Quantifying ionospheric effects on time-domain astrophysics with the Murchison Widefield Array

TL;DR

This study empirically evaluates how ionospheric refraction and scintillation affect radio source observations with the Murchison Widefield Array, finding minimal impact on time-domain astrophysics at 154 MHz.

Contribution

The paper provides the first detailed empirical assessment of ionospheric effects on MWA data, quantifying position shifts and amplitude variability under quiet geomagnetic conditions.

Findings

Ionospheric refraction causes small angular displacements of 10-20 arcsec.

Amplitude scintillation modulation index is limited to 1-3%.

Ionospheric effects are not a major obstacle for MWA time-domain science.

Abstract

Refraction and diffraction of incoming radio waves by the ionosphere induce time variability in the angular positions, peak amplitudes and shapes of radio sources, potentially complicating the automated cross-matching and identification of transient and variable radio sources. In this work, we empirically assess the effects of the ionosphere on data taken by the Murchison Widefield Array (MWA) radio telescope. We directly examine 51 hours of data observed over 10 nights under quiet geomagnetic conditions (global storm index Kp < 2), analysing the behaviour of short-timescale angular position and peak flux density variations of around ten thousand unresolved sources. We find that while much of the variation in angular position can be attributed to ionospheric refraction, the characteristic displacements (10-20 arcsec) at 154 MHz are small enough that search radii of 1-2 arcmin should be sufficient for cross-matching under typical conditions. By examining bulk trends in amplitude variability, we place upper limits on the modulation index associated with ionospheric scintillation of 1-3% for the various nights. For sources fainter than ~1 Jy, this variation is below the image noise at typical MWA sensitivities. Our results demonstrate that the ionosphere is not a significant impediment to the goals of time-domain science with the MWA at 154 MHz.