  This paper presents the results from one of the first observations of
ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The
observation was of the strong natural radio source Cas A, taken overnight on
18-19 August 2013, and exhibited moderately strong scattering effects in
dynamic spectra of intensity received across an observing bandwidth of
10-80MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the
first hour of observation showed two discrete parabolic arcs, one with a steep
curvature and the other shallow, which can be used to provide estimates of the
distance to, and velocity of, the scattering plasma. A cross-correlation
analysis of data received by the dense array of stations in the LOFAR "core"
reveals two different velocities in the scintillation pattern: a primary
velocity of ~30m/s with a north-west to south-east direction, associated with
the steep parabolic arc and a scattering altitude in the F-region or higher,
and a secondary velocity of ~110m/s with a north-east to south-west direction,
associated with the shallow arc and a scattering altitude in the D-region.
Geomagnetic activity was low in the mid-latitudes at the time, but a weak
sub-storm at high latitudes reached its peak at the start of the observation.
An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data
from the time reveals a larger-scale travelling ionospheric disturbance (TID),
possibly the result of the high-latitude activity, travelling in the north-west
to south-east direction, and, simultaneously, a smaller--scale TID travelling
in a north-east to south-west direction, which could be associated with
atmospheric gravity wave activity. The LOFAR observation shows scattering from
both TIDs, at different altitudes and propagating in different directions. To
the best of our knowledge this is the first time that such a phenomenon has
been reported.
