Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

E.E. Titova et al. upper band of chorus is due to large T0 which increases with increasing frequency (see Eq.5). The average values of the q parameter, obtained from the CLUSTER experimental data, are in agreement with simulation results by Demekhov and Trakhtengerts [9]. Fig.3 shows results of a numerical simulation of the chorus dynamic spectrum based on the BWO model for different values of the parameter q = S/S,hr quantifying excess of the energetic electron flux S over the generation threshold S,hr It is seen that a significant excess of the flux over the threshold value (q ~ 10) is required for the generation of discrete elements. For smaller q values (e.g., q ~ 3), the dynamic spectrum shows only some broadening near the initial generation frequency and possibly some sidebands. Significant scatter o f the q values was observed during each Cluster passage through the generation region. Fig.4 illustrates this scatter by showing the distributions of the q parameter for the lower and upper bands of the chorus emissions. The distributions of q have an asymmetric shape with a long tail towards higher values. This scatter of the q values is related to the scatter in the frequency sweep rate o f the chorus elements, which is typical for CLUSTER observations [4]. 0.05 M/fB 0.05 -0.05 -0.05 Fig. 3. Dynamic spectrums on exit of the BWO generation region for different values o f parameter q = S/Sthr (a) q = 3, i.e., smaller electron flux, (b) q =12, i.e., larger electron flux. 1000 N 1000-a N Fig. 4. The distribution of the q parameter for chorus elements with frequencies below (a) and above (b) one half of the gyrofrequency for 7 orbits of the CLUSTER spacecraft. 4. Conclusions We determined a dimensionless parameter q for the magnetospheric BWO generating chorus VLF emissions. This parameter quantifies an excess of the 92 electron flux over the absolute-instability threshold. The parameter q is related to the step feature on the electron distribution function. Using the measured spectral characteristics of VLF chorus elements detected onboard the CLUSTER spacecraft and the electron