Saturation of IMF B_z influence on the position of dayside magnetopause

The dayside magnetopause moves closer to the Earth with increasing southward IMF B_z. Is the response of magnetopause to solar wind parameters, southward IMF B_z, and dynamic pressure D_p linear or nonlinear? GOES observations on 6 April 2000 shows that the magnetopause is still outside of geosynchronous orbit even though the southward IMF B_z is greater than 25 nT and D_p is near 8 nPa. We suggest that the earthward motion of the dayside magnetopause saturates for large southward IMF B_z. 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 B_z for saturation occurring as a function of D_p, 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 (D_p + 1) is obtained where B′_z is the threshold of IMF B_z 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.