Fundamental accuracy limits of ionospheric characterization by radio interferometers

The ionosphere has a profound effect on the propagation of radio waves. This will blur radio astronomical images, in particular at low radio frequencies (< 300 MHz), if the ionospheric layer above an interferometric array is not properly characterized and its effects compensated during imaging. Radio interferometers are insensitive to common propagation delays but can determine differential delays very accurately using self-calibration. An astronomical imaging interferometer is therefore a very accurate probe for TEC differences in the ionosphere over distances defined by the extent of the array. In the self-calibration process, readily available astronomical sources are used for calibration and define the directions of ionosphere patches. Since we cannot control the strength of these sources while the ionosphere variability dictates the calibration update rate, the SNR and source configuration may limit our accuracy. In this presentation, I provide a quantitative discussion of these limitations and discuss the implications for ionospheric calibration for the Low Frequency Array (LOFAR) and the Low Frequency Aperture Array (LFAA) system for the Square Kilometre Array (SKA).