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 ofa ground image ofthe surface oscillations at the magnetopause pressure increases, showed that waves with 1.8 mHz frequency are excited whose global properties cannot be explained by other known ULF wave modes [Hartinger et al., 2015]. Thus, magnetopause surface eigenmodes could be a potential source of magnetospheric dayside ULF waves with f< 2 mHz driven by quasi-periodic fluctuations of the magnetoshcath dynamic pressure. So far, magnetospheric field line oscillations have been modeled in a plasma with steady parameters. However, a realistic plasma medium is highly fluctuating because of small-scale plasma processes. Therefore, the eigenfrequency of the field line resonator Q should experience inherited fluctuations because of stochastic variations of B, p, and field line length. Here we have considered the effect of such background fluctuations on driven field line standing oscillations. Our estimates indicate the necessity to consider seriously a possible effective damping of ULF waves due to background magnetospheric fluctuations. The effect considered here may explain a lack of narrowband ULF waves during magnetically disturbed periods, when the magnetospheric turbulence level is substantially elevated. More specifically, this effect may be responsible for suppression of field line standing oscillations at the magnetopause or very close to it. Deeper in the magnetosphere, away from the magnetopause, the magnetospheric magnetic field and plasma becomc more regular, and the level o f their fluctuations decreases. As a result, a resonant response to an external driving becomes evident. The proposed mechanism may interpret the puzzling lack of ground response to surface modes or last field line oscillations near the OCB projection, and it merits further validation and verification. The suppression of resonant field line oscillations is dependent not only on the level o f fluctuations but on their spectral form as well. For more definitive conclusions a more detailed information about the power and spectra of background magnetospheric turbulence is necessary. Conclusion. The latitudinal structure of dayside broadband Pe5-6 pulsations recorded by magnetometers covering near-cusp latitudes has the maximum of the pulsation power -2° deeper in the magnetosphere than the OCB determined either with SuperDARN radar or scanning photometer. To interpret a puzzling lack of ground response to last field line oscillations or surface mode at magnetopause, we suggest that stochastic fluctuations of the magnetospheric plasma and magnetic field can suppress the excitation of standing MHD oscillations in a close vicinity o f the magnetopause. To quantify this hypothesis, we have considered the model of a driven field line resonator with stochastic fluctuations of the eigenfrequency. The results of analytical calculations has shown the deterioration of resonant properties owing to background fluctuations. Acknow ledgements. This work was supported by the grant #246725 from the POLARPROG program o f the Norwegian Research Council (VP, LB, OK) and the RFBR grant 16-35-00095 mol a (VB). References Archer M.O., et al. (2013) Magnetospheric “magic” frequencies as magnetopause surface eigenmodes, Geophys. Res. Lett., 40, 5003-5008. Baker K.B., et al. (1995) HF radar signatures of the cusp and low-latitude boundary layer, J. Geophys. Res., 100 7671-7695. Coult N., et al. (2007) Suppression of resonant field line oscillations by a turbulent background, Planet. Space Sci 55, 694-700. Dimentberg M.F. (1980) Nonlinear stochastic problems of mechanical oscillations, Nauka, М., p. 368 (in Russian). Hartinger M.D., et al. (2015) The global structure and time evolution of dayside magnetopause surface eigenmodes, Geophys. Res. Lett., 42, 2594-2602. Hollweg, J.V. (1997) A simple mechanical model for resonance absorption: The Alfven resonance, J. Geophys. Res., 102,24127-24137. Johnsen M.G., et al. (2012) The dayside open/closed field line boundary as seen from space and ground-based instrumentation, J. Geophys. Res., 117, A03320, doi:10.1029/2011JA016983. Kleimenova N.G., et al. (1985) Two types of long period geomagnetic pulsations near equator border of dayside polar cusp, Geomagn. Aeronomy, 25, 163-166. Lanzerotti L.J., et al. (1999) Long-period hydromagnetic waves at verv high geomagnetic latitudes, J. Geophys. Res., 104,28423-28435. Pilipenko V., et al. (2015) Are dayside long-period pulsations related to the cusp? Ann. Geophys., 33, 395-404. Plaschke F., et al. (2009) Standing Alfven waves at the magnetopause, Geophys. Res. Lett., 36, L02104, doi:10.1029/2008GL03 6411. Plaschke F., et al. (2011) Properties of standing Kruskal-Schwarzschild modes at the magnetopause, Ann Geophys., 29, 1793-1807. Urban K.D., et al. (2011) Quiet time observations of the open-closed boundary prior to the CIR-induced storm, Space Weather, 9, SI 1001. 57