M. Mevius et al., "Probing ionospheric structures using the
LOFAR radio telescope," Radio Science, V51, no. 7, pp927941, July
2016.
abstract:
LOFAR is the LOw Frequency Radio interferometer ARray located at
midlatitude (52d30' 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 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 1 rad^{2}, 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 60% of
the observations.
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