Search

A network of ionosondes in Europe has been established to monitor travelling ionospheric disturbances (TIDs) by simultaneously making vertical and oblique incidence HF sounding measurements. This network is the outcome of the Net-TIDE project, a collaboration between European Digisonde operators that have synchronised the sounding schedules of the Digisondes in order to record vertical and oblique ionogram traces simultaneously, and have added Digisonde-to-Digisonde (D2D) fixed frequency oblique-incidence measurements to the measurement schedule. The distances between the observatories involved in the project range from 500 km to over 2000 km. The technical feasibility of this network approach is explored. The challenge for the fixed-frequency D2D skymap measurements is the automatic selection of the sounding frequencies depending on the geometry of the sounding paths, the diurnal and seasonal ionospheric changes, and space weather induced events.

It is well established that the sunrise and sunset periods are of particular importance to ionospheric research and modelling because of the rapid changes in the ionospheric plasma density, temperature, and dynamics. In particular, the sharp increase in the ionisation following sunrise results in a quick increase in the ionospheric peak density, N m F 2 , and a decrease in the peak height, h m F 2 . Changes in plasma temperature, scale height and transport processes add further complexity which makes it difficult to investigate and model the ionospheric behaviour during this transitional period from night to day. One of the aspects contributing to this difficulty is that not all ionospheric altitudes are exposed to the first sunlight of the day at the same time. During sunrise, the upper part of the ionosphere is illuminated prior to the lower part which is still in the dark. The boundary between sunlit and dark regions moves downwards until it reaches the surface of the Earth, which is commonly taken as the moment of sunrise at certain geographical coordinates. This means that the sunrise at surface level does not occur until after the entire ionosphere has been illuminated. During sunset, the same process happens in reverse order. This paper addresses the issue of these altitude-dependent times of sunrise and sunset and reports on our study of some of the effects on the diurnal variations in the ionospheric characteristics.

The ionospheric behavior over Belgium during the partial solar eclipse of 20 March 2015 is analyzed based on high-resolution solar radio flux, vertical incidence sounding, and GPS TEC measurements. First results of ionosonde-based ionospheric plasma drift and tilt observations are presented and analyzed, including some traveling ionospheric disturbances caused by the eclipse. Also, collocated ionosonde and GPS measurements are used to reconstruct the time evolution of the vertical electron density distribution using the Royal Meteorological Institute (RMI) ionospheric specification system, called Local Ionospheric Electron Density profile Reconstruction (L iedr ).

In relation to the development of an operational ionospheric monitoring and imaging system, the most frequently used analytical ionospheric profilers (Chapman, Epstein, Exponential) were investigated in terms of suitability for topside ionosphere modelling. For the purpose, topside sounder measurements onboard Alouette and ISIS satellites have been analysed. We have come to the conclusion that the use of the Chapman profiler should be exercised with precaution as there are evidences that there are conditions when other profilers are better fit for modelling purposes. This is highlighted during ionospheric disturbances (e.g. during geomagnetic storms), when the shape of the topside electron density distribution might be better described by an Epstein profiler rather than a Chapman profiler.

We present a data correction algorithm for real-time data processing for the NM64 galactic cosmic ray neutron monitor at the Royal Meteorological Institute (RMI) in Dourbes, Belgium. The correction is based on three main tests: a continuity test, tube ratios test and a derivative test. The continuity test works as a high pass filter with a threshold based on the entire recorded dataset. Additionally, it monitors whether the logging takes place at regular intervals (continuously). The ratios test identifies noisy sections and the final derivative test criterion will identify single or double spikes by testing them against the median increase of the intensities. Using these criteria, all data from the cosmic ray station at Dourbes is corrected in real time. Test results have been compared with data from verified neutron monitor stations with a similar geomagnetic cutoff rigidity.

The ionospheric topside sounder measurement database developed at the US National Space Science Data Center (NSSDC) is a valuable source of information when investigating the composition and complex dynamics of the upper ionosphere. The database is increasingly used by many scientists around the world for both research and development of empirical models. However, there is always a danger of indiscriminately using the data without properly assessing the data quality and applicability for a given purpose. This paper is concerned with the issue of data screening and pre-processing of the Alouette/ISIS topside sounder database. An overview of the original database availability and formatting is given and the use of solar and geomagnetic indices is discussed. Data screening procedures, concerning detection and handling of erroneous profiles, are also presented. Special attention is drawn to the systematic biases observed in the database and the possibilities for their removal.

The purpose of the LIEDR (local ionospheric electron density profile reconstruction) system is to acquire and process data from simultaneous ground-based total electron content (TEC) and digital ionosonde measurements, and subsequently to deduce the vertical electron density distribution above the ionosonde’s location. LIEDR is primarily designed to operate in real time for service applications and, for research applications and further development of the system, in a post-processing mode. The system is suitable for use at sites where collocated TEC and digital ionosonde measurements are available. Developments, implementations, and some preliminary results are presented and discussed in view of possible applications.

Global Navigation Satellite System (GNSS) measurements of the Total Electron Content (TEC) from local (Dourbes, 50.1°N, 04.6°E) and European IGS (International GNSS Service) stations were used to obtain the TEC changes during the geomagnetic storms of the latest solar activity cycle. A common epoch analysis, with respect to geomagnetic storm intensity, season, and latitude, was performed on data representing nearly 300 storm events. In general, the storm-time behaviour of TEC shows clear positive and negative phases, relative to the non-storm (median) behaviour, with amplitudes that tend to increase during more intense storms. The most pronounced positive phase is observed during winter, while the strongest and yet shortest negative phase is detected during equinox. Average storm-time patterns in the TEC behaviour are deduced for potential use in ionosphere prediction services.

Ionospheric disturbances are known to have adverse effects on the satellite-based communication and navigation. One particular type of ionospheric effects, observed during major geomagnetic storms and threatening the integrity performance of both ground-based and space-based GNSS augmentation systems, is the sharp increase/decrease in the ionospheric delay that propagates in horizontal direction, thus called for convenience ‘moving ionospheric wall’. This paper presents preliminary results from researching such anomalous ionospheric delay gradients at European middle latitudes during the storm events of 29 October 2003 and 20 November 2003. For the purpose, 30-s GPS data from the Belgian permanent network was used for calculating and analysing the slant ionospheric delay and total electron content values. It has been found that, during these two particular storm events, substantial gradients did occur in Europe although they were not so pronounced as in the American sector.

The topside ionospheric scale height (Ts) and the O + –H + transition height (Th) are key ionospheric characteristics that are of special interest when studying and modelling the plasma composition and dynamics. Recently, a new Topside Sounder Model (TSM) has been developed which provides the Ts and Th quantities together with the ratio between them, Ts/Th. The database for this model has been built upon thousands of Ts and Th values deduced from electron density profiles retrieved from topside sounding measurements. For validation purposes, it would be interesting to compare calculations of the ionospheric scale/transition heights from TSM with corresponding calculations from two other well-known models – the NeQuick Model (NeQ) and the Parameterized Ionospheric Model (PIM). For the purpose, electron density profiles have been computed with both the NeQuick and PIM models over suitable grids of input parameters such as month, local time, geomagnetic latitude, solar activity, and in the altitude range between 200 and 2000 km. The topside ionospheric scale height and the O + –H + transition height values have been extracted from each profile in the same manner as previously done for the TSM development database. Finally, the Ts and Th values deduced from the NeQuick and PIM profiles have been compared with the respective values provided by the TSM. Results of this comparison are analysed and suggestions are put forward for further improving the models in question. TSM applications are discussed as well. 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

To better and faster quantify the strength and impact of the ionospheric perturbations, we propose a new index for operational use in communication/navigation systems. Presented here is the analysis of selected events of strong disturbances observed with techniques based on Global Navigation Satellite Systems (GNSS) in order to show some insufficiencies when using existing geomagnetic indices. We found that the GNSS Total Electron Content (TEC) measurements and/or derivatives are outstanding candidates for defining a new ionospheric perturbation index, more specialised and therefore more effective. It is believed that the standardization and usage of the proposed perturbation index, together with other indices of similar nature, can prove helpful in reducing the space weather impact on the GNSS-based navigation and positioning.

A mathematical model is used to study the relative abundance of H+ and He+ ions in the topside ionosphere. It is found that the daytime light-ion densities are strongly coupled with the neutral densities. This fact arises difficulties in modelling the ion composition for IRI without taking into account any particular reference atmosphere. As an example, the transition heights between O+H+ and O+He+ are shown, plotted against the neutral densities. The supposed linear dependance gives a clear evidence that all light-ion ionization below these heights will experience stronger influence by the neutral atmosphere.