Maria Rioja (CASS/ICRAR-UWA)

Maria Rioja colloquium: Ionospheric Calibration Schemes for m- to cm-wavelengths

Abstract

will discuss two new calibration methods for the low frequency domain, from meters to cm-wavelengths, that I have been developing. Both address the fundamental problem arising from the ionospheric propagation effects; that is, the spatial (i.e. direction dependent) and temporal fluctuations imposed on the phase observable, which degrade the image quality and ultimately will limit the achievable science.

The next generation of low frequency radio interferometers, such as SKA-Low, which have high sensitivities and extremely wide FoV, represent the most challenging case. For these metre wavelengths we have developed a highly parallel calibration method that shrinks the array response to a tightly beamed direction and therefore allows independent processing across the FoV. We call this method Low-frequency Excision of the Atmosphere in Parallel (LEAP) to emphasize the fundamental difference from the traditional PEELing approach. LEAP allows the measurement and characterisation of the ionospheric distortions across the array, and I will show examples of this using observations with MWA and LOFAR pathfinders.

For longer baselines and at cm-wavelengths we have developed MultiView calibration. In this case, we approximate the local atmospheres as planar "wedges" and use multiple calibrators to solve for the atmospheric contributions in the exact direction of the target of interest. This has been designed for VLBI observations with e.g. ASKAP, where the multiple beams allow for simultaneous observations in multiple directions. MultiView is the driver for the design requirements for SKA, in the sense that these must provide a minimum of 4 VLBI tied array beams. However, the demonstrations I will
present are from VLBA observations, using fast switching between the sources on the sky. Nevertheless MultiView has been very successful in mitigating the ionospheric distortions and achieving high astrometric accuracy, even surpassing in-beam astrometry. This will reopen the field of parallax measurements of OH-masers, which has been in abeyance for a decade.