Center for Astrophysics Research and Technologies Seminar Series
Measurements and Models of Ionospheric Faraday Rotation
Presented by Joe Malins (UNM)
The ionosphere imparts Faraday rotation (FR) to radio signals passing through it that can perturb the properties of the emission, and correction for the effect of the ionosphere is therefore important for deriving magnetic field information from FR observations of polarized cosmic radio sources. In addition, measurements of ionospheric FR can provide valuable diagnostics of the structure of the ionosphere. Measurements of ionospheric FR can be used test the accuracy of models commonly used to correct for its effects. Observations of pulsars at low frequencies provide higher-precision rotation measures (RM) than have been previously available. We test our ability to predict the incoming ionospheric contribution to RM using GPS receivers, which can measure the column density of electrons along the line of site from a receiver to the satellite, together with predictions using the profile provide by a modern digital ionosonde. We find that the ionospheric density profiles generated by current modern digisondes do not reproduce the measured FR, because they use models that fail to represent the unmeasured top-side electron density profile adequately. Modifications to the profiles that incorporate the local TEC measurements do a much better job of matching the observations. These modifications produce profiles similar to a thin-shell model, and we confirm that such a thin shell with a fixed height of 350 km performs equally well in reproducing the measured rotation measure, to within about 0.1 rad/m^2. We emphasize that local TEC measurements are essential for good reproduction of ionospheric FR: global ionospheric models interpolated to the location of the observatory perform poorly for this task.
2:00 pm, Thursday, September 21, 2017
PAIS-2540, PAIS
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