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.

"Physics o fAuroral Phenomena", Proc. XXXIVAnnual Seminar, Apatity, pp. 79 -81 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute ON EX TR EM E STA T IST IC A L CH ARAC TER IST IC S OF CHORUS EL EM EN T GENERATION B.V. Kozelov1, E.E. Titova1,2, A.G. Demekhov3, O. Santolik4 1 Polar Geophysical Institute o fRAS, Apatity, Murmansk region, 184209 Russia 2 Space Research Institute o fRAS, Profsoyuznaya str. 84/32, 117997, Moscow, Russia 3 Institute o fApplied Physics, Nizhny Novgorod, Russia 4 Charles University, and IAP/CAS, Prague, Czech Republic Abstract. A generation mechanism for VLF chorus was suggested by V. Yu. Trakhtengerts [1, 2] on the basis of the backward wave oscillator (BWO) regime of magnetospheric cyclotron maser. Up to now many predictions of the BWO regime were supported by observations. In this report we discuss a statistical distribution of the dimensionless parameter q, quantifying an excess of the electron flux over the absolute-instability threshold. Typically the q parameter values deduced from VLF chorus elements observed by WBD instrument onboard the CLUSTER spacecraft exhibit significant scatter, while their mean values were ~10. We suppose that so large excess of the instability threshold cannot be permanently supported in the magnetospheric plasma. On the basis of the discrete numerical model we demonstrate that if the noised “on- off’ intermittency regime generation is realized, then the observed q values deduced from chorus elements should be extreme ones, but the average value over the entire event can be much smaller. We stress an importance of taking the noise-induced type of chorus generation into account. 1. Introduction While the ELF-VLF chorus emissions are studied during more than 50 years, the only mechanism of their generation is well supported by theory. This the is the backward wave oscillator (BWO) regime of chorus generation which was proposed by V. Trakhtengerts [1, 2] and is based on the analogy with operation of the backward wave oscillator in laboratory electronic. The BWO regime of chorus generation occurs when the conditions of absolute cyclotron instability are satisfied for radiation belt electrons near the equatorial region. Up to now many predictions of the BWO regime were supported by observations [3-5]. However to explain statistical features of succession of chorus elements within the BWO model it is necessary in addition to take into account fluctuations of the electron distribution function which determines the excess over generation threshold. Kozelov et al. [6] presented a discrete numerical model which allowed them to reproduce statistical peculiarities of the ELF-VLF emission dynamics in ground-based observations. The model is based on the assumption that due to the occurrence of noise the magnetospheric BWO generator should operate in the so-called "on-off intermittency regime. Further, the same model with parameters deduced from CLUSTER data allowed us to reproduce the dynamics of the ELF- VLF emissions observed by CLUSTER spacecraft in the magnetosphere near the generation region [3]. In this report we use the model adopted from [6] to explain recently obtained statistical distribution of the dimensionless parameter q, quantifying an excess of the electron flux over the absolute-instability threshold. According to [3]: 4 ~ y 2stcp/y thr • ( 1 ) Here ystep is growth rate of waves in a homogeneous unbounded medium when a step-like deformation exists on the velocity distribution of energetic electrons, y2stcp being proportional to the step height; y2thr = n/T0 is a threshold value of y2step for the transition to the BWO regime; T0 =lB wo(l/vg + l/vstep) is the BWO characteristic time scale, /BW0 is the BWO length, vg is the group velocity of the whistler-mode waves, and vslep is the absolute value of the parallel velocity of resonant electrons. 2. Experimental distribution of the q parameter Figure 1 presents a typical distribution of (/-parameter values deduced from VLF chorus elements observed by WBD instrument onboard the Cluster spacecraft during the event on December 6, 2003. This distribution is non-symmetrical one, it has a maximum at q=l 0-15 and a longer wing to higher values. So large values obtained from the data agree well to results of numerical calculations [7], where it was shown that this is the values when continuous generation with slow frequency variation is changed by generation of discrete chorus-like elements. However, it would be difficult to assume that so large excess over the instability threshold can be permanently supported in the magnetospheric plasma in the generation region. The problem can be resolved if we take into account the noise-induced type of chorus generation. Then it is clear that the q values observed during chorus element generation should be extreme ones and, therefore, their distribution deduced from chorus elements should differ significantly from the distribution over the entire 79