An unusual observation of a plasma structure in the mid-latitude ionosphere

The LOFAR (Low Frequency Array) is one of the world’s leading radio telescopes, operating across the frequency band 10-250 MHz. As radio waves from astronomical sources pass through the ionosphere, they can undergo refraction and/or diffraction. The variations in the intensity of the received signal are caused by irregularities with a spatial scale size ranging from the Fresnel dimension to an order of magnitude below this value. The received signal can therefore be used to infer information on plasma structures in the ionosphere. As the frequencies used are significantly lower than the 1.4 GHz typically associated with Global Navigation Satellite Systems (GNSS), the plasma structures that affect the signals received by LOFAR are significantly larger, typically of the order of kilometres.

On 14^th July 2018 the Dutch stations of LOFAR observed the strong natural radio sources Cassiopeia A and Cygnus A between 17:00 UT and 18:05 UT at a frequency range of 20-80 MHz. During the observation, the signal intensity received by many of the stations underwent a substantial reduction across all frequencies, lasting approximately 10 minutes. Immediately before and after this, periodic enhancements in the signal strength were observed. These enhancements showed a noticeable frequency dependence, with longer period oscillations at lower frequencies. The feature was not observed simultaneously by the stations and evolved during the observations. Such a feature is most likely to be the result of a large-scale density structure in the ionosphere, which appears to move west and north over the northern Netherlands.

The deep fading of the received signal may be due to the presence of sporadic-E, which is a consequence of variations in the neutral wind speed with altitude in the presence of the geomagnetic field, resulting in plasma accumulating in a thin layer. This can cause incident radio waves to be strongly refracted, affecting the strength of the received signal. The wave-like structure immediately before and after the deep fade is a likely consequence of scattering of the observed signal.