Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsa21a1871s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SA21A-1871
Other
[2423] Ionosphere / Ionization Processes, [2479] Ionosphere / Solar Radiation And Cosmic Ray Effects
Scientific paper
Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (geomagnetic lat = 140 55'N, longitude = 1540 E, dip. angle = 37.30) was investigated using ELF/VLF radio signal. The solar eclipse started at 05:30:04.4 hrs IST and lasted up to 07:27 hrs IST with totally from 6.25 IST to 6.27 IST.The changes in D-region ionospheric VLF reflection heights and electron density during eclipse have been estimated from tweek analysis. The reflection height increased from ~90 km from the first occurrence of tweek to about 93-94 km at the totality and then decreased to ~89 km at the end of the eclipse. The reflection heights are lower by 2-3 km as compared to the usual nighttime tweek reflection heights. The electron density is found to vary between 25-27 cm-3 at the reflection heights. The significant increase in tweek reflection height of about 15 km during the eclipse as compared to the daytime (morning) reflection heights of ~ 78 km is observed. Observations suggest that about 30-40% obscuration of solar disc can lead to the tweeks occurrence which otherwise occur only in the nighttime. A significant increase of 3dB in the strength of the amplitude of VLF signal is observed around the time of TSE as compared to a control day. These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days. During a solar eclipse, the decrease in solar flux due to moon's shadow causes sudden change in the D-region physical and chemical processes. During the totality due to blocking of Lyman-α 1215Å (major D-region ionizing radiation) by moon's umbral shadow, the electron density decreases drastically towards the nighttime values [Smith, 1972]. During the TSE, there was no production of ionization in the ionosphere and the ions and electrons in the lowest part of it recombined at a rapid rate resulting a depletion in the electron density in the 'D' region of the Ionosphere and hence an increase in the effective width of the earth-ionosphere waveguide [Clivered et al., 2001] hence VLF reflection height. An increase in the height of this reflecting surface causes the waveguide "cutoff" frequency to decrease with a resulting decrease in phase velocity and increase in phase delay. Thus, the phase of the signal "lags," in accord with the observations. However, because of the 'decrease in "cutoff" frequency, the attenuation in the waveguide decreases, i.e., the amplitude of the signals increases. This is in accordance with the observed enhancement of the atmospherics/tweeks and increased amplitude strength. REFERENCES: Clilverd, M.A., C. J. Rodger, N. R. Thomsone, J. Lichtenberger, P. Steinbach, P. Cannon, and M. J. Angling, Total solar eclipse effects on VLF signals: Observations and modeling, Radio Sci. 36(4), 773 -788, 2001. Smith, L.G. (1972), Rocket observations of solar UV radiation during the eclipse of 7 March 1970, J. Atmos. Terr. Phys., 3(1), 601-611.
Singh Ambuj K.
Singh Rahul
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