SISTED and SOLERA

The availability of real-time Global Navigation Satellite Systems (GNSS) networks of dual frequency receivers distributed world-wide allows to monitor ionosphere in real-time. In particular, major solar flares can be detected as they cause rapid strong ionisation in the dayside ionosphere. This opportunity is used in SISTED (Sunlit Ionosphere Sudden TEC Enhancement Detector) and SOLERA (SOLar Euv flux RAte GNSS proxy) products, which have been developed and are operated by the Technical University of Catalonia (UPC-IonSAT research group). These services are integrated in the Ionospheric Weather Expert Service Centre, which is operated under the ESA Space Safety Programme.

SISTED product monitors simultaneous sudden enhancements in the ionospheric Total Electron Content (TEC) using the drift rate (second difference in time) of the carrier phase ionospheric combination (LI) which can be derived from GNSS signals gathered in real time by permanent receivers distributed world-wide. LI is linearly related to the Slant TEC (STEC) along the satellite-receiver ray path under consideration. The drift rate is used to generate a set of three Impact Parameters (IPs). Each SISTED IP tells (in %) how many satellite-receiver pairs are affected by an abrupt over-ionisation (positive drift rate) simultaneously in a certain region. Then, the ray paths between satellites and receivers are categorised into three groups according to the Solar Zenith Angle (SZA) of their Ionospheric Piercing Point (IPP): Sunlit (0°≤SZA<70°), Dawn/Dusk (70°≤SZA<110°) or Night (110°≤SZA<180°). A solar flare affecting Earth's ionosphere is considered detected if the Sunlit IP exceeds 74% with a minimum number of IPPs in the Sunlit region is at least 100.

SOLERA-drift product is also based on the impact of the ionospheric electron content as a response to solar flare activity. The ionospheric response appears as a change in the Vertical Total Electron Content (VTEC) whose time derivative has a linear dependency on the cosine of the Solar Zenith Angle (SZA). The slope of this linear trend, i.e. SOLERA product, can be used as a proxy for the time derivative of Solar EUV flux. Comparison studies with direct EUV rate measurements from dedicated instrumentation on-board space probes, such as the SOHO-SEM (in the spectral band of 21-34 nm), have shown that SOLERA gives good estimates for the EUV flux rate in the cases of X- M- and C-class solar flares. SOLERA-drift is also capable of distinguishing solar flare effects in VTEC data from rapid variations generated by other space weather activity (e.g. by geomagnetic storms).

IGS UQRG VTEC Global Ionospheric Maps

UQRG-GIM rapid 15-minute resolution Vertical Total Electron Content (VTEC) Global Ionospheric Maps (GIMs) are computed by UPC-IonSAT TOMION-v2 software. TOMION-v2 tomographic model relies on multi-GNSS dual-frequency phase measurements from a world-wide network of permanent GNSS receivers. UQRG-GIMs is one of the outperforming GIMs, or even the best (RMS-wise) behaving GIM when compared to other International GNSS Service (IGS) VTEC GIMs. In fact, it enable detecting realistic features of the polar ionosphere and providing a realistic and sensitive storm index.