  We present broadband ionospheric scintillation observations of highly defined
symmetric quasi-periodic oscillations (QPO: Maruyama 1991) caused by plasma
structures in the midlatitude ionosphere using the LOw Frequency ARray (LOFAR:
van Haarlem et al., 2013). Two case studies are shown, one from 15th December
2016, and one from 30th January 2018, in which well-defined main signal fades
and secondary diffraction fringing are observed. In particular, the broadband
observing capabilities of LOFAR permit us to see considerable frequency
dependent behaviour in the QPOs which, to our knowledge, is a new result. We
extract some of the clearest examples of scintillation arcs reported in an
ionospheric context, from delay-Doppler spectral analysis of these two events.
These arcs permit the extraction of propagation velocities for the plasma
structures causing the QPOs ranging from 50 - 200 ms$^{-1}$, depending on the
assumed altitude. The spacing between the individual plasma structures ranges
between 5 - 20 km. The periodicities of the main signal fades in each event
and, in the case of the 2018 data, co-temporal ionosonde data, suggest the
propagation of the plasma structures causing the QPOs is in the E-region. Each
of the two events is accurately reproduced using a Gaussian perturbation phase
screen model. Individual signal fades and enhancements were modelled using
small variations in total electron content (TEC) amplitudes of order 1 mTECu,
demonstrating the sensitivity of LOFAR to very small fluctuations in
ionospheric plasma density. To our knowledge these results are among the most
detailed observations and modelling of QPOs in the literature.
