Mitra, S N

Rao, B C Narasinga

English

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EFFECT OF ANNULAR SOLAR ECLIPSE OF I9TH APRIL, 1958 (AT SUNRISE) ON THE Fj LAYER OF THE IONOSPHERES. N. MITRARebeabch Depaktment, All I ndia E adio, New Delhi ANDB. C. NARASTNGA RAOSenioh  Scienttfic A ssistant , O .S.I.R ., All I ndia U adio , T hivandbum[RGcHvvd, OctobM' 30, 1959)ABSTRACT. Tho papor doscnbea tlio offoct of the annual Solar Eclipso of 19th April, 1958 on tho ionisation density of tho l<';. layer over 7’rivandi‘uin, Tiriiehirapalli and Madras (South India) Tho oclipso oreurrod nnar sunrise at all tho throe places and its magnitude at maximum phase was 75 to 83%, Analysis of (ioF.j)2 values diirinf' control period ard  ochiiae day .showed a marked doereaso in the loiiisatioii density with tho jirogross of the eclipse at all the thieo places. Theoretical oonsidenitionfl ot the effect of an ociipse at sunrise on tho ionisation density of the layer are discussed which have also led lo tho dotormination of tho value of attachmont co-offiGient at 350 km. as approxmiately lO-^ soc-^ over Trivan­drum.I N T R O D U C T I O NI t  IS wel] known that an eclipse of the sun produces a decrease in the ionisa­tion density in tho E  and layers. But such an effetjt is not so clearly marked in the F™ it is expected to be so from theoretical considerations. How­ever, when tho eclipse occurs around ground sunrise at any particular location, interesting results can be obtained of its optical effect on the Fg layer. The annular solar eclipse of 19th April, 1958 occurred near sunrise in tho south of India and special investigations of the ionosphere were carried out at Trivandrum, Tiruchirapalli and Madras Stations of All India Radio. The present paper deals with these observations. The circumstances of the eclipse are shown in Table I below.I t  would be noted from Table I that the eclipse occurred very near to ground sunrise at all the three locations. Ionospheric observations have also shown interesting resultsE X P E K J M E N T A L  P E O C E D U R EThe equipment for iouosijheric observations at all the three places are exactly identical. The pulse transmitter is manually operated and is capable of sweeping65540through a frequency range of 1.5 to 18 Mc/s. The radiated power is of the order of One kilowatt. The echoes are received in a receiver modified for pulse reception and displayed on a cathode ray oscillograph in the usual way. A height calibrator provides height marks at hitervals of 40 or 20 km. A suppressor unit has been provided to suppress the strong ground pulse for facilitating the measurement of E layer characteristics. The transmitting and the receiving aerials are con­ventional inverted deltas oriented at right angles to each other.The effect on the Eg layer of the solar eclipse of 19th April 1958 was investi­gated in the following way. The control period was taken as 8 days each pre­ceding and following the eclipse day. The half hourly measurements of layer heights and critical froqueiicies of E and Eg layers for this period were utilised as (iontrol observations. On the eclipse day itself, measurements were taken once every 15 minutes, (The accuracy in height measurements was ‘>r. ±10km and critical frequency ±0.1 Mc/s). Measurements on E layer were vitiated by the appearance of Es and those on the Eg layer will only bo discussed.TABLE 1Circumstances of the eclipse of 19tli Ajiril ’58.Effect of Annuls Solar Eclipse of hWi Aprils etc. 5AlTrivandrum76°57'E)Tiruohirapalh (10^49'N, 78°42'E)Madras(13"0r/N, 80"17'E)Eclipse begins 0604 1ST 0606 1ST 0609 1STEclipse tnaxiinum 0711 .. 0714 „ 0718 „Eclipse oriels 0829 0834 „ 0838 „Ground sunriao 0612 0603 „ 0555 „Magnitude 8a% 7y% 7ri%(18T =  +  houisj.R E S U L T L SThe average behaviour of the Eg layer is determined from the observations during the control days (±8 days centred on the eclipse day) and compared with those on the eclipse day. The layer heights did not show any significant bohavioui- and therefore, only Eg values will bo considered.(a)^  TrivandrumEigui'e 1 shows the variation of (foEa)“ from 0430 to 0930 1ST on the control days and on the eclipse day. The eclipse and sunrise timings are indicated in the figure. I t  would be seen from this figure that there waa a steady and rapid increase in ionisation from about 0530 to 0830 1ST during the control period. This type of rapid increase in early morning is well-known at equatorial latitudes542 8. N, Mitra and B. 0, Narasinga Bctoand 18 also regularly observed at all the three stations. The day-to-day variation IB only in the level of ionisation. On the eclipse day, the initial level of ionisationKip 1 V a r ia t io n  oi ion  deuRity, T n v u n d ru inwas itself high as would be seen from figure I . At about 0600 1ST, the iomsatioii density started ineieasmg and followed its usual trend npto about 0630 1ST Thereafter, it remained more or less stationary as the eclipse jirogressed. It showed a slight decrease at 0715 1ST Thus, the usually observed rapid rise in the ionisation density was arrested Avith the obscjuration of the solar disc. During the recovery period i.e after the maximum of the eclipse, there was a rapid in­crease in ionisation density to reach the normal level by about 0900 1ST. I t  may be noted that the decrease in ionisation density near the maximum of the eclipse, as compared to the level on control days, is quite considerable.(6) TiruchirajyaUiFigure 2 shows the variation of (foFjj)® for Tiruchirapalli. The ellipse timings were a few minutes later than those at Trivandrum. The same trend, as observed in the case of Trivandrum, is also apparent here; the initial level of ionisation was high on the eclipse day, it remained practically constant from 0646 to 0745 1ST, ‘ a small decrease at 0800 1ST and thereafter a, rapid rise to a constant level at 0845 1ST. During the control period, there was a steady but rapid increase in ionisation density from 0600 to 0830 1ST which is, however, a regular feature at this station. The mmimum in ionisation density occurred about 45 minutes after the eclipse maximum. Here also, the decrease in the ionisatian density with the onsei- and progress of eclipse was quite considerable.(o) MtidmsFigure 3 shows the variation of (foFa)® for Madras and the same trend in the behaviour of the ionisation density was observed as was in the case of Tri­vandrum and Tiruohirapaill. On the eclipse day, the values of foF^ at 0430,of Annular Solar Eclipse of 19^ fe April, etc, 6i3Time (1. S. T.) ^Fig. 2. Vitrintion of ioii (ionsity, Tiruchnapalli-Time (I. S, T .)-^Pip. 2 Variation of ion density, Madras.0445, 0515, 0545 and 0615 1ST were doubtful and have been excluded from the L curve. During the control period, the ionisation density increased rapidly 0530 to 0900 1ST. On the eclipse day, the increase was maintained till544 S. N. Mitrd and B. 0 . Ndrasinga Rdo0645 and then the ionisation decreased gradually to reach a minimum i»t- 0745 1ST. Thereafter, the increase was rapid till a constant level was reached at 0900 1ST. The minimum in the ionisation density was quite considerable and wap displaced from the eclipse maximum by about 27 minutes.D I S C U S S I O NI t  is apparent from the variation of (foFo)’^ as shown in figures 1, 2 and 3 that the eclipse did bring down the ionisation density along with the obscuration of the solar disc. I t  is also noticed that the ionisation density did not vary very appreciably as the solar disc was gradually covered and its minimum did not coincide with the maximum obscuration. But in all the tlnee cases, the increase hi ionisation density had been very rapid soon after the maximum of the eclipse. As the eclipse had occiuTed shortly after sunrise, the difference in the behaviour of 'the ionisalion before and after the maximum of the eclipse can be explained (jualitativoly in the following way. During the obscuration of the solar disc, though the intensity of the solar radiation was gradually decreasing Cos x ix ~  solar zenith angle) was on the increase. These two effects wore apparently balanc­ing each other so as to retain the ionisation density at a fairly constant value. On the other hand, after the maximum of the eclipse, both the intensity of radia­tion as well as cosjy were increasing with the result that the ionisation density showed a rapid rise.We have mentioned earlier that the effect of eclipse in the Fg layer of the ionos­phere has not boon so regulai'ly observed as in the case with the E and layers But when the eclipse occurs near ground sunrise the situation is different and a clear effect on the Fg layer should be observable. Wells (1052) has reported well- marked effects of solar eclipse of September 1, 1951 which occurred near sunrise along the coast of United States.I t  is interesting to note that the variation in the ionisation density during the progress of the eclipse, as observed by Wells, is very similar to what has been observed by us and described in this pjipor. Theoretical interpretation of the effect of eclipse near sunrise is discussed below.The ionisation density of the Fg layer at times other than sunrise is not wholly determined by x  is greatly influenced by large scale movements of electrons. Ratcliffc (1956) has showm that if the electrons in the Fg layer disappear by attach­ment process, its co-efficient y varying with height, being given byy(Z) =  10-‘ exp [ j goc-i . . .  (1)then the time constant of the electrons at different heights w^ ould be 3 “lioiirs Ml 300 km, 7.6 hours at 350 km and 20 hours at 400 km. In the pressntthe eclipBo lasted for about 2 hours and the layer height was about 350 km. Thus, if the same considerations are to hold good, the effect of the eclipse should be nil.But the eclipse occurring near sunrise brings about a significant differ­ence. Assuming that the peaks electron production rate is at the height(Z^) of maximum electron density {N^) as m a Chapman layer, the variation of with time during solar eclipse is governed by :Effect of Annular Solar Eclipse of 19^ /*. Aprils etc, 545dN ^ (/) dt mWhere A =-- fraction of the solar disc exposed,(^0 “  l)eak electron production rate when the solar rays are vertical.The motion of electrons in the Fj layer represented by a function givesan additional term (Ratcliffe, 1956). But near sunrise, ia largeand y{Z„,)Nj,i(t) and N„, M{t) are small, so that Nfn is mostly determined by AQ^ cos X Thus, when an eclipse occurs near sunrise, its effect should manifest itself m a distinct decrease of the ionisation density in synchronism with the obscuration of the solar disc.I t  will be noted from figure I that -dN^dt0 from 0630 to 0715 1ST.Fi'oni equation (2) we get, for this time interval,A Q q cos X r i Z m )  ■ ■ ■ ■ (3)Assuming probable values of y(Z^) was found to be of the order of 10 sec~  ^when the height of the maximum ionisation density was 350 kra. This value of the attachment co-efficient is in good agreement with the same obtained elsewhere.A C ^ K N O W L E D G M E N T SThe work described in this paper forms a part of programme of ionospheric research of Research Department of All India Radio, and also forms a part of the programme of the Indian National Committee for International Geophysical Year. The paper is published by permission of the Chief Engineer, All India Radio.We are grateful to Mr. P. Ramankutty Nair, Mr. Y.N.G. Rao and Mr. S. S. Kohli, Station Engineers, All India Radio at Trivandrum, Tiruchirapalli and Madras for the supervision and to the CSIR staff at the stations for taking obser­vations. The eclipse pai'ameters have been obtained from the Indian Ephemeris and Nautical Almanac, 1958, published by the Director General of Observatories, New Delhi,R E F E R E N C E SRatclifFo, J. A., 195Q, Solar Eclipses and the Ionosphere, Pergamon Frees, 1.WoHs, H. W., 1952, J .  O e o p h y a . ,  R e s . ,  57, 291.

Effect of Annular Solar Eclipse of 19th April, 1958 (at Sunrise) on the F2 Layer of the Ionosphere

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Effect of Annular Solar Eclipse of 19th April, 1958 (at Sunrise) on the F2 Layer of the Ionosphere

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