Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

Faraday rotationmeasurements using the current and next generation of low-frequency radio telescopeswill provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotationcontribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotationfor a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron contentmaps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotationsusing LOFARpulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotationmeasures ever achieved. Precision rotationmeasures can be used to monitor rotationmeasure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionospherecan contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotationcalibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.