Annals 48, 3, 2005+app1 453 ANNALS OF GEOPHYSICS, VOL. 48, N. 3, June 2005 Key words ionosphere – ionospheric nowcasting – ionospheric forecasting – space weather – digisonde – ionospheric monitoring networks – ionogram scaling 1. Introduction Knowledge of the state of the upper atmos- phere is very important in several applications. The space effects on Radio Frequency (RF) communications and satellite positioning and navigation applications are determined by the ionospheric electron density structure and the Total Electron Content (TEC). Ionospheric- space weather effects can also cause time-vary- ing ionospheric currents causing problems in ground systems such as systems for power gen- eration and supply, oil and gas pipeline distribu- tion, aerial surveying for minerals oil and gas, drilling for oil and gas, railways, especially at the higher latitudes. Lately there has been an in- creasing interest of the seismic hazards research community in identifying electromagnetic phe- nomena in the ionosphere as predecessors of seismic events. Systematic real-time measurements of the ionosphere in Europe are obtained by ground digisondes (Reinisch, 1986) suitable for re- search and commercial use. The geographic distribution of these digisondes provides almost full coverage of the ionosphere over Europe. These digisondes have the capability of auto- matically scaling and transmitting in real-time all-important parameters characterising the state of the upper atmosphere and the propaga- tion of radio waves in the ionosphere. At the Real time monitoring for nowcasting and forecasting ionospheric space weather in Europe with ground digisondes Anna Belehaki Ionospheric Group, Institute for Space Applications and Remote Sensing, National Observatory of Athens, Palaia Penteli, Greece Abstract The Earth’s ionosphere largely determines space weather effects on radio wave communications, navigation and surveillance systems. Lately there has been an increasing demand for ionospheric nowcast and accurate forecast services by various groups of users, including European industry. The paper reviews research activities in Eu- rope based on the exploitation of real-time ground digisondes for the provision of nowcasting and forecasting ionospheric space weather information and useful products and services to support operational applications. During the last few years, important progress in databasing, modelling and forecasting ionospheric disturbances based on real-time data from ground digisondes was achieved in the frames of COST Action 271 «Effects of the Upper Atmosphere on Terrestrial and Earth-Space Communications». Further developments are expected to be deployed with the new COST Action 724 on «Developing the basis for monitoring, modelling and predicting space weather», as well as through the Space Weather Pilot Project of the European Space Agency and through projects funded by the European Commission programmes. Mailing address: Dr. Anna Belehaki, Ionospheric Group, Institute for Space Applications and Remote Sensing, National Observatory of Athens, Metaxa and Vas. Pavlou str., 15236 Palaia Penteli, Greece; e-mail: belehaki@space.noa.gr 454 Anna Belehaki Fig. 1a,b. Ionograms from Athens Digisonde station (a) for the case of undisturbed ionosphere and (b) for a case of spread F appearance. moment these digisondes operate independent- ly, and knowledge on the state of the ionosphere is generated for a limited area around each sta- tion only. This independent operation creates several barriers for transforming this informa- tion into usable data, products and services. Considering the increasing demand for upper- atmosphere nowcast and forecast services by the scientific community and various commer- cial users, the need to develop a European net- work is pressing, especially since similar sys- tems already exist in the US and Australia. Ground digisondes perform the following operations: – Automatic scaling of ionograms (with ARTIST software). – Real-time derivation and transmission of ionospheric parameters (densities and heights of E-, Es-, F1-, F2-layers peaks) – Real-time derivation of electron density profile up to 1000 km and estimation of the ionospheric electron content (ITEC). – Real-time derivation of MUF for specific distances. – Real-time derivation of spread F status. The performance of the ARTIST automatic scaling software (Reinisch and Huang, 1983), which is integrated to the digisonde computer system, is the key for the exploitation of real- time autoscaling characteristics. The results of the autoscaling program are presented in fig. 1a for the case of undisturbed ionosphere and in fig. 1b for the case of spread F appearance. The ARTIST performance is very accurate in both cases. To better support the previous result, the foF2 values derived from automatic scaled ionograms from Athens Digisonde are com- pared with manual validated foF2 values from the corresponding ionograms for a time period of 11 days (fig. 2a) during which a geomagnet- ic storm occurred on 24 September 2001 and lasted for three days. It is obvious that the foF2 values from the automatic scaling follow the manually validated data very closely. Figure 2b shows the differences between the values of foF2 as scaled by ARTIST and by the manual method from 2019 ionograms recorded at Athens during 20-30 September 2001 as his- tograms. For the period under study, 97.6% of the comparisons for foF2 lay within ± 0.5 MHz. This agrees with the comparison published by Gilbert and Smith (1988). The distribution is weighted in favor of negative values. This fea- ture results from the effect of interference on the echo trace in the vicinity of foF2, which of- ten causes premature termination of the ARTIST trace. The same effect appears in the line plot of a b 455 Real time monitoring for nowcasting and forecasting ionospheric space weather in Europe with ground digisondes the foF2 (fig. 2a) as sharp peaks in the auto- matic scaled foF2. This effect was also reported by Gilbert and Smith (1988) in their compari- son of ARTIST and manual scaled ionospheric parameters observed at Slough. A new version of ARTIST was recently released from the Center of Atmospheric Research of the University of Massachusetts Lowell, and the first results are very promising for successful performance on premature terminated traces (Reinisch, 2003, 2004, pers. comm.). The accuracy of real-time data, the satisfac- tory geographic coverage of the European real- time digisondes, and the availability of long term series of historical manual validated data that in some cases cover more than six decades provide the possibility for the development of a system for high quality ionospheric space weather serv- ices, for the European Region. Currently two major research projects are implemented by the European scientific commu- nity to study space weather and develop models and forecasting techniques: – COST271 «Effects of the Upper Atmos- phere on Terrestrial and Earth Space Communica- tions» (http://www.cost271.rl.ac.uk), which puts the emphasis on Space Weather effects on Tele- communications (Zolesi and Cander, 2004). – COST724 «Developing the Scientific Basis for Monitoring, Modelling and Predicting Space Fig. 2a,b. a) Comparison of automatic scaled (gray line) and manually scaled validated (black line) foF2 from Athens Digisonde station for a time period of 11 days. b) Differences between the values of foF2, as scaled by ARTIST and by manual method from 2019 ionograms recorded at Athens during the period 20-30 September 2001. a b 456 Anna Belehaki Weather» (http://cost724.obs.ujf-granoble.fr), which addresses wider issues in modelling the sun-Earth system (Lilensten et al., 2004). While COST724 Action is in its initial phase, COST271 is in progress and major efforts have been reported in the development of a compre- hensive database of prompt ionospheric parame- ters for nowcasting and forecasting purposes and of forecasting algorithms to predict the ionos- pheric and space plasma effects on communica- tions up to a few days ahead of the present. 2. Nowcasting ionospheric space weather One of the most important results of COST271 Action is the development of the Prompt Ionospheric Database at Rutherford Appleton Laboratory (Stamper, 2003). This database receives prompt data on regular basis from the European Digisondes. Developments can be followed at the web site of the database . This tool can be used by re- searchers to access prompt and past ionospher- ic data from several European stations. Besides this basic tool, noticeable progress has been re- ported during the last few years, developed mainly in the frames of COST271 Action and of the ESA Pilot Space Weather Project, aiming to map the ionospheric characteristics in real- time over Europe and to monitor and forecast the space environment including the ionosphere and plasmasphere. 2.1. Ionospheric mapping The effective use of automatic scaled ionos- pheric parameters derived from ground digison- des was demonstrated in the frames of COST271 Action and the ESA Pilot Space Weather Project with the development of a method for real-time mapping of the state of the ionosphere over Europe, proposed by Zolesi et al. (2003). The objective of the proposed method is to update the Simplified Ionospheric Regional Model (SIRMUP) in real-time using au- tomatic scaled ionospheric data from the four European Digisondes operated in Athens, Rome, Juliusruh and Chilton. The achieved re- sult is monitoring the state of the ionosphere over Europe with sufficient accuracy to be of use from earth-space communications and navi- gation systems (as GALILEO). SIRM (Zolesi et al., 1993) provides ionos- pheric parameters that represent monthly medi- an values and vary as a function of geographic location, local time, and R12. The updated SIRM (SIRMUP) takes as input the real-time au- tomatic scaled values from the four Digisondes to compute an effective sunspot number (Reff) that the method is using instead of R12. Reff is the sunspot number for which minn foF foF 1 2 2 i n obs calci i 2 1 - = = !^ h (2.1) where n is the number of the reference stations, foF2obsi is the observed foF2 at the reference sta- tion i, foF2calci is the calculated foF2 from SIRM at the coordinated of the reference station i. To test the reliability of the method, the rel- ative errors (Houminer et al., 1993) between the observed values (manually validated) at some test stations and the calculated foF2 val- ues by the model, were determined using the following equations: e foF foF foF 1 2 2 2 obs obs SIRM = - (2.2) e foF foF foF 2 2 2 2 obs obs SIRMUP = - (2.3) where foF2obs is the observed foF2 at the test station, foF2SIRM is the SIRM calculated foF2 at the test station using the observed R12 sunspot number, and foF2SIRMUP is the SIRMUP calculated foF2 at the test station using Reff. According to the above description, when the criterion e2 < e1 (2.4) is valid, the method of real-time updating is successful in the sense that the resulting map in the specific area is more representative of the real ionosphere than the corresponding map re- sulting from the use of monthly median values. 457 Real time monitoring for nowcasting and forecasting ionospheric space weather in Europe with ground digisondes The performance of SIRMUP method was test- ed by Zolesi et al. (2004) under all possible ionos- pheric conditions, including quiet intervals, large- scale disturbances such as storm negative phases and daytime positive effects of long duration as well as small-scale effects such as Travelling At- mospheric Disturbances (TADs) and nighttime positive effects, by presenting simulation results for several time intervals. An indicative interval is presented in fig. 3 for the storm period of 16 to 24 August 2001. The storm started with an initial compression phase in Dst observed on 17 August 2001 at 12:00 UT. The European region was al- ready in the noon sector and therefore was affect- ed by TADs, causing the strong daytime positive effect, which was observed from Sofia during lo- cal afternoon hours. At 22:00 UT, Dst reached its minimum and consequently the recovery phase started, although considerable substorm activity was recorded by the AE-index (not shown) until midnight. It is possible that this substorm activity caused the generation of a new composition dis- turbance zone (Tsagouri et al., 2000), producing the strong negative phase in the early hours of the 18 August. Consequently, the adjacent TADs gen- erated in the daylight sector probably caused the noticeable positive effect in the morning sector. As the geomagnetic activity recovered in the fol- lowing days, there was a remaining ionospheric disturbance recorded in Sofia of small magnitude. An intense substorm activity recorded by the AE- index from the morning of 21 August until noon of 23 August produced TAD’s on the 21 and 22 August in the daylight sector and a noticeable negative phase on 23 August. Most of the time the quantity e1-e2 was positive indicating that the performance of SIRMUP was particularly success- ful during periods of large scale ionospheric dis- turbances causing the negative effects recorded on the 18 and 23 August 2001, as well as small scale ionospheric disturbances generated by TADs on 17 August 2001. In the following the performance of the SIRMUP model is presented during the relatively Fig. 3. Simulation results for the storm period 16-24 August 2001 at the test station in Sofia. 458 Anna Belehaki quiet geomagnetic interval from 6 to 10 De- cember 2001. The simulation results are pre- sented in fig. 4 for Sofia test station. During this period a different type of ionospheric activity was observed, caused probably by changes in the global wind circulation (Prölss, 1995), which resulted in the considerable increase in the foF2 value during day hours, for the first three days of the interval. Although the Dst-in- dex did not present any disturbance, the source Fig. 5. The scatter plots of e2 versus e1 for the two simulated time periods. The best-fit line and its equation are also shown. Fig. 4. Simulation results of the quiet period 6-10 December 2001 at the test station in Sofia. 459 Real time monitoring for nowcasting and forecasting ionospheric space weather in Europe with ground digisondes of the observed ionospheric disturbance is the continuous substorm activity recorded by the AE-index. From the inspection of the differ- ences between the relative errors (e1-e2) plot- ted in the bottom of fig. 4 it is concluded that SIRMUP performance is particularly successful during large-scale positive effects observed on the first three days of this interval. During nighttime positive effects the performance is worse but still better than SIRM. To obtain a quantitative estimate of the SIRMUP performance, the scatter plots of e2 ver- sus e1 are shown in fig. 5 for the two simulated time periods. The best-fit line is overplotted in each diagram. In both cases the criterion e2 < e1 is statistically satisfied. Especially for the dis- turbed period (August 2001) a significant im- provement in the mapping performance is not- ed since e2 is much smaller than e1. Neverthe- less this is not the case for the quiet period (De- cember 2001) as well, since the high correlation between the two error values e1 and e2 indicate an almost equivalent performance of the two mapping techniques. Overall, the results presented by Zolesi et al. (2003) show a significant improvement of SIRMUP performance comparing to SIRM, dur- ing quiet intervals and also during large-scale ionospheric disturbances. A marginal improve- ment during localised ionospheric disturbances is also reported. This very detailed analysis demonstrates that the use of real-time automatic data can im- prove existing mapping techniques in order to be useful for operational applications. 2.2. Use of ITEC parameter for nowcasting the ionospheric and plasmaspheric electron content Another service that could be generated from the exploitation of real-time digisondes is the real-time availability of the ionospheric electron content (ITEC). Total Electron Content (TEC) is a key parameter that describes the ma- jor impact of the ionized atmosphere on the propagation of radio waves, which is crucial for terrestrial and Earth-space communications in- cluding navigation satellite systems such as GPS, GLONASS, and the future GALILEO system. A standard technique to determine TEC is from dual frequency GPS (Global Positioning Sys- tem) measurements. This technique measures the electron content along a slant signal path, from which a vertical TEC is estimated by sim- ple geometric corrections (see for example Jakowski, 1996). As the orbit altitude of GPS satellites is ∼20 000 km, GPS derived TEC cor- responds to the total electron content (bottom- side and topside ionosphere, and plasmasphere) and it is sensitive to topside ionospheric and plasmaspheric processes. Recently, Reinisch and Huang (2001) pro- posed a new technique to determine the vertical ionospheric electron content from ground- based ionogram measurements. The ionogram provides the information to directly calculate the vertical electron density profile up to the peak of the F2-layer. The profile above the peak is approximated by an α-Chapman function with a constant scale height HT derived from the bottomside profile shape at the F2 peak, ac- cording to the equation z- expN NmF z e2 2 1 1T = - -^ h; E (2.5) where z h hmF H2 T= -^ h , NmF2 is the maxi- mum electron density and hmF2 the height of the F2-layer peak. The ionosonde TEC, or ITEC, is then calculated as an integral from 0 to ∼1000 km over the entire profile. From the above definition it is obvious that ITEC is not equivalent to the GPS derived TEC parameter (GPSTEC). Since the ITEC parameter could develop into a very useful tool for now- casting ionospheric space weather, it is impor- tant to determine with accuracy the relationship between ITEC and TEC. A preliminary ITEC validation based on case studies was performed by comparing ITEC with TEC values derived from GPS, incoherent scatter radar, and geosta- tionary satellite beacon measurements at middle latitudes and with TOPEX measurement at the equator (Reinisch and Huang, 2001; Belehaki and Tsagouri, 2002a). It was shown that ITEC is generally within ∼10% of the satellite TEC. As a first approach the difference between GPSTEC and ITEC may be interpreted as the plasmas- pheric content, but this interpretation needs to 460 Anna Belehaki be verified on a statistical basis (Reinisch et al., 2001; Belehaki and Jakowski, 2002). Recently, Belehaki et al. (2003) presented a systematic analysis of the ITEC values from digisonde measurements at Athens and of GP- STEC data from TEC maps produced by the German Aerospace Center (DLR/IKN) for Eu- rope using GPS measurements, for Athens co- ordinates. It was shown that the residuals of the two independent quantities, ∆TEC, provide qualitative information on the plasmaspheric dynamics as deduced from their diurnal and seasonal behavior and their variation during ge- omagnetic storms and this conclusion is based on three findings: a) in general, the daily varia- tions of electron content above 1000 km for Athens have the same qualitative characteris- tics as the plasmaspheric content daily variation (fig. 6). There is some diurnal interchange be- tween the ionosphere and plasmasphere with Fig. 6. The daily variation of the monthly median values of ∆TEC over four months. A morning minimum and an evening maximum characterize the daily variation of ∆TEC. Fig. 7. Superposed epoch analysis on 13 geomag- netic storms of the daily averages of the difference between GPSTEC and ITEC as a function of number of days since storm initiation. 461 Real time monitoring for nowcasting and forecasting ionospheric space weather in Europe with ground digisondes Fig. 8. The daily variation pattern of the slab thickness parameters τt (black-dotted line) and τi (black-solid line) at Athens, extracted from the monthly median values, for the four seasons. downward diffusion from the latter helping maintain the nighttime F2-layer (Lunt et al., 1999; Belehaki and Tsagouri, 2002b) and day- time refilling. The evening peak in ∆TEC resid- uals, which is the striking feature in ∆TEC vari- ation over Athens and is also observed over Chilton, is attributed to the evening plasmas- pheric bulge. b) The effect of geomagnetic ac- tivity in ∆TEC residuals, presented in fig. 7, is qualitatively the same as the effect on the plas- masphere, showing a drastic depletion immedi- ately after the start of the storm and a conse- quent replenishment that lasts for 9 days. c) The daily variation of the ionospheric slab thickness (τi = ITEC/Nm F2) is compatible with the varia- tion of the daytime thermospheric temperature (fig. 8). There is, however, a significant night- time increase in the total slab thickness (τt = = GPSTEC/Nm F2), which is most prominent in winter and is due to the lowering of the O+/H+ transition height. The study demonstrates that the ITEC pa- rameter is a qualitative measure of the ionos- pheric electron content and that comparing the ionosonde derived TEC (ITEC) with GPS de- rived TEC (GPSTEC) at a given location it is pos- sible to estimate experimentally the effect of the plasmasphere on GPS measurements. As a next step, in order to determine the quantitative relation between the ITEC parame- ter and the Total Electron Content a statistical study was recently presented by Belehaki and Kersley (2003) using Incoherent Scatter Radar (ISR) profiles from Malvern site (52.1°N, 2.3°W). The TEC estimates from ISR profiles were compared with ITEC estimates, extracted from the same profiles, computed by extrapolat- ing the bottomside ISR electron density profile to the topside according to the method proposed by Reinisch and Huang (2001). A case showing the results of the topside extrapolation of the bot- tomside ISR ionograms is presented in fig. 9. The first results from the comparison study between TEC (from ISR profiles) and ITEC (computed from the extrapolated ISR profiles us- ing the H-R method) are presented in fig. 10. Here the statistics are limited to 317 cases of day- time profiles and 368 cases of nighttime profiles. In summary, the results show a very good agreement especially during nighttime hours and support the original derivation of the ITEC as a measure of the ionospheric electron con- 462 Anna Belehaki tent. Statistical analysis over the whole sample of 4000 profiles is needed before extracting fi- nal conclusions. 3. Forecasting ionospheric space weather Systematic ionospheric space weather fore- casts are provided by the Rutherford Appleton Laboratory (http://ionosphere.rcru.rl.ac.uk) using the Short-Term Ionospheric Forecasting (STIF) tool for the European Region based on continu- ous monitoring of the ionosphere (Cander, 2003). This operational system provides forecasting maps for up to 24 h ahead and archive measure- ment maps of the critical frequency of ionospher- ic F2-layer foF2, the Maximum Usable Frequen- cy for a 3000 km range MUF(3000)F2, NeQuick modelled vertical Total Electron Content (TEC) and Frequency of Optimum Traffic (FOT) for the area of interest at each UT hour. Accuracy of forecast of foF2, MUF(3000)F2 and TEC has been studied through several statistical compar- isons between measured and forecast values of foF2 and MUF(3000)F2. These results clearly suggest that STIF tool is a very reliable forecast- ing technique in relatively quiet geomagnetic conditions. Although these conditions prevail in the normal Earth’s ionosphere, the conditions during the geomagnetic storms are of more im- portance for current and future radio communica- tions services as well as space weather opera- tional requirements and scientific studies and un- der such conditions STIF has been proved inade- quate (Cander et al., 2002). A new approach to forecast ionospheric space weather is attempted based on the idea that solar wind disturbances can determine the timing and the magnitude of ionospheric disturbances (Bele- haki and Tsagouri, 2002b), with the primary goal to develop the understanding and the means to forecast how the ionospheric F2-layer will re- spond to abruptly and dramatically changing so- lar and geomagnetic conditions. Real-time measurements of the critical fre- quency of the F2-layer, foF2, and the propagation factor for a 3000 km range, M(3000)F2 from four European Digisondes operating in Athens, Rome, Chilton and Juliusruh and being available from WDC-C1 at RAL (http://www.wdc.rl.ac.uk/cgi- Fig. 9. Comparison between the ISR profile (gray line) and the extrapolated topside profile after the Huang and Reinisch (2001) method. The resulted to- tal electron content is also noted. Fig. 10. Statistical results from the comparison be- tween TEC (from ISR profiles) and ITEC (computed from the extrapolated ISR profiles using the H-R method) for daytime cases (upper panel) and night- time cases (bottom panel). 463 Real time monitoring for nowcasting and forecasting ionospheric space weather in Europe with ground digisondes bin/digisondes/cost_database.pl) and the Bz-com- ponent of the interplanetary magnetic field, Bz- IMF, from NASA Advanced Composition Explor- er (ACE) spacecraft mission (http://sec.noaa.gov/ Data) are combined for the development of a re- al-time dynamic system, oriented to monitor the ionospheric propagation conditions over Europe. Currently such an automated data analysis at 30 min intervals is operational at RAL . The latest charts show the modified Bz-IMF rate of change over the preceding 30 min inter- val, with the corresponding percentage devia- tion from the median of foF2, dfoF2 (%), and percentage deviation from the median of M(3000)F2, dmF2 (%), for Juliusruh, Chilton, Rome and Athens ionosonde stations. A very representative example is shown in fig. 11, where after a very strong disturbance in the Bz component of the IMF at midnight, a strong ionospheric depletion is observed over Athens with a time delay of 3 h. The presented facility is available for both operational purposes and scientific studies. Fu- ture developments will include the introduction of other solar wind parameters from ACE such as the rate of change of solar wind number den- sity (dn/dt), and the rate of change in solar wind bulk speed velocity (dU/dt) to fully monitor so- lar-terrestrial conditions in purpose to issue a preliminary warning. Moreover, the statistical analysis of the future results could help in mod- elling and forecasting the ionospheric response during storm conditions few hours ahead, which is of crucial importance for the reliable perform- ance of technologically advanced radio commu- nications systems under disturbed propagation conditions (Johnson et al., 1997), since the ionospheric forecasting models currently in op- eration have shown a high degree of reliability during quiet conditions, but have been proved in- adequate during storm events (Cander et al., 2002). 4. Conclusions Significant progress has been reported dur- ing the last few years by the European ionos- pheric community towards the development of tools and models for the nowcasting and fore- casting ionospheric space weather based on re- al-time measurements acquired from European Digisondes. Most of this progress is part of the COST271 Action and of the ESA Space Weath- er Pilot Project. The new staring COST724 Ac- Fig. 11. Bz-IMF rate of change over the preceding 30 min interval, with the corresponding percentage devia- tion from the median of foF2, dfoF2 (%), and percentage deviation from the median of M(3000)F2, dmF2 (%), for Athens Digisonde. 464 Anna Belehaki tion will contribute in this direction, improving our basic knowledge on space weather model- ling and forecast. Europe has the major advantage of possess- ing modern ionospheric stations capable of pro- viding in real-time accurate automatic scaled ionospheric parameters that characterize the propagation of radiowaves (Cander et al., 2004). The existing digisondes are reliable, operate on a continuous basis and their geographic distribu- tion provides an almost full coverage of the Eu- ropean region. The systematic exploitation of au- tomatic validated ionospheric data from the net- work of real-time digisondes in Europe can i) lead to the development of space weather, com- munications, navigation models; ii) improve our understanding of the physical processes in- volved in the ionosphere and plasmasphere; iii) lead to the development of an operational sys- tem for nowcasting and forecasting the state of the ionosphere over Europe. An important step in this direction can be achieved through the successful implementation of DIAS project (European Digital Upper At- mosphere Server). DIAS is a project co-funded by the European Commission eContent pro- gramme, and has as the main objective to devel- op a pan-European digital data collection on the state of the upper atmosphere, based on real-time information and historical data collections pro- vided by most operating ionospheric stations in Europe. DIAS system will distribute the infor- mation required by various groups of users for the specification of upper atmospheric condi- tions over Europe suitable for nowcasting and forecasting purposes. The successful operation of DIAS system will lead to the development of new European added-value products and servic- es and to the effective use of observational data in operational applications. The participating in- stitutes working on the acquisition, processing, storage and dissemination of data and services and on the development of the technological sys- tem are the National Observatory of Athens, the Rutherford Appleton Laboratory, the Istituto Nazionale di Geofisica e Vulcanologia, the Swedish Institute of Space Physics, the Leibniz Institute of Atmospheric Physics, the Space Re- search Centre of the Polish Academy of Science, and the Department of Informatics and Telecom- munications of the University of Athens. The of- ficial web site can be accessed in the address . 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