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.

The ionospheric Nighttime Winter Anomaly (NWA) was first reported more than three decades ago based on total electron content (TEC) and vertical sounding data. The aim of this paper is to provide further evidence that the NWA effect is a persistent feature in the Northern Hemisphere at the American and in the Southern Hemisphere at the Asian longitude sector under low solar activity conditions. The analysis of ground‐based GPS derived TEC and peak electron density data from radio occultation measurements on Formosat‐3/COSMIC satellites confirms and further supports the findings published in earlier NWA papers. So it has been confirmed and further specified that the NWA appears at longitude sectors where the displacement between the geomagnetic and the geographic equator maximizes. Here NWA peaks at around 40°–50° geomagnetic midlatitude supporting the idea that wind‐induced plasma uplifting in the conjugated summer hemisphere is the main driving force for the accumulation of ionospheric plasma in the topside ionosphere and plasmasphere. In parallel, the midsummer nighttime anomaly (MSNA) is caused at the local ionosphere. Simultaneously, interhemispheric coupling causes severe downward plasma fluxes in the conjugated winter hemisphere during night causing the NWA at low solar activity. With increasing solar activity, the downward plasma fluxes lose their impact due to the much stronger increasing background ionization that masks the NWA. It is assumed that MSNA and related special anomalies such as the Weddell Sea Anomaly and the Okhotsk Sea Anomaly are closely related to the NWA via enhanced wind‐induced uplifting of the ionosphere.

-Space weather can strongly affect trans-ionospheric radio signals depending on the used frequency. In order to assess the strength of a space weather event from its origin at the sun towards its impact on the ionosphere a number of physical quantities need to be derived from scientific measurements. These are for example the Wolf number sunspot index, the solar flux density F10.7, measurements of the interplanetary magnetic field, the proton density, the solar wind speed, the dynamical pressure, the geomagnetic indices Auroral Electrojet, Kp, Ap and Dst as well as the Total Electron Content (TEC), the Rate of TEC, the scintillation indices S4 and s(') and the Along-Arc TEC Rate index index. All these quantities provide in combination with an additional classification an orientation in a physical complex environment. Hence, they are used for brief communication of a simplified but appropriate space situation awareness. However, space weather driven ionospheric phenomena can affect many customers in the communication and navigation domain, which are still served inadequately by the existing indices. We present a new robust index, that is able to properly characterize temporal and spatial ionospheric variations of small to medium scales. The proposed ionospheric disturbance index can overcome several drawbacks of other ionospheric measures and might be suitable as potential driver for an ionospheric space weather scale.

The solar eclipse on March 20, 2015 was a fascinating event for people in Northern Europe. From a scientific point of view, the solar eclipse can be considered as an in situ experiment on the Earth's upper atmosphere with a well-defined switching off and on of solar irradiation. Due to the strong changes in solar radiation during the eclipse, dynamic processes were initiated in the atmosphere and ionosphere causing a measurable impact, for example, on temperature and ionization. We analyzed the behavior of total ionospheric ionization over Europe by reconstructing total electron content (TEC) maps and differential TEC maps. Investigating the large depletion zone around the shadow spot, we found a TEC reduction of up to 6 TEC units, i.e., the total plasma depletion reached up to about 50%. However, the March 20, 2015 eclipse occurred during the recovery phase of a strong geomagnetic storm and the ionosphere was still perturbed and depleted. Therefore, the unusual high depletion is due to the negative bias of up to 20% already observed over Northern Europe before the eclipse occurred. After removing the negative storm effect, the eclipse-induced depletion amounts to about 30%, which is in agreement with previous observations. During the solar eclipse, ionospheric plasma redistribution processes significantly affected the shape of the electron density profile, which is seen in the equivalent slab thickness derived by combining vertical incidence sounding (VS) and TEC measurements. We found enhanced slab thickness values revealing, on the one hand, an increased width of the ionosphere around the maximum phase and, on the other, evidence for delayed depletion of the topside ionosphere. Additionally, we investigated very low frequency (VLF) signal strength measurements and found immediate amplitude changes due to ionization loss at the lower ionosphere during the eclipse time. We found that the magnitude of TEC depletion is linearly dependent on the Sun's obscuration function. By modelling TEC depletion and knowing the Sun's obscuration function in advance, Global Navigation Satellite System (GNSS) operators may improve the broadcast ionospheric correction during a solar eclipse day.

This paper presents a review on the PECASUS service, which provides advisories on enhanced space weather activity for civil aviation. The advisories are tailored according to the Standards and Recommended Practices of the International Civil Aviation Organization (ICAO). Advisories are disseminated in three impact areas: radiation levels at flight altitudes, GNSS-based navigation and positioning, and HF communication. The review, which is based on the experiences of the authors from two years of running pilot ICAO services, describes empirical models behind PECASUS products and lists ground- and space-based sensors, providing inputs for the models and 24/7 manual monitoring activities. As a concrete example of PECASUS performance, its products for a post-storm ionospheric F2-layer depression event are analyzed in more detail. As PECASUS models are particularly tailored to describe F2-layer thinning, they reproduce observations more accurately than the International Reference Ionosphere model (IRI(STORM)), but, on the other hand, it is recognized that the service performance is much affected by the coverage of its input data. Therefore, more efforts will be directed toward systematic measuring of the availability, timeliness and quality of the data provision in the next steps of the service development.