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.
@article{arxiv.1508.00965,
title = {Quantifying ionospheric effects on time-domain astrophysics with the Murchison Widefield Array},
author = {Shyeh Tjing Loi and Tara Murphy and Martin E. Bell and David L. Kaplan and Emil Lenc and Andre R. Offringa and Natasha Hurley-Walker and G. Bernardi and J. D. Bowman and F. Briggs and R. J. Cappallo and B. E. Corey and A. A. Deshpande and D. Emrich and B. M. Gaensler and R. Goeke and L. J. Greenhill and B. J. Hazelton and M. Johnston-Hollitt and J. C. Kasper and E. Kratzenberg and C. J. Lonsdale and M. J. Lynch and S. R. McWhirter and D. A. Mitchell and M. F. Morales and E. Morgan and D. Oberoi and S. M. Ord and T. Prabu and A. E. E. Rogers and A. Roshi and N. Udaya Shankar and K. S. Srivani and R. Subrahmanyan and S. J. Tingay and M. Waterson and R. B. Wayth and R. L. Webster and A. R. Whitney and A. Williams and C. L. Williams},
journal= {arXiv preprint arXiv:1508.00965},
year = {2015}
}