In thispap er, the changesin 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 itspropagation over 567 km between Ankara and Elazıˇg

on the eclipse day (11 August 1999) exhibited a very marked decrease

Aydovgdu, M.

Canyilmaz, M.

Güzel, E.

Ipekciovglu, E.

Tulunay, E.

Tulunay, Y.

Yeşil, A.

Özcan, O.

Özkaptan, C.

Ünal, I.

Şenalp, E.T.

English

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 Elazig on the eclipse day (11 August 1999) exhibited a very marked decrease

Tulunay, E

Tulunay, Yurdanur

Ozkaptan, C

Senalp, ET

Aydogdu, M

Ozcan, O

Guzel, E

Yesil, A

Unal, I

Canyilmaz, M

Ipekcioglu, E

OpenMETU (Middle East Technical University)

Two solar eclipses observations in Turkey

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO Vol. 25 C, N. 2 Marzo-Aprile 2002Two solar eclipses observations in TurkeyE. Tulunay(1), Y. Tulunay(2), C. O¨zkaptan(1), E. T. S¸enalp(1)M. Aydogˇdu(3), O. O¨zcan(3), E. Gu¨zel(3), A. Yes¸il(3), I˙. U¨nal(3)M. Canyilmaz(3) and E. I˙pekciogˇlu(3)(1) Electrical and Electronic Engineering Department, Middle East Technical UniversityAnkara, Turkey(2) Faculty of Aeronautics and Astronautics, Istanbul Technical UniversityMaslak, Istanbul, Turkey(3) Department of Physics, Faculty of Science and Arts, Firat UniversityElazıgˇ, 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 twosolar eclipses are reported. TEC on the eclipse day (26 April 1976) and the intensityof an HF radio wave during its propagation over 567 km between Ankara and Elazıgˇon 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 (includingsolar wind).PACS 41.20.Jb – Electromagnetic wave propagation; radiowave propagation.1. – IntroductionA 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 thetotal ionospheric electron content (TEC) over Ankara from the measurements of theFaraday 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 theionosphere on HF radio wave propagation, an experiment is set up during the total solareclipse of 11 August 1999. An HF radio wave was transmitted between Ankara (40◦N,33◦E) and Elazıgˇ (38◦N, 39◦E) over an approximately 567 km distance. This experimentprovided 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 directlyrelated to each other, the authors believe that it is a unique occasion since there weretwo solar eclipses that can be associated with such data during solar eclipse.c© Societa` Italiana di Fisica 251252 E. TULUNAY, Y. TULUNAY, C. O¨ZKAPTAN, E. T. S¸ENALP ETC.2. – Ionospheric total electron content measurements from Turkey during thesolar eclipse of 29 April 1976 over AnkaraA station was set up in Ankara to observe variations of the total electron content(TEC) between December 1975 and June 1976. The METU station was equipped witha switched polarimeter consisting of a 1inear cross yagi antenna, a receiver and a penrecorder recording a Faraday polarization twist of VHF radio waves which are trans-mitted at the 140 MHz from the geostationary satellite ATS-6. The location of thesub-ionospheric point is 36.4◦N, 33.2◦ E. The polarization angle was measured at 15minutes interval, and continuous data were received for almost all days except duringlate December and partly in January due to chart recorder failure. There were also someoff signal periods due to the interruptions of the signal from the satellite. The data havebeen reduced by employing the method referred to in Klobuchar and Rastogi’s work [2].After being encoded the data are being sorted into hourly group. In order to remove the“nπ” ambiguity the critical frequencies (f0F2) of the nearest ionosonde stations to thesubionospheric point have been obtained from the WDC-C1.The diurnal variations in the ionospheric TEC are shown in fig. 1 for three days inApril 1976 prior to the solar eclipse. In these curves the effect of increasing magneticactivity is also seen. As Kp (daily sum of the three-hour planetary magnetic activityindex) increased enhancements did occur in the TEC. Maximum TEC enhancementswere observed around noon hours.In order to investigate the changes observed during the eclipse a non-eclipse or a“control” curve is constructed by taking the arithmetic mean of TEC data of the 27thand 28th of April 1976. The Kp index was 18 during the control days, whereas it was 23on the eclipse day. Figure 2 shows the diurnal variations in the TEC data obtained on theeclipse day. The control curve is also drawn in the same figure for comparison purposes.The TEC values were greater than those of the control curve during the morning period.The solar obscuration was clearly felt in the TEC as the data obtained on the eclipse dayexhibited a very marked decrease. After 15:00 the TEC values again became greater thanthose of the control curve and this behavior continued until 22:00. In order to investigatethe effects of the eclipse on f0F2 data, the diurnal variations of the f0F2 measured onFig. 1. – The diurnal variations in TEC obtained at the METU station.TWO SOLAR ECLIPSES OBSERVATIONS IN TURKEY 253Fig. 2. – The diurnal variations in TEC obtained during the solar eclipse of 29 April 1976.a magnetically quiet day (26 April 1976) and on eclipse (29 Apri1 1976) are plotted infig. 3. As can be seen a very similar behavior is observed in the f0F2 data. On the eclipseday the f0F2 values started to decrease 18 minutes earlier than the first contact point andthey continued to decrease till 50 minutes past the maximum obscuration. Whereas, thecorresponding time intervals were 33 minutes and 5 minutes respectively in the eclipsedTEC data. Figure 4a shows the difference in TEC between the eclipse and control dayvalues. In fig. 4b the time rates of change of both the TEC and the control curve areillustrated. The TEC on the eclipse day started to decrease at 10:45, well before thefirst contact time (11:18). The TEC values continued to decrease till 5 minutes past amaximum solar obscuration which took place at 13:10. The maximum change in TECwas approximately −9 × 1016 el/m2 with respect to the control curve. Recovery tookplace at a rapid rate near the fourth contact and within 25 minutes following the fourthcontact the TEC reached the control curve values. The increase in TEC continued until18:15. Following this the TEC started to decrease reaching the control curve values at20:00.Fig. 3. – The diurnal variations in f0F2.254 E. TULUNAY, Y. TULUNAY, C. O¨ZKAPTAN, E. T. S¸ENALP ETC.Fig. 4. – a) The difference in TEC between the eclipse and control day values. b) The time ratesof change of both the TEC and the control curve.3. – HF relative signal strength measurement between Ankara and ElazıgˇThe objective of the experiment between Ankara (40◦N, 33◦E) and Elazıgˇ (38◦N,39◦E) was to observe the relative variations in the signal strength during the 11 August1999 total solar eclipse. Table I shows the times relevant to the 11 August 1999 totalsolar eclipse.TWO SOLAR ECLIPSES OBSERVATIONS IN TURKEY 255Table I. – Times (UT) of partial and total (or maximum) eclipse in Elazıgˇ and Ankara duringthe August 11, 1999 total solar eclipse.Beginning of Maximum End of partialpartial eclipse 96.7% eclipseAnkara 10:58:13 12:24:58 13:45:01Beginning of Beginning of End of total End ofpartial eclipse total eclipse eclipse partial eclipseElazıgˇ 11:12:23 12:36:14 12:38:18 13:2-54:02A map showing the regions related to the total eclipse of date 11 August 1999 ispresented in fig. 5. The receiver and transmitter used were both Harris RF 3200 type150W HF transceivers. The transmitter was located in Ankara, while the receiver waslocated in Elazıgˇ. The transmitting antenna was a yagi-type antenna and could bedirected in any direction with remote control. The receiving antenna was a Rohde &Schwartz HE 011 calibrated active monopole. The data were recorded by laptop personalcomputer running software specific to the prolab multimeter, which gave an RS 232 serialoutput to the computer. A block diagram of the experiment set up is shown in fig. 6.The radio was tested and an appropriate point in the circuitry of the radio was foundFig. 5. – Map of regions affected by the total solar eclipse. Black region shows total eclipse area.256 E. TULUNAY, Y. TULUNAY, C. O¨ZKAPTAN, E. T. S¸ENALP ETC.Fig. 6. – Set-up of equipment in HF relative signal strength experiment.from which it was possible to obtain a measure of the relative strength of the receivedsignal. A suitable point which provides a DC voltage between 0.7 and 6.5 V representingthe relative signal strength in the range from 0 to −120 dBm was located on the receiverelectronic card.Fig. 7. – Plot of relative signal strengths measured during the total solar eclipse.TWO SOLAR ECLIPSES OBSERVATIONS IN TURKEY 257Fig. 8. – Plot of running mean of relative signal strengths measured during the total solar eclipse.A link was made at 18.111 MHz and a 12000 seconds recording of data was made.The results are displayed in fig. 7. The time was synchronized with universal time (UT)through an atomic clock timeserver on the Internet. When the running mean of therecorded data shown in fig. 8 is taken in order to eliminate the oscillations observed onfig. 7, a clear decrease of strength during the eclipse became very obvious. The HF relativefield strength reached a minimum near the times at which the total eclipse occurred atElazıgˇ and it began to rise to a value near the value observed at the beginning of theeclipse.4. – DiscussionHow difficult is to interpret the effects of an eclipse on the topside ionosphere is aknown fact. However, in general terms, the TEC level decrease well before the firstcontact and the ionosphere recovers after the fourth contact on 29 April 1976. Similarly,Jakowski et al. [3] reported that TEC level decreased considerably on 11 August 1999reacting abruptly the switching off of the solar radiation.The HF radio wave experiment as revealed in fig. 8 indicates that the relative signalstrength decreases due to the total solar eclipse of 11 August 1999. The opposite typeof variation would have been expected since the rate of absorption in the ionospheredecreases due to decrease of photoelectrons in the shadow of the moon. It is believedthat since the HF radio wave frequency of 18.111 MHz is near to the natural ionosphericcritical frequency, the probable thinning of the ionosphere caused the part of the HFwave to be transmitted into space above the ionosphere instead of being reflected backto the Earth.It is hoped that these cases reported here could provide some experimental insight,which are rare to be experienced.258 E. TULUNAY, Y. TULUNAY, C. O¨ZKAPTAN, E. T. S¸ENALP ETC.∗ ∗ ∗The HF experiment is partially funded by the Turkish State Planning Office as anMETU DPT project, AFP-03-01-DPT.98K 122690. The transmitter-receiver and HFpropagation were provided by the General Directory of Wireless Communication. Thesignal corps of the Turkish Army helped the first reception of the signal. Dr. G. Gootof the UMIST, UK, provided the active calibrated receiving antenna. Late Prof. Dr. H.Oranc is the one who took initiative in setting up the METU Faraday rotation signaldata. All are gratefully acknowledged.REFERENCES[1] Ertac E. and Tulunay Y., Ionospheric total electron content measurements from Turkeyduring the solar eclipse of 29 April 1976, Advanced Study Institute on Dynamical AndNeutral and Ionized Atmospheres, Nord Torpa, Norway 1979.[2] Klobuchar J. A. and Rastogi R. G., Radio Sci. (USA), 23 (1988) 292.[3] Jakowski N., Schlueter S., Haise S. and Feltens J., Auswirkungen derSonnenfnsternis wom 11. August 1999 auf die Ionosphere, Allegemeine Vermessungs-Nachrichten (AVN), 11-12/1999 (Wichmann Verlag) 1999, pp. 370-373.

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Two solar eclipses observations in Turkey

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