C. Raghava Reddi

Mean winds and tidal components during counter electrojet events

The first observations of mean winds and the amplitude and phases of the tidal components measured with a meteor wind radar located at Trivandrum (8.5°N, 77°E) during five consecutive days of counter electrojet events identified in the horizontal component of the earths magnetic field from January 27–31, 1987 are described. The mean zonal winds in the altitude region of 90–105 km are in general westward during counter electrojet days and eastward during no counter electrojet days. The amplitudes and phases of the tidal components on the counter electrojet days are found to be substantially different from those on the no counter electrojet days.

Night-time ionospheric scintillations at the magnetic equator

The nocturnal and seasonal variations of equatorial ionospheric scintillations are presented. Scintillations are classified into two classes, namely, class I and class II depending on their fading rates and association with bottomside spread-F. Power spectra and frequency indices for class I and class II scintillations are presented and their theoretical implications are discussed.

Climatologies of tidal winds in the radio-meteor region over Trivandrum (8°N)

Abstract The climatologies of tidal wind fields in the 80–105 km region using Meteor Wind Radar (MWR) observations during June 1984–1988 at Trivandrum (8.5°N) are presented. The most significant feature revealed by the amplitude and phase profiles of the tidal winds is the simultaneous presence of two modes, differing in vertical wavelengths, for each oscillation. In contrast to the expected (1,1) mode, the (1,−2) diurnal mode is found to be stronger. In addition to the stronger (2,2) semi-diurnal mode, higher order modes are also found to be significant. The 24h oscillation is found to be the strongest component and 12 h and 8 h oscillations are equally significant. The tidal wind amplitudes over Trivandrum are found to be larger during the equinoctial months. The amplitudes and vertical wavelengths of the two interfering modes of the 24 h, 12 h, 8 h oscillations are deduced and compared with our current theoretical understanding on tidal winds and observations from other low latitude stations.

Meteor trail induced backscatter in MST radar echoes

The amplitude variations and Doppler spectra of the 50 MHz radar echoes from meteor trails are investigated. The radar echoes from underdense meteor trails particulary during nighttime do not always fit the conventional specular reflection model. The echo was found to spread, sometimes, over as large as 8–10 km in range. In each range bin, the spread echo from the trail is produced with an altitude dependent time delay after the passage of the meteoroid producing the ionized trail. The Doppler spectra of the spread echo in each range bin is asymmetric with a high frequency wing comprised of narrow spectral peaks and successive spectral peaks amplitude decrease with frequency. These observations are interpreted as due to multiple reflections of the radar pulse between the striations generated parallel to the geomagnetic field along the length of the meteor trail. There is evidence to show that within the dynamo region the EW and NS components of the currents at lower and higher altitudes could be in opposite directions during the early morning hours of Sq electric field reversal.

The annual and semi-annual wind fields in low latitudes

Abstract The annual oscillations (AO) and semi-annual oscillations (SAO) of the monthly mean zonal and meridional winds in the troposphere, stratosphere and mesosphere have been studied using simultaneous data from balloonsondes, rocketsondes and Meteor Wind Radar (MWR) acquired at Trivandrum. A study is also made of the AO and SAO of the diurnal, semi-diurnal and ter-diurnal wind oscillations in the 80–102 km altitudes using MWR observations. The same observations have been used to study the altitude profiles of the annual mean winds, and the annual mean tidal winds. It is found that the AO phase in the NS wind leads with increasing altitude in the stratosphere and mesosphere while SAO phase lags with altitude. The SAO of EW tidal wind amplitudes are slightly stronger than AO. For the NS wind, the AO of the diurnal tide has the largest amplitude and the SAO of the semi-diurnal tide also has a relatively large amplitude. The results are compared with earlier reports from other low latitude stations and with the CIRA 1986 model for 8.5 °N latitude. Further, they are discussed in the light of tidal dissipation in and above the mesopause ( > 70 km).

Spatial domain interferometric VHF radar observations of spread meteor echoes

Abstract This paper is concerned with short lived ( s ) meteor trails which produced VHF radar echoes, spread in range >10 km . The Doppler spectrum of these echoes in each range bin is very broad and asymmetric with many discrete peaks. Spatial domain Interferometer observations of Spread Meteor Echoes were made at the Indian National MST Radar Facility to locate the echoing regions on the trail producing the discrete peaks in the broad Doppler spectra of each range bin. We report that the mean direction of arrival was found to be similar for all Doppler frequencies in each range bin and varies for successive bins by much less than one degree. The echoing regions on the trail are located nearly parallel to the radar beam direction. Further, the directions of arrival were found to be not necessarily normal to the geomagnetic field. The observations are discussed in terms of ionization structures developed within the meteor trail.

Annual and semiannual temperature oscillations at the mesopause levels over Trivandrum (8.5°N, 78°E)

The ambipolar diffusion coefficient (D) of the atmosphere, obtained from the exponential amplitude decay of 62,081 underdense radio meteor trails, recorded by the meteor wind radar operated at Trivandrum, was analyzed to obtain the annual (AO) and semiannual oscillations (SAO) of the monthly mean “D”. The amplitudes of AO and SAO of the mean D were interpreted in terms of AO and SAO of the mesopause temperature. The results show excellent agreement with the AO and SAO of mesopause temperature reported earlier using rocket data in low latitudes. While the COSPAR International Reference Atmosphere (CIRA) 1972 models reproduce the SAO observed in the present study, the more recent CIRA 1986 models do not reproduce it. Both CIRA 1972 and CIRA 1986 do not reproduce AO of mesopause temperature at low latitudes. It is of significance to note that AO of mesopause temperature is much stronger than SAO.

Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

Abstract A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s −1 (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s −1 also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere. The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.

Long period wind oscillations in the meteor region over Trivandrum (8°N, 77°E)

Abstract The spectra of long period wind oscillations in the meteor zone over Trivandrum are presented. The spectral amplitudes were found to be much larger during June 1984 when the QBO in the stratospheric zonal wind was in a strong easterly phase compared with June 1987 when the zonal winds at the altitude of maximum QBO were weak westerlies. Zonal wind amplitudes for periods of 15 and 5 days were found to be most significant during these two June months. The amplitudes of these two oscillations in meridional wind were found to be as large as the amplitudes in the zonal wind. The vertical wavelength in both zonal wind and meridional winds of the 15-day oscillation is very large whereas for the 5-day oscillation the vertical wavelengths were 80 and 65 km during June 1984 and June 1987, respectively. The results are discussed.

Quasi-periodic fluctuations in electron content during a partial solar eclipse

CHIMONAS and Hines1,2 have suggested that a solar eclipse might generate gravity waves in the atmosphere. During solar eclipse the localised cooling of the atmosphere in the lunar shadow causes an energy imbalance and the shadow moving at supersonic speed across the Earths surface could be a continuous source of gravity waves propagating to great distances in the atmosphere. These quasi-periodic wave perturbations in the ionospheric electron density, caused by the coupling between the ionised and the neutral particles, have been detected by various investigators3–5 at middle and high latitude stations away from the path of the eclipse shadow. Hajkowicz6 has reported the observation of perturbations of quasi-peniods of less than 2 min, after the October 1976 eclipse. We report here the observation of quasi-periodic fluctuations in Faraday rotation angle Ω and the 1 MHz modulation phase delayφ of 40 MHz transmissions from ATS-6 geosynchronous satellite recorded at Trivamdrum (dip 0°57′S, geographical longitude 76°57′E) during the partial solar eclipse on 29 April 1976. φ directly gives columnar electron density integrated along the radio ray path from the satellite to the receiver; whereas Ω gives columnar integrated electron density only up to an altitude of 2,000 km because of the weightage by the component of the geomagnetic field along the ray path. For ATS-6 to Trivandrum ray path geometry, a change of 10° in Ω will be produced by a change of 0.47×1016 el. m–2 of electron (el.) content. In contrast to this, 10° change in φ will be produced by a change of 0.34 × 1015el. m–2. The accuracy of measurement of both Ω and φ is better than 1°.