Precision correction of radioastronomical polarization measurements for ionospheric faraday rotation

A widely used model for ionospheric faraday rotation is the model contained within the AIPS package. This model is based on a phenomenological model by Y.T. Chiu and reduces the electron density profile of the ionosphere into a slab of constant height, thickness and density near the observing site. The height of the slab is taken to be the height of the F2 layer. The rotation measure due to the ionosphere is then calculated using a simple dipole for the Earth's magnetic field. The accuracy of the Chiu model is claimed to be 20 percent but it is also noted that the model performs poorly during the daytime around solar maximum. Hence, a model that reproduces the electron density profile and is capable of performing well at solar maximum is desirable. The International Reference Ionosphere (IRI) meets these criteria. Comparison of these two models against measured values of the total electron content (TEC) have been carried out at high and low solar activity with sunspot number being the only geophysical parameter entered into the models. At low solar activity the models essentially have the same distribution around the real TEC values with an error of about 35 percent of themore » observed TEC value. At high solar activity, however, the IRI model is much better than the AIPS model, with an error of about 20 percent compared to an error of about 30 percent for AIPS. The error can be reduced further, to about 15 percent, in the IRI model by specifying the peak plasma frequency of the F2 layer. These errors in the value of the TEC are expected to be approximately the same as those in the rotation measure due to the ionosphere. In calculating the rotation measure in the ionosphere, there can be differences as great as 2 to 3 radians/m[sup 2] between the two models.« less