Physics of auroral phenomena : proceedings of the 40th annual seminar, Apatity, 13-17 March, 2017 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2017. - 143 с. : ил., табл.

“Physics o f Auroral Phenomena”, Proc. XL Annual Seminar, Apatity, pp. 54-57, 2017 © Polar Geophysical Institute, 2017 Polar Geophysical Institute IN SEARCH OF A GROUND IMAGE OF THE SURFACE OSCILLATIONS AT THE MAGNETOPAUSE V. Pilipenko1, D. Lorentsen2, V. Belakhovsky3, and O. Kozyreva1 1 Institute o f Physics o f the Earth, Moscow, Russia; e-mail: pilipenko_va@mail.ru 2Kjell Henriksen Observatory, Svalbard, Norway гРо1аг Geophysical Institute, Apatity, Russia Abstract. Magnetopause surface eigenmodes were suggested as a potential source o f dayside high-latitude broadband pulsations in the Pc5-6 band (~l-2 mHz) and as a mechanism of the "magic" frequency occurrence. However, the search for a ground signature o f these modes has not provided encouraging results. The comparison of Svalbard SuperDARN radar data with the latitudinal structure of Pc5-6 pulsations recorded by magnetometers covering near-cusp latitudes showed that often the latitudinal maximum of the pulsation power maximizes ~2°-3° deeper in the magnetosphere than the dayside open-closed field line boundary (OCB). Here the OCB-ULF correspondence is further examined by comparison of the latitudinal profile of the near-noon pulsation power with the equatorward edge of the auroral red emission from the scanning photometer data. In most analyzed events the "epicenter" of the Pc5-6 power is at ~2° lower latitude than the optical OCB proxy. Therefore, the dayside Pc5-6 pulsations cannot be associated with the ground image of the magnetopause surface modes or last field line oscillations. A lack of ground response to these modes beneath the ionospheric projection o f the OCB is puzzling. As a possible explanation, we suggest that a high variability of the outer magnetosphere near the magnetopause region may suppress the wave excitation efficiency. This additional mechanism of damping of field line oscillations is caused by stochastic fluctuations of the magnetospheric plasma and background magnetic field. To quantify this hypothesis, we consider a driven field line resonator terminated by conjugate ionospheres with stochastic fluctuations of the eigenfrequency. The solution of this problem predicts a substantial deterioration of resonant properties of the MHD resonator even under a relatively low level of background fluctuations. This effect may explain why a ground response to magnetopause surface modes or last field line oscillations is lacking at the OCB latitude, but can be seen at somewhat lower latitudes with more regular and stable magnetic and plasma structure. With an account of ~2° offset, the maximum of Pc5-6 power can be used as a simple indicator of the dayside OCB latitude. 1. Introduction. Long-period pulsations in the Pc5-6 band (743-15 min) are known to be a persistent feature of the ULF activity at dayside high latitudes. A potential source of these high-latitude long-period pulsations could be the magnetopause surface eigenmodes [Plaschke et al., 2009; Hartinger et al, 2015] or oscillations of the last field line [Lanzerotti et al., 1999; Urban et al., 2011]. The "magic" pulsation frequencies were reinterpreted in terms of standing AlfVenic surface mode discrete eigenfrequencies (Kruskal-Schwarzschild modes) at the magnetopause [Archer et al., 2013]. A characteristic frequency Q a of Alfven field line oscillations in a dipole-like magnetic field can be estimated as where V a is the AlfVen velocity near the top of a field line, and L R e is the radial distance to a field line. The Alfven wave continuum is terminated by frequency of the last closed field line. Properties of MHD surface mode are similar to that of AlfVen waves: both are non-compressive disturbances, and are guided along background magnetic field B. Both AlfVen field line oscillations and surface waves on a steep gradient are modes of the MHD resonator terminated by the conjugate ionospheres. The frequency of a surface mode Qs lies between the Alfven frequencies ( ) at both sides of the interface between two media with magnetic field В 1.2 and plasma density p)j2'(e.g., magnetosphere and magnetosheath) oscillations. The energy of discrete spectrum mode (e.g., surface mode) is irreversibly converted into the energy of Alfven continuum. This process is most effective at a resonant shell L where со—»£2(Z). Pumping of wave energy into the resonator results in the growth and narrowing of the spatial resonant peak, terminated by a dominant dissipation mechanism. Here we present a typical example of the correspondence between dayside ULF power structure and OCB proxy, In a realistic inhomogeneous magnetosphere global MHD disturbances are coupled to local standing field line Alfven determined from the auroral optical data. To interpret the observational results we present a simplified model of the MHD resonator with fluctuating eigenfrequency driven by an external source. 54