Monitoring and Forecasting the Ionosphere Over Europe: The DIAS Project

Knowledge of the state of the upper atmosphere, and in particular its ionospheric part, is very important in several applications affected by space weather, especially the communications and navigation systems that rely on radio transmission. To better classify the ionosphere and forecast its disturbances over Europe, a data collection endeavour called the European Digital Upper Atmosphere Server (DIAS) was initiated in 2004 by a consortium formed around several European ionospheric stations that transmit in real-time ionospheric parameters automatically scaled. The DIAS project is a collaborative venture of eight institutions funded by the European Commission eContent Programme. The project seeks to improve access to digital information collected by public European institutes and to expand its use. The main objective of the DIAS project is to develop a pan-European digital data collection describing the state of the upper atmosphere, based on real-time information and historical data collections provided by most of the operating ionospheric stations in Europe. Various groups of users require data specifying upper atmospheric conditions over Europe for nowcasting and forecasting purposes. The DIAS system is designed to distribute such information. The successful operation of DIAS is based on the effective use of observational data in operational applications through the development of new added-value ionospheric products and services that best fit the needs of the market. DIAS is a unique European system, and its continuous operation will efficiently support radio propagation services with the most reliable information. DIAS began providing services to users in August 2006

Stationarity, homogeneity and isotropy in the coherence theory of the electromagnetic fields

The concepts of stationarity, homogeneity and isotropy are introduced in terms of the invariance of certain expectation values under time translations, under space translations and under space rotations respectively. Some consequences of homogeneity and isotropy on the forms and relations among second-order correlation tensors of the electromagnetic field are derived. It is shown that, if the second-order correlation tensors are invariant under space translations, they are also invariant under time translations. Further it is also shown that in such cases the trace of the electric correlation tensor is identical to the trace of the magnetic correlation tensor. General forms of the second-order electromagnetic correlation tensors are derived for fields which are both isotropic and homogeneous. Both quantum and classical descriptions of coherence are considered and the restrictions which stationarity, homogeneity or isotropy impose on the density operator in quantum description and on the probability distributions in classical description are deduced

Dynamic phenomena in the chromospheric layer of a sunspot

We have studied running penumbral waves, umbral oscillations, umbral flashes and their interrelations from Hα observations of a large isolated sunspot. Using a subtraction image processing technique we removed the sharp intensity gradient between the umbra and the penumbra and enhanced the low contrast, fine features. We observed running penumbral waves which started in umbral elements with a size of a few arcseconds, covered the umbra and subsequently propagated through the penumbra. The period of the waves was 190 s and the mean propagation velocity was about 15 km s-1. We detected intense brightenings, located between umbral elements from where waves started, which had the characteristics of umbral flashes. There are indications that umbral flashes are related to the propagation of the waves through the umbra and their coupling. The subtraction images also show considerable fine structure in the chromospheric umbra, with size between 0.3″ and 0.8″. © 1992 Kluwer Academic Publishers