Direction of ionospheric structures in LOFAR calibration data

<pre>The Low Frequency Array (LOFAR) interferometer is a radio telescope network that provides the radio astronomical observations with the highest up-to-date sensitivity in the frequency regime between 10 and 240 MHz. As these frequencies approach the ionospheric plasma frequency, ionospheric perturbation of propagating electromagnetic signal is the main environmental factor affecting the quality of observations. Removal of ionospheric influence is a part of routinely conducted data calibration, resulting in high sensitivity differential Total Electron Content (dTEC) values between LOFAR stations. In this study we present a method for medium scale ionospheric structures detection applied to interferometric data obtained from calibration solutions of one of the key LOFAR projects- the Epoch of Reionization. Each observation spans 110-250 MHz of frequency range and lasts 6-8 hours during winter nighttime. Due to operating frequency and sensitivity of interferometric data, studies conducted with LOFAR can complement GNSS research with medium scale structures.</pre>