LOFAR observations of refractive scattering from substructure within a
traveling ionospheric disturbance at mid-latitude.
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.