An all sky optical imaging system was operated from Sriharikota rocket range (SHAR) (14• N, 80• E, 5.5• N dip latitude) during January-March, 1993 to observe ionospheric plasma depletions through 630 nm and 777.4 nm night glow emissions. Strong plasma depletions were observed only on four nights viz., 14, 17, 19 and 21 February, 1993. Except the 17 February, which was a magnetically disturbed day, all the other nights pertained to magnetically quiet period. A number of plasma depletion parameters such as, degree of depletion, east-west extent, tilt with respect to the geomagnetic field, inter-depletion distance, drift velocity and plasma enhancements or brightness patterns were estimated. Some of the important results are: (a) It was found that the east-west extent of plasma depletions varied with the degree of depletion; for the 630 nm images the degree of depletion ranged between 6-9% per 100 km east-west extent and for 777.4 nm images it was 3% per 100 km east-west extent, (b) The average inter-depletion distance (IDD) was in the range of 1500 ± 100 km during the magnetically disturbed period and 740 ± 60 km during quiet periods. This is suggestive of gravity wave modulation of the bottom side of the F-region. While the large scale gravity waves (1500 ± 100 km) of auroral origin could be responsible during magnetically disturbed period, smaller scale gravity waves (740 ± 60 km) having their origin in the lower atmosphere could produce initial perturbation in the bottom side of the F-region, (c) Plasma depletions are observed to have an eastward tilt in the range of 10-15• with respect to the geomagnetic field. It has been suggested here that these tilts are associated with the variation of plasma drift with altitude, (d) plasma depletions are observed to be moving eastwards with drift velocities in the range of 40-190 ms −1 , and (e) Strong plasma enhancements or brightness patterns were observed on three nights. The degree of enhancement was by a factor of 1.4-3.8. These enhancements lasted for more than 15 minutes. Although, prima facie, these observations look similar to the transient brightness wave reported by Mendillo et al. (1997a), the high degree of enhancement and an extended duration of more than 15 minutes, observed in the present case, need to be understood.

[1] In this paper we study the off-electrojet low-latitude daytime E region plasma irregularities using first multi-instrument observations in India made during July 2004 by the MST radar from Gadanki (13.5°N, 79.2°E, magnetic latitude 6.4°N), Langmuir probe on board the RH-300 Mk II rocket, and ionosonde from Sriharikota (13.6°N, 80.2°E, magnetic latitude 6.4°N). Radar echoes were confined to altitudes below 105 km and were observed in the form of a descending echoing layer with the descent rate of 1 km/h. Virtual height of the E layer, as observed by ionosonde, shows identical descending behavior. A detailed analysis based on the radar and ionosonde observations shows that the radar echoes are related to the range spread in the ionogram. Rocket observations made on 23 July 2004 revealed weak plasma irregularities with scale sizes more than 100 m and no noticeable irregularity at shorter scales. The spectral slope of the irregularities observed by the rocket probe is found to be À4 for scales in between 1 km and 100 m. During the rocket launch, radar did not detect any echo conforming that the small-scale irregularities were not present. Examination of concurrent observations of neutral wind made by TIMED Doppler interferometry suggests that zonal wind plays a crucial role in forming electron density layers, which become unstable via the gradient drift instability with background electric field or/and zonal neutral wind generating low-latitude E region plasma irregularities.Citation: Patra, A. K., N. Venkateswara Rao, D. V. Phanikumar, H. Chandra, U. Das, H. S. S. Sinha, T. K. Pant, and S. Sripathi (2009), A study on the low-latitude daytime E region plasma irregularities using coordinated VHF radar, rocket-borne, and ionosonde observations,

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.

Combining the data of in-situ measurements of ionospheric current, J m by rocket-borne instruments and the ground based geomagnetic H field close to the magnetic equator a linear relation has been found between the peak current density J m and the daily range of H, (R H ). This relationship has been used to convert long series of R H data into J m . Combining J m and the E-region peak electron density N m , the electron velocity in the ionosphere, V E has been calculated. It is shown that after all corrections are made of the solar zenith angle variations, the ionospheric current as well as electron drift in American and Indian sectors show strong equinoctial maxima, the mean values of both the parameters are larger at American than in Indian sector. The solar cycle variation of the electrojet current is primarily due to the variations of N m E, and not due to the variations of electric field. The diurnal variation of the electric field with peak at 09-10 LT interacting with noon peak of N m E making ΔH to peak at an hour earlier than noon. It is stressed to realise the importance of electric field in diurnal, seasonal and longitudinal variations of the equatorial electrojet current.at Yauca, Chimbote, Chiclayo and Talara covered the whole width of the EEJ. In India new observatories were established at Trivandrum and Annamalainagar. EEJ observatories were organised at Koror, Jarvis and Ibadan. Table 1 gives the coordinates and the magnetic field parameters of some of these observatories.Chapman and Rajarao (1965) showed that the seasonal variations of the range in H as well as in Z at the equatorial electrojet stations have strong semi-annual wave with maxima during equinoctial months. Analysing the lunar tides in the critical frequency of the F 2 layer, f o F 2 , at equatorial stations in Peru and in India during the IGY period, Rastogi (1962a) predicted that the equatorial electrojet current strength must be stronger in Peru than in India. Later using the geomagnetic data from the equatorial observatories during IGY, Rastogi (1962b) confirmed that the electrojet current is indeed stronger in Peru than in India. The strength of the electrojet current was shown to vary roughly inversally to the background mean value of the geomagnetic field H at the station. It was suggested that the electrical conductivity over the magnetic equator is inversally proportional to the strength of the H field.If equatorial electrojet is a part of the global S q current system, modified close to the magnetic equator, the day to day variations in the strength of the equatorial electrojet should correlate with the corresponding variations in the strength of the S q currents at low latitudes outside the electrojet belt. However the daily ranges of H at different latitudes were found not to correlate well (Osborne, 1966;Schlapp, 1968;Kane, 1971). James et al. (1996) has shown that the correlation of ΔH at an equatorial station, Trivandrum, with ΔH at other stations in India decreases with increasing latitude. It becomes almost zero ar...

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