We have studied the data strings for the planetary index Ap, the interplanetary magnetic field intensity (B), and the solar wind bulk velocity (V) for the period 1963 to 1998. The study covers three sunspot number cycles (20, 21, 22). We report the presence of a long-term trend in the available B data (1963 to 1998) that corresponds in time with the three-cycle quasiperiodicity discovered by us earlier in Ap data for the 1932 to 1996 period. A set of regression calculations has been carried out correlating V and B with Ap. Our analysis brings out some of the steady state characteristics of the long-term changes in the parameters and their interrelationships. Among other things, our phenomenological study indicates that solar polar field reversals may be responsible for the existence of the Gnevyshev gap in Ap data. A function proportional to BV2 describes the time variations of Ap quite well, as does the function proportional to the product BV. The latter represents the interplanetary electric field responsible for the magnetosphere-solar wind coupling. We ascribe the Ap time variations to this fluctuating electric field. We suggest that B is a more fundamental parameter than V. ¿ 2000 American Geophysical Union