Inferring Magnetic Field Structure of Flux Rope CMEs from STEREO Imaging and In Situ Observations

In its four year lifetime, STEREO has observed many CMEs that have an appearance in white light images that can be interpreted as being indicative of a magnetic flux rope. Some of these CMEs have also been observed in situ at 1 AU, after hitting one of the STEREO spacecraft, or the Wind and ACE spacecraft closer to Earth. We atttempt to reconstruct the magnetic structure of flux rope CMEs considering both white light imaging and in situ measurements. We use STEREO imaging both close to the Sun and in the interplanetary medium to infer the flux rope's size, trajectory, and orientation. This analysis establishes the path of whatever spacecraft is hit by the CME through the flux rope. We then insert a model magnetic field into the reconstructed flux rope using a force-free approximation and see if we can find a combination of field parameters that can reasonably fit the in situ measurements of field strength and orientation observed at the spacecraft's location. Events modeled include a well studied CME from 2008 June 1, which yielded a particularly clean flux rope in situ signature at STEREO-B when it arrived on June 6, and a more recent event from 2010 April 3, which produced the first significant geomagnetic storm of the new solar cycle on April 5.