The numerical simulation of the generation of lower-band VLF chorus using a quasi-broadband Vlasov Hybrid Simulation code

David Nunn

doi:10.1186/s40623-021-01549-3

Abstract

In this paper, we perform the numerical modelling of lower-band VLF chorus in the earth’s magnetosphere. Assuming parallel propagation the 1d3v code has one spatial dimension z along the ambient magnetic field, which has a parabolic z dependence about the equator. The method used is Vlasov Hybrid Simulation (VHS) also known in the literature as the method of Kinetic Phase Point Trajectories (Nunn in Computer Physics Comms 60:1–25, 1990, J Computational Phys 108(1):180–196, 1993; Kazeminezhad et al. in Phys Rev E67:026704, 2003). The method is straightforward and easy to program, and robust against distribution function filamentation. Importantly, VHS does not invoke unphysical smoothing of the distribution function. Previous versions of the VLF/VHS code had a narrow bandwidth ~ 100 Hz, which enabled simulation of a wide variety of discrete triggered emissions. The present quasi-broadband VHS code has a bandwidth of ~ 3000 Hz, which is far more realistic for the simulation of chorus in its entirety. Further, the quasi-broadband code does not require artificial saturation, and does not need to employ matched filtering to accommodate large spatial frequency gradients. The aim of this paper which has been achieved is to produce VLF chorus Vlasov simulations employing a systematic variety of triggering input signals, namely key down, single pulse, PLHR, and broadband hiss.Graphical

Highlights

Very low frequency (VLF) chorus is currently a topic of considerable scientific interest
In this paper, we have simulated lower-band VLF chorus using the method of Vlasov Hybrid Simulation (VHS) called the method of Kinetic Phase Point Trajectories (Kazeminezhad et al 2003)
Simulation particle trajectories are followed forwards in time continuously, which is synonymous with a phase space trajectory

Summary

Introduction

VLF chorus is currently a topic of considerable scientific interest. It presents as a strong self-sustaining whistler mode emission, normally found at dusk/dawn outside the plasmapause. The hill or hole in distribution function located at the particle trap in phase space will immediately give a sizeable nonlinear current, perpendicular to the ambient magnetic field, whose phase relative to the wave electric field will be controlled by the local inhomogeneity factor These codes have one spatial dimension and assume the VLF wave field is parallel propagating, a reasonable approximation at least for lower-band chorus.

Results

Conclusion