Tuhi Ram Tyagi

A multi-station satellite radio beacon study of ionospheric variations during total solar eclipses

Faraday rotation data obtained at Delhi, Kurukshetra, Hyderabad, Bangalore, Waltair, Nagpur and Calcutta during the total solar eclipse of 16 February 1980 and at Delhi during the total solar eclipse of 31 July 1981 have been analysed to detect the gravity waves generated by a total solar eclipse as hypothesized by Chimonas and Hines (1970, J. geophys. Res. 75, 875). It has been found that gravity waves can be generated by a total solar eclipse but their detection at ionospheric heights is critically dependent on the location of the observing station in relation to the eclipse path geometry. The distance of the observing station from the eclipse path should be more than 500 km in order to detect such gravity waves.

Some geographic and geophysical aspects of electron content measurements from satellite radio beacon transmissions

Geographic distribution of the ionospheric electron content has been studied from the Faraday rotation of satellite radio transmissions recorded at several stations in the Indian zone. The effect of large scale ionospheric irregularities on the Faraday rotation rate and rotation angle has been demonstrated. The integrated electron production rate has been obtained as a function of solar activity from the rate of change of electron content around sunrise for the period October (1964)–September (1968) (the rising half of the present solar cycle) and compared with other estimates, both theoretical and experimental. There are days of unusually high electron content apparently unconnected with any observed geophysical event; these days also show high 30 MHz cosmic radio noise absorption and the two are linearly related.

Comparison of Faraday rotation observations at Delhi with IRI model

Abstract Comparison of Faraday rotation (FR) observations made at Delhi with FR computed from option II of the International Reference Ionosphere (IRI) shows that during low solar activity (1975–1976) the IRI model grossly overstimates daytime Faraday rotation in all seasons by 185 to 320%. The premidnight and postmidnight values are also overestimated by 130 to 205% and 95 to 200% respectively. During high solar activity (1980), the daytime FR values predicted by the IRI are overestimated by 70 to 110%. The premidnight and postmidnight values are also overestimated by 55 to 115% and 100 to 280% respectively. However, it may be noted that in the 05 to 06 hour time slot, the estimates are within 20% in all seasons during low solar activity and in winter and autumnal equinox of high solar activity. It is suggested that instead of the CCIR set, a different set of co-efficients as applied in option I and II be derived based on the observed data obtained in the low latitude region.

On the determination of ‘mean field height’ from the Faraday fading of satellite transmissions

A method of determining the ‘mean field height’ is described. This method is applicable only for the stations where the quasi-transverse (Q-T) position can be located on the Faraday fading record of the satellite transmissions.

Lunar and solar daily variations of ionospheric electron content at Delhi

Abstract Ionospheric electron content measurements obtained at Delhi during the period 1975–1980 have been analysed by the Chapman-Miller method to compute lunar and solar daily variations. The results show that the magnitude of the lunar harmonic components is about one-tenth that of the solar harmonic components. Significant seasonal and solar cycle variations were observed for both the lunar and the solar terms. The lunar semi-diurnal component, the most significant term, can be explained as due to the additional ‘fountain’ effect caused by the lunar semi-diurnal variation of the electric field at the equatorial region. The lunar semi-diurnal variations were found to have significant oceanic and ionospheric components.

Time lag in occurrence of Q-T point on Faraday rotation records at widely spaced frequencies

Abstract An explanation of the non-occurence of the Q-T points simultaneously on the Faraday rotation records of the two widely spaced frequencies, has been given in terms of the “refraction” theory. It is found with some simplified assumptions, that the time delay in Q−T points at the two frequencies ƒ 1 and ƒ 2 ( ƒ 1 , > ƒ 2 ) is proportional to μ √(k 2 −μ 1 2 sin 2 j − μ 2 √(k 2 −μ 2 2 sim 2 j where μ1 and μ2 are the refractive indices for the frequencies ƒ 1 and ƒ 2 respectively, k is a constant depending on the satellite height and the mean field height and j the satellite zenith angle at the mean field height. The theory explains the observed time difference between the two Q-T points reasonably well.

Satellite radio beacon monitoring of the troposphere

Abstract In this communication some effects of characteristic enhancements and/or fluctuations of the satellite radio beacon transmissions are described. It is convincingly shown that these effects are of tropospheric origin. The satellite radio beacon can thus be used for monitoring the tropospheric events.

On the determination of mean field height using Q-T position on Faraday fading records

Abstract Ionospheric refraction of HF signals from low orbit satellites is investigated and it is shown that its effect is insignificant in the determination of mean field height using the quasi-transverse ( Q - T ) point on 40 MHz Faraday fading records.

Clock Synchronization Experiment in India Using Symphonie Satellite

A recent clock synchronization experiment between the National Physical Laboratory (NPL), New Delhi and Space Applications Centre (SAC), Ahmedabad via geostationary satellite symphonie-II, stationed at 49°E longitude, is reported in this paper. As only one satellite transponder was available for this experiment, the two-way transmission of the clock pulses was carried out by switching the transmit—receive roles at the two stations at 5 minute intervals to achieve a nearly simultaneous two-way transmission. Taking into account all the additional delays, the results demonstrated a clock—synchronisation accuracy of better than 0.5 μs. A crystal-based portable clock flown aboard an aircraft confirmed this clock-synchronization to within a microsecond.