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

Abstract

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