SOLAR-CYCLE VARIATION OF THE INTERPLANETARY MAGNETIC-FIELD SPIRAL

Interplanetary measurements spanning the time interval from 1965 through 1987 and a distance range from 0.7 to 15.9 AU are employed to test the Parker theory for the large-scale structure of the interplanetary magnetic field. Examination of data recorded by Earth-orbiting spacecraft reveals that the interplanetary magnetic field spiral depends upon the phase of the solar cycle, such that the annual mean winding angle in the years surrounding solar maximum is approximately 10-degrees larger than in the years surrounding solar minimum. An analysis of Pioneer-Venus Orbiter observations from 1979 through 1987 confirms this result. The observed variation of the solar wind speed with the solar cycle is shown to account for much of the recorded variation in the winding angle, but it is shown that the Parker theory requires excessive solar rotation rates to fully account for the observed winding of the field. Both Pioneer-Venus Orbiter and Voyager 2 observations confirm this conclusion. It is suggested that nonzero azimuthal field components arising from differential solar rotation may be convected into the corona to provide a steady source of azimuthal magnetic fields at the source point of the solar wind. A straightforward extension of the Parker theory shows how such seed fields would account for the observed discrepancy in the interplanetary spiral winding. Implications of this theory for high-latitude interplanetary fields are also discussed. It is shown that differential solar rotation may lead to significantly greater winding of the high-latitude interplanetary magnetic field than the Parker theory predicts.