Precision Meter-wavelength Polarimetry with the Jansky Very Large Array

Abstract:
Accurate imaging polarimetry at centimeter — meter wavelengths requires removal of the highly variable ionospheric Faraday rotation (IFRM) with an accuracy of ~0.1 rad/m^2 for 1 meter wavelength observations. Models to estimate the IFRM require estimates of the spatial and temporal vertical total electron content (VTEC) combined with global terrestrial magnetic field models. Over the past 25 years, estimates of the VTEC using GNSS timing data have been developed and are now readily available from a number of agencies.

Estimating the accuracy of these models requires observations of a source of known intrinsic electric vector position angle (EVPA) over a wide rage of ionospheric conditions. The only astronomical sources with a-priori known values of the EVPA are the terrestrial bodies Mars, Venus, and the Moon. Using eight VLA polarimetric observations of the Moon spanning ten years using the VLA’s P-band (295 — 445 MHz) receivers, and of the Moon and the planet Venus with the VLA’s L-band (1 — 2 GHz) we have tested the accuracy of seven global IONEX models produced by the AIPS program TECOR, and of the regional model derived by the stand-alone program ALBUS.

All seven global models overestimate the IFRM, typically by 0.5 to 1.0 rad/m² — a value which remains constant over many hours, including day — night transitions. The regional ‘ALBUS’ model, which utilizes GNSS ground station data within ~300 km of the VLA site, generates predictions which are always the closest — usually within 0.15 rad/m² — to the observed values. This is sufficiently accurate for the VLA’s P-band system, but not for longer wavelength polarimetric observations. We emphasize that these conclusions are valid for the VLA site — and may not be applicable to others.

As a byproduct of this program, we have determined the intrinsic EVPA values for the stable standard calibrator 3C286 for frequencies from 520 MHz through 48 GHz, (utilizing both VLA and MeerKAT data) and of three low frequency highly polarized sources — DA240, 3C303, and 3C345.

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