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

annales geophysicae

Effects of solar eclipse on the electrodynamical processes of the equatorial ionosphere: a case study during 11 August 1999 dusk time total solar eclipse over India