Determining the Magnetic Field Orientation of Coronal Mass Ejections from Faraday Rotation

We describe a method with which to measure the magnetic field orientation of coronal mass ejections (CMEs) using Faraday rotation (FR). Two basic FR profiles, Gaussian-shaped with a single polarity or N-shaped with polarity reversals, are produced by a radio source occulted by a moving flux rope, depending on its orientation. These curves are consistent with Helios observations, providing evidence for the flux rope geometry of CMEs. Many background radio sources can map CMEs in FR onto the sky. We demonstrate with a simple flux rope that the magnetic field orientation and helicity of the flux rope can be determined 2–3 days before it reaches the Earth, which is of crucial importance for space weather forecasting. An FR calculation based on global magnetohydrodynamic (MHD) simulations of CMEs in a background heliosphere shows that FR mapping can also resolve a CME geometry that curves back to the Sun. We discuss implementation of the method using data from the Mileura Widefield Array (MWA). Subject headingg s: magnetic fields — Sun: coronal mass ejections (CMEs) Online material: color figures