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.

N.G. Kleimenova et al. Fig. 7. The auroral oval location with the FCC and GILL location and the projection of the THEMIS satellites foot print. All stations under study were located at the closed magnetosphere. The morning arcs, observed by NAL ASC, supported its location inside of the closed field lines. The estimated auroral oval location based on the OVATION data is shown in Fig. 7. The long bar indicates the beam direction of the Tromso radar. 3. Discussion and conclusion We presented a detailed case study of morning Pc5 pulsations observed at IMAGE stations on January 18, 2008 in 04-06 UT (-07-09 MLT). The considered event was observed under high values of the solar wind speed (680-720 km/s), positive IMF Bz, and the slightly varied dynamic pressure from 1.6 to 2.6 nPa. The pulsations exhibited the typical for field line resonance (FLR) latitudinal structure. The high values of the solar speed provide conditions for coupling of a surface wave produced by the Kelvin- Helmholtz (K-H) instability to shear Alfven wave. We suppose that the trigger of FLR generation was the sharp increasing of IMF Bz and Bx accompanied by a small jump of the solar wind velocity. The discussed resonant Pc5 geomagnetic pulsation “died” with the onset of the night side substorm, timing by the ground-based Pi2 pulsation burst and auroral WTS. This substorm onset was documented by magnetic field and particle observations by three THEMIS probes (TH-A, TH- E, TH-D), and was accompanied by a sharp enhancement of the electron density (Ne) at EISCAT Svalbard and Tromso radars, while the IMF Bz was positive. The constant length of a resonant magnetic field line and the magnetic field strength along this line as well as cold plasma density at the equatorial plane of the magnetosphere should be a necessary condition for stable FLR processes (at least, several oscillations with similar amplitudes). The conditions of the wave reflection from both ionospheres also should be stable. We suppose that magnetic field stretching in the substorm growth phase and following dipolarization change the plasma and magnetic field configurations even in the inner magnetosphere. This change breaks the FLR processes. The standing wave nature of the FLR provides the electric field nodes in the northern and southern ionosphere. The increasing of the high-latitude ionosphere conductance (due to particle precipitation) results an additional factor of the ionosphere loss which could lead to the destruction of the resonant wave reflection. One more plausible reason of the sudden Pc5 amplitude drop could be sharp increasing of the solar wind dynamic pressure. The relative value of this increasing was not big, but it could produce a significant strong nonlinear effect due to very high solar wind speed. Acknowledgement. This work was partly supported by RFBR grant No. 10-05-00247. References Glassmeier, K.H., Lester, М., Mier-Jedrzejowicz, W.A.C., et al., Pc5 pulsations and their possible source mechanisms: a case study, J. Geophys., 55, 108-119, 1984. Kessel, R. L., Solar wind excitation of Pc5 fluctuations in the magnetosphere and on the ground, J. Geophys. Res., 113, A04202, doi: 10.1029 /2007JA012255,2008. Kleimenova N.G., Kozyreva O.V., Manninen J., and Ranta A., Unusual strong quasi-monochromatic ground geomagnetic Pc5 pulsations in the recovery phase of November 2003 superstorm, Annale Geophysicae, 23, 2621- 2634,2005. Kleizing, C., Berg, G.A., Kelley, M.C., Primdahl, F., and Torbert, R.B. The electrical and precipitation characteristics of morning sector Sun-aligned auroral arcs, J. Geophys. Res., 101, 17,175-17,190, 1996. Motoba, Т., Kikuchi, T. Okuzawa, T. and Yumoto, K. Dynamical response of the magnetosphere- ionosphere system to a solar wind dynamic pressure oscillation, J. Geophys. Res., 108(A5), 1206, doi:10.1029/2002JA009696, 2003. Ober, M, Maynard, N.C., Burke, W.J., et al., Mapping prenoon auroral structures to the magnetosphere, J. Geophys. Res., 105, 27,519-27,530, 2000. Rostoker, G., Spadinger, I, and Samson, J.C. Local time variations in the response of Pc5 pulsations in the morning sector to substorm expansive phase onsets near midnight, J. Geophys. Res., A89, 6749-6757,1984. Samson, J.C., and Rostoker, G., Response of dayside Pc5 pulsations to substorm activity in the nighttime magnetosphere, J. Geophys. Res., A86, 733-752, 1981. Shiokawa, K , Yumoto, K , Hayshi, K , Oguti, Т., and McEwen, D.J. A ststistical study of the motions of auroral arcs in the high-latitude morning sector, J. Geophys. Res., 102, 24,325-24,332, 1997. 78