  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.
