Excitation of chorus with small wave normal angles due to beam pulse amplifier (BPA) mechanism in density ducts

Peter A Bespalov,Olga N Savina

doi:10.5194/angeo-37-819-2019

Abstract

Abstract. We examine specific features of the realisation of the beam pulse amplifier (BPA) mechanism of chorus excitation in the density ducts that have 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 fulfilment of the Wentzel–Kramers–Brillouin (WKB) approximation and refractive reflection from the “walls” of the duct are analysed. 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 of the electron cyclotron frequency. In the depleted duct at frequencies somewhat higher than half of 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.

Highlights

In accordance with the experimental data of the CLUSTER probes and Van Allen Probes, chorus emissions are excited in the “cigar-shaped” region with a length of the order of l = (1–2) 108 cm and average diameter d = 3 × 107 cm (Bell et al, 2009; Agapitov et al, 2017) near the local magnetic field minimum
The beam pulse amplifier (BPA) mechanism of chorus excitation was first studied for the homogeneous plasma
The proposed model with waveguide propagation explains the possibility of excitation of chorus with small angles of the wave normal when the BPA mechanism is implemented

Summary

Introduction

In the theoretical and experimental studies of chorus, much attention has been given to determination of the wave normal angles (θ ) in the region of excitation of emissions (Muto et al, 1987; Hayakawa et al, 1990; Santolik et al, 2009). According to experimental data and the results of numerical calculations (Li et al, 2011), for the achievement of a cyclotron instability growth rate of this order, high anisotropy of the distribution function of energetic electrons in the excitation region is necessary. We will explain the experimental data about typical wave normal angles in the chorus excitation region.

Results

Conclusion