Radial and latitudinal gradients in the interplanetary magnetic field: Evidence for meridional flux transport

Abstract

An analysis is presented of the magnetic field data obtained by the Pioneer 10 and 11 spacecraft in the outer heliosphere, with regard to radial and latitudinal gradients in the field components and magnitude. At first sight the field data appear to conform fairly well with the predictions of the Parker model. However, when the data obtained by Pioneer are compared with data from earth orbiting satellites, it is found that time variations are significantly affecting the field throughout the low‐latitude heliosphere. Removing these time variations allows a significant departure from the Parker model to emerge. The field magnitude and directions are found to be consistent with meridional flux transport from the equatorial plane towards higher heliographic latitudes. Such meridional transport could be driven by higher pressure near the solar equator. Enhanced pressure would be expected on the basis of the increased intensity of the spiral field in that region. However, another possible source of increased pressure is associated with the heating that occurs in compressive solar wind interaction regions and the large field magnitudes that occur in these regions both of which may be enhanced at low latitudes at least at the radial distances sampled thus far. Finally, evidence is presented which suggests a solar cycle dependence in the degree of meridional flux transport.