Abstract.A campaign to study turbulence in the mesosphere, over low latitudes in India, using rocket-borne measurements and Indian MST radar, was conducted during July 2004. A rocket-borne Langmuir probe detected a spectrum of electron density irregularities, with scale sizes in the range of about 1 m to 1 km, in 67.5-78.0 km and 84-89 km altitude regions over a low latitude station Sriharikota (13.6 • N, 80.2 • E). A rocket-borne chaff experiment measured zonal and meridional winds about 30 min after the Langmuir probe flight. The MST radar located at Gadanki (13.5 • N, 79.2 • E), which is about 100 km west of Sriharikota, also detected the presence of a strong scattering layer in 73.5-77.5 km region from which radar echoes corresponding to 3 m irregularities were received. Based on the region of occurrence of irregularities, which was highly collisional, presence of significant shears in zonal and meridional components of wind measured by the chaff experiment, 10 min periodicity in zonal and meridional winds obtained by the MST radar and the nature of wave number spectra of the irregularities, it is suggested that the observed irregularities were produced through the neutral turbulence mechanism. The percentage amplitude of fluctuations across the entire scale size range showed that the strength of turbulence was stronger in the lower altitude regions and decreased with increasing altitude. It was also found that the amplitude of fluctuations was large in regions of steeper electron density gradients. MST radar observations showed that at smaller scales of turbulence such as 3 m, (a) the thickness of the turbulent layer was between 2 and 3 km and (b) and fine structures, with layer thicknesses of about a km or less were also embedded in these layers. Rocket also detected 3-m fluctuations, which were very strong (a few percent) in lower altitudes (67.5 to 71.0 km)Correspondence to: H. Chandra (hchandra@prl.res.in) and small but clearly well above the noise floor at higher altitudes. Rocket and radar results also point to the possibility of existence of thin layers of turbulence (<450 m). The turbulence parameters estimated from rocket-borne measurements of electron density fluctuations are consistent with those determined from MST radar observed Doppler spectra and the earlier works.

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Abstract.Observations of ionospheric plasma depletions were made over Kavalur (12.56 • N, 78.8• E, Mag. Lat 4.6• N), India during March-April 1998 using an all sky optical imaging system operating at 630 nm, 777.4 nm and 557.7 nm. Out of 14 nights of observations, plasma depletions were seen only on 9 nights. Except for 21 March 1998, which was a magnetically disturbed period, all other nights belonged to a magnetically quiet period. Some of the important results obtained from these observations are: (a) After the onset of the equatorial spread F (ESF), plasma depletions take typically about 2 hrs 40 min to come to a fully developed state, (b) There are three distinct types of plasma depletions: type 1 have an east-west (e-w) extent of 250-350 km with an inter-depletion distance (IDD) of 125-300 km; Type 2 have an e-w extent of 100-150 km and IDD of 50-150 km; Type 3 have smallest the e-w extent (40-100 km) and IDD of 20-60 km, (c) Most of the observed plasma depletions (> 82%) had their eastward velocity in the range of 25-125 ms −1 . Almost stationary plasma depletions (0-25 ms −1 ) were observed on one night, which was magnetically disturbed. These very slow moving depletions appear to be the result of a modification of the F-region dynamo field due to direct penetration of the electric field and/or changes in the neutral winds induced by the magnetic disturbance, (d) On the night of 21/22 March 1998, which was a magnetically disturbed period, plasma depletions could be seen simultaneously in all three observing wavelengths, i.e. in 630 nm, 777.4 nm and 557.7 nm. It is believed that this simultaneous occurrence was due to neutral density modifications as a result of enhanced magnetic activity. (e) Well developed brightness patterns were observed for the first time in 777.4 nm images. Earlier, such brightness patterns were observed only in 630 nm and 557.7 nm images. These brightness patterns initially appear as very small regions in the northern part of the image and then in about 90 min time, they attain their peak brightness and encompass the entire field-of-view in about 2 hrs 30 min. In some cases, brightnessCorrespondence to: H. S. S. Sinha (hsinha@prl.ernet.in) patterns contain one or two well developed plasma depletions within them. (f) The brightness patterns reported here differ from the earlier observations in that they do not show any differential behaviour in the direction of movement before and after the midnight, and that they are present for extended periods of time as large as 6 hrs.

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