Solar Extreme Events of 2003:
Fundamental Science and Applied Aspects

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Manifestations of solar extreme events of October-November 2003 in the ionosphere over Europe imaged by radio tomography
Kunitsyn V.E., Tereshchenko E.D., Andreeva E.S., Kozharin M.A., Kozlova M.O., Melnitchenko Yu.A., Nesterov I.A., Khudukon B.Z.,

Using the approaches developed earlier, tomographic images of electron density distribution in the ionosphere over Europe are obtained for time interval 28-31 October, 2003. Tomographic results show that a "spot" of enhanced electron concentration appears in the evening and night hours (approximately from 20 UT on 30 October to 2 UT on 31 October) at latitudes of middle Europe. The spot moves from East to West approximately along the geomagnetic latitude. Duration of the structure motion within the region under study was of the order of 5-6 hours. The dimensions of the spot were 500-1000 km in the North-South direction and 2000-5000 km in the East-West direction. During two hours the size of the spot were comparable with the size of the region of signal registration, therefore it is not impossible that real dimensions of the structure were larger than those seen in the reconstruction. Levels of maximum electron densities in the spot region, as a rule, exceeded the daytime values of neighboring days. TEC values reached 30 TECU, which is also higher than the neighboring daytime values. The passage of a spot of increased ionization was also noticed in the night on 29/30 October, but at that time the dimensions of the structure were not larger than 1000 km. For the period under study, the data from a series of European ionosondes were compared with plasma frequencies foF2 calculated from RT reconstructions. It was shown that in the North (Tromso) and in the South of Europe (Athens, Rome and San Vito) - outside the spot - the daily behavior of foF2 on 28-31 October was close to the regular variation typical of weakly disturbed ionosphere: plasma frequency values increase in the daytime and decrease by night. foF2 behavior obtained from ionosondes data is quite similar to that calculated from RT reconstruction. In the ionosphere over central part of Europe within the spot of enhanced electron density the foF2 behavior was rather different: on 29 and 30 October, a significant (one-half as great) decrease of daily foF2 values as compared to the typical values of quiet interval; during the evening and night hours on 30/31 October, a sudden increase of plasma frequency was observed, the gradient approaching a few MHz per 15-20 minutes. foF2 behavior as obtained from British (Chilton and Fairford) and German (Juliusruh) ionosondes during this time interval was, on the whole, also much similar to plasma frequency changes retrieved from RT reconstruction. For the first time, 4-dimensional tomographic images of the ionosphere under strongly disturbed helio-geophysical conditions are obtained. Several specific features of ionospheric behavior are revealed. Comparison of tomographic results with independent ionosondes measurements has demonstrated a high quality and efficiency of the developed RT approaches and algorithms.

Skobeltsyn Institute of Nuclear Physics, Moscow State University, 2004