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Ann. Geophys., 20, 1977-1985, 2002
www.ann-geophys.net/20/1977/2002/
© European Geosciences Union 2002
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
R. Sridharan, C. V. Devasia, N. Jyoti, Diwakar Tiwari, K. S. Viswanathan, and K. S. V. Subbarao
Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, India
Correspondence to: R. Sridharan
(r sridharan@vssc.org)
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 F-regions in terms of (i)
the sudden intensification of a weak blanketing ES-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 ES ,
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 E-region
loading of the F-region dynamo, resulting in a larger post sunset F-region
height (h' F) rise. These aspects of E-and F-region behaviour on the eclipse
day are discussed in relation to those observed on the control day.
Key words. Ionosphere (electric
fields and currents; equatorial ionosphere; ionospheric irregularities)
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