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 ofAuroral Phenomena", Proc. XXXIVAnnual Seminar, Apatity, pp. 75 - 78 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute COMPREHENSIVE ANALYSIS OF THE MORNING Pc5 GEOMAGNETIC PULSATIONS GROUND-BASED OBSERVATIONS, EISCAT AND THEMIS DATA N.G. Kleimenova1’3, O.V. Kozyreva1, K. Kauristie2, M.V. Uspensky2, A.A. Vlasov2 1. Institute o f the Earth Physics RAS, Moscow, Russia 2. Finnish Meteorological Institute, Helsinki, Finland 3. Space Research Institute RAS, Moscow, Russia Abstract. The case study of Pc5 pulsations (f-1-6 mHz at Scandinavian IMAGE magnetometer network on January 18, 2008, as well as ionosphere disturbances measured by the EISCAT Svalbard Radar (ESR) and VHF EISCAT radar at Tromso, and magnetic field and particles data from THEMIS satellites have been discussed. This event was developed under the positive IMF Bz and strong solar wind velocity (~700 km/s). The Pc5 pulsations exhibited the typical resonant nature with the latitude maxima and the correspondent phase reverse. The Pc5 onset was triggered by the sharp change in IMF and a small jump in the solar wind velocity. The morning-side Pc5 pulsations suddenly “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 satellites (TH- A, TH-E, TH-D), and was accompanied by a sharp enhancement of the electron density (Ne) at EISCAT Svalbard and Tromso radars (that could be interpreted as a simultaneous particle precipitation in a large high-latitude area), and visible aurora intensification by NAL ASC. We suppose that the magnetic field line stretching in the substorm growth phase and following dipolarization may change the plasma and magnetic field configurations even in the inner magnetosphere and violate the ULF resonance exiting. We believe the Pc5 pulsations suddenly “died” due to the magnetic field line reconfiguration. Moreover, the enhanced ionosphere conductance near the Pc5 FLR electric field node(s) may disturb an ionosphere FLR wave reflection. The third 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. 1. Introduction Geomagnetic pulsations Pc5 (f= 1.6-6.6 mHz) are one of the most commonly encountered and widely discussed in the literature types of the ULF waves in the geomagnetic field. The Pc5 pulsations are typical for the morning sector of the auroral latitudes and often interpreted as an excitation of resonance oscillations of geomagnetic field lines (FLR). These pulsations are usually observed under large values of the solar wind speed, suggested the Kelvin-Helmholtz (K-H) instability as a source of the resonance energy. Many authors discussed the interplanetary medium conditions, particularly, solar wind dynamic pressure variations, which result the Pc5 pulsations generation [e.g., Motoba et al., 2003; Kessel, 2008]. However, nobody studied the magnetosphere and ionosphere state in the time of sudden drop of Pc5 activity, which often observed by ground stations. The aim of this paper is to study the high-latitude ionosphere disturbances associated with the sudden drop of Pc5 amplitude. The suitable interval of the January 18, 2008, 0400-0600 UT, has been chosen, when the large amplitude morning Pc5 geomagnetic pulsations were observed at the IMAGE magnetometer array. In our analysis we used the simultaneous ionosphere data from EISCAT Svaldbar radar (ESR), VHF EISCAT radar at Tromso, and Svaldbar ASC data at NAL as well the THEMIS satellites and ground-based Canadian stations. jf H K n J V л ' ч г ■,'гу Ч \Л -у 700 J- I ft.---- J/"T— <W / v w P " 5 ; ^ r v -—'— \A WAl* . . У . i ................. Fig. 1. The solar wind and IMF (ONMI) data and some IMAGE magnetograms shifted to 20 min. 75