LOFAR is the LOw Frequency Radio interferometer ARray located at mid-latitude (52∘53′N). Here, we present results on ionospheric structures derived from 29 LOFAR nighttime observations during the winters of 2012/2013 and 2013/2014. We show that LOFAR is able to determine differential ionospheric TEC values with an accuracy better than 1 mTECU over distances ranging between 1 and 100 km. For all observations the power law behavior of the phase structure function is confirmed over a long range of baseline lengths, between 1 and 80 km, with a slope that is in general larger than the
Nearby in the stack
5/3
expected for pure Kolmogorov turbulence. The measured average slope is
1.89
with a one standard deviation spread of
0.1
. The diffractive scale, i.e. the length scale where the phase variance is
1rad2
, is shown to be an easily obtained single number that represents the ionospheric quality of a radio interferometric observation. A small diffractive scale is equivalent to high phase variability over the field of view as well as a short time coherence of the signal, which limits calibration and imaging quality. For the studied observations the diffractive scales at
150
MHz vary between
3.5
and
30
km. A diffractive scale above
5
km, pertinent to about
90%
of the observations, is considered sufficient for the high dynamic range imaging needed for the LOFAR Epoch of Reionization project. For most nights the ionospheric irregularities were anisotropic, with the structures being aligned with the Earth magnetic field in about