Excitation of chorus with small wave normal angles due to BPA mechanism into density ducts

Abstract

Abstract. We examine specific features of the realization of the beam pulse amplifier mechanism (BPA) of chorus excitation in the density ducts having a width of the order of 100–300 km with refractive reflection. The dispersion characteristics of whistler emissions in a planar duct under conditions for the fulfillment of the WKB approximation and refractive reflection from the walls of the duct are analyzed. It is shown that in the enhanced duct, discrete spectral elements of chorus with a narrow angular spectrum along the external magnetic field can be excited at frequencies somewhat lower than half the electron cyclotron frequency. In the depleted duct at frequencies somewhat higher than half the electron cyclotron frequency chorus with a narrow angular spectrum along the magnetic field can be excited. The proposed model explains the possibility of excitation of chorus with small angles of the wave normal when the BPA mechanism is implemented. It is noted that the properties of chorus, such as the intensity and a typical angle of the wave normal, can be different for the lower and upper band chorus.