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.

In search o fa ground image o fthe surface oscillations at the magnetopause 1.7- 2.0 mHz 199AM 2V30 2. Dayside OCB and ULF activity. To unambiguously resolve the association o f dayside high-latitude ULF activity with the OCB, the advantage of the Svalbard complex can be used. It comprises the latitudinal IMAGE magnetometer chain along the geomagnetic longitude Л~И0°, SuperDARN radar, and the meridian scanning photometer at Longyearbyen (LYR). A ground response to the magnetopause surface modes is expected to be beneath the ionospheric projection of the open-closed field line boundary (OCB). The dayside OCB proxy can be determined either as an enhanced spectral width of the SuperDARN radar return signal [Baker et al., 1995], or as the equatorward boundaries of the cusp aurora determined by meridian scanning photometer [Johnsen and Lorentzen, 2012]. Irregular Pulsations at Cusp Latitudes (IPCL) and narrow-band Pc5 waves were found to be a ubiquitous element of ULF activity in the dayside high-latitude region [Kleimenova et al., 1985]. The comparison of the latitudinal structure of broadband Pc5-6 pulsations recorded by magnetometers covering near-cusp latitudes with the OCB radar proxy showed that the maximum o f the IPCL power maximized somewhat deeper in the magnetosphere [Pilipenko et al., 2015]. The spatial structure o f broadband dayside Pc5-6 pulsation spectral power was found to ha"ve a localized latitudinal peak, but not under the cusp proper as was previously thought, but several degrees southward from the equatorward cusp boundary. Therefore, these pulsations cannot be associated with the ground image of the magnetopause surface modes or last field line oscillations. The earlier claims of the dayside monochromatic Pc5 wave association with the OCB [Lanzerotti et al., 1999] also seems doubtful. To verify the results obtained with the SuperDARN radar, we use the meridian scanning photometer to identify the cusp aurora equatorward boundary (dayside OCB proxy). We present a typical example of the correspondence between the OCB from LYR photometer and the latitudinal structure of wave power in the Pc5-6 band from magnetometer data. We consider the 1994, Dec. 30 event, when the Svalbard complex is around noon and auroral activity is observed above LYR. We will concentrate on the time interval near the geomagnetic noon, -09-10 UT (MLT noon is -09 UT). During this interval, quasi-periodic irregular pulsations are observed along the IMAGE magnetometer chain with two dominating spectral peaks: at -1.4 mHz and -2.2 mHz (not shown). A magnetic keogram, constructed from the X- component magnetometer data (Fig. 1), enables us to monitor the time evolution o f the latitudinal distribution of 1.7-2.0 mHz band-integrated spectral power along the geomagnetic meridian. The superposition o f the magnetic keogram with the equatorward and poleward boundaries of the cusp aurora identified from photometer data indicates that the Pc5-6 pulsation power is located -2° equatorward of the optical OCB proxy. The presented example illustrates a commonly observed regularity: a ground response to driving of the outer magnetosphere is observed not beneath the last closed field line, but somewhat deeper in the magnetosphere. In attempt to comprehend these observational results, we take into account that the magnetospheric plasma and background magnetic field experience substantial stochastic fluctuations, especially in the outer magnetosphere near the magnetopause. Hence, the eigenfrequency of the MHD resonator also must experience stochastic fluctuations. Below we consider a simple model of homogeneous MHD resonator with stochastic fluctuations of its eigenfrequency. This model enables us to examine possible deterioration of field line resonant response to an external monochromatic driving. 3. A resonator with fluctuating eigenfrequency. We consider a simple model of homogeneous resonator terminated by conjugated ionospheres. Let us suppose that its eigenfrequency Q (either f2A or Qs) experiences stochastic fluctuations n 2( 0 = f i 2[ i+ a f ( 0 ] where %{t) is the stationary stochastic function with vanishing time-average <^> =0 and unit dispersion <£2>=1. The relevant auto-correlation function K(x) satisfies the condition <%2>=ЛТ(0)=1. The parameter 5 characterizes the amplitude o f eigenfrequency fluctuations, such as |Afi2/Q2|~5. The equation for field line oscillations driven by large-scale compressional waves or for surface waves buffeted by the magnetosheath disturbances is formally reduced to the equation for a driven harmonic oscillator with eigenfrequency Q [Hollweg, 1997]. This modeling equation describes oscillations characterized by a variable x(t) xtt+ 2 y x t + n 2\\ + 8%(t)]x = AD2 cos cot (l) Here A is the amplitude of a driver, and у is the damping factor o f the resonator. Figure 1. The 2002, Jan. 04 event: magnetic keogram, constructed from the X-component magnetometer data, and superposed the equatorward and poleward boundaries of the cusp aurora (red and blue lines). 55