Towards the possibility to combine LOFAR and GNSS measurements to sense ionospheric irregularities

Inhomogeneities within the ionospheric plasma density affect trans-ionospheric radio signals, causing radio wave scintillation in the amplitude and phase of the signals. The amount of scintillation induced by ionospheric irregularities typically decreases with the radio wave frequency. As the ionosphere affects a variety of technological systems (e.g., civil aviation, financial operations) as well as low-frequency radio astronomy observations, it is important to detect and monitor ionospheric effects with higher accuracy than currently available. Here, a novel methodology for the detection and characterization of ionospheric irregularities is established on the basis of LOFAR (Low-Frequency Array) scintillation measurements at Very High Frequency (VHF) that take into account the lack of ergodicity in the intensity fluctuations induced by scintillation. The methodology estimates the S₄ scintillation index originating from irregularities with spatial scales in the inertial sub-range of electron density fluctuations in the ionosphere. The methodology is illustrated by means of observations that were collected through the Polish LOFAR stations located in Bałdy, Borowiec and Łazy: its validation was carried out by comparing LOFAR VHF scintillation observations with independent Global Navigation Satellite Systems (GNSS) observations that were collected through a high-rate receiver located near the LOFAR station in Bałdy as well as through geodetic receivers from the Polish ASG-EUPOS network. Two case studies are presented: 31 March 2017 and 28 September 2017. The comparison between LOFAR S₄ observations and independent ionospheric measurements of both scintillation and rate of change of Total Electron Content (TEC) from GNSS reveals that the sensitivity of LOFAR and GNSS to ionospheric structures is different as a consequence of the frequency dependency of radio wave scintillation. Furthermore, it can be noticed that observations of LOFAR VHF scintillation can be utilised to detect plasma structures forming in the mid-latitude ionosphere, including electron density gradients occurring over spatial scales that are not necessarily detected through traditional GNSS measurements: the detection of all spatial scales is important for correct monitoring and modelling of ionospheric processes. Hence, the different sensitivity of LOFAR to ionospheric structures, in addition to traditional GNSS ionospheric measurements, allows us to expand the knowledge of ionospheric processes.