R. Narayanan

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.

A multiwavelength daytime photometer—a new tool for the investigation of atmospheric processes

Technological innovations in optics in the form of a spiral mask system and in electronics in the form of on-line gate scanning of Fabry-Perot fringes and critical balancing of gate widths for complete background rejection have allowed unambiguous detection of faint dayglow emission features of multiple wavelengths buried in the bright daytime background continuum. The technical details of this unique multiwavelength daytime photometer (MWDPM) and its performance characteristics together with its potential application for the investigation of several geophysical phenomena are presented and discussed.

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.

O I 630.0‐nm dayglow in the region of equatorial ionization anomaly: Temporal variability and its causative mechanism

Results of the continuous measurements of O I 630.0-nm dayglow from Mount Abu (20° 20′N dip latitude), a station in the region of equatorial ionization anomaly (EIA) by means of a recently developed ground-based dayglow photometer are presented. The measurements on magnetically quiet and moderately active days reveal that there are strong noontime bite-outs in 630.0-nm intensities. The temporal variabilities in the intensities show a good correlation with those in electron densities in the lower F region over Ahmedabad (18° 23′N dip latitude) with a time delay of 1 hour. Theoretical estimation of the intensities indicates that the dissociative recombination of O2+ with ambient electrons is mainly responsible for the observed temporal variability in the dayglow emission. The time delay between the theoretically evaluated and the measured noontime bite-outs is discussed in the context of the development of the EIA.

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.

Ionization hole campaign-a coordinated rocket and ground-based study at the onset of equatorial spread-F: first results

Abstract A comprehensive multi-technique campaign involving the launch of two high altitude RH-560 rockets was carried out from Sriharikota (SHAR), India, a near-equatorial rocket launching station at the onset of equatorial spread-F, along with a host of ground-based complementary experiments at other locations spread over the country. The main objectives were to obtain the background ionospheric and thermospheric conditions at the onset of equatorial spread-F, and to be able to evaluate the relative importance of the various agencies, the neutral dynamics in particular. Multiple barium cloud releases, in situ plasma diagnostic measurements, along with complementary optical and radio probing experiments were carried out as a part of this campaign. The presence of large scale gradients in the ambient electric fields at ~185 km altitude and also of vertical winds of significant magnitudes at higher altitudes were found. The presence of large scale irregularities in the ion densities at heights above 250 km in a region of negative background density gradient is one of the significant new results. The details of the Ionization hole campaign along with the first results are presented and discussed in the context of the present knowledge of the phenomenon of equatorial spread-F.

Novel mask design for multiwavelength dayglow photometry.

We present and discuss a novel mask design that provides multiwavelength measurement capability for the dayglow photometer.

Improved chopper mask for the dayglow photometer

The chopper mask of the new dayglow photometer has been redesigned for optimum performance. The minimum detectability improves at least by a factor of 2.

First results of OI 630.0 nm dayglow measurements from equatorial latitudes

Abstract First results of the campaign to measure the OI 630.0 nm dayglow emission continuously from an equatorial station, Sriharikota (5.5° dip latitude), India, during January–February 1989 are presented. Results from zenith observations during five days reveal close coupling with the F -region electron densities controlled by the electrodynamical processes, showing features like the noon-time ‘bite-outs’ and several other fine features. These new results give us clues for identifying the significant processes that result in OI 630.0 nm dayglow emission.

Winds, wind-shears and plasma densities during the initial phase of a magnetic storm from equatorial latitudes

Abstract Results of a sodium vapour release experiment carried out from SHAR (India), an equatorial rocket launching station, immediately after (⩽ 2 h) a storm sudden commencement (SSC) during the initial phase of a magnetic storm, followed by electron density measurements are presented. Many of the relevant atmospheric parameters, namely, neutral winds and their altitude variation, the magnitude of the shears in them, the neutral temperature with altitude by spectroscopic methods, diffusion measurements on the released trail, clues on the turbopause level and the electron densities including the structures (irregularities) in them were obtained. The results of the temperature measurements carried out independently on the sodium trail by means of a ground-based Fabry-Perot spectrometer, operating on the sodium D 1 line, resonantly scattered by the trail have already been reported by us ( Ranjan Gupta et al ., 1986). In this paper the effects of the excess temperature reported earlier and the rest of the measured parameters on the electron density profiles are evaluated using models and compared with the measurements. The formation of sharp layers of ionization have been explained on the basis of the electro-dynamical processes associated with the wind shears at a location, close to the edge of the equatorial electrojet belt. The significance of the changes in the neutral composition due to the enhanced neutral temperature and the low turbopause level, in increasing the base-level plasma densities by a factor of 3–5 are demonstrated and the possible role of plasma dynamics discussed.