Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1968
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1968ap%26ss...2..417z&link_type=abstract
Astrophysics and Space Science, Volume 2, Issue 4, pp.417-430
Astronomy and Astrophysics
Astrophysics
16
Scientific paper
The variation in the polarization of radiation propagating in a magnetoactive plasma due to the Faraday effect and differential absorption of ordinary and extraordinary waves is considered. This problem is especially important for polarization studies of the distributed cosmic emission, the radiation of discrete sources, etc. An Equation (1.10) describing the variation of the polarization tensor (1.2) (or (1.2a)) along the direction of propagation is formulated. This equation correctly accounts for the effect of absorption in distinction to the corresponding equation of Kawabata (1964). Equation (1.10), which was obtained for a homogeneous medium, is also true for an inhomogeneous plasma when the geometrical optics approximation is valid for the radiation, the difference between the refractions of ordinary and extraordinary waves is negligible, and inequalities (1.13) are satisfied. In this case, however, the tensorsS iq ,R iqlm , andK iqlm in (1.10) will depend on the coordinate. The case of quasi-longitudinal propagation for circularly polarized ordinary and extraordinary waves is treated in detail by means of (1.10). In this case, which is frequently realized in a cosmic plasma, the equations of transfer written in terms of the Stokes parameters (1.3) take the form of (2.3). Their solution for the case of a uniform plasma is obtained as (2.8) (2.10). From the analysis of these solutions it follows that, if absorption is neglected, the orientation of the polarization ellipse of the radiation emitted in a layer of thicknessz of a magnetoactive plasma varies according to (2.20), i.e. twice as slowly as the angle χ of radiation incident on the layer (see (2.15)). In the presence of absorption the polarization ellipse ceases to rotate at a distancez ≫ 1/(2kappa e,{}_0) from the beginning of the layer (K e, 0 is the amplitude of the absorption coefficient of the extraordinary wave). If the Faraday effect is not important (see (2.24)), the angle χ is close to the ellipse orientation of sources in the plasma χ S . For a strong Faraday effect (2.24a) the angle χ is displaced relative to χ S by ±π/4. The character of polarization of radiation in a plasma changes abruptly if the conditions for negative re-absorption are satisfied (K e, o<0). For strong amplification within a source of dimensionsL (when - 2kappa e,{}_0L ≫ 1) and a marked difference in re-absorption of ordinary and extraordinary waves(2|kappa e,kappa _0 |L ≫ 1), the radiation emitted by the source belongs entirely to one type of wave; the polarization of this radiation is completely defined by the polarization of waves of this type in a cosmic plasma and does not depend directly on the polarization of radiation emitted by individual electrons of this source. The latter circumstance is of great importance for a treatment of the polarization characteristics of radio emission from cosmic sources with negative re-absorption.
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