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.
Ionization of the Earth's atmosphere by sunlight forms a complex, multi-layered plasma environment within the Earth's magnetosphere, the innermost layers being the ionosphere and plasmasphere. The ...plasmasphere is believed to be embedded with cylindrical density structures (ducts) aligned along the Earth's magnetic field, but direct evidence for these remains scarce. Here we report the first direct wide-angle observation of an extensive array of field-aligned ducts bridging the upper ionosphere and inner plasmasphere, using a novel ground-based imaging technique. We establish their heights and motions by feature-tracking and parallax analysis. The structures are strikingly organized, appearing as regularly-spaced, alternating tubes of overdensities and underdensities strongly aligned with the Earth's magnetic field. These findings represent the first direct visual evidence for the existence of such structures.
Low-frequency, wide field-of-view (FoV) radio telescopes such as the Murchison Widefield Array (MWA) enable the ionosphere to be sampled at high spatial completeness. We present the results of the ...first power spectrum analysis of ionospheric fluctuations in MWA data, where we examined the position offsets of radio sources appearing in two datasets. The refractive shifts in the positions of celestial sources are proportional to spatial gradients in the electron column density transverse to the line of sight. These can be used to probe plasma structures and waves in the ionosphere. The regional (10-100 km) scales probed by the MWA, determined by the size of its FoV and the spatial density of radio sources (typically thousands in a single FoV), complement the global (100-1000 km) scales of GPS studies and local (0.01-1 km) scales of radar scattering measurements. Our data exhibit a range of complex structures and waves. Some fluctuations have the characteristics of travelling ionospheric disturbances (TIDs), while others take the form of narrow, slowly-drifting bands aligned along the Earth's magnetic field.
PDF