IL NUOVO CIMENTO Vol. 25 C, N. 2 Marzo-Aprile 2002 Two solar eclipses observations in Turkey E. Tulunay( 1 ), Y. Tulunay( 2 ), C. ¨ Ozkaptan( 1 ), E. T. S¸enalp( 1 ) M. Aydoˇ gdu( 3 ), O. ¨ Ozcan( 3 ), E. G¨ uzel( 3 ), A. Yes ¸il( 3 ), ˙ I. ¨ Unal( 3 ) M. Canyilmaz( 3 ) and E. ˙ Ipekcioˇ glu( 3 ) ( 1 ) Electrical and Electronic Engineering Department, Middle East Technical University Ankara, Turkey ( 2 ) Faculty of Aeronautics and Astronautics, Istanbul Technical University Maslak, Istanbul, Turkey ( 3 ) Department of Physics, Faculty of Science and Arts, Firat University Elazıˇg, Turkey (ricevuto il 2 Luglio 2001; revisionato il 30 Gennaio 2002; approvato il 20 Marzo 2002) Summary. — In this paper, the changes in the ionosphere over Turkey due to two solar eclipses are reported. TEC on the eclipse day (26 April 1976) and the intensity of an HF radio wave during its propagation over 567 km between Ankara and Elazıˇg on the eclipse day (11 August 1999) exhibited a very marked decrease. PACS 94.20.-y – Physics of the ionosphere. PACS 94.30.Va – Magnetosheath; interaction with interplanetary space (including solar wind). PACS 41.20.Jb – Electromagnetic wave propagation; radiowave propagation. 1. – Introduction A station was set up in Ankara to observe variations of the total electron content (TEC) between December 1975 and June 1976. Ertac and Tulunay [1] determined the total ionospheric electron content (TEC) over Ankara from the measurements of the Faraday rotation of a plane polarized wave that have been returned from the geostation- ary satellite ATS 6 transmitting at a frequency of 140 MHz. To show the effect of the ionosphere on HF radio wave propagation, an experiment is set up during the total solar eclipse of 11 August 1999. An HF radio wave was transmitted between Ankara (40 ◦ N, 33 ◦ E) and Elazıˇ g (38 ◦ N, 39 ◦ E) over an approximately 567 km distance. This experiment provided an opportunity to observe the direct effect of the eclipse on the ionosphere. Although the TEC behavior and the propagation of an HF radio wave are not directly related to each other, the authors believe that it is a unique occasion since there were two solar eclipses that can be associated with such data during solar eclipse. c  Societ`a Italiana di Fisica 251