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.
Simultaneous observations of E and F region irregularities made using the Gadanki MST radar are presented. The observations show that the E region echoes weaken or disappear during the growth phase of the topside F region irregularities. Unlike Jicamarca observations, no valley region echoes are observed during this phase. It is shown that the weakening or disappearance of E region signals are not directly coupled with the F region irregularities just overhead, but linked with the instability processes over the magnetic equator through the magnetic field lines. It is proposed that the fringe fields present in the valley region in association with the equatorial F region plasma bubbles, in the presence of appropriate background electric field conditions, are responsible candidates. It is shown that these fringe fields and the electric fields associated with the irregularities in the valley region can map to the low latitude E region and thereby inhibit the growth of the E region instability processes as revealed by the Gadanki radar observations.
[1] Simultaneous observations of equatorial spread F irregularities made with an 18 MHz radar from Trivandrum, located at the geomagnetic equator, and a 53 MHz radar from Gadanki, located at a magnetic latitude of 6.5°N, corresponding to nearly the same longitude zone, are presented. The observations correspond to 8.3 and 2.8 m irregularities, respectively. The spread F irregularities at both the locations are found to occur nearly at the same time but are observed for longer duration at Gadanki than at Trivandrum. The spread F structures as observed in the intensity maps corresponding to Gadanki are characterized by multiple periodic plumes in contrast to a limited number of plumes observed over Trivandrum. The Doppler velocities associated with these irregularities corresponding to Trivandrum are in the range of À100-150 m s
À1, whereas they are in the range of À100-250 m s À1 at Gadanki. Further, the fluctuating velocity fields are much stronger in the Gadanki observations than in the Trivandrum observations. Remarkably, the spectral widths are <100 m s À1 in Trivandrum observations in contrast to those observed at Gadanki with values as high as 300 m s À1 . The observations are compared with those made elsewhere and are discussed in the light of current understanding of the meter-scale irregularities responsible for the radar backscatter.
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