Coronal Faraday rotation of occulted radio signals.

Faraday rotation (FR) observations of radio sources near solar conjunction yield information on the coronal magnetic field at heliospheric distances not reached by in situ exploration. Measurements of FR yield the rotation measure (RM), a wavelength independent quantity defined as the integral along the raypath of the product of the electron density times the raypath-parallel component of the magnetic field. Independent observations or models of the coronal electron density are required in order to extract information about the magnetic field. The radio sounding sources can be either artificial (spacecraft) or natural, but they must be at least partially linearly polarized. The most extensive campaign of coronal radio sounding polarization measurements using a spacecraft was the Helios Faraday Rotation Experiment, which was conducted over the duration of the Helios 1 (1974-1984) and Helios 2 (1976-1980) missions. Other coronal FR experiments have been carried out using natural continuum sources recently at the VLA and even as early as 1962 at the Pulkovo Radio Telescope. Pulsars were exploited to measure coronal RM at the MPIfR Effelsberg 100-m Telescope. Extending these single raypath observations, an ambitious project has been proposed to use the future LOFAR facility for constructing a coronal RM 'image'. Different time scales of FR variations are related to different physical phenomena. Among the observed effects are: (a) slow variations associated with the changing geometry and rotation of the corona; (b) random oscillations probably arising from a rich spectrum of coronal Alfvén waves; (c) rapid changes in RM caused by transient events such as coronal mass ejections (CMEs). [ABSTRACT FROM AUTHOR]