission spectra and the calculated wave growth rates shows their good agreement.