  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 hr of the passage
of the TID, attaining their peak prominence 1-2 hr later and persisting for
several more hours until observations ended at local dawn. Analyses of the next
two 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 three. These
observations represent the broadest angular coverage of the ionosphere by a
radio telescope to date.
