Probing Coronal Mass Ejections with Faraday Rotation Measurements

Probing Coronal Mass Ejections with Faraday Rotation Measurements Steven R. Spangler and Catherine A. Whiting University of Iowa

Importance of CMEs • Associated with destabilization of coronal loops • Basic physics questions remain as to how this occurs • CMEs are a rare case of an astronomical phenomenon with practical importance

Measuring Magnetic Fields in CMEs Radioastronomical polarization observations at GHz frequencies, and solar elongations of 1 -3 degrees

Literature on Faraday Rotation Observations of Coronal Mass Ejections • M. Bird et al 1985, Solar Physics 98, 341 • Y. Lui et al 2007, Ap. J 665, 1439 (model predictions for theoretical CMEs) • Ingleby, Spangler, Whiting 2007, Ap. J 668, 520

Coronal Faraday rotation measurements with the VLA • Simultaneous L band measurements at 1465 and 1665 MHz • Easy measurements of RM ~ 1 rad/m 2 • Imaging extended radio sources allows measurements of “differential Faraday rotation”

VLA observations of CME-associated, Faraday Rotation anomaly on March 12, 2005 Constellation of radio sources LASCO coronagraph image during observing session

Progress of the March 12, 2005 CME 22: 36 UT 23: 12 UT Position of CME relative to radio sources

Polarization position angle time series for 2337 -025 and 2335 -015 1465 MHz 2337 -025 C 2 frames

Rotation measure transient • No change for 2335 -015 • D(RM)=10 rad/m 2 (25 deg pa at 1465 MHz) • D(RM) apparently started before occultation of source by outer loop

Model predictions for CME-Faraday Rotation at 10 solar radii Prediction of 9 rad/sqm At greater solar elongation, so in the ballpark Fig. 6. — Mapping of the rotation measure corresponding to the four configurations of a flux rope onto the sky. The color shading indicates the value of the rotation measure. The arrows show the directions of the azimuthal and axial magnetic fields, from which a left‐handed (LH) or right‐handed (RH) helicity is apparent. Each configuration of the flux rope has a distinct rotation measure pattern.

Future radioastronomical CME studies • VLA/LASCO C 2 CME patrol (exploiting dynamic scheduling • Mileura Widefield Array (80 -300 MHz) could see CMEs at great heliocentric distances. Issue is polarization of extragalactic sources at these frequencies