Correcting Ionospheric Faraday Rotation for the VLA and MeerKAT

TL;DR

This study evaluates methods to correct Ionospheric Faraday Rotation in radio telescopes, finding that using GNSS data with ALBUS significantly improves accuracy over traditional models, and also establishes EVPA standards for calibration.

Contribution

The paper introduces the ALBUS software for more accurate IFRM estimation using GNSS data, surpassing traditional thin-shell models, and provides EVPA calibration standards across a broad frequency range.

Findings

Traditional models overestimate IFRM by 0.5 to 1.1 rad/m^2 for VLA.

ALBUS achieves IFRM accuracy of 0.1 rad/m^2 for both VLA and MeerKAT.

Established EVPA standards for 3C286 and 3C138 from 500 MHz to 50 GHz.

Abstract

We report here on studies to determine the accuracy of estimated corrections of Ionospheric Faraday Rotation Measure (IFRM) using observations of the Moon with the Very Large Array (VLA) and MeerKAT telescopes. To estimate the IFRM requires an estimate of the total electron content along the line-of-sight to the observed sources (the so-called Slant Total Electron Content, or STEC). Estimating the STEC requires an estimate of the global 2-D map of Vertical Total Electron Content (VTEC) along with the ray path from the telescope to the source. Traditionally, these global VTEC maps have been utilized along with an assumption that the electrons are in a thin shell at a given altitude to provide an estimate of the IFRM as a function of time. We find that this traditional technique significantly overestimates the IFRM - typically by 0.5 to 1.1 rad/m^2 for the VLA, and -0.3 rad/m^2 for MeerKAT. Alternatively, the software package ALBUS utilizes raw data from nearby Global Navigation Satellite System (GNSS) stations, to generate a local estimate of the IFRM as a function of time. ALBUS provides considerably better estimates of the IFRM - accurate to 0.1 rad/m^2 for both the VLA and MeerKAT, provided the stations utilized have known receiver bias values. A byproduct of our study is the establishment of the intrinsic electric vector position angle (EVPA) of the standard polarized calibrators 3C286 and 3C138 from 500 MHz to 50 GHz, using additional VLA observations of the Moon, Venus, and Mars.