Temporal variation of median foEs

Adv. Space Res. VoL. 10, No. 11. pp. (11)35-.(11)38, 1990 Printed in Great Britain. All rights reserved.

0273—1177/90 S0.00 + .50 Copyright © 1990 COSPAR

TEMPORAL VARIATION OF MEDIAN foEs G. A. Moraitis National Observatory of Athens, Ionospheric Institute, P.O. Box 20048, GR 11810 Athens, Greece

ABSTRACT The temporal variation of foEs is examined for monthly median data from Athens Ionospheric Station (38.17 N°, 23.6 E°) over the periods Jan. 1961 to Nov. 1965 and June 1966 to Dec. 1973. The median values, for each hour during the above periods are Fourier analysed for 12—months running intervals; 130 sets of coefficients (amplitudes and phases) are calculated. The analysis shows that: a. The amplitudes and the 12—months mean running average value of sunspot number are linearly related; b. The daily variations of the Y—intercept and slopes for the amplitudes show a characteristic behaviour; c. A stability in phases with a~decrease in higher components is found. The possibility of evaluating median foEs values, using smoothed coefficients, mean monthly sunspot number, time of the day and number of the day is discussed. I NTEODUCT ION One of the most important transient phenomena observed in the ionosphere is “ Sporadic F “, clouds of abnormally high density localised at 95 to 110 Km. It is generally thought that Es clouds are produced as a result of :on compression arising from the action of tidal winds in the presence of the geomagnetic field in the dynamo region at lower F region heights /1,2/. Gravity waves and turbulence are also thought to play a role in Es formation /3,4/. The major problem with Sporadic E is the irregularity in its temporal and spatial variations. One manifestation of the random production of Es ionization is the large fluctuations that occur in the probability of cumulative counts of Es; this holds for midlatitude stations in both hemispheres /5,6/. On some days and in certain locations, Sporadic E provides better than normal propagation conditions with stronger signals. The signal strength at frequencies reflected from Sporadic E layers can differ greatly from that obtained by reflection from other layers /7/. Statistical results /8—10/ obtained from monthly median ionosonde data show a summer maximum, an enhanced winter activity in the temperate zones and an increase during spring compared with the autumn decrease. Paul /11/ using digital ionosonde and three receiving antennas measured the angle of arrival of an echo reflected from Sporadic E by phase comparison. He demonstrated that these layers are frequently tilted in different directions with deviations from the overhead of up to 30 Km; horizontal structure sizes were found to be of the order of 50 Km.

Fig. 1 Characteristics of E—region echoes recorded over two days and one night (centre). (11)35

(11)36

G. A. Moraitis

In our geographical area the ptesence of Sporadic E is important from April to September as a daytime phenomenon with increasing magnitude and converirig the hole day and night in June (Figure 1). DATA ANALYSIS For each hour of the day, the experimental median values, during the time periods January 1961 to November 1965 and June 1966 to December 1973 were selected and Fourier—analyzed (up to the fifth order) in 12—months running intervals. 130 sets of coefficients for every hour were determined. The results are presented in Figure 2 showing typical noon behaviour of all orders (amplitudes) depending on the 12—months mean running sunspot number Ri 2. 6

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Zero order component A(O): Slight positive correlation with R12. The [positive correlation is about as good as that observed for the normal E—layer. — 1st to 5th order amplitudes: All these show a slight negative correlation with R12. The most interesting results are the daily behaviour of the Y—intercept (Y) and the slopes (S) of the component amplitudes. Y of A(O) (Figure 3) presents a characteristic diurnal variation with a maximum before local noon and a minimum around 05.00 L.T. A rapid increase is observed during the morning period and a slight decrease from the noon maximum to the early morning minimum. A second smaller maximum of no importance, is observed around 21.00 L.T. Y of C(1) shows two distinct maxima, the first around 09.00 L.T. and the second around 20.00 L.T. Two minima are also observed around 05.00 and 14.00 L.T. Y of C(2) is stable from around 01.00 to 12.00 L.T. and then shows a slight decrease after noon, a rapid increase from around 14.00 to 23.00 L.T. Y of C(3),C(4) and C(S) show a rather stable curve during the day and a slight increase in the first hours of the night. 1 ‘r

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The slope (5) (Figure 4) of the zero order component A(0) is positive from 02.00 to 19.00 L.T., has a maximum around 14.00 L.T. and a minimum around 23.00 L.T.

24

Temporal Variation of Median foEs

(11)37

S of the first order component is negative during whole day and shows a slight increase with fluctuations, from around 03.00 to 24.00 L.T. The 2nd to 5th order components show almost no dependence on R12 (slope zero). 0.01

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Fig. 4 In order to obtain smoothed curves Fourier analysis (to the 4th order) was then applied to the diurnal variation. The four terms correspond to the 24, 12,8 and 6 periods. While the daily variations of the Y values represent the diurnal behaviour of Sporadic—E, the phases Theta represent the yearly morphology of the phenomenon, one degree corresponding to about one day. Mean Theta (1) (Figure 5) is almost constant (varies between 1800 and 200°). This means that the maximum of the yearly component occurs between the last days of June and first days of -July. Mean Theta (2): Two minima are observed at 06.00 and i8.O0 L.T. with a value about 00 and two maxima at around 03.00 and 14.00 L.T. with a value around 400. The six months component has maxima between the last days of ~ecember and first days of February. Mean Theta (3), Theta (4) show daily variations between 140° to 230° and the mean Theta (5) daily variation between 1100 and 210°. Note that the spread increases from Theta (3) to Theta(5). 250 -

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DISCUSSION AND RESULTS In order to calculate the predicted top reflecting frequency foEs, the inverse procedure must be followed: A(O),C(i),C(2),C(3),C(4) and C(5) are calculated using smoothed Y,S and R12 values. Phases (30n for every month, where n is the order of the components) must be used for a fixed yearly period. The most interesting period is May to August. A good agreement between observed and predicted values is observed in most cases the predicted values are near to the lower envelope of the

G. A. Moraitis

(11)38

observed ones (Figure 6). Resulting from zonal winds and gravity waves, Sporadic—E must be a phenomenon with complex properties, depending on logitude, latitude and season, so that its variability is difficult to predict. Nevertheless thirteen years of observations show a rather stable morphology which may permit HF users to include Sporadic—E in their calculations. REFERENCES 1. 2. 3. 4. 5.

6.

7. 8. 9. 10. 11.

J.D.Whitehead, J.Atm.Terr.Phys. 20, 49—58 (1961) W.I.Axford, J.Geophys.Res. 68(3), 769—779 (1963) J.D.Whitehead, Radio Science 7, #3, 355—358 (1972) M.Anastassiades, G.A.Moraitis and A.Anastassiades, Nature, #5257, 479—480 (1970) G.A.Moraitis and K.B.Serafimov, C.R.de l’Academie bul.des Sc. 39, #11, 53—56 (1986) W.J.Baggaley, J.Atm.Terr.Phys. 47, #6,611—614 (1985) K.Bibl, A.Paul and K.Rawer, J.Atm.Terr.Phys. 16,324—339 (1959) E.K.Smith and S.Matsushita, Ionospheric Sporadic F, Pergamon Press, Oxford, (1962) E.K.Smith, E.K.Smith and S.Matsushita, Ionospheric Sporadic E, Pergamon Press, Oxford, 3—12 (1962) K.Rawer, E.K.Smith and S.Matsushita, Ionospheric Sporadic E, Pergamon Press, Oxford, 151—165 (1962) A.Paul, Radio Science 21, #3, 304—308 (1986)