Waves in the sky: Probing the ionosphere with the Murchison widefield array

Abstract

Summary form only given. Low-frequency, wide-field radio telescopes such as the Murchison Widefield Array (MWA) enable the dense spatial sampling of the ionosphere and plasmasphere on regional scales. For a physically compact array such as the MWA, the refractive shifts in the positions of celestial sources in the synthesised radio images are proportional to spatial gradients in the total electron content (TEC) transverse to the line of sight. By measuring the angular position shifts of celestial radio sources, one can probe waves and disturbances in the intervening plasma. Radio telescopes differ fundamentally from other techniques for measuring plasma fluctuations in that they are sensitive to TEC gradients/differences rather than absolute TEC. This makes them sensitive specifically to fluctuations about the ambient density, and therefore powerful probes of plasma density waves and irregularities. The authors present the results of an analysis of plasma fluctuations detected by the MWA, which can measure TEC gradients to a precision of ~1 mTECU/km at observing frequencies of ~150MHz. Around 2000-3000 point sources are visible instantaneously to the MWA, each functioning as a measurement point for the TEC gradient across the field-of-view (FoV). The spatial sampling completeness achieved by the MWA is unparalleled among interferometer observations of the ionosphere/plasmasphere to date, which have been limited both to smaller fields of view and at most several tens of measurement points (e.g. J. F. Helmboldt, W. M. Lane & W. D. Cotton, 2012, Radio Sci., 47, RS5008). This ~100-fold improvement in sampling completeness has permitted the first detailed imaging of the near-Earth plasma by a radio telescope.