Driving the mid-latitude ionosphere from below: A statistical study using LOFAR

Kurzfassung

The Low Frequency Array (LOFAR) is one of the most advanced radio telescopes in the world. When radio waves from a distant astronomical source traverse the ionosphere, structures in this plasma affect the signal. Results are presented from a statistical study using 2,810 hours of observations of Cassiopeia A from a LOFAR station located in the Netherlands (station CS032, located at 52.9° N; 6.9° E) between 28th June 2014 and 27th November 2016. Ionospheric structures were identified in 469 (~17 %) of these observations. A comparison with proxies for geomagnetic activity (the Kp index) and solar activity (the F10.7 cm solar radio flux) showed that geomagnetic or solar effects were not the primary driver of these ionospheric structures. Ionospheric structures were more common in summer and between ~21 LT and 02 LT. These patterns in season and local time showed similarities to the occurrence of lightning strikes. When ionospheric structures were present, the mean number of lightning strikes in a spatial region close to the LOFAR observations (51.9° – 56.5° N; 3.9° – 9.9° E) two hours prior to the LOFAR observations was (70 ± 25) per hour. This was substantially larger than the mean value of (19 ± 5) per hour when the ionospheric structures were absent. This suggests that quasi-upward propagating Atmospheric Gravity Waves (AGWs) launched by thunderstorm activity could be one of the sources of the ionospheric structures. Collectively, these observations show that LOFAR can be used to infer ionospheric signatures of vertical coupling processes in the mid-latitude atmosphere. Such observations have the potential to be used to develop or validate existing Global Circulation Models (GCMs) or new models of vertical coupling within the terrestrial atmosphere.