In this work, we investigate various types of ionospheric disturbances observed over Europe after the earthquake in Turkey on 6 February 2023.By combining observations from Doppler sounding systems, ionosondes, and GNSS receivers, we are able to discern different types of disturbances, propagating with different velocities and through different mechanisms. We can detect the co-seismic disturbances produced in the ionosphere close to the epicenter, as well the ionospheric signatures of acoustic waves propagating as a consequence of propagating seismic waves.

The 15 January 2022 eruption of the Hunga volcano provides a unique opportunity to study the reaction of the ionosphere to large explosive events. In particular, this event allows us to study the global propagation of travelling ionospheric disturbances using various instruments. We focus on the detection of the ionospheric disturbances caused by this eruption over Europe, where dense networks of both ionosondes and GNSS receivers are available. This event took place on the day of a geomagnetic storm. We show how data from different instruments and from different observatories can be combined to clearly distinguish the TIDs produced by the eruption from those caused by concurrent geomagnetic activity. By comparing observations obtained from multiple types of instruments, we also show that TIDs produced by various mechanisms are present simultaneously, with different types of waves affecting different physical quantities.

To model ionospheric climate and to study its long-term changes we need solar activity proxies, because long and homogeneous data series of solar ionizing flux are not available. Here we use solar activity proxies/indices F10.7, sunspot numbers, F30, Mg II, He II and solar Lyman-α flux, and yearly average foF2 values from six midlatitude ionospheric stations from four continents (Juliusruh, Pruhonice, Roma, Boulder, Kokubunji and Canberra) over 1976-2014. Main results are as follows: (1) Relationships among solar activity indices/proxies differ between solar cycles 21 and 22 (represented by years 1976-1995) on the one hand, and cycles 23 and 24 (represented by years 1996-2014) on the other hand. (2) Consequently the relationships between midlatitude foF2 and solar activity proxies also differ except for F30. (3) The variability of yearly values of foF2 at middle latitudes appears to be described best by solar activity proxies F30 and Mg II.