The paper reviews the current state of GNSS-based detection, monitoring and forecasting of ionospheric perturbations in Europe in relation to the COST action ES0803 ''Developing Space Weather Products and Services in Europe''. Space weather research and related ionospheric studies require broad international collaboration in sharing databases, developing analysis software and models and providing services. Reviewed is the European GNSS data basis including ionospheric services providing derived data products such as the Total Electron Content (TEC) and radio scintillation indices. Fundamental ionospheric perturbation phenomena covering quite different scales in time and space are discussed in the light of recent achievements in GNSS-based ionospheric monitoring. Thus, large-scale perturbation processes characterized by moving ionization fronts, wave-like travelling ionospheric disturbances and finally small-scale irregularities causing radio scintillations are considered. Whereas ground and space-based GNSS monitoring techniques are well developed, forecasting of ionospheric perturbations needs much more work to become attractive for users who might be interested in condensed information on the perturbation degree of the ionosphere by robust indices. Finally, we have briefly presented a few samples illustrating the space weather impact on GNSS applications thus encouraging the scientific community to enhance space weather research in upcoming years.

No abstract

Local variability in total electron content can seriously affect the accuracy of GNSS real-time applications. We have developed software to compute the positioning error due to the ionosphere for all baselines of the Belgian GPS network, called the Active Geodetic Network (AGN). In a first step, a reference day has been chosen to validate the methodology by comparing results with the nominal accuracy of relative positioning at centimeter level. Then, the effects of two types of ionospheric disturbances on the positioning error have been analyzed: (1) Traveling ionospheric disturbances (TIDs) and (2) noiselike variability due to geomagnetic storms. The influence of baseline length on positioning error has been analyzed for these three cases. The analysis shows that geomagnetic storms induce the largest positioning error (more than 2 m for a 20 km baseline) and that the positioning error depends on the baseline orientation. Baselines oriented parallel to the propagation direction of the ionospheric disturbances are more affected than others. The positioning error due to ionospheric small-scale structures can be so identified by our method, which is not always the case with the modern ionosphere mitigation techniques. In the future, this ionospheric impact formulation could be considered in the development of an integrity monitoring service for GNSS relative positioning users.

We provide the first comparison of the ICON-EUV O + density profile with radio wave datasets coming from GNSS radio-occultation, ionosondes and incoherent scatter radar. The peak density and height deduced from those different observation techniques are compared. It is found that the EUV-deduced peak density is smaller than that from other techniques by 50 to 60%, while the altitude of the peak is retrieved with a slight bias of 10 to 20 km on average. These average values are found to vary between November 2019 and March 2021. Magnetic latitude and local time are not factors significantly influencing this variability. In contrast, the EUV density is closer to that deduced from radio-wave techniques in the mid latitude region, i.e. where the ionospheric crests do not play a role. The persistent very low solar activity conditions prevailing during the studied time interval challenge the EUV O + density profile retrieval technique. These values are consistent, both in magnitude and direction, with a systematic error on the order of 10% in the data or the forward model, or a combination of both. Ultimately, the EUV instrument on-board ICON provides the only known technique capable of precisely monitoring the ionospheric peak properties at daytime from a single space platform, on a global scale and at high cadence. This feature paves the way to transpose the technology to the study of the ionosphere surrounding other planets.

On October 11, 2019, NASA's ICON satellite was launched into a circular orbit at about 590 km altitude, inclined by 27°. The spacecraft carries four scientific instruments dedicated to the study of the coupling between the lower atmosphere, the upper atmosphere, and the solar wind. Besides the in-situ plasma measurement performed by the Ion Velocity Meter (IVM) (Heelis et al., 2017), the remaining three instruments remotely sense the neutral and ionized atmosphere at altitudes ranging from about 90-600 km by observing airglow emissions in several wavelength ranges. In the visible domain, the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) observes the red and green oxygen airglow lines for wind speed retrieval and the 𝐴𝐴 O2 A-band in the near-infrared to measure the thermospheric temperature (Englert et al., 2017;Harding et al., 2017;Stevens et al., 2017). 𝐴𝐴 O + density profiles are retrieved at the 12-s measurement cadence by the two complementary instruments operating in the ultraviolet: the Far Ultra Violet Imaging Spectrograph (FUV) and the Extreme Ultra Violet Spectrograph (EUV). The first one simultaneously measures the 𝐴𝐴 OI -135.6 nm emission of atomic oxygen and the Lyman-Birge-Hopfield (LBH) band of 𝐴𝐴 N2 near 157 nm (Mende et al., 2017). During nighttime, the 135.6-nm channel is used alone to infer the 𝐴𝐴 O + density profile by observing the radiative recombination of oxygen ions with ambient electrons (Kamalabadi et al., 2018). On the dayside, both the 135.6 nm and LBH emissions are measured and combined to determine 𝐴𝐴 O and 𝐴𝐴 N2 altitude profiles and column 𝐴𝐴 O∕N2, used to monitor the atmospheric composition changes (Stephan et al., 2018). The EUV spectrograph records limb altitude profiles of terrestrial emissions in the extreme ultraviolet spectrum from 54 to 88 nm (Sirk et al., 2017). Specifically, the 𝐴𝐴 OII -61.7 and 83.4 nm emissions are used to retrieve daytime 𝐴𝐴 O + altitude profiles (Stephan et al., 2017).The radio-occultation space mission program COSMIC-2 (C2) currently provides up to 3,000 electron density profiles on a daily basis since October 1, 2019, using six spacecraft orbiting above low latitudes at similar altitudes as ICON. Additionally, ground-based ionosondes allow retrieving precise and accurate measurements of the electron density profile up to the peak altitude. These two data sets provide a large and robust