  The turbulent ionosphere causes phase shifts to incoming radio waves on a
broad range of temporal and spatial scales. When an interferometer is not
sufficiently calibrated for the direction-dependent ionospheric effects, the
time-varying phase shifts can cause the signal to decorrelate. The ionosphere's
influence over various spatiotemporal scales introduces a baseline-dependent
effect on the interferometric array. We study the impact of baseline-dependent
decorrelation on high-redshift observations with the Low Frequency Array
(LOFAR). Datasets with a range of ionospheric corruptions are simulated using a
thin-screen ionosphere model, and calibrated using the state-of-the-art LOFAR
Epoch of Reionisation pipeline. For the first time ever, we show the
ionospheric impact on various stages of the calibration process including an
analysis of the transfer of gain errors from longer to shorter baselines using
realistic end-to-end simulations. We find that direction-dependent calibration
for source subtraction leaves excess power of up to two orders of magnitude
above the thermal noise at the largest spectral scales in the cylindrically
averaged auto-power spectrum under normal ionospheric conditions. However, we
demonstrate that this excess power can be removed through Gaussian process
regression, leaving no excess power above the ten per cent level for a $5~$km
diffractive scale. We conclude that ionospheric errors, in the absence of
interactions with other aggravating effects, do not constitute a dominant
component in the excess power observed in LOFAR Epoch of Reionisation
observations of the North Celestial Pole. Future work should therefore focus on
less spectrally smooth effects, such as beam modelling errors.
