doi: 10.3319/TAO.2012.09.26.02(SEC) * Corresponding author E-mail: dalex@jupiter.ss.ncu.edu.tw Terr. Atmos. Ocean. Sci., Vol. 24, No. 2, 225-232, April 2013 The Shape of Strongly Disturbed Dayside Magnetopause Alexei V. Dmitriev 1, * and Alla V. Suvorova 2 1 Institute of Space Science, National Central University, Jhongli, Taiwan 2 Center for Space and Remote Sensing Research, National Central University, Jhongli, Taiwan Received 5 April 2012, accepted 26 September 2012 AbSTrAcT During strong geomagnetic disturbances, the Earth’s magnetosphere exhibits unusual and nonlinear interaction with the incident flow of magnetized solar wind plasma. Global Magneto-hydro-dynamic (MHD) modeling of the magnetosphere predicts that the storm-time effects at the magnetopause result from the abnormal plasma transport and/or extremely strong field aligned currents. In-situ observations of the magnetospheric boundary, magnetopause, by Geosynchronous Operational Environmental Satellite (GOES) allowed us to find experimentally such effects as a saturation of the dayside reconnection, unusual bluntness and prominent duskward skewing of the nose magnetopause. The saturation and duskward skewing were at- tributed to the storm-time magnetopause formation under strong southward interplanetary magnetic field (IMF). The unusual bluntness was observed during both high solar wind pressure and strong southward IMF. We suggest that these phenomena are caused by a substantial contribution of the cross-tail current magnetic field and the hot magnetospheric plasma from the asymmetrical ring current into the pressure balance at the dayside magnetopause. Key words: Magnetopause, Magnetospheric currents, Geomagnetic storm Citation: Dmitriev, A. V. and A. V. Suvorova, 2013: The shape of strongly disturbed dayside magnetopause. Terr. Atmos. Ocean. Sci., 24, 225-232, doi: 10.3319/TAO.2012.09.26.02(SEC) 1. InTroDucTIon The magnetopause shape is usually represented by a surface of revolution around the aberrated Sun-Earth line, or X-GSM axis. A functional form of the shape and its depen- dence on solar wind driving parameters vary from one mod- el to another (e.g., Shue et al. 2000). Key driving parameters of the magnetopause are solar wind dynamic pressure (P d ) and B z component of interplanetary magnetic field (IMF). Different parts of the magnetopause are driven by the solar wind parameters in different manners. Here we focus on the dayside magnetopause. For quiet and moderately disturbed conditions, practi- cally all the empirical models demonstrate similar accuracy in prediction of the dayside magnetopause (Šafránková et al. 2002). During strong geomagnetic disturbances, the magne- topause exhibits unusual and nonlinear behavior such as re- connection saturation and prominent dawn-dusk asymmetry (e.g., Dmitriev et al. 2004, 2011; Suvorova et al. 2005). It is widely accepted that the subsolar magnetopause moves earthward due to the reconnection of geomagnetic field with a southward component (negative B z ) of the IMF. Stronger southward IMF results in a smaller distance to the subsolar magnetopause. However, when the magnitude of negative B z exceeds a certain threshold, the magnetopause distance no longer decreases, i.e., the southward IMF influ- ence (reconnection) is saturated (e.g., Yang et al. 2003; Su- vorova et al. 2005). The threshold for saturation is estimated to be about -20 nT (i.e., E ~ 10 mV m -1 ) and its magnitude increases with solar wind pressure. The dawn-dusk asymmetry in the shape of dayside magnetopause has been convincingly shown on the ba- sis of geosynchronous magnetopause crossings (GMCs) (Dmitriev et al. 2004). They found that the GMCs occur most often in a prenoon sector. Moreover, higher (lower) solar wind pressure is required to push the magnetopause inside the geosynchronous orbit in postnoon (prenoon) and dusk (dawn) sectors. As a result, during magnetic storms the magnetopause is located closer to the Earth in the dawn and prenoon sectors and is more distant in the postnoon and