Rapidly changing plasma structures observed with the International LOFAR Telescope

Radio waves from distant astronomical sources can be affected by any plasma through which they propagate, and so the signal received by a ground-based observer contains information about the source, the interstellar medium, the solar wind, the magnetosphere and the terrestrial ionosphere. The Low Frequency Array (LOFAR) is designed to observe the early universe at radio wavelengths. However, when radio waves from a distance astronomical source traverse the terrestrial ionosphere, structures in this plasma affect this signal and cause variations in both the signal intensity and the phase. 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, and further opportunities will be provided by the SKA.
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 feature was observed by multiple receiving stations across The Netherlands at slightly different times, indicating that it was caused by a structure located in the terrestrial ionosphere. The structure evolved in time and in space and it also exhibited frequency dependent behaviour. It will be shown how this structure can be represented by a simple model and the potential to use such models to correct astronomical observations for ionospheric effects will be discussed.