Strongly underwound magnetic fields in co-rotating rarefaction regions: Observations and implications

Abstract

Observations of the magnetic field orientation in corotating rarefaction regions (CRRs) reveals that the field can be significantly more radial than predicted by the Parker model. In particular, CRRs sampled by the Ulysses spacecraft beyond 4 AU from the sun often show average field orientations deviating by more than 30° from the expected Archimedian spiral and lasting many days. These observations are explained by a model combining footpoint motion between fast and slow solar wind streams at the source surface with the effects of velocity shear across coronal hole boundaries.