Ionospheric Response to the X9.3 Flare on 6 September 2017 and Its Implication for Navigation Services Over Europe

On 6 September 2017, an X‐class flare of the magnitude 9.3 occurred around noon UT, being the strongest flare event in a decade. The flare itself was the highlight of a quite interesting phase of solar‐terrestrial interactions caused by the active region known as the Catania sunspot group 46 or active region number 2673 on the NOAA catalog. From 3 to 13 September 2017 strong flare activities occurred, accompanied by a number of radio bursts and earthward‐directed coronal mass ejections. Solar wind influences at Earth were modest during the flare activity and limited to the polar regions (Linty et al., 2018, https://doi.org/10.1029/2018SW001940). But, the strong X9.3 flare itself had impacts on the dayside ionosphere causing some problems in navigation services as we present within this paper. The event data observed and analyzed give us the opportunity to improve our understanding of strong and extreme space weather events and allow us to distinguish between the influence of the different event classes on our technological infrastructure within periods of strong solar activity. Here we will discuss our observations with special focus on the X9.3 flare event and provide examples how the flare itself influenced services in the domains of aviation and maritime navigation in the European sector. Key Points Solar flare events can have a significant impact on GNSS positioning The critical effects are based on rapid changes of the dayside ionosphere during the peak phase of the flare event The knowledge of the flare spectrum is very important in order to estimate its significance for GNSS users