Understanding Instrumental Stokes Leakage in Murchison Widefield Array Polarimetry

TL;DR

This paper investigates the causes of instrumental Stokes leakage in the Murchison Widefield Array, proposing array theory models to understand and mitigate polarization errors caused by mutual coupling and element patterns.

Contribution

It introduces simple and rigorous electromagnetic models, including the full and average embedded element patterns, to explain and reduce instrumental polarization leakage in aperture arrays.

Findings

The simple model explains Q leakage due to current imbalance.

More rigorous models replicate observed leakage levels of 2-5%.

Upgrading the Jones matrix estimate can reduce leakage to a few percent.

Abstract

This paper offers an electromagnetic, more specifically array theory, perspective on understanding strong instrumental polarization effects for planar low-frequency "aperture arrays" with the Murchison Widefield Array (MWA) as an example. A long-standing issue that has been seen here is significant instrumental Stokes leakage after calibration, particularly in Stokes Q at high frequencies. A simple model that accounts for inter-element mutual coupling is presented which explains the prominence of Q leakage seen when the array is scanned away from zenith in the principal planes. On these planes, the model predicts current imbalance in the X (E-W) and Y (N-S) dipoles and hence the Q leakage. Although helpful in concept, we find that this model is inadequate to explain the full details of the observation data. This finding motivates further experimentation with more rigorous models that account for both mutual coupling and embedded element patterns. Two more rigorous models are discussed: the "full" and "average" embedded element patterns. The viability of the "full" model is demonstrated by simulating current MWA practice of using a Hertzian dipole model as a Jones matrix estimate. We find that these results replicate the observed Q leakage to approximately 2 to 5%. Finally, we offer more direct indication for the level of improvement expected from upgrading the Jones matrix estimate with more rigorous models. Using the "average" embedded pattern as an estimate for the "full" model, we find that Q leakage of a few percent is achievable.