Inferences About the Magnetic Field Structure of a CME with Both In Situ and Faraday Rotation Constraints

Abstract

On 2012 August 2, two coronal mass ejections (CMEs; CME-1 and CME-2) erupted from the west limb of the Sun as viewed from Earth, and were observed in images from the white-light coronagraphs on the SOlar and Heliospheric Observatory and Solar TErrestrial RElations Observatory (STEREO) spacecraft. These events were also observed by the Very Large Array (VLA), which was monitoring the Sun at radio wavelengths, allowing time-dependent Faraday rotation observations to be made of both events. We use the white-light imaging and radio data to model the 3D field geometry of both CMEs, assuming a magnetic flux rope geometry. For CME-2, we also consider 1 au in situ field measurements in the analysis, as this CME hits STEREO-A on August 6, making this the first CME with observational constraints from stereoscopic coronal imaging, radio Faraday rotation, and in situ plasma measurements combined. The imaging and in situ observations of CME-2 provide two clear predictions for the radio data: VLA should observe positive rotation measures (RMs) when the radio line of sight first encounters the CME, and the sign should reverse to negative within two hours. The initial positive RMs are in fact observed. The expected sign reversal is not, but the VLA data unfortunately end too soon to be sure of the significance of this discrepancy. We interpret an RM increase prior to the expected occultation time of the CME as a signature of a sheath region of deflected field ahead of the CME itself.