Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

V. Pilipenkoet al. For the identification of the physical nature of electromagnetic burst the method of an apparent impedance has been used. For orthogonal components, E2 and B l, the relevant impedance \J(f) is shown in Fig. 3. The obtained U(f) turns out to be frequency-dependent, increasing from -1580 km/s at f~ 0.1 Hz to -3160 km/s at/ - 5 Hz. magnetic pulsation power from northword rrognetic field component (contour lines) equivolent ionospheric current from northword magnetic field component (color coded) Mognelec tocol Time [hrs] Fig-1. The distribution of the ionospheric east-west current, reconstructed from the ground H (upper panel) and Z (middle panel) magnetic data and the spatial-temporal distribution of ULF total power in the band 1-10 mHz (contour lines). The moment of the Astrid-2 crossing the magnetometer network is shown with thin vertical lines. 6 > e 9 10 >< 12 13 14 IS IS 17 Ig Un.versci time (h'sj magnetic pulsation power from verticol magnetic field component (contour lines) equivolent ionospheric current from ve'ticol mognetic field component (color coded) Mognoiic Local Time (hrs) 2 3 4 5 6 7 В 9 Ю 11 12 13 I* >5 >6 Small-scale Periodic Structure o f Electromagnetic Bursts: Possible Physical Mechanism Localized electromagnetic disturbances in the upper ionosphere can be manifestations of small-scale Alfvenic structures. In non-ideal MHD there are two competing effects leading to transverse dispersion of small-scale ( k]_ ) dispersive Alfven wave (DAW) [Lysak, 1998; Strelsov and Lotko, 1999]: (a) electron inertia, characterized by the electron plasma skin length Xe, and (b) finite Larmor radius of ions ps. In summary, the significance of dispersive effects may be characterized by the characteristic dispersive radius pd [. Leonovich and Mazur, 1989], p~d - A* + p \ , comprising both effects. Transverse scales detected by Astrid-2, -few km, are an order of magnitude larger than typical dispersive radius. Al low altitudes pd ~XC is very small, less 100 m, and it will produce during observations at satellite moving across the spatial structure with velocity V0 a response a t/ ~XC/V0 > 70 Hz!? However, even the seemingly discrepancy between this estimate and Astrid-2 observations does not exclude DAW from possible mechanisms. Ground-based observations evidence that the AEJ, being the ionospheric projection of the auroral region, at the same time, is the region with a highest Pc5 wave activity. Thus, the MHD wave energy, pumped from remote parts of the magnetosphere, is accumulated and eventually absorbed in a magnetic shell at auroral latitude. The absorption may be accompanied by a partial conversion of large-scale MHD disturbances into small-scale DAWs. It is essential, that the actual transverse scale of these "secondary" DAWs is to be determined not by pd, but by other parameter. The typical spatial transverse scale 8 of the ULF wave structure in a resonant region is controlled by a dominant dissipation mechanism of Alfven wave in the magnetospheric Alfven resonator (MAR). A pumping of wave energy into the resonant region causes the growth of amplitude and narrowing of 8 . In the steady state this growth is terminated at some level, determined by the quality factor QAof MAR. Parameters of MAR can be estimated via the damping rate y, as QA~(2y/co)'' and 8 - (у/co )L, where L is the typical scale of Alfven frequency spatial variations. Diminishing of the transverse scale and the amplitude growth in the Alfven resonance region may be saturated either due to the ionospheric dissipation or to the energy leakage from the resonant region caused by the transverse dispersion. The ionospheric Joule dissipation is essential for the fundamental field-aligned Pc5 mode, but for harmonics dispersive mechanisms may come into play. Transverse Spatial Structure of Localized DAW We consider DAW, described by the wave equation for the potential A=AZ[Hasegawa and Chen, 1976] д2А дгА 2 2 d2A V2dt2 dz1 1 dz2 72