VARIABILITY OF TEC OVER AN EQUATORIAL STATION

Transcription

1 XA VARIABILITY OF TEC OVER A EQUATORIAL STATIO J.O. Adeniyi 1 and S.M. Radicella 2 1 Physics Department, University f Ilrin, Ilrin, igeria 2 The Abdus Salam Internatinal Centre fr Theretical Physics, Trieste, Italy Abstract: Variability in TEC btained by the Faraday rtatin technique at an equatrial statin is investigated. Diurnal, seasnal and slar cycle effects were bserved. Bth abslute and relative variability were cnsidered. The trend f variatins in abslute variability is cmpletely different frm thse f relative variability. Intrductin Ttal clumnar electrn cntent (TEC) data is very imprtant in radi signal transmissin via satellite. When experimental TEC data is nt available, theretical TEC mdels are relied upn. A number f investigatins n the ability f sme mdeling t predict TEC have been undertaken (e.g. Radicella, and Zhang, 1995; Kailang and Jianming, 1994; Ezquer et al., 1999). The assessment f predictin accuracy f ttal clumnar electrn cntent (TEC) mdels requires a well-established variability pattern based n experimental data. In this study, we investigate the variability f TEC ver an equatrial statin. Data and Methd f analysis The data used are thse btained by means f the Faraday rtatin technique frm Legn, Ghana, (Ge. Latitude 5.63, Ge. Lngitude E) MHz signals were transmitted frm the synchrnus satellite ATS-C, which was lcated at apprximately 70 west f lngitude. The satellite was bserved at an elevatin f apprximately 12, azimuth 267 (Kster, 1972). The technique used t cnvert slant t vertical values assumes a fixed crrectin height f abut 400 km. Data frm bth high and lw slar activity perids were selected frm available recrds. Fr lw slar activity, data cnsidered are thse f January, April, July and Octber fr the year Fr high slar activity, we used data fr January and March in 1971 and July and September in The mnths chsen are the representative mnths f winter (December slstice), March equinx, summer (June slstice) and September equinx respectively. All available hurly TEC values fr each f the days f the mnths cnsidered were used fr the study. The number f data used fr the cmputatin fr each f the hurs f the mnths f lw slar activity was nt

2 76 less than. At high slar activity, the number is nt less than 27 fr the December slstice and September equinx. Thse f the ther tw seasns fr this slar activity perid ranged frm 18 t 22. Standard deviatins (SD) frm the hurly mnthly means (TECAVE) were used as the abslute variability index. Relative variability; (SD/TECAV) x 100 was als cnsidered. Results 1. Abslute Variability (a) Diurnal and seasnal variatin The results f abslute variability fr high and lw slar activity perids are shwn in figures la and b respectively. The value and diurnal range f variatin f variability is cnsistently lwer in the June slstice than at ther seasns at bth high and lw slar activities. At high slar activity it varied mst f the time frm abut 1.3 t 6 TEC unit and frm abut 1.5 t 2.7 TEC unit at lw slar activity. Fr all the seasns f bth slar activities, abslute variability is lwer between 0100 and 0800LT than at ther times f the day. A minimum ccurs in the variability arund 1800LT at all seasns f lw slar activity. At high slar activity, the minimum ccurred at abut 1900LT except fr the equinx seasn when it is seen at 2200LT. Between lloolt and 1900LT variability is highest during the March equinx f high slar activity. (b) Slar activity effects Figure 2a - d demnstrates that variability is higher at high slar activity than at lw slar activity at all seasns and fr all times f the day. The difference in variability between high and lw slar activity is mst prminent during the March equinx and least during the September equinx. 2. Relative variability The results fr relative variability are shwn in figure 3a-d. Generally, relative variability is higher at night ( LT) than during the daytime ( LT) and the values fr bth slar activities perids are quite clse mst f the time. The daytime relative variability tends t maintain a cnstant value, which is belw 20 percent mst f the time. The value is abut 10 percent fr mst f the daytime during the June slstice and September equinx f bth slar activities. Cnclusins The majr cnclusins frm this study are given belw. 1. Abslute day t day variability in the June slstice seasn f bth high and lw slar activity is generally lesser than at ther seasns.

3 77 2. Abslute variability is generally greater at high than at lw slar activity. 3. The values f relative variability at bth slar activity perids are quite clse during mst parts f the day at all seasns. 4. Relative variability is higher at night than during the daytime at bth high and lw slar activity perids. Acknwledgments. One f the authrs (JOA) wuld like t thank the Abdus Salam Internatinal Centre fr Theretical Physics, Trieste, Italy, fr hspitality where this wrk was dne. He wuld als like t acknwledge the financial supprt f SIDA during his visit t IOTP under the Assciateship Scheme.

4 78 References Ezquer Rdlf G., Miguel A. Carbrera, Jse R. Manzan, Predicted and measured electrn density at 600 km altitude in the Suth American peak f the equatrial anmaly, Jurnal f Atmspheric and Slar-terrestrial Physics (61)5 (1999) pp Kailang D. and Jianming M., Cmparisn f ttal electrn cntent calculated using the IRI with bservatins in China. Jurn. Atms. And Terres. Phys., 56(3), Kster J.R., Inspheric research using satellites: The equatrial Faraday evening minimum. Interim scientific reprt, University f Ghana. Radicella S.M. and M-L. Zhang, The imprved DGR analytical mdel f electrn density height prfile and ttal electrn cntent in the insphere, Annali di Gefisica, 38, 1, pp35-41.

5 79 Figure Captins Figure 1. Diurnal and seasnal variatin f abslute variability (a) at high slar activity and (b) at lw slar activity Figure 2. Slar cycle effect n abslute variability during (a) December slstice (b) March equinx (c) June slstice and (d) September equinx Figure 3. Relative variability during (e) December slstice (f) March equinx (g) June slstice and (h) September equinx

6 80 (a) HIGH SOLAR ACTIVITY -Jan-71 -Mar-71 -Jul-70 -Sep (b) LOW SOLAR ACTIVITY Figure 1

7 Z 9-mSij STDEV (TEC UIT) O to JS* O> 00 STDEV (TEC UIT) C C en II Ol c_ Z -J I Ul a) STDEV (TEC UIT) STDEV (TEC UIT) r * > > O to *. O5 m II CD i II >! > T3 ->l Ol II 18

8 , ' f 82 i a. O a. < A If in r a> II c a UJ t I-- n. <D O CO I + rm vv r <. CM CD CM CO CO OU.VIA3Q % OUVIA3Q % CO S r in DC in s II < m T[ CC (O 1O * (O W <- 0U.VIA3Q % Figure 3