Observations of large scale spatial gradients in the heliospheric magnetic field

Abstract

Magnetic field observations by the interplanetary probe Pioneer 11 are used to investigate large scale spatial gradients in the heliospheric magnetic field. The distance of Pioneer 11 ranges from 1 AU to 24 AU radially, and from −5° to +16° in heliocentric latitude, providing a view of a small but significant fraction of the three dimensional heliosphere. To remove the solar cycle variations the data are normalized using measurements obtained at 1 AU at the corresponding times. To first order, the observations agree with the Parker model for spherically symmetric, radial solar wind flow. However, a second order deficit in the magnitude and azimuthal component of the magnetic field has been confirmed. Specific issues are addressed which have arisen recently: an apparent absence of the deficit in the Voyager measurements, the possible influence on the deficit of time and/or latitude variations in the solar wind speed and the possible effect of latitude asymmetries in the magnetic field strength. This recent analysis supports the earlier conclusions that the deficit is correlated with radial distance and involves a divergence of magnetic flux away from the equatorial region.