A mechanism of formation of pulsating aurorae

Abstract

We consider a mechanism of pulsating aurora formation relevant for the substorm recovery phase. It is based on the spikelike regime of whistler cyclotron instability in auroral ducts with enhmacecl plasma density, elongated with geomagnetic field lines. We present the rigorously derived self-consistent quasi-linear theory of flow cyclotron maser operation, which describes spikelike regimes of whistler wave generation and energetic particle precipitation in a duct. We show that pu]m•tions are formed due to broadening of the resonance region in velocity space in the course of irmtabillty development. We have obtained theoretical estimations for temporal characteristics of pu]m•ing auroral patches. Pulsating regime is realized when the energetic electron number density is below some critical value depending on background plasma density and particle source paratneters. Pulsation "on" time is determined by the nonlinear instability dynamics whereas "off" time is nearly equal to the time of reaching the wave excitation threshold. We compare our analytical and numerical results with known experimental data and show that the model explains reasonably most observations concerning pulsation time pattern, electron precipitation fluxes, VLF wave spectIaxm dynamics, latitudinal and longitudinal dependencies of pulsation characteristics, etc. We have made also preliminary estimations of nonlinear particle trapping effects during the maximum of the on phase that can give rise to a fine temporal structure of pulsations.

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

“…Admittedly the magnetospheric origins of the observed PsAs would be inward of the THEMIS probe locations (L∼11) in our event. Therefore, though in situ measurements show clear dominance of the ECH emissions, considerations of other candidate mechanisms leading to the pitch angle diffusion and PsAs, such as the whistler-FCM model [Demekhov and Trakhtengerts, 1994], in the intermediate CPS (L∼8-10) are necessary. The efficiency of whistler mode chorus in resonantly scattering auroral electrons depends on the wave amplitude and the ambient plasma density, both of which tend to decrease substantially beyond 7 R E .…”

“…The PsA usually occurs in the postmidnight sector during the late expansion and recovery phase of a substorm, and manifests in forms of geostable or slowly convecting luminous patches with scale sizes 10-200 km [Royrvik and Davis, 1977;Johnstone, 1983]. Classical theories usually designate an equatorial magnetosphere origin of the PsA generation; the processes typically involve [e.g., Davidson, 1990;Demekhov and Trakhtengerts, 1994] (1) fast-drifting energetic electrons that provide the energy and/or particle source of the PsA, (2) flux tubes containing localized concentrations of low-energy plasma that control the dimension and bulk motion of the PsA patches, and (3) nonlinear interactions between one (or both) of the above noted ELF/ VLF waves and the electron loss cone distribution during the passing of the energetic electrons through the lowenergy plasma blobs. However, some observation evidences, in particular from the nonconjugacy of the PsAs in two hemispheres [Sato et al, 1998[Sato et al, , 2004Watanabe et al, 2007] suggest an independent modulation source for each hemisphere that is located away from the equatorial plane [Sato et al, 2004].…”

“…[4] The flow-cyclotron-maser (FCM) theory [Demekhov and Trakhtengerts, 1994] has been one of the leading models of the PsA generation in the inner magnetosphere so far. In this model, a flux tube with enhanced cold plasma density acts as a duct "resonator" for ELF/VLF waves, preferentially a whistler mode in the scenario.…”

Modeling whistler wave generation regimes in magnetospheric cyclotron maser

“…(6) and (7) below). Due to that, the growth rate of ducted whistler waves is determined by integral parameters of the energy distribution, such as characteristic energy (Bespalov and Trakhtengerts, 1976;Demekhov and Trakhtengerts, 1994). This approximation allows us to obtain correct integral characteristics, such as the particle flux and the wave amplification without analyzing the evolution of the particle energy spectrum.…”

“…This mechanism was taken into account in the flow cyclotron maser (FCM) model developed by Demekhov and Trakhtengerts (1994). Periodic regimes of whistler wave generation obtained in this model were used for explanation of pulsating auroras.…”