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

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 K p < 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.

AARTFAAC flux density calibration and Northern hemisphere catalogue at 60 MHz

“…Because we do this every second and monitor the variations, we are provided with an automatic assessment of ionospheric conditions, which is also used by LOFAR. These effects are much less in MWA at somewhat higher frequencies, see (Loi et al 2015c), indicating that even within the LOFAR low band the strength will vary significantly with frequency.…”

Limits on Polarized Leakage for the PAPER Epoch of Reionization Measurements at 126 and 164 MHz

“…In P14 and J15, we ignored the effects of the ionosphere, since for Stokes I these are largely changes in source position induced by refraction (for a recent study, see Loi et al 2015) and these are negligible on the large angular scales considered (Vedantham & Koopmans 2015a. However, both spatial and temporal fluctuations in the Faraday depth of the Earth's ionosphere will have a strong effect on polarized signal.…”

A Large-Scale, Low-Frequency Murchison Widefield Array Survey of Galactic H ii Regions between 260 < l < 340

“…At low frequency, the ionosphere generates slow astrometric changes in source position that can result in shifts on the order of 10-20 arcsec in 112 s GLEAM snapshots at 154 MHz (Loi et al 2015). In an effort to correct for these shifts in the apparent position of sources, we cross match the position of compact sources detected in each of the GLEAM snapshots with MRC sources to determine an average astrometric correction and update the headers in each snapshot accordingly.…”