The shape and location of the sector boundary surface in the inner solar system

Simultaneous observations by Helios 1 and Helios 2 over four solar rotations, between January 20 and May 23, 1976, were used to determine the latitudinal dependence of the polarity of the interplanetary magnetic field within ±7.23° of the solar equator and within 1 AU. The longitudinal and latitudinal positions of the sector boundary crossing are consistent with a warped sector boundary which extended from the sun to 1 AU and was inclined ≈ 10° with respect to the heliographic equator. This is consistent with simultaneous Pioneer 11 observations, which showed unipolar fields at latitude ≈ 16° at heliocentric distances greater than 3.5 AU. Two sectors were observed at southern latitudes; however, four sectors were observed at northern latitudes on two rotations, indicating a distortion from planarity of the sector boundary surface.