Density duct formation in the wake of a travelling ionospheric disturbance: Murchison Widefield Array observations

Abstract

Geomagnetically aligned density structures with a range of sizes exist in the near‐Earth plasma environment, including 10–100 km wide VLF/HF wave‐ducting structures. Their small diameters and modest density enhancements make them difficult to observe, and there is limited evidence for any of the several formation mechanisms proposed to date. We present a case study of an event on 26 August 2014 where a travelling ionospheric disturbance (TID) shortly precedes the formation of a complex collection of field‐aligned ducts, using data obtained by the Murchison Widefield Array (MWA) radio telescope. Their spatiotemporal proximity leads us to suggest a causal interpretation. Geomagnetic conditions were quiet at the time, and no obvious triggers were noted. Growth of the structures proceeds rapidly, within 0.5 h of the passage of the TID, attaining their peak prominence 1–2 h later and persisting for several more hours until observations ended at local dawn. Analyses of the next 2 days show field‐aligned structures to be preferentially detectable under quiet rather than active geomagnetic conditions. We used a raster scanning strategy facilitated by the speed of electronic beamforming to expand the quasi‐instantaneous field of view of the MWA by a factor of 3. These observations represent the broadest angular coverage of the ionosphere by a radio telescope to date.

Assessment of Ionospheric Activity Tolerances for Epoch of Reionization Science with the Murchison Widefield Array

“…Of the two primary Types of ionospheric activity identified at the Murchison Radioastronomy Observatory in MWA EoR datasets (Type I: isotropic small-scale turbulence, and Type IV: turbulent, anisotropic structure) and in other MWA datasets (Loi et al 2016;Hurley-Walker et al 2017;Rioja et al 2018), the scale of the turbulence is such that it should not affect EoR power spectra (where power is principally on larger scales), while anisotropic behaviour (observed to occur in 8% of observations) imprints residual foreground power that can affect parameter estimation. We therefore conclude that data with anisotropic ionospheric activity should be omitted from deep EoR observations, and data showing turbulent behaviour should be monitored to ensure the spatial scales are not of relevance to the EoR.…”

“…The analysis of these data resulted in the development of a metric to quantitatively describe the degree of ionospheric activity, and identification and classification of four distinct ionospheric types. The metric has two independent components, which capture the two primary classes of activity observed: isotropic turbulent-like increases in the source position variations compared with thermal noise, and anisotropic structured waves of plasma density with a spatially periodic structure (cf, Loi et al 2016, who discovered these density ducts in MWA datasets). The relevant components of the metric for these quantify the median absolute source offset, relative to a catalogue position, and the anisotropy of position offset vectors (quantified via a Principal Component Analysis of the ensemble source vectors), respectively.…”

Ionospheric Detection of Explosive Events

“…With the development of methods to monitor several (~20–50) moderately bright sources over the relatively wide 74‐MHz field of view of the VLA, Cohen and Röttgering () and Helmboldt, Lane, and Cotton () were able to perform larger‐scale studies of both wavelike and turbulent ionospheric fluctuations. With the extremely wide field of view of the MWA, Loi, Murphy, et al (), Loi, Trott, et al (), and Loi et al () expanded upon these methods to monitor and track hundreds of cosmic radio sources at once, allowing for a kind of imaging of ionospheric disturbances. This led to some of the first direct evidence of high altitude (~600–700 km) plasma troughs in the topside ionosphere, which were directly imaged with the MWA (Loi, Murphy, et al, ; Loi et al, ).…”

“…With the extremely wide field of view of the MWA, Loi, Murphy, et al (), Loi, Trott, et al (), and Loi et al () expanded upon these methods to monitor and track hundreds of cosmic radio sources at once, allowing for a kind of imaging of ionospheric disturbances. This led to some of the first direct evidence of high altitude (~600–700 km) plasma troughs in the topside ionosphere, which were directly imaged with the MWA (Loi, Murphy, et al, ; Loi et al, ).…”

A new angle for probing field‐aligned irregularities with the Murchison Widefield Array

“…Acoustic-gravity waves (AGWs), which couple to charged species via collisions, are believed to be the source of energy driving the formation of smaller-scale density structures, for example, via the spatial resonance mechanism [Whitehead, 1971;Klostermeyer, 1978] or upon passage through a critical layer [Booker and Bretherton, 1967;Staquet and Sommeria, 2002], with further cascades possible through various plasma instabilities and nonlinear processes [Perkins, 1973;Fejer and Kelley, 1980;Maruyama, 1990;Nicolls and Kelley, 2005]. Observational evidence for the association between AGWs and FAIs was recently presented by Sun et al [2015] and Loi et al [2016].…”