On the numerical modelling of VLF chorus dynamical spectra
- 1School of Electronics and Computer Science, Southampton University, Southampton SO17 1BJ, UK
- 2Charles University, Faculty of Maths and Physics,V Holesovickach 2,18000, Praha 8, Czech Republic
- 3Institute of Atmospheric Physics, BOCNI II 1401, 14131, Praha 4, Czech Republic
- 4CAESAR Consultancy, 35 Millington Road, Cambridge CB3 9HW, UK
- 5Institute of Applied Physics, Russian Academy of Sciences, 46 Ulyanov Street, 603950 Nizhni Novgorod, Russia
Abstract. This paper presents a study of the use of a one-dimensional Vlasov Hybrid Simulation (VHS) computer code to simulate the dynamical spectra (i.e. frequency versus time spectrograms) of ELF/VLF chorus signals (from ~a fraction to ~10 kHz). Recently excellent measurements of chorus have been made in the source region close to the geomagnetic equator aboard the four spacecraft Cluster mission. Using Cluster data for wave amplitude, which is up to 300 pT, local gyrofrequency, cold plasma density, and L-shell, observed chorus signals are reproduced with remarkable fidelity and, in particular, sweep rates in the range 1–10 kHz result as observed. Further, we find that the sweep rate is a falling function of increasing cold plasma density, again in accord with observations. Finally, we have satisfactorily simulated the rather rare falling frequency elements of chorus which are sometimes observed aboard Cluster in the generation region. For both rising and falling chorus we have presented detailed structural analyses of the generation regions. The main contributor to the frequency sweep rate is primarily the establishment of wave number/frequency gradients across the generation region by the out of phase component of the resonant particle current. The secondary contributor is the shortening of the wavelength of resonant particle current relative to that of the wave field. In view of the close agreement between observation and simulation, we conclude that nonlinear electron cyclotron resonance is indeed the mechanism underlying the generation of chorus signals just outside the plasmasphere.