Meteorological effects observed in the D-region of the equatorial ionosphere

69, 1984 Adv. Spaca Printed in Res. Great Vol.4, Britain.No.4, All pp.167—I rights reserved.

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METEOROLOGICAL EFFECTS OBSERVED IN THE D-REGION OF THE EQUATORIAL IONOSPHERE S. P. Gupta and Amarendra Narayan Physical Research Laboratory, Ahmedabad-380009, India

ABSTRfiCT Electron density values were measured during morning hours over Thunba, The results show that electron density in mesosphere is more during sunnier than during winter for sane solar zenith angle. The temperature measurements carried out on the sane day during night hours show that mesosphere is hotter in winter and cooler in sunmer over Thumba. The electron density ar.d temperature are anti—correlated. The results are explained in terms of temperature effects arid other meteorological effects. INT~DWT ]ON The equatorial D region has several special features, for example, the meteorological processes are different over low latitudes than over temperate latiti4es~. The earth’s magnetic field is horizontal over the magnetic equator so the contribution of cosmic rays in producing ionisation over the equator is negligible. Recently Taubenheim /1/ has found a meteorological link between D region electron density and meteorological paraneters. The so called winter anomaly is a well known feature of temperate latitudes. Over mid— latitudes the electron density is higher in the D region in winter than in sunnier. This feature may be related to stratospheric warming as suggested by Offerman et al /2/. For low latit~4e zones, we do not know what is the seasonal effect in D region ionisation. To understand the seasonal effects we have carried out electron density measurements from Thunba (dip.O.6°S). Neutral gas temperature measurements were also carried out during sane day. The results of five rocket flights are presented. F~SULTS The electron density was measured by D.C. Langmuir probe. A bias voltage was applied to the sensor with respect to the rocket body. The current collected by the sensor has been converted into electron density values. The effect of photo—electron was negligible since the rockets were launched during morning hours. Figure 1 shows the rea~lts of two flights carried out in sunzner and winter. The rockets were launched at times when the solar zenith angles were similar /3/. Figure 2 shows results for August and April flights. The solar zenith angle for the flight on 27 April 1.980 was 85° and it was 65° for the 12 August 1972 and 12 Au~ust1982 flights. We can see that the electron density in April was higher than in August, even though the solar zenith angle for the August flights was enaller. Figure 3 shows the observations of neutral gas temperature. The temperature measurements were carried out during evening hours. In December 1978 the temperature was measured on the day before the electron density flight while in April 1980 the temperature was measured one day after the electron density. The temperature data are from &issian M-100 rocket flights which carry Russian payloads. The rockets are launched weekly from Thunba in evening hours. Here we assune that inesos— pheric temperatures are the sane during evening and morning hours which is a reasonable assunption. DI9~ USEION On comparing Figures 1 and 2, we see clearly that over Thunba the mesospheric electron densities are areater in sunnier than in i~d.riteror in autunn. These

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S.P. Gupta and A. Narayan

results are completely different from what has been observed at temperate latitudes where we have winter anomaly. Over Thunba we have sunnier anomaly. On comparing the temperature values, Figure 3, we find that mesosphere is cooler in sunnier and hotter in winter. The lower temperature will reduce the recombination coefficient and hence increase the electron density /11. An other explanation may be that, since the ionisation in the daytime mesosphere is mainly due to photo—ionisation of nitric oxide molecules, we can argue that over Thunba the nitric oxide concentration is greater in suniner than in winter. So far we do not have any measurements of nitric oxide density during different seasons. Such measurements are planned in near future. In conclusion, our results show that the electron density in the mesosphere over Thunba is greater in summer than in winter suggesting the existence of a “suotner anomaly” over Thuinba. This we attribute to an observed seasonal variation in gas temperature. CKNOWLED(~MENTS Authors are grateful to Director, PRL, who took a keen interest in this project. The temperature data were supplied to us by Dr.V.Narayanan of VSS~, Trivandrum for which we are grateful to him. The project was financed by Dept. of Space, Govt. of India. ~PE~I’CES 1. J.Taubenhei.m, Space Science, Rev. ~, 397 (1983). 2. D.Offerman et al, Jour.Atoos. Terr. Phys., 41, 1C51 (1979). 3. S.P. Gupta, Mvance Space Res. ~, 217 (1983~3T

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Fig. 3. Neutral gas temperature in stratosphere and mesosphere during winter and summer.