New models for the size and shape of the Earth's magnetopause and bow shock are derived, based on a criterion for selecting the crossing events and their corresponding up-stream solar wind parameters. In this study, we emphasize the importance of accurate interplanetary parameters for predicting the size and shape of the magnetopause and bow shock. The time lag of the solar wind between the solar wind monitor and the location of crossings is carefully considered, ensuring more reliable up-stream solar wind parameters. With this database new functional forms for the magnetopause and bow shock surfaces are deduced. In this paper, we briefly present the preliminary results. For a given up-stream solar wind dynamic pressure D p , an IMF north-south component B z , a solar wind β and a magnetosonic Mach number M ms , the parameters that describe the magnetopause and bow shock surfaces r 0 and α can be expressed in terms of a set of coefficients determined with a multi-parameter fitting. Applications of these models to extreme solar wind conditions are demonstrated. For convenience, we have assumed that r 0 , B z and D p retain their units, except in equations where they are normalized by 1 R E (Earth radius), 1 nT and 1 nPa, respectively.

[1] The International Solar Terrestrial Physics database of the magnetic measurements on GOES and plasma measurements on Los Alamos National Laboratory (LANL) geosynchronous satellites is used for selection of 169 case events containing 638 geosynchronous magnetopause crossings (GMCs) in 1995 to 2001. The GMCs and magnetosheath intervals associated with them are identified using advanced methods that take into account (1) strong deviation of the magnetic field measured by GOES from the magnetospheric field, (2) high correlation between the GOES magnetic field and interplanetary magnetic field (IMF), and (3) substantial increase of the midenergy ion and electron fluxes measured by LANL. Accurate determination of the upstream solar wind conditions for the GMCs is performed using correlation of geomagnetic activity (Dst (SYM-H) index) with the upstream solar wind pressure. The location of the GMCs and associated upstream solar wind conditions are ordered in an aberrated GSM coordinate system (aGSM) with X-axis directed along the solar wind flow. In the selected data set of GMCs the solar wind total pressure Psw varies up to 100 nPa and the southward IMF Bz reaches 60 nT. We study the conditions necessary for geosynchronous magnetopause crossings using scatterplots of the GMCs in the coordinate space of Psw versus Bz. In such a representation the upstream solar wind conditions show a sharp envelope boundary beyond which no GMCs are observed. The boundary has two straight horizontal branches where Bz does not influence the magnetopause location. The first branch is located in the range of Psw = 21 nPa for large positive Bz and is associated with a regime of pressure balance. The second branch asymptotically approaches the range of Psw = 4.8 nPa under strong negative Bz, and it is associated with a regime in which the Bz influence saturates. The intermediate region of the boundary ranges from moderate negative to moderate positive IMF Bz and can be well approximated by a hyperbolic tangent function. We interpret the envelope boundary as a range of necessary upstream solar wind conditions required for the magnetopause to reach geosynchronous orbit at its closest approach to the Earth (its ''perigee'' location).

Abstract. Although the average magnetopause is •010 RE from the Earth, the magnetopause moves inside the geosynchronous orbit during extreme solar wind conditions. Under these circumstances some geosynchronous satellites suddenly enter the magnetosheath and are exposed to the plasma and fields of the magnetosheath. In this study we evaluate the predictive capabilities of magnetopause location models in forecasting geosynchronous magnetopause crossings. We predict periods during which geosynchronous satellites enter the magnetosheath using the model). The probability of detection is very high for both prediction models. These results suggest that both models work well in predicting magnetosheath periods for geosynchronous satellites. Predictions from the models provide a prerequisite condition for geosynchronous magnetopause crossings. Further examination of unsuccessful events indicates that preconditioning by the interplanetary magnetic field Bz needs to be included in the forecasting procedure for a better forecast. This finding provides us with a guide to improving future magnetopause location models.

[1] Geosynchronous magnetopause crossing (GMC) data were collected from literature sources from 1967 and from the experimental data on magnetic measurements on GOES (129 GMCs) and plasma measurements on LANL (197 GMCs) geosynchronous satellites in 1994 to 2001. The dawn-dusk asymmetry of the magnetopause at geosynchronous orbit was examined by two independent methods using the collected data set of 515 GMCs. First, the large amount of accumulated GMCs permitted the revealing of a substantial dawn-dusk asymmetry in the local time (LT) distribution of the GMC occurrence probability, with a statistically significant maximum in the range from 1000 LT to 1100 LT. Second, an analysis of the dawn-dusk asymmetry dependence on the upstream solar wind conditions was performed using a scatter plot of the solar wind total pressure versus local time for various IMF Bz. There was no asymmetry revealed for large positive Bz. Under strong negative Bz we found a prominent magnetopause dawn-dusk asymmetry. The asymmetry is characterized by a shifting of the GMCs with the minimal required solar wind total pressure toward the dawn and by a significantly lower (about 3 times) solar wind pressures required for the GMCs in the dawn sector relative to the dusk sector. We found that the asymmetry cannot be attributed to the IMF orientation along the Parker spiral, which is not revealed for strongly disturbed solar wind conditions accompanying the GMCs. An application of the dawn-dusk asymmetry effect for the Chao et al. [2002] model provided a substantial increase in the model predictive capability interim of the geosynchronous magnetopause crossings. The standard deviation decreased by 20% from 0.55 R E for the initial version to 0.45 R E for the asymmetrical version of the model, with the magnetopause axis rotated by an angle of about 15°toward the dawn. The physical processes responsible for the magnetopause dawn-dusk asymmetry are discussed. We indicate the two most probable magnetospheric phenomena, which would contribute to the substantial dawn-dusk asymmetry of the magnetopause under disturbed solar wind and geomagnetic conditions. The first one is that magnetopause erosion would operate more intensively in the prenoon sector. The second phenomenon is an asymmetrical terrestrial ring current that would develop during geomagnetic storms.

The dayside magnetopause moves closer to the Earth with increasing southward IMF Bz. Is the response of magnetopause to solar wind parameters, southward IMF Bz, and dynamic pressure Dp linear or nonlinear? GOES observations on 6 April 2000 shows that the magnetopause is still outside of geosynchronous orbit even though the southward IMF Bz is greater than 25 nT and Dp is near 8 nPa. We suggest that the earthward motion of the dayside magnetopause saturates for large southward IMF Bz. Magnetosheath encounters observed by GOES satellites during 1999–2000 are used as a database for selecting a functional form of the saturation effect based on the calculations of the modified magnetopause model of Chao et al. [2002]. To obtain a relationship of the threshold of southward IMF Bz for saturation occurring as a function of Dp, an iteration procedure is used to minimize the false alarm rate (FAR) and maximize the probability of prediction (PoP). The relationship B′z = −8.1 − 12.0 × log (Dp + 1) is obtained where B′z is the threshold of IMF Bz for saturation. This relationship is applied to a modified Chao et al. [2002] model and the new model is compared against magnetosheath encounters observed by the LANL MPA instruments on 31 March 2001.