Speaker
Description
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. The high temporal resolution available (~10 ms), the range of frequencies observed (10-90 MHz & 110-250 MHz) and the large number of receiving stations (currently 52 across Europe) mean that LOFAR can also observe the effects of the midlatitude and sub-auroral ionosphere at an unprecedented level of detail.
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 52.9o N; 6.9o E) from 28th June 2014 to 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 – 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 (51.9o – 56.5o N; 3.9o – 9.9o 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 upward propagating Atmospheric Gravity Waves (AGWs) launched by deep convection above thunderstorms could be a source of the ionospheric structures. Collectively, these observations suggest that LOFAR can be used to infer ionospheric signatures of vertical coupling processes in the mid-latitude atmosphere.
LOFAR is currently being upgraded to LOFAR 2.0, which will increase the sensitivity of the telescope, but it will also be more vulnerable to ionospheric variability. The Dynamic Ionospheric Notifications for Operations and Scheduling (DINOS) project is using ionospheric results to attempt to mitigate the effects upon LOFAR. These approaches include using ionosondes, magnetometers and HF Continuous Doppler Sounding Systems. A model based upon ionosonde observations is presented. Such a model could reduce the number of observations which later need to be discarded due to the ionospheric conditions, optimizing the usage of telescope time, and making the operations more sustainable by reducing the computational and storage resources required.
