MAGNETIC-FLUX REDISTRIBUTION IN THE STORM TIME MAGNETOSPHERE

The commonly used formula H = DCF + Dd, where DCF and DR are the effec ts of the magnetopause and ring current, respectively, neglects contri bution of the cross-tail current to the Dst variation. The formula all ows us to explain satisfactorily the observed relation of the Dst vari ation to the ring current intensity but faces difficulties in explaini ng other experimental facts. First, the equatorward shift of the amora l oval cannot be caused by the sole enhancement of the ring current. S econd, the observed relation of the Dst growth rate to the southward I MF component [Burton et al., 1975] does not have any quantitative expl anation up to now. We suggest using a different formula, H = (2 mu(o)p (sw))(1/2) + DR - F-out/2S. The formula is obtained from the condition s of magnetic flux conservation and pressure balance. The flux F-out i s directed mainly to the nightside of the magnetosphere. Hence the ter m F-out/2S describes the effect of the cross-tail current and a part o f the magnetopause currents. During quiet periods, each term in the ri ght-hand side of our formula is of the order of tens of nanoteslas. Du ring storm time, each term can rise to hundreds of nanoteslas. The flu x F-out grows after the interplanetary magnetic field (IMF) becomes so uthward owing to the flux transport from the dayside to the magnetotai l. The growth rate is described by the formula dF(out)/df = U - F-out/ tau(F) + eta(F), where U is the electric potential difference between the dawnside and duskside of the magnetosphere and tau(F) and eta(F) a re constant. The voltage U depends linearly on the IMF southward compo nent. Combining the latter formula with the expression for H yields a relationship between the Dst growth rate and the IMF southward compone nt close to the observed one. Since the auroral oval is mapped predomi nantly to the plasma sheet of the magnetotail, the growth of F-out dur ing 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 F-out by the equation S-1/2[S(2 mu(o)p(sw))(1/2) + F-out] = 3 pi(3/2)(M(E) + M(RC)), where M(E) and M(RC) are the magnetic moments of the Earth and ring current, respectively. Growth of F-out leads to the decrease of S and to the earthward movement of the dayside magneto pause. During storms this effect can be stronger than that of the regi on 1 Birkeland current, also moving the magnetopause earthward.