J. N. Desai

In situ measurements of winds, electric fields and electron densities at the onset of equatorial spread-F

Abstract The results of barium-strontium(Ba-Sr) releases and Langmuir probe measurements at the onset time of equatorial spread- F by two RH-560 rockets launched from SHAR on 16 February 1982 are discussed. Winds and electric fields are measured by Ba-Sr blob releases at heights of 139, 182, 225 and 294 km. The horizontal winds reveal the presence of vertical shears in both the zonal and meridional components located around 150 and 160 km, with minimum shear strengths of 1.7 and 1.9 m s −1 km −1 . respectively. The largest zonal wind observed was 112±3 m s −1 at 225 km. Significant magnitudes (10–40 m s −1 ) of vertical winds were also measured at altitudes of 182, 225 and 294 km, which are below the altitude of the F -layer. Electron density irregularities were also observed in a localized height region at 240–300 km by a Langmuir probe flown by the second RH-560 rocket. The causative mechanisms of the observed density irregularities are discussed and comparison between the various possible destabilizing mechanisms are made.

Dayglow photometry:a new approach

A unique photometer capable of measuring line intensities of <0.1% of the bright background continuum (5 x 10(6)/A) has been developed and successfully commissioned for the measurement of daytime OI 630.0-nm emission from the thermosphere. The photometer employs a low resolution (10(4)) Fabry-Perot etalon, temperature tuned narrowband (3-A) interference filter, radial chopper, and up/down counting system. The optimum choice of the photometer parameters and a unique method of eliminating the background enabled us to overcome the limitations of the earlier attempts to make measurements on such phenomena as dayglow and has opened up a wide range of uses. However, at present, it is being discussed only in the context of dayglow measurements.

Vertical winds and turbulence over Thumba

Abstract Results of the analysis of barium cloud released over Thumba at an altitude 93 km are presented. In the initial stage, the cloud was in the form of two rings coupled together by a knot and later on the rings were distorted to become elongated loop-like structures. A strong north-south wind shear of 30 m/s/km and a vertical wind gradient are observed which explain the distortion of the initial rings into the loops. The lower loop showed development of turbulence, 280s after the release. However, the upper loop did not show any evidence of turbulence throughout the observed period (10 min), but expanded by wind. The delay of 280 s in the onset of turbulence for the lower loop suggests co-existence of turbulent and non-turbulent regions in the 93–95 km altitude range. The top portion of the upper loop moved upward upto an altitude of about 100km where it recorded maximum upward velocity of 19 ±7 m/s. The values of vertical velocities in the 95–100 km height region coupled with the vertical velocities recorded from other barium clouds released by the same rocket at the lower thermospheric altitudes reveal a wavy profile that is suggestive of the presence of an internal atmospheric gravity wave. The altitude of the turbopause observed in the present experiment is 95 ±2 km, about 10 km lower than that observed earlier. It is suggested that gradients in the vertical wind could cause lowering of turbopause level.

Evidence for a large scale electric field gradient at the onset of equatorial spread-F

Abstract Barium-strontium release experiments were conducted at Sriharikota Rocket Range (SHAR. 5.5 N dip latitude) at the onset of equatorial spread-F. We report here an unusual phenomenon of the development of two barium ion clouds from a single release around 200 km altitude, moving with different speeds indifferent directions. This is the first experimental evidence for the presence of large scale electric field gradients with a scale size of 15km. By incorporating neutral wind measured during spread-F into a numerical model for equatorial electrojet we interpret these gradients to be the manifestation of effects due to the meridional winds and wind shears. It is possible that the electric field gradients observed may lead to the generation of plasma holes during the onset of equatorial spread-F.

Interstellar extinction and polarization by spheroidal dust grains

Using the recently available exact computations of the scattering efficiencies of spheroidal particles numerical calculations of the extinction and polarization curves have been made for a distribution of particle sizes, shapes and orientations. The results are presented and compared with the observed interstellar extinction and polarization. Possible models for interstellar dust with nonspherical grains have been discussed.

Twilight optical studies of the El Chichón volcanic dust over Ahmedabad, India

Following the spectacular twilights associated with the El Chichon volcanic eruption, noticed in Ahmedabad, India (23°03′N, 72°30′E) since late May 1982, twilight sky brightness measurements were carried out from the same place for about a year between June 1982 and May 1983. The measurements made mainly in the near infrared I band (λeff = 0.87 μm) show a novel feature representative of the volcanic aerosol layer in the plot of sky brightness against solar depression. The height of the lower end of the aerosol layer (HL) derived from the measurements has remained at ∼ 18 ± 2 km over a period of about one year. The strength of the aerosol feature, determined by an index Q, shows a gradual decrease, suggesting a slow dispersal of the layer with a detectable existence in the I band till the middle of 1984. Multispectral observations indicate that the near infrared transmission window (λeff = 2.2 μm) may be a more sensitive indicator of aerosol layer than visible wavelengths.

Temperature distribution in the corona from λ 5303 Å line-width observations: Eclipse of 1980 February 16 – Tentative evidence for a temperature maximum

Fabry-Perot interferometfic observations in the green coronal line during the total eclipse of 1980 February 16, have yielded line-width temperatures up to 1.5Rė over a wide range of position angles. Least square analysis of the data indicates a tentative temperature maximum near 1.2Rė from the centre of the Sun. The ratio of peak line intensity to square of the continuum intensity Eline/E2cont appears to be inversely correlated to the temperature. Turbulent velocities calculated using this ratio and the observed line-width temperatures show a peak at ~ 10 km s-1.

Doppler shift measurements on the green coronal line—evidence for largescale macroscopic mass motion

Fabry-Perot interferometric observations on the green coronal line (λ 5303 å) carried out during the total solar eclipse of 1980 February 16 have yielded relative Doppler shift velocities with an accuracy of ± 7 km s-1. The values show a peak in the 30–50 km s-1 range indicating largescale macroscopic mass motion in the solar maximum corona.

The excitation mechanism of [Fe XIV] 5303 å line in the inner regions of solar corona

The line intensity of the green coronal line and the continuum intensity are derived from the filter and white light photographs of the solar corona obtained during the 1980 total solar eclipse. Ratio of the line to continuum intensity is plotted against the radial distancer(=R/R0,R0 is the solar radius), in various position angles. A simple model assuming an electron density dependence of the line and continuum intensities suggests a dominant collisional mechanism for the excitation of the line in the innermost regions (∼ 1.4R0). The measured line to continuum ratio tends to a constant value at different radial distances in different position angles. The constancy of the measured line to continuum ratio indicates significant radiative excitation beyond 1.4 R0, in some of the position angles.

Porous and Fluffy Grains in the Regions of Anomalous Extinction

It has long been established that the ratio of total to selective extinction is anomalously large (>- 5) in certain regions of the interstellar medium. In these regions of anomalous extinction the dust grains are likely to be irregular in shape and fluffy in structure. Using discrete dipole approximation (DDA) we calculate the extinction for porous and fluffy grains. We apply DDA first to solid spheroidal particles assumed to be made of a certain (large) number of dipoles. Then we systematically reduce the number of dipoles to model the porous grains. The aggregates of these particles are suggested to form the fluffy grains. We study the extinction for these particles as a function of grain size, porosity and wavelength. We apply these calculations to interpret the observed extincttion data in the regions of star formation (e.g. the Orion complex).