[1] 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 ( f o F 2 , f o E, M 3000 F 2 , h m f 2 ), 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.
The paper outlines a method allowing to compute the TEC with a precision of about 2-3 TECU and to detect Travelling Ionospheric Disturbances using GPS measurements. We describe the solar cycle dependance of the TEC and TIDs. Since the beginning of 1998, we have observed a stronger ionospheric activity due to the increasing solar activity. This ionospheric activity is characterized by larger TEC values which are regularly reaching the level of 60 TECU and by a larger number of Travelling Ionospheric Disturbances. During the winter 1999-2000, the mean daily TEC was above 45 TECU; at solar minimum the mean daily TEC is ranging from 4 TECU to 12 TECU. In January 2000 (close to solar maximum) more than 1300 events due to TID's were detected: it is 6.5 more than in January 1996 (at solar minimum).
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.
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