Terrestrial drivers of rapidly changing plasma structures observed with the International LOFAR Telescope

The Low Frequency Array (LOFAR) is designed to observe the early universe at radio wavelengths. When radio waves from a distance astronomical source traverse the ionosphere, structures in this plasma affect this signal. The high temporal resolution available (~100 ms), the large range of frequencies observed (10-80 MHz & 120-240 MHz) and the large number of receiving stations (currently 52 across Europe) mean that LOFAR can observe the effects of the midlatitude ionosphere in a level of detail never seen before.
On the 14th July 2018 LOFAR stations across the Netherlands observed Cygnus A between 17:00 UT and 18:00 UT. At approximately 17:40 UT a deep fade in the intensity of the received signal was observed, lasting some 15 minutes. Immediately before and after this deep fade rapid variations of signal strength were observed, lasting less than five minutes. This structure was observed by multiple receiving stations across the Netherlands. It evolved in time and in space. It also exhibited frequency dependent behaviour.
The geomagnetic conditions at the time of the observation were quiet, as were the solar conditions. It is suggested that this structure is driven by a source within the Earth system. Observations from lower in the atmosphere are used to identify possible drivers.