Faraday Rotation

Faraday Rotation

Faraday Rotation (FR) is the apparent change in the observed plane of polarization of a linearly polarized electomagnetic wave on passage through a magnetized plasma. Using extragalactic radio sources as background polarized source, the observed variations in FR can be used to study the variations in the magnetized solar wind plasma.

This figure shows the quiescent solar wind. The plasma density is encoded in the colors, while the magnetic field direction is shown by the arrows. The line of sight (in red) from a distant radio source passes through this medium, and experiences Faraday rotation. The amount of rotation is large at MWA-LFD frequencies, and measurements can yield constraints on the magnetoionic properties of the plasma. By observing many independent lines of sight it will be possible to build up a 3-D representation.

In particular, the astronomers at Haystack Observatory intend to use FR observations to track Coronal Mass Ejections (CMEs) and determine their magnetic field geometry. The CME magnetic field is widely regarded as the holy grail in space weather, as the geo-effectiveness of a CME is entirely dependent on the orientation of its magnetic field. FR observations provide the only means of obtaining this information, apart from the in-situ sampling by spacecraft. Given MWA-LFD frequency and sensitivity, the FR technique will also be able to investigate the magnetic field topology of the corona out to about 40-50 solar radii.

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