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Detailed investigations have been carried out on the occurrence of bottom side Equatorial Spread F (ESF) and the thermospheric meridional wind characteristics just before the former's initiation using ground based ionospheric data corresponding to the equinoctial periods of 1993–1998, from Trivandrum (8.5°N, 76.5°E, dip = 0.5°N) and Sriharikota (13.7°N, 80.2°E, dip ∼ 10°N) in the Indian longitudes. Critical analysis of the base height of the F‐region h′F at the time of triggering of ESF and the polarity of the meridional winds revealed that, if the h′F is above a certain level ESF occurred under both equatorward and poleward wind conditions. Below that level, ESF occurred only when equatorward winds were present implying that the equatorward winds must somehow be able to offset the reduced growth rate of the plasma instability responsible for ESF. A plausible explanation linking Equatorial Ionization Anomaly (EIA) and the consequent Equatorial Temperature and Wind Anomaly (ETWA) and the consequent neutral dynamics effectively enabling the instability even at lower height has been offered. The threshold height (h′F)c gleaned out on the basis of the polarity of the meridional winds has been shown to bear a linear relation to the solar activity and sheds light on the enigmatic short and long term variability of ESF.

Abstract. The effects on the electrodynamics of the equatorial E-and F-regions of the ionosphere, due to the occurrence of the solar eclipse during sunset hours on 11 August 1999, were investigated in a unique observational campaign involving ground based ionosondes, VHF and HF radars from the equatorial location of Trivandrum (8.5 • N; 77 • E; dip lat. 0.5 • N), India. The study revealed the nature of changes brought about by the eclipse in the evening time E-and Fregions in terms of (i) the sudden intensification of a weak blanketing E S -layer and the associated large enhancement of the VHF backscattered returns, (ii) significant increase in h F immediately following the eclipse and (iii) distinctly different spatial and temporal structures in the spread-F irregularity drift velocities as observed by the HF radar. The significantly large enhancement of the backscattered returns from the E-region coincident with the onset of the eclipse is attributed to the generation of steep electron density gradients associated with the blanketing E S , possibly triggered by the eclipse phenomena. The increase in F-region base height immediately after the eclipse is explained as due to the reduction in the conductivity of the conjugate E-region in the path of totality connected to the F-region over the equator along the magnetic field lines, and this, with the peculiar local and regional conditions, seems to have reduced the Eregion loading of the F-region dynamo, resulting in a larger post sunset F-region height (h F ) rise. These aspects of Eand F-region behaviour on the eclipse day are discussed in relation to those observed on the control day.

[1] The characteristics of different types of Sporadic E (E S ) layers and the associated plasma density irregularities over the magnetic equator have been studied in a campaign mode using VHF backscatter radar, digital ionosonde, and ground magnetometer data from Trivandrum (dip latitude 0.5°N, geographic latitude 8.5°N, geographic longitude 77°E), India. The presence of blanketing type E S (E Sb ) in the ionograms with varying intensity and duration were observed in association with afternoon Counter Equatorial Electrojet (CEEJ) events. E Sb was associated with intense backscatter returns and with either very low zonal electric field and/or with distortions present in the altitude profile of the drift velocity of the type II irregularities. The results of the coordinated study indicate the possible role of vertical electron density gradients in E Sb layers in addition to providing evidence for the local winds to be responsible for the vertical gradients themselves.

Abstract. In this paper, we present observations of equatorial spread F (ESF) irregularities made using a newly installed 18MHz radar located at Trivandrum. We characterize the morphology and the spectral parameters of the 8.3-m ESF irregularities which are found to be remarkably different from that observed so extensively at the 3-m scale size. We also present statistical results of the irregularities in the form of percentage occurrence of the echoes and spectral parameters (SNR, Doppler velocity, Spectral width). The Doppler spectra are narrower, less structured and less variable in time as compared to those observed for 3-m scale size. We have never observed the ESF irregularity velocities to be supersonic here unlike those at Jicamarca, and the velocities are found to be within ±200ms–1. The spectral widths are found to be less than 150ms–1. Hence, the velocities and spectral width both are smaller than those reported for 3-m scale size. The velocities and spectral widths are further found to be much smaller than those of the American sector. These observations are compared with those reported elsewhere and discussed in the light of present understanding on the ESF irregularities at different wavelengths. Key words. Ionoshphere (equatorial ionosphere, plasma waves and instabilities; ionospheric irregularities)

[1] Observations of equatorial electrojet (EEJ) plasma irregularities made using a newly established 18 MHz coherent backscatter radar at Trivandrum are presented. Also presented are the simultaneous observations of the electrojet plasma irregularities made using a collocated 54.95 MHz radar. These correspond to 8.3 m and 2.7 m scale irregularities, respectively. The statistics of spectral parameters observed at the two scale sizes are also presented and compared in detail. An interesting and new aspect of these observations is that the 8.3 m irregularities have significant anisotropy in the plane perpendicular to magnetic field in contrast to that reported for 3 m irregularities. While type-1 and type-2 spectra could be distinctly observed at 2.7 m, it is rather difficult to distinguish at 8.3 m. The type-2 velocities for the two scale sizes are pretty close to each other below 102 km, but the velocities for 8.3 m are less than that of 2.7 m above. Type-1 velocities are quite close to each other only at altitudes of the peak electrojet when clear type-1 echoes are observed. Spectral widths are remarkably different at the two scale sizes. The ratio of spectral width corresponding to 2.7 m to that at 8.3 m is close to 1.4, quite similar to that reported earlier from Jicamarca. The observed difference in the velocities and spectral width at the two scale sizes are attributed to the effects of anomalous collision of electrons caused by the large-scale turbulence present in the EEJ. The above observations are found to be consistent with the EEJ instability theories and related simulation works.