Statistical analysis and modeling of the local ionospheric critical frequency: a mid-latitude single-station model for use in forecasting

The hourly values of the F-layer critical frequency from the ionospheric sounder in Dourbes (50.1°N, 4.6°E) during the time interval from 1957 to 2010, comprising five solar cycles, were analyzed for the effects of the solar activity. The hourly time series were reduced to hourly monthly medians which in turn were used for fitting a single station foF2 monthly median model. Two functional approaches have been investigated: a statistical approach and a spectral approach. The solar flux F10.7 is used to model the dependence of foF2 on the solar activity and is incorporated into both models by a polynomial expression. The statistical model employs polynomial functions to fit the F-layer critical frequency while the spectral model is based on spectral decomposition of the measured data and offers a better physical interpretation of the fitting parameters. The daytime and nighttime foF2 values calculated by both approaches are compared during high and low solar activity. In general, the statistical model has a slightly lower uncertainty at the expense of the larger number of fitting parameters. However, the spectral approach is superior for modeling the periodic effects and performs better when comparing the results for high and low solar activity. Comparison with the International Reference Ionosphere (IRI 2012) shows that both local models are better at describing the local values of the F-layer critical frequency

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

A new method for reconstruction of the vertical electron density distribution in the upper ionosphere and plasmasphere

Ground-based ionosphere sounding measurements alone are incapable of reliably modeling the topside electron density distribution above the F layer peak density height. Such information can be derived from Global Positioning System (GPS)-based total electron content (TEC) measurements. A novel technique is presented for retrieving the electron density height profile from three types of measurements: ionosonde (foF2, foE, M3000F2, hmf2), TEC (GPS-based), and O+-H+ ion transition level. The method employs new formulae based on Chapman, sech-squared, and exponential ionosphere profilers to construct a system of equations, the solution of which system provides the unknown ion scale heights, sufficient to construct a unique electron density profile at the site of measurements. All formulae are based on the assumption of diffusive equilibrium with constant scale height for each ion species. The presented technique is most suitable for middle- and high-geomagnetic latitudes and possible applications include: development, evaluation, and improvement of theoretical and empirical ionospheric models, development of similar reconstruction methods utilizing low-earth-orbiting satellite measurements of TEC, operational reconstruction of the electron density on a real-time basis, etc

Ionospheric behaviour over Europe during the solar eclipse of 3 October 2005

An annular eclipse occurred over Europe in the morning hours of 3 October 2005. The well-defined obscuration function of the solar radiation during the eclipse provided a good opportunity to study the ionospheric/thermospheric response to solar radiation changes. Since the electron density peak behaviour of the ionospheric F2 layer follows the local balance of plasma production, loss and transport, the ionospheric plasma redistribution processes significantly affect the shape of the electron density profile. These processes are discussed here based on a comparison of vertical sounding and vertical total electron content (TEC) data above selected ionosonde stations in Europe. The equivalent slab thickness, derived with a time resolution of 10 minutes, provides relatively good information on the variation of the electron density profile shape during the eclipse. The computations reveal an increased width of the ionosphere before the maximum phase. As indicated by the available measurements over Spain, the photo production is significantly reduced during the event leading to a slower increase of the total ionization in comparison with the neighbouring days. The supersonic motion of the Moon’s cool shadow through the atmosphere may generate atmospheric gravity waves that propagate upward and are detectable as travelling ionospheric disturbances in ionospheric heights. High frequency (HF) Doppler shift spectrograms were recorded during the eclipse showing a distinct disturbance in the band of maximum eclipse. However, it is almost not distinguishable outside this band, which is a different result obtained from previous eclipses. Whereas ionosonde and HF Doppler measurements show enhanced wave activity, the TEC data analysis doesn’t, indicating that the wave amplitudes are too small for detecting. The total ionization reduces up to about 30%. Comparisons with similar observations from the solar eclipse of 11 August 1999 revealed quite different horizontal distribution of the depletion which is assumed to be related to differences in the meridional thermospheric wind system

Multi-instrument observations of the solar eclipse on 20 March 2015 and its effects on the ionosphere over Belgium and Europe

A total solar eclipse occurred on 20 March 2015, with a totality path passing mostly above the North Atlantic Ocean, which resulted in a partial solar eclipse over Belgium and large parts of Europe. In anticipation of this event, a dedicated observational campaign was set up at the Belgian Solar-Terrestrial Centre of Excellence (STCE). The objective was to perform high-quality observations of the eclipse and the associated effects on the geospace environment by utilising the advanced space- and ground-based instrumentation available to the STCE in order to further our understanding of these effects, particularly on the ionosphere. The study highlights the crucial importance of taking into account the eclipse geometry when analysing the ionospheric behaviour during eclipses and interpreting the eclipse effects. A detailed review of the eclipse geometry proves that considering the actual obscuration level and solar zenith angle at ionospheric heights is much more important for the analysis than at the commonly referenced Earth’s surface or at the plasmaspheric heights. The eclipse occurred during the recovery phase of a strong geomagnetic storm which certainly had an impact on (some of) the ionospheric characteristics and perhaps caused the omission of some “low-profile” effects. However, the analysis of the ionosonde measurements, carried out at unprecedented high rates during the eclipse, suggests the occurrence of travelling ionospheric disturbances (TIDs). Also, the high temporal and spatial resolution measurements proved very important in revealing and estimating some finer details of the delay in the ionospheric reaction and the ionospheric disturbances