2. Abstract/INTRODUCTION Electron density (ED) data returned by the ARIEL 3 and ARIEL 4 Satellites have been separated into seasonal, diurnal, longitudinal and latitudinal groups. This worldwide and continues coverage had allowed an extensive examination of the behavior of the ionosphere at mid-latitudes. One of the important results revealed that the mid-latitude ED Trough is the main gross feature of the ionosphere between 40 and 60 invariant magnetic latitude (Λ). In short, its appearance is affected by the relative position of the sub-solar point with respect to magnetic equator. The trough, in terms of this longitudinal dependence, is always observed both day and night in the magnetic winter hemisphere. Otherwise it is usually observed at night. The trough is not observed at all during magnetic equinox or in equinox summer. Its appearance seems to be correlated with the reduction in altitude of (O + - H + ) transition level [1].

3. Since 1990’s Tulunay, Y and her group have been involved and developed data driven models on the temporal and spatial forecasting of the ionospheric critical frequency (foF2) values up to 24 hours in advance [2]. Day-to-day variability of the height distribution of F region ionospheric ED greatly influences the propagation characteristics of HF waves. To predict the day-to-day variability of foF2 has been an important goal. The unpredictable variability greatly limits the efficiency of the operations of communication, radar and navigation systems which employ HF radio waves. The METU-NN Model followed the spatial variations. For terrestrial HF systems, the ED depletion in the trough region reduces the maximum usable frequency that can be reflected by the ionosphere along the great circle [3]. In practical applications of HF radio communication, any model that does not include the ED Trough is not complete [4].

4. Figure1 illustrates the range positions and shapes of the mid-latitude ED Trough in the INTERCOSMOS–19 foF2 data with reasonable limits [5, 6]. The results of the METU-NN model revealed that the Model based system approached the desired operating point, giving rise to some small errors. Accordingly, high correlation coefficients were desired between the observed and predicted foF2 values [6]. Since it is essential to understand the role of the trough in ionospheric variability, a comparison between the ambient ED data returned by the ARIEL 3 satellite from May 1967 to April 1968 and foF2 values observed over the COST251 Area between 1972 and 1976 is made. The results obtained for the December and June Solstices are presented herewith. In particular, foF2 values were obtained from the ionosondes stations at Sofia (42 Λ); Bekescsaba (45 Λ); Miedzeszyn (50 Λ); Kaliningrad (52 Λ); Uppsala (57 Λ); Lycksele (61 Λ); Kiruna (65 Λ).

5. RESULTS and CONCLUSION During the magnetic winter solstice, the signature of the ED Trough is in phase with the foF2 depletions observed in the foF2 data over Europe around the dusk hours at between 42Λ – 65Λ. The electron density trough minimum position (MP) and the minimum position observed in the foF2 values were found at 60 Λ. At the significant level of 0.05, the cross correlation coefficient between latitudinal distributions of the electron densities (ED) and ionospheric critical frequencies (foF2) is 0.95 at dusk hours. Whereas, the signature of the trough is clearly observed towards the dawn hours with a spatial phase shift of ~5 Λ with respect to the dusk hour distributions.

6. The trough is not explicitly observed at the magnetic summer June solstice neither in the ED data nor in the foF2 data as expected since the sub-solar point relative to the magnetic equator does not vary very much during magnetic equinox and magnetic summer [1].

7. References [1]Tulunay, Y. K., 1973. Global electron density distributions from the Ariel 3 satellite at mid- latitudes during quiet magnetic periods. J. Atmosph. Terr. Phys., 35, 233-254, doi:10.1016/0021-9169(73)90090-1. [2]Tulunay, E., Özkaptan, C. and Tulunay, Y., 2000. Temporal and spatial forecasting of the foF2 values up to twenty four hours in advance. Phys. Chem. Earth (C), 25:4, 281-284, doi:10.1016/S1464-1917(00)00017-9. [3]Warrington, E. M., Stocker, A. J., 2003. Measurements of the Doppler and multipath spread of HF signals received over a path oriented along the midlatitude trough. Radio Science, 38:5, 1080, doi:10.1029/2002RS002815. [4]Tulunay, Y. K., Stanislawska, I. and Rothkaehl, H., 2003. Revisiting the Ariel trough work for HF telecommunication purposes. Cosmic Research Journal, 41:4, 1-13, doi:10.1023/A: 1025093223687. [5]Stamper, R., Belehaki, A., Buresová, D., Cander, L. R., Kutiev, I., Pietrella, M., Stanislawska, I., Stankov, S., Tsagouri, I., Tulunay, Y. K. and Zolesi, B., 2004. Nowcasting, forecasting and warning for ionospheric propagation: tools and methods. Annals of Geophysics, Supplement to Vol. 47:2/3, 957-983. [6]Tulunay, Y., Karpachev, A., Tulunay, E., 2003. Spatial Prediction of foF2 in Modeling the Influence of Trough on HF Communication by Using Neural Networks, 3rd COST 271 Workshop Proceedings CD and the web, Spetses, Greece. AND, Tulunay, E., Senalp, E. T., Cander, L. R., Tulunay, Y. K., Bilge, A. H., Mizrahi, E., Kouris, S. S., Jakowski, N., 2004. Development of algorithms and software for forecasting, nowcasting and variability of TEC, COST271 Final Report, Annals of Geophysics, Supplement to Vol. 47:2/3, 1201-1214.