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

Observations of the magnetic field orientation in co-rotating 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 30degrees 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.