Identification of potential precursors for the occurrence of Large-Scale Traveling Ionospheric Disturbances in a case study during September 2017

Traveling Ionospheric Disturbances (TIDs) reflect changes in the ionospheric electron density which are caused by atmospheric gravity waves. These changes in the electron density impact the functionality of different applications such as precise navigation and high-frequency geolocation. The Horizon 2020 project TechTIDE establishes a warning system for the occurrence of TIDs with the motivation to mitigate their impact on communication and navigation applications. This requires the identification of appropriate indicators for the generation of TIDs and for this purpose we investigate potential precursors for the TID occurrence. This paper presents a case study of the double main phase geomagnetic storm, starting from the night of 7th September and lasting until the end of 8th September 2017. Detrended Total Electron Content (TEC) derived from Global Navigation Satellite System (GNSS) measurements from more than 880 ground stations in Europe was used to identify the occurrence of different types of large scale traveling ionospheric disturbances (LSTIDs) propagating over the European sector. In this case study, LSTIDs were observed more frequently and with higher amplitude during periods of enhanced auroral activity, as indicated by increased electrojet index (IE) from the International Monitor for Auroral Geomagnetic Effects (IMAGE). Our investigation suggests that Joule heating due to the dissipation of Pedersen currents is the main contributor to the excitation of the observed LSTIDs. We observe that the LSTIDs are excited predominantly after strong ionospheric perturbations at high-latitudes. Ionospheric parameters including TEC gradients, the Along Arc TEC Rate (AATR) index and the Rate Of change of TEC index (ROTI) have been analysed for their suitability to serve as a precursor for LSTID occurrence in mid-latitude Europe, aiming for near real-time indication and warning of LSTID activity. The results of the presented case study suggest that the AATR index and TEC gradients are promising candidates for near real-time indication and warning of the LSTIDs occurrence in mid-latitude Europe since they have a close relation to the source mechanisms of LSTIDs during periods of increased auroral activity

Lensing from small-scale travelling ionospheric disturbances observed using LOFAR

Observations made using the LOw-Frequency ARray (LOFAR) between 10:15 and 11:48 UT on the 15th of September 2018 over a bandwidth of approximately 25-65 MHz contain discrete pseudo-periodic features of ionospheric origin. These features occur within a period of approximately 10 min and collectively last roughly an hour. They are strongly frequency dependent, broadening significantly in time towards the lower frequencies, and show an overlaid pattern of diffraction fringes. By modelling the ionosphere as a thin phase screen containing a wave-like disturbance, we are able to replicate the observations, suggesting that they are associated with small-scale travelling ionospheric disturbances (TIDs). This modelling indicates that the features observed here require a compact radio source at a low elevation and that the TID or TIDs in question have a wavelength <~30 km. Several features suggest the presence of deviations from an idealised sinusoidal wave form. These results demonstrate LOFAR-s capability to identify and characterise small-scale ionospheric structures

pilot ionosonde network for identification of traveling ionospheric disturbances

Travelling Ionospheric Disturbances (TIDs) are the ionospheric signatures of atmospheric gravity waves (AGWs). Their identification and tracking is important because the TIDs affect all services that rely on predictable ionospheric radio wave propagation. Although various techniques have been proposed to measure TID characteristics, their real-time implementation still has several difficulties. In this contribution, we present a new technique, based on the analysis of oblique Digisonde-to-Digisonde (D2D) "skymap" observations, to directly identify TIDs and specify the TID wave parameters based on the measurement of angle-of-arrival, Doppler frequency, and time-of-flight of ionospherically reflected high-frequency (HF) radio pulses. The technique has been implemented for the first time for the Net-TIDE project with data streaming from the network of European Digisonde DPS4D observatories. The performance is demonstrated during a period of moderate auroral activity, assessing its consistency with independent measurements such as data from auroral magnetometers and electron density perturbations from Digisondes and GNSS stations. Given that the different types of measurements used for this assessment were not made at exactly the same time and location, and that there was insufficient coverage in the area between the AGW sources and the measurement lo cations, we can only consider our interpretation as plausible and indicative for the reliability of the extracted TID characteristics. In the framework of the new TechTIDE project (European Commission H2020), a retrospective analysis of the Net-TIDE results in comparison with those extracted from GNSS TEC-based methodologies is currently being attempted, and the results will be the objective of a follow up paper

On the dynamics of large-scale traveling ionospheric disturbances over Europe on 20 November 2003

Ionospheric disturbances, often associated with geomagnetic storms, may cause threats to radio systems used for communication and navigation. One example is the super storm on 20 November 2003, when plenty of strong and unusual perturbations were reported. This paper reveals additional information on the dynamics in the high-latitude ionosphere over Europe during this storm. Here analyses of wavelike traveling ionospheric disturbances (TIDs) over Europe are presented, based on estimates of the total electron content (TEC) derived from ground-based Global Navigation Satellite System (GNSS) measurements. These TIDs are ionospheric signatures of thermospheric surges initiated by space weather events. The source region of these TIDs is characterized by enhanced spatial gradients, TEC depression, strong uplift of the F2 layer, the vicinity of the eastward auroral electrojet, and strong aurora E layers. Joule heating is identified as the most probable driver for the TIDs observed over Europe during 20 November 2003. The sudden heating of the thermosphere leads to strong changes in the pressure and thermospheric wind circulation system, which in turn generates thermospheric wind surges observed as TID signatures in the TEC. Either the dissipation of the eastward auroral electrojet or particle precipitation are considered as the source mechanism for the Joule heating. In the course of the storm, the TEC observations show a southward shift of the source region of the TIDs. These meridional dislocation effects are obviously related to a strong compression of the plasmasphere. The presented results demonstrate the complex interaction processes in the thermosphere-ionosphere-magnetosphere system during this extreme storm

Properties and Generation of Large Scale Travelling Ionospheric Disturbances during 8 September 2017

Large Scale Travelling Ionospheric Disturbances (LSTIDs) are a frequent phenomenon during ionospheric storms, indicating strong electrodynamic processes in high latitudes. LSTIDs are signatures of Atmospheric Gravity Waves (AGW) observed in the changes of the electron density in the ionosphere. During ionospheric storms, large scale AGWs are often generated in the vicinity of the auroral region, where sudden strong heating processes take place. Many LSTIDs are observed during the ionosphere storm during the September 2017 Space Weather event. In this presentation, the LSTID occurrence on 8th September 2017 is analysed in more detail, based on a TID detection method using ground based Global Navigation Satellite System (GNSS) measurements. Fast LSTIDs are observed in midlatitudes between 0-3 UT and 13-16 UT. Slow LSTIDs are observed between 3-12 UT. A significant strong wave-like TEC perturbation occurred in high latitudes at noon, which vanished at around 50°N. A strong single LSTID in mid-latitudes generated in high latitudes around 18 UT. Consulting IMAGE magnetometer data, ionosonde measurements and Swarm field aligned current measurements, strong heating processes, the extension of the Auroral oval and unusual electrodynamic processes are discussed as source mechanisms for these LSTIDs

Large scale ionospheric gradients over Europe observed in October 2003

It is well known that ionospheric perturbations are characterized by strong horizontal gradients and rapid changes of the ionization. Thus, space weather induced severe ionosphere perturbations can cause serious technological problems in Global Navigation Satellite Systems (GNSS) such as GPS. During the severe ionosphere storm period of 29-31 October 2003, reported were several significant malfunctions due to the adverse effects of the ionosphere perturbations such as interruption of the WAAS service and degradation of mid-latitudes GPS reference services. To warn users of such services in a proper way, a quick evaluation of the current signal propagation conditions effectively expressed in a suitable ionospheric perturbation index would be of great benefit. To investigate possibilities of describing ionospheric perturbations by such an index, a task force group was established in the COST 296 activity. Preliminary results of a comparative study of ionospheric gradients including vertical sounding and TEC data are presented. Strong enhancements of latitudinal gradients and temporal changes of the ionization are observed over Europe during the 29-30 October storm period. The potential use of spatial and temporal gradients for characterizing the actual perturbation degree of the ionosphere is discussed

Statistical analysis of the results: assessment of the impact on aerospace and ground systems

The aim of this work package (WP) 5 is to specify the impact of TID in EGNOS, N-RTK and HF systems through statistical correlation of the performance data recorded from system operators and of TID detection results. Within the HF-TID-method in TechTIDE WP2 we use Digisonde-to-Digisonde measurements to determine characteristics of TIDs appearing in the region of the ionospheric reflection area between a transmitting and a receiving Digisonde. One of the TID characteristics is the Angle of Arrival (AoA) of the received radio signal, which can be divided into azimuth and elevation. In this analysis, we look for a correlation between TIDs and the variation of the azimuth of the received radio signal. In N-RTK, statistical tests have been applied over the entire data tests showing NRTK solutions are close to the solutions implementing a dual frequency solution methodology. Additionally, it was possible to correlate presence of MSTID with the degradation in the NRTK solutions.The aim of this WP5 is to specify the impact of TID in EGNOS, N-RTK and HF systems through statistical correlation of the performance data recorded from system operators and of TID detection results.Within the HF-TID-method in TechTIDE WP2 we use Digisonde-to-Digisonde measurements to determine characteristics of TIDs appearing in the region of the ionospheric reflection area between a transmitting and a receiving Digisonde. One of the TID characteristics is the Angle of Arrival (AoA) of the received radio signal, which can be divided into azimuth and elevation. In this analysis, we look for a correlation between TIDs and the variation of the azimuth of the received radio signal.In N-RTK, statistical tests have been applied over the entire data tests showing NRTK solutions are close to the solutions implementing a dual frequency solution methodology. Additionally, it was possible to correlate presence of MSTID with the degradation in the NRTK solutionsPreprin

Investigating the effect of large solar flares on the ionosphere based on novel Digisonde data comparing three different methods

Increased solar radiation during solar flare events can cause additional ionization and enhanced absorption of the electromagnetic (EM) waves in the ionosphere leading to partial or even total radio fade-outs. In this study, the ionospheric response to large solar flares has been investigated using the ionosonde data from Juliusruh (54.63° N, 13.37° E), Průhonice (49.98° N, 14.55° E) and San Vito (40.6° N, 17.8° E) Digisonde (DPS-4D) stations. We studied the effect of 13 intense (>C4.8) solar flares that occurred between 06:00 and 16:30 (UT, daytime LT = UT+1 h) from 04 to 10 September 2017 using three different methods. A novel method based on the amplitude data of the measured EM waves is used to calculate and investigate the relative absorption changes (compared to quiet period) occurring during the flares. The amplitude data are compared with the variation of the fmin parameter ( fmin , the minimum measured frequency, it is considered as a qualitative proxy for the “non-deviative” radio wave absorption). Furthermore, the signal-to-noise ratio (SNR) measured by the Digisondes was used as well to quantify and characterize the fade-out events and the ionospheric absorption. In order to compare the three different methods, residuals have been defined for all parameters, which provide the percentage changes compared to the selected reference periods. Total and partial radio fade-outs, increased values (+0.4%–318%) of the fmin parameter, and +20%–1400% amplitude changes (measured at 2.5 and 4 MHz) were experienced during and after the investigated flares. Generally, the observed changes depended on the intensity, solar zenith angle and duration of the flare events. Although the three different methods have their own advantages/disadvantages and their limitations, the combination of them seems to be an efficient approach to monitor the ionospheric response to solar flares

Capabilities of SWACI as an essential component of EURIPOS

EURIPOS as a European research network of an ionospheric and plamaspheric observation system relies on the well established European vertical sounding network and an equivalent network of numerous receivers of signals from Global Navigation Satellite Systems (GNSS) such as GPS and Galileo. According to this base line EURIPOS is assisted by current ionospheric services such as DIAS (Digital Upper Atmosphere Server) and SWACI (Space Weather Application Center Ionosphere) operated by NOA and DLR, respectively. This paper shall demonstrate the capabilities of SWACI providing an essential basis for supporting the enhanced service by EURIPOS. The current SWACI service operates a powerful data processing system working both in real-time and postprocessing modes in order to provide actual information to the customers (http://w3swaci.dlr.de). Typical data products include ground based GNSS derived European maps of the Total Electron Content (TEC) and corresponding derivatives such as latitudinal and longitudinal gradients and rate of change, updated every 5 minutes. Space based retrievals include radio occultation data as well as a 3D reconstruction of the topside ionosphere between CHAMP orbit and GPS satellite orbit height. The different types of products are described and discussed in relation to EURIPOS requirements and objectives. Emphasis is given to new products which may be created by combining vertical sounding and TEC data. The resulting equivalent slab thickness is discussed by combining ionosonde data from Juliusruh with corresponding TEC data extracted from the TEC maps. EURIPOS shall provide this valuable shape parameter in near real time at several vertical sounding stations. The dynamics of the profile shape is important for studying ionospheric storms. The capabilities of SWACI for monitoring and studying ionospheric storms observed since 2000 will be demonstrated

Large-scale ionospheric gradients over Europe observed in October 2003

It is well known that ionospheric perturbations are characterised by strong horizontal gradients and rapid changes of the ionisation. Thus, space weather induced severe ionosphere perturbations can cause serious technological problems in Global Navigation Satellite Systems (GNSS) such as GPS. During the severe ionosphere storm period of 29–31 October 2003, reported were several significant malfunctions due to the adverse effects of the ionosphere perturbations such as interruption of the WAAS service and degradation of mid-latitudes GPS reference services. To properly warn service users of such effects, a quick evaluation of the current signal propagation conditions expressed in a suitable ionospheric perturbation index would be of great benefit. Preliminary results of a comparative study of ionospheric gradients including vertical sounding and Total Electron Content (TEC) data are presented. Strong enhancements of latitudinal gradients and temporal changes of the ionisation are observed over Europe during the 29–30 October storm period. The potential use of spatial gradients and rate of change of foF2 and TEC characterising the actual perturbation degree of the ionosphere is discussed. It has been found that perturbation induced spatial gradients of TEC and foF2 strongly enhance during the ionospheric storm on 29 October over the Central European region in particular in North–South direction exceeding the gradients in East–West direction by a factor of 2