Investigation of the Nighttime MSTIDs' Source Based on LOFAR Calibration Phase Solutions

The LOw Frequency ARray (LOFAR) is a radio interferometer located primarily in the Netherlands, with a number of international stations. Dutch stations are divided into dense core (stations separated by few tens of metres to 3 kilometers) and remote stations (up to 80 kilometers). The layout of the interferometer and high sensitivity due to proximity of its operating frequency to the ionospheric plasma frequency (LOFAR operates in the frequency range between 30-240 MHz) make it an excellent tool to study small- and medium-scale ionospheric plasma density disturbances, including Medium-scale Travelling Ionospheric Disturbances (MSTIDs). MSTIDs are prominent wave-like disturbance patterns observable in the ionosphere at mid-latitudes. In this study we investigate a high-latitude forcing as a source mechanism for their formation. We use calibration solutions of the Epoch of Reionization project obtained for Dutch stations. Our dataset includes 25 nights of the winter season 2012/2013, all of them characterized by Kp index value below 4 for the timespan of observation, which was typically 8 hours with few cases of 6 hours. As an output of calibration processing we obtain phase solutions referenced to the station in the center of the LOFAR field, which are then converted into differential Total Electron Content (dTEC) values at the 150 MHz central frequency with 10 seconds time resolution. We detect wave-like disturbances occurance by using a wavelet transform and calculate their propagation direction. We compare the results with keograms recorded at the Sodankyla observatory, global geomagnetic indices, as well as geomagnetic field changes from the 3 nearest magnetometers.