Probing ionospheric structures using the LOFAR radio telescope

Abstract

LOFAR is the LOw‐Frequency Radio interferometer ARray located at midlatitude (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 total electron content values with an accuracy better than 0.001 total electron content unit = 1016m−2 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 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 60% of the observations.

The LOFAR Two-metre Sky Survey

“…It will also require further work on the calibration and understanding of ionospheric effects, which is currently under way (e.g. Mevius et al 2016 …”

“…This complex direction-dependent calibration procedure, which corrects for the varying ionospheric conditions (e.g. Mevius et al 2016) and errors in the beam models, is crucial to create high-fidelity images at full resolution and sensitivity. Several approaches are being developed to minimise these 1 https://science.nrao.edu/science/surveys/vlass 2 http://www.ing.iac.es/weave/weavelofar/ direction-dependent effects (e.g.…”

H I 21‐cm Cosmology and the Bispectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays

“…A real interferometer will introduce both thermal noise and complex gain terms, G i (amplitude and phase terms due to the instrument response; note that this response includes path length differences due to the ionosphere above each antenna; Mevius et al, ), which will alter the sky visibility, resulting in a measured quantity, : where is the true sky visibility amplitude, is the sky visibility phase, and θ i is the phase introduced to the visibility by the antenna electronics, optics, or ionosphere, N i , j is the noise added to the visibility, is the effective sky visibility (the true sky visibility set by the sky intensity distribution multiplied by the primary beam power pattern ), and a i is the gain amplitude of the antenna plus electronics. This assumes that the complex gain on a given visibility is separable into antenna‐based terms.…”

Constraining the intergalactic medium at z ≈ 9.1 using LOFAR Epoch of Reionization observations

“…This then requires exquisite calibration of the system as any left-over artefacts from strong sources will make a measurement impossible (Barry et al 2016;Patil et al 2017). This implies calibrating the many hardware components of the telescope (see e.g., Kern et al 2019) while a further complication is added by the presence of the temporally and spatially varying ionosphere (see e.g., Mevius et al 2016).…”