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

The large-scale temporal behavior and spatial gradients in the heliospheric magnetic field were investigated using magnetic field observations by Pioneer 11 and ISEE 3/ICE satellites, including observations out to a radial distance of 24 AU and over a time interval that includes the recent solar minimum. It was found that, to first order, the gradients in the magnetic field magnitude and in the azimutal component agree well with the Parker (1958) model. The analysis of the Pioneer data confirms previous reports of a magnetic field flux deficit of about 1 percent/AU, so that at 20 AU the relative deficit is of the order of 20 percent. It is argued that most of the magnetic field deficit is in the radial gradient.