A document by Gareth Dorrian. Click on the document to view its contents.

Radio interferometers used to make astronomical observations, such as the LOw Frequency ARray (LOFAR), experience distortions imposed upon the received signal due to the ionosphere as well as those from instrumental errors. Calibration using a well-characterised radio source can be used to mitigate these effects and produce more accurate images of astronomical sources, and the calibration process provides measurements of ionospheric conditions over a wide range of length scales. The basic ionospheric measurement this provides is differential Total Electron Content (TEC, the integral of electron density along the line of sight). Differential TEC measurements made using LOFAR have a precision of <1 mTECu and therefore enable investigation of ionospheric disturbances which may be undetectable to many other methods. We demonstrate an approach to identify ionospheric waves from these data using a wavelet transform and a simple plane wave model. The noise spectra are robustly characterised to provide uncertainty estimates for the fitted parameters. An example is shown in which this method identifies a wave with an amplitude an order of magnitude below those reported using GNSS TEC measurements. Artificially generated data are used to test the accuracy of the method and establish the range of wavelengths which can be detected using this method with LOFAR data. This technique will enable the use of a large and mostly unexplored dataset to study travelling ionospheric disturbances over Europe.

The large scale morphology and finer sub-structure within a slowly propagating traveling ionospheric disturbance (TID) are studied using wide band trans-ionospheric radio observations with the LOw Frequency ARray (LOFAR: van Haarlem, et al., 2013). The observations were made under geomagnetically quiet conditions, between 0400-0800 UT on 7 January 2019, over the UK. In combination with ionograms and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) anomaly data we estimate the TID velocity to ~65ms-1, in a NorthWesterly direction. Clearly defined substructures with oscillation periods of ~300 seconds were identified within the LOFAR observations of the TID, corresponding to scale sizes of ~ 20 km. At the geometries and observing wavelengths involved, the Fresnel scale is between 3-4 km, hence these substructures contribute significant refractive scattering to the received LOFAR signal. The refractive scattering is strongly coherent across the LOFAR bandwidth used here (25-64 MHz). The size of these structures distinguishes them from previously identified ionospheric scintillation with LOFAR in Fallows et al., 2020 where the scale sizes of the plasma structure varied from ~500 m – 5 km.