The ring current buildup depending on the solar wind density and the time-dependent convection field is examined by using a particle tracing scheme. To obtain the current density, energetic protons injected from the night side boundary at L=9 are traced under the dipolar magnetic field and the Kp dependent model of the Volland-Stern type convection field. The source distribution function at the boundary is assumed to be isotropic Maxwellian with a temperature of 5 keV and a number density depending on the solar wind density. We derive the corrected Dst (Dst*) from the Biot-Savart integral over the whole three-dimensional distribution of the calculated current density. The results are compared with the observed Dst* in the three successive storms in April 1997; the major variation of the Dst* is mainly due to the convection electric field and the solar wind density as well. ¿ 1998 American Geophysical Union