Journal of Armospheric and Terrestrial Phvmcs, Vol. 51, No. I. pp. 1%27, 1989. OOZI-9169/89S3.00+ .W Printed zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in Great hitam. Pergamon Pressplc Fine scale structure and turbulence parameters in the equatorial middle atmosphere D. K. CHAKRABARTY, G. BEIG, J. S. SIDHU and S. R. DAS Physical Research Laboratory, Ahmedabad 380 009, India (Receivedfor publication 22 July 1988) Abstract-Two rockets carrying identical spherical probe payloads were launched from Thumba to measure positive ion density of the mesosphere and lower thermosphere over an equatorial location. Data obtained show the presence of strong irregularities in the ion density. From the measured positive ion current, the spectra of the spatial density fluctuation, turbulent velocity, energy dissipation rate and eddy diffusion coefficients have been derived in the altitude range of 7&100 km. The results are found to be different from those at middle and high latitudes. 1. INTRODUCTION Several measurements made by rocket and ground based techniques in the past have shown the presence of fine scale structures in the ionization density profiles of the lower ionosphere (MECHTLY et al., 1967 ; ZIMM- ERMAN et al., 1972 ; MAN.SON et al., 1969 ; BELROSE et al., 1972; HAUC et al., 1977). However, the nature and cause of these irregularities are still not properly known. In this part of the atmosphere, ions are in the minority (1 part in 10 million by volume) and the collision frequency is large, hence a close coupling between ions and the neutral air is expected. Also, since the atmosphere in this region is under turbulence mixing, one can assume that the irregularities in the ionization density are caused by the neutral air tur- bulence. Based on this assumption, Thrane and his co-workers have derived information on various tur- bulent parameters from the positive ion density profile measured by rocket borne probe (THRANE and GRAN- DAL, 1981 ; THRANE et al., 1985, 1987). It is worth mentioning here that very recently, during the MAP/ WINE campaign, ion and neutral density were mea- sured simultaneously by neutral mass spectrometer and positive ion probe on board several rockets. An intercomparison of the results supports the assump- tion that under certain circumstances positive ions can be used as passive tracers for neutral gas number density fluctuations (L~BKEN et al., 1987). It may be interesting to mention here that PRAKASH et al. (1980), in reviewing the results of several rocket measure- ments of electron density from Thumba, identified the presence of irregularities in the electron density distribution due to the turbulence in the neutral den- sity distribution. The middle atmosphere is likely to be dynamically very active. The turbulent state of this region may exhibit latitudinal and seasonal variations. Several authors (HESSTVEDT, 1968 ; BLUM and SCHUCHARDT, 1978) made attempts in the past to study these varia- bilities of turbulence, but no consistent picture has emerged so far. Measurements made by Thrane and his co-workers are for Andsya (69”N, 16”E), a high latitude station. Our aim is to study how far the tur- bulence characteristics at the equatorial station differ from those at a high latitude station, when studied by the same technique. With this aim in mind, two spherical probes were flown on two rockets from Thumba. The ion current measured by one of the probes has been used to derive various turbulent parameters such as turbulent velocity, energy dis- sipation rate and eddy diffusion coefficient. The results thus obtained have also been compared with the values given by other workers. 2. THE EXPERIMENT Two spherical probes were flown on Centaure and Rohini 300 rockets from Thumba (8”N, 76”E) on 7 and 10 March 1986, respectively. The Centaure reached an apogee of about 130 km and the Rohini 300 reached 98 km. The probes were mounted at the top of the rocket payload under the nose cone, which was ejected around 51 km. The spherical probe con- sists of a solid sphere of diameter 1.4 cm surrounded by another concentric gridded sphere of diameter 4.0 cm. Both the spheres were gold plated. The outer sphere was kept at the rocket potential and the inner sphere was given a fixed bias of - 6 V. An electronics package was connected to the probe which could mea- sure current from lo-’ to lo-” A. Linear ampli- 19