Stochastic Faraday rotation

Different ray paths through a turbulent plasma can produce stochastic Farad ay rotation leading to depolarization of any linearly polarized component. Simple theory predicts that the average values of the Stokes parameters dec ay according to [Q] and [U] proportional to exp (-delta(l)), with delta(l) proportional to lambda(4). It is pointed out that a definitive test for suc h depolarization is provided by the fact that [Q(2) + U-2] remains constant while [Q](2) + [U](2) decreases proportional to exp (-2 delta(l)). The ave rages to which this effect, called polarization covariance, should apply ar e discussed; it should apply to spatial averages over a polarization map or temporal averages over a data set, but not to beamwidth and bandwidth aver ages that are intrinsic to the observation process. Observations of depolar ization would provide statistical information on fluctuations in the turbul ent plasma along the line of sight, specifically, the variance of the rotat ion measure. Other effects that can also cause depolarization are discussed . Favorable data sets to which the test could be applied are spatial averag es over polarization maps of the lobes of radio galaxies and temporal avera ges for the binary pulsar PSR B3.259-63 as it goes into and comes out of ec lipse by the wind of its companion.