Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

“Physics o fAuroral Phenomena", Proc. XXXV Annual Seminar, Apctity, pp. 45- 48, 2012 © Kola Science Centre, Russian Academy of Science, 2012 MAGNETIC FIELD PARAMETERS NEAR THE SUBSOLAR MAGNETOPAUSE IN ACCORDANCE WITH THEMIS DATA M.S. Pulinets1, M.O. Riazantseva1’2, E.E. Antonova1’2,1.P. Kirpichev2,1 1Skobeltsyn Institute o f Nuclear Physics Moscow State University, Moscow, Russia 2Space Research Institute RAS, Moscow, Russia Abstract. We analyze the crossings of the magnetopause near the subsolar point using FGM and ESA devices of THEMIS mission. Variations of the magnetic field near magnetopause measured by one of THEMIS satellites are compared with simultaneous measurements in the solar wind by another THEMIS satellite. 30 and 90 s averaging of magnetic field in the magnetosheath is produced. The results of averaging are compared with the results of measurement just after the magnetopause crossing. It is shown, that Bx component of the magnetic field near magnetopause is near to zero, which supports the possibility to consider the magnetopause as the tangentional discontinuity. Comparatively good correlation of By component in the solar wind and near the magnetopause is observed. The correlation of Bz component near the magnetopause and IMF is practically absent. It is shown, that in many cases the sign of the Bz component of magnetic field near the subsolar point does not coincide with the sign of IMF Bz component. The results of the analysis create definite difficulties to simplified theories of magnetic reconnection of solar wind magnetic field at the magnetopause. Introduction The development of the solar wind-terrestrial interaction theory requires taking into account the existence of a high level of turbulence in the magnetosheath. Therefore, comparison of the magnetic field parameters directly in front of the magnetopause with the magnetic field in the solar wind (IMF) is of great interest. A large number of measurements of IMF were carried out in the libration point. The solar wind propagation time from the libration point to the Earth is about one hour. Solar wind is itself a turbulent medium (see Riazantseva et al. [2005, 2007] and references in these papers). Therefore, its parameters may change during the propagation to the Earth's orbit. To assess the effect of magnetosheath turbulence on the magnetic field parameters changing during the propagation through the magnetosheath to the magnetopause these parameters should be compared directly in front of the shock wave and near the magnetopause. At the same time measuring of the solar wind should be carried out upstream the foreshock region which makes a strong disturbance in the solar wind prior to the shock front. The opportunity of such a comparison has appeared only with the start of the five-satellite THEMIS mission ( Angelopoulos, [2008]; Sibeck and Angelopoulos , [2008]). One of THEMIS satellites during summer times performed measurements in the solar wind, while the other occasionally crossed the magnetopause on the dayside. In this study, a comparison of the magnetic field parameters near the magnetopause measured every 3 seconds (spin resolution of the probe), 30 seconds (which is ~ the correlation time, in accordance with estimations of Gutynska et al. [2008]) and 90 seconds (~ three times greater than characteristic correlation time) with the same parameters before the bow shock is made. Average dependences of the magnetic field parameters near the magnetopause on the corresponding parameters in the solar wind at selected averaging intervals are obtained. It is shown that the orientation of the magnetic field at the magnetopause may differ significantly from the orientation in the solar wind including even change of sign. Method and results of data analysis Data for analysis were taken from the website http://cdaweb.gsfc.nasa.gov/. In this paper we analyze THEMIS data for years 2008, 2009 when the orbits of spacecrafts deployed by the precession in such a way that their apogees were located close to the Earth-Sun line, i.e. the configuration convenient for studying the interactions on the dayside of the Earth's magnetosphere takes place. The intervals when one of the spacecrafts was located in the solar wind, and another crossed the magnetopause near the subsolar point were picked out. The events were selected when the deviation of the probe from the x-axis did not exceed 7 RE. The moment of crossing of the magnetopause was fixed by the distinctive changes in plasma parameters and magnetic field, determined according to the Electrostatic Analyzer ESA and the Flux Gate Magnetometer FGM on the probe. Parameters of the interplanetary magnetic field (IMF) were determined according by FGM. The events in which the solar wind did not suffer significant variations were chosen. The value of the standard deviation of the absolute value of the magnetic field from the average for the selected periods does not exceed 2 nT, the flow velocity was no more than 650 km/sec. The parameters of the magnetic field, measured by one of the spacecraft after crossing the magnetopause, were compared with the IMF parameters observed by another spacecraft. The time resolution of the measurements was 3 seconds which is equal to the spin resolution of the probe. The following quantities were used as analyzed parameters: the magnitude and the three components of the magnetic field and the IMF clock angle. The 'clock angle' 45 Polar Geophysical Institute