In Situ Observations of Corotating Rarefaction Regions with STEREO

It has been observed that the in situ magnetic fields of corotating rarefaction regions (CRRs) are underwound (i.e. closer to radial) compared to the nominal Parker spiral angle. One suggested explanation for the underwinding is footpoint motion of the solar wind source region. Schwadron (2002) presents a model that can be used to calculate the angular speed of the footpoint motion using in situ observations of magnetic field components and solar wind speed. This model is applied to 12 CRRs observed by both STEREO observatories from March through May 2007, when temporal and spatial separation between the spacecraft is minimal. We calculate the effective angular speed of the source region (solar rotation plus footpoint motion) at both STEREO-A and STEREO-B using the Schwadron (2002) model. An alternative geometric technique (possible with multiple in situ observations of the same CRR) is also used to calculate the effective angular speed of the source region. We find the three effective angular speeds calculated for each CRR generally do not agree with one another, suggesting CRRs have not reached an equilibrium configuration by 1 AU.