Spatial gradients in the heliospheric magnetic field: Pioneer 11 observations between 1 AU and 24 AU, and over solar cycle 21

Abstract

Magnetic field observations by Pioneer 11 and ISEE 3/ICE are used to investigate the large scale temporal behavior and spatial gradients in the heliospheric magnetic field. We extend and refine previous analyses by expanding the data set to include observations out to a radial distance of 24 AU and over a longer time interval which includes the recent solar minimum. The spatial gradients are investigated by removing the solar cycle variations with data obtained at 1 AU at the corresponding time. To first order, the gradients in the magnetic field magnitude and in the azimuthal component are found to agree with the Parker model. Furthermore, this analysis of the Pioneer data confirms previous reports of a magnetic field flux deficit of approximately 1%/AU, so that at 20 AU the relative deficit is of the order of 20%. The solar wind speed measured at 1 AU as well as at Pioneer 11 has been analyzed, but it is found that the small variations observed between the two locations affect this result only minimally. The average field strength at Pioneer 11 at a distance of 16 AU has been compared with a Voyager 2 average obtained at nearly the same radial distance and time interval but at a significantly different latitude. They agree very closely and both exhibit a deficit of ≈ 20% relative to the 1‐AU baseline. This agreement demonstrates that B did not vary significantly with latitude at this time (1984–1985 near solar minimum). Furthermore, the 1‐AU baseline has been adjusted to allow for an observed dependence of the field on latitude near solar maximum (1978–1982) inferred from a comparison of measurements in the inner heliosphere at 0.7 AU (PVO) and 1.0 AU (ISEE 3). This adjustment of the 1‐AU baseline did not significantly affect the deficit, again confirming that the Pioneer measurements were not strongly influenced by the latitude dependences. We thus argue that most of the magnetic field deficit is in the radial gradient.