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# Propagation Through a Homogeneous Plasma

Even above the cutoff frequency there are various propagation effects that are important for a radio wave passing through a plasma. Let us start with the most straightforward ones. Consider a radio signal passing through a homogeneous slab of plasma of length L. The signal is delayed (with respect to the propagation time in the absence of the plasma) by the amount The magnitude of the propagation delay can hence be written as The propagation delay can also be considered as an excess path length'' = c . Further since ( and differ only in sign16.1, the magnitude of the excess phase'' (viz. ) is given by . Note that since the propagation delay is a function of frequency , waves of different frequencies get delayed by different amounts. A pulse of radiation incident at the far end of the slab will hence get smeared out on propagation through the slab; this is called dispersion''. If the plasma also has a magnetic field running through it then it becomes birefringent - the refractive index is different for right and left circularly polarized waves. A linearly polarized wave can be considered a superposition of left and right circularly polarized waves. On propagation through a magnetized plasma the right and left circularly polarized components are phase shifted by different amounts, or equivalently the plane of polarization of the linearly polarized component is rotated. This rotation of the plane of polarization on passage through a magnetized plasma is called Faraday rotation''. The angle through which the plane of polarization is rotated is given by and is called the rotation measure. For the second equality is in meters, is in cm , is in and the length is in parsecs.

#### Footnotes

... sign16.1
to first order for , as can be easily verified.    Next: Propagation Through a Smooth Up: Ionospheric effects in Radio Previous: Introduction   Contents
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