ON THE LATITUDE AND SOLAR-CYCLE DEPENDENCE OF THE INTERPLANETARY MAGNETIC-FIELD STRENGTH

Using 20 years of solar magnetograph and in-ecliptic interplanetary magnetic field (IMF) measurements, we verify that the radial IMF component (B(r)) can be approximated by a superposition of the Sun's inclined magnetic dipole moment and a current sheet normal to the dipole axis. The net field is found empirically to be approximately 3 times stronger along the dipole axis than near the current sheet, whose effect is to redistribute flux toward the dipole equator. The radial IMF intensity at a given latitude and phase of the sunspot cycle is determined by the changing strength and inclination of the dipole, which attains its maximum amplitude when its axis is aligned with the solar poles near sunspot minimum. The model predicts that over the sunspot cycle, \B(r)\ should undergo the least variation near the heliographic equator and the greatest variation above the Sun's poles, where it decreases by a factor of 10 between sunspot minimum and sunspot maximum. The latitudinal gradients in B(r) are expected to be steepest near sunspot minimum and flattest near maximum. The model suggests that Ulysses will encounter very strong fields when it flies over the solar poles during the declining phase of sunspot cycle 22.