The commonly used formula H=DCF+DR, where DCF and DR are the effects of the magnetopause and ring current, respectively, neglects contribution of the cross-tail current to the Dst variation. The formula allows us to explain satisfactorily the observed relation of the Dst variation to the ring current intensity but faces difficulties in explaining other experimental facts. First, the equatorward shift of the auroral oval cannot be caused by the sole enhancement of the ring current. Second, the observed relation of the Dst growth rate to the southward IMF component [Burton et al., 1975> does not have any quantitative explanation up to now. We suggest using a different formula, H=(2&mgr;opsw)1/2+DR-Fout/2S. The formula is obtained from the conditions of magnetic flux conservation and pressure balance. The flux Fout is directed mainly to the nightside of the magnetosphere. Hence the term Fout/2S describes the effect of the cross-tail current and a part of the magnetopause currents. During quiet periods, each term in the right-hand side of our formula is of the order of tens of nanoteslas. During storm time, each term can rise to hundreds of nanoteslas. The flux Fout grows after the interplanetary magnetic field (IMF) becomes southward owing to the flux transport from the dayside to the magnetotail. The growth rate is described by the formula dFout/dt=U-Fout/&tgr;F+&eegr;F, where U is the electric potential difference between the dawnside and duskside of the magnetosphere and &tgr;F and &eegr;F are constant. The voltage U depends linearly on the IMF southward component. Combining the latter formula with the expression for H yields a relationship between the Dst growth rate and the IMF southward component close to the observed one. Since the auroral oval is mapped predominantly to the plasma sheet of the magnetotail, the growth of Fout during a storm allows us to explain the equatorward shift of the auroral oval. Another prediction from our theory is the erosion of the stable trapping region in which the equatorial cross section S is related to the flux Fout by the equation S1/2[S(2&mgr;opsw)1/2+Fout>=3&pgr;3/2(emph type='1'>ME+MRC), where ME and MRC are the magnetic moments of the Earth and ring current, respectively. Growth of Fout leads to the decrease of S and to the earthward movement of the dayside magnetopause. During storms this effect can be stronger than that of the region 1 Birkeland current, also moving the magnetopause earthward. ¿ American Geophysical Union 1996