Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

“P hysics o f Auroral P henom ena”, Proc. XXXUI A n n u a l Sem inar, A patity, pp. 6 3 - 66, 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute CALCULATION OF THE MAGNETOPAUSE STAND-OFF DISTANCE A.E. Levitin, L.I. Gromova, L.A. Dremukhina, E.G. Avdeeva, A.U. Burtsev (Pushkov Institute o f Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk, Moscow region, Russia) Abstract. Two magnetopause stand-off distance R0 models using statistics of the dayside magnetopause locations deduced from the satellite measurements in crossing o f the magnetopause, and basing on balance of the solar wind plasma’s pressure and that of the Earth’s magnetic field, are examined. Results predicted according to different models have been compared with satellite measurements of the magnetopause crossings close to the subsolar point. A discrepancy between the models becomes more significant, when R0 becomes less than 7 Re. As data set of satellite crossings of the magnetopause close to the subsolar point don’t contain IMF and the solar wind parameters that are typical for strong magnetic storms, analytical models o f the magnetopause stand-off distance don’t allow to calculate R0 reliably, when R0 becomes less than 7 Re. Introduction The magnetopause stand -off distance ( Ro ) is an important input parameter of the modem magnetospheric magnetic field models. The R0 calculating can be based either on the dynamic balance of the solar winds plasma’s pressure and that of the Earth's magnetic field or can response on statistics of the dayside magnetopause location deduced from the satellite measurements. Choosing the first option, it’s necessary to calculate the near-Earth’s magnetic field taking into account the instant spatial-temporal distribution of the basic magnetosphere current systems. However in this case, a reliable conclusion is hardly achievable. In the other way, the analytical relations between R0 and parameters of the solar wind and interplanetary magnetic field fixed by patrol satellites should be applied. These relations are deduced from statistics of satellite crossing of the dayside magnetopause and transfer the crossing points into initial point of the geometry figure describing the model magnetopause (ellipsoid, paraboloid and others). Data of satellite crossings of the dayside magnetopause, especially close to the subsolar point, are few. As the magnetospheric disturbances occur rather rarely satellites cross the magnetopause under quiet or moderate magnetospheric conditions substantially. Correspondingly, statistical data sets used for constructing R0 analytical models don’t practically contain great amplitudes of the southern IMF component Bz and the solar wind dynamic pressure. Therefore such models don’t allow to calculate the magnetopause stand-off distance R0 reliably, when R0 becomes less than 7 Re. We have presented short description of modem R0 models and comparison o f their results with satellite measurements o f the magnetopause crossings close to the subsolar point under different magnetospheric conditions. Model calculation of the magnetopause stand-off distance In magnetospheric physics, it is important to have an accurate model for the determination of the size and shape of the magnetopause. In the absence of solar wind coupling to the magnetosphere, these parameters could be predicted by the dynamic and static pressures of the solar wind and the magnetic pressure of the magnetosphere. Based on this assumption, various models have been developed. The earlier statistical study and following empirical model o f the average magnetopause shape and size was carried out by Fairfield in 1971. Other empirical models followed; Formisano in 1979 adopted Fairfield’s approach and used nearly all magnetopause crossings available at that time to develop a new model. Detailed studies of magnetopause processes have shown that dayside reconnection leads to the changes of the magnetopause shape and location. For this reason, Sibeck et al. (1991) fitted magnetopause crossings as either a function of dynamic pressure or as a function of the Bz component of the interplanetary magnetic field (IMF) and Petrinec et al. (1991) fitted the magnetopause as a function of dynamic pressure for strongly northward and strongly southward IMF separately. Further empirical magnetopause models are already bivariate with respect to both dynamic pressure and IMF Bz (e.g. Roelof and Sibeck, 1993; Petrinec and Russel, 1993, 1996; Kuznetsov and Suvorova, 1998; Shue et al., 1997). The Petrinec and Russell (1996) model o f the nightside magnetopause use inverse trigonometric functions. The other mentioned models adopted either the general equation of an ellipsoid with two parameters (eccentricity and standoff distance) or the general quadratic equation; Shue et al. (1997) used the standoff distance and the level of tail flaring. From this short survey, it follows that these models use various functional forms to represent the shape and location of the magnetopause and are usually parametrized by solar wind dynamic pressure and IMF Bz. The basic findings of these studies were that the magnetopause scales are roughly with pressure as p 1/6 ( p l/6‘6 in Shue et al., 1997) and that for decreasing IMF Bz, the magnetopause displaces inward near the nose and outward down the tail. 63