Abstract
Abstract. This work is concerned with the numerical modelling of VLF emissions triggered in the equatorial region of the Earth's magnetosphere, using a well established 1-D Vlasov Hybrid Simulation (VHS) code. Although this code reproduces observed ground based emissions well there is some uncertainty regarding the magnitude of simulation parameters such as saturation wave amplitude, cold plasma density, linear growth rate and simulation bandwidth. Concentrating on emissions triggered by pulses of VLF radio waves from the transmitter at Siple Station, Antarctica (L=4.2), these parameters, as well as triggering pulse length and amplitude, are systematically varied. This parametric study leads to an understanding of the physics of the triggering process and also of how the properties of these emissions, particularly their frequency time profile, depend upon these parameters. The main results are that weak power input tends to generate fallers, intermediate power input gives stable risers and strong growth rates give fallers, hooks or oscillating tones. The main factor determining the frequency sweep rate - of either sign - turns out to be the cold plasma density, lower densities giving larger sweep rates.
Highlights
The magnetosphere is a marvellous 3-D plasma physics laboratory in which Nature demonstrates many fascinating time varying phenomena
The simulation code for the numerical modelling of triggered VLF emissions, termed Vlasov Hybrid Simulation (VHS)/VLF, is a 1-D code which realistically assumes that the VLF waves are “ducted” through the magnetosphere
In a duct of enhanced cold plasma density the waves propagate with their wave vector approximately parallel to the ambient magnetic field
Summary
The simulation code for the numerical modelling of triggered VLF emissions, termed VHS/VLF, is a 1-D code which realistically assumes that the VLF waves are “ducted” through the magnetosphere. It was shown by Nunn (1974) that in a parabolic magnetic field geometry a minimum wavefield amplitude (∼2 pT in the case of Siple triggered emissions (TEs)) is necessary for nonlinear trapping to take place This value the wave particle interaction behaviour is quite linear. There is direct modification of the wave phase due to the component of resonant particle current parallel to the wave magnetic field; this is the root cause of the sweeping frequency of emissions Another noteworthy feature of nonlinear wave particle interactions in an inhomogeneous medium is that the nonlinear growth rates are larger than the linear ones but that, unlike the homogeneous case, there is no obvious saturation mechanism in the ducted one dimensional problem. The only current present is that given by the above expression
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