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Ann. Geophys., 22, 3129-3136, 2004
www.ann-geophys.net/22/3129/2004/
© European Geosciences Union 2004
Characterization of VHF radar observations associated with equatorial Spread F by narrow-band optical measurements
R. Sekar1, D. Chakrabarty1, R. Narayanan1, S. Sripathi2, A. K. Patra2, and K. S. V. Subbarao3
1Physical Research Laboratory, Ahmedabad, India
2National MST Radar Facility, Gadanki, Tirupati, India
3Space Physics Laboratory, Vikram Sarabhai Space Center, Thiruvananthapuram, India
Abstract. The VHF radars have been extensively used to
investigate the structures and dynamics of equatorial Spread F
(ESF) irregularities. However, unambiguous identification of the nature of
the structures in terms of plasma depletion or enhancement
requires another technique, as the return echo measured by VHF radar
is proportional to the square of the electron
density fluctuations. In order to address this issue, co-ordinated radar backscatter
and thermospheric airglow intensity measurements were carried out
during March 2003 from the MST radar site at Gadanki.
Temporal variations of 630.0-nm and 777.4-nm emission intensities
reveal small-scale ("micro") and large-scale ("macro")
variations during the period of observation. The micro
variations are absent on non-ESF nights while the macro
variations are present on both ESF and non-ESF nights. In addition to
the well-known anti-correlation between the base height of the F-region
and the nocturnal variation of thermospheric airglow intensities,
the variation of the base height of the F-layer, on occasion,
is found to manifest as a bottomside wave-like structure, as seen by VHF radar on an ESF
night. The micro variations in the airglow intensities are
associated with large-scale irregular plasma structures and found to be in
correspondence with the "plume" structures obtained by VHF
radar. In addition to the commonly observed depletions with upward movement,
the observation unequivocally reveals the presence of plasma enhancements
which move downwards. The observation of enhancement in 777.4-nm airglow intensity,
which is characterized as plasma
enhancement, provides
an experimental verification of the earlier prediction based on numerical
modeling studies.
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