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 f Auroral Phenom ena", Proc. XXXTVAnnual Seminar, A patity, pp. 4 7 - SO 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute POLARIZATION OF SMALL-SCALE MAGNETIC PERTURBATIONS IN THE TOPSIDE AURORAL IONOSPHERE IN THE EVENTS OF BROADBAND ELF TURBULENCE I. V. Golovchanskaya, B.V. Kozelov, Y.V. Fedorenko ( Polar Geophysical Institute, Apatity, Russia ) Abstract. Magnetic perturbations in the topside auroral ionosphere registered by the FAST satellite at frequencies 0.5-8 Hz (scales from 14 km down to 0.9 km) in the events of broadband ELF turbulence are investigated. It is demonstrated that their polarization features, along with large amplitudes and power-law scaling, is strongly inconsistent with the approximation of plane linear waves often used for their description. Introduction With a few exceptions it is widely believed that electric and magnetic perturbations registered in the topside ionosphere and lower magnetosphere during the events of the BB ELF turbulence are intrinsically related to shear Alfven waves or their dispersive modifications. To explain the observed characteristics of the perturbations, such as phase and group velocities, the ratios of field components, electromagnetic or electrostatic perturbation type, the predictions of linear analysis are often used, where the perturbations are considered as small-amplitude plane waves of the form ~exp (ik2z-icot) for shear Alfven waves propagating along B g or ~exp (i(kj-rj_-+ k2z) -icot) for dispersive Alfven waves, which propagate obliquely to B0. Figure 1. Transverse to B 0 magnetic perturbations by (solid) and bx (dashed) (bz is assumed zero in the incompressible case) in the form of sine waves with different amplitudes and phase shifts, x pointing northward, and у eastward (left column); the phase angle of the transverse magnetic perturbation computed as atan2(6r/6>.) (middle column); hodograms of the perturbations on the left (right column); [Stasiewicz and Potemra, 1998]. On the other hand, polarization patterns expected for propagating or standing plane linear Alfven waves have never been demonstrated for the fields observed in the BB ELF turbulence events. Instead, highly disordered patterns with changeable sense of polarization and unsteady phase relations, as those shown in Figure 2 from [Stasiewicz et a l, 1998], were presented. This is not surprising, since large amplitudes and power-law scaling of the perturbations under study suggest non-linear processes involved. HBHtwaim. «Нимпа— » ч . а л а \ А ' \ 1 ^ 42 44 Ш и Щ(comISOSUT Figure 2. (left) Large-amplitude multiscale magnetic components by (solid), and bx (dashed) observed by FREYA in a 6 s time interval from orbit 2396 (altitude ~ 1500 km), and (right) phase angles of wavelet decomposed magnetic signals in the top panel; [Stasiewicz et al., 1998]. 47