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Title: Nighttime VHF ionospheric scintillation characteristics near the crest of Appleton anomaly station, Udaipur (24.6°N, 73.7°E)
Authors: Vyas, B M
Dayanandan, B
Keywords: Ionospheric irregularities
Ionospheric propagation
Ionospheric scintillations
VHF scintillations
Issue Date: Aug-2011
Publisher: NISCAIR-CSIR, India
PACS No.: 94.20.Vv; 92.60 hx
Abstract: The paper presents variation of percentage occurrences of nighttime VHF ionospheric scintillation characteristics in amplitude of radio beacon signals of 244/250 MHz transmitted from Fleet Satellite (positioned at 73°E longitude) received over Udaipur (24.6°N, 73.7°E, dip angle 35°) during different levels of solar activity from March 1986 to April 2000. These long time observations of VHF scintillations phenomenon, spanning a solar cycle (1986 to 2000), which covers low, mid as well as high solar activity period, have shown nighttime temporal hourly, seasonal and solar cycle variations in scintillations occurrences. The percentage occurrences of nighttime VHF radio wave scintillations have been observed with maximum percentage occurrences mostly during the equinoxes months, less during winter and the least during summer months in different phases of solar cycle. The average monthly variations of percentage occurrences of VHF ionospheric scintillations activities have been seen to be enhanced during high solar activity year and reduced during low solar activity year. The peculiar feature of discrete or patchy nature of nighttime VHF amplitude scintillations, specifically over Appleton anomaly region, has also been discussed in the light of seasonal and solar activity dependence of scintillations occurrences and their patch duration. The present observations have been compared with the earlier results reported by researchers at Indian stations specifically over similar Appleton anomaly zone and almost similar behaviour of ionospheric scintillations occurrence has been found. An attempt has been made to interpret the present findings on the basis of similarities observed between seasonal solar activity dependence of scintillations events in the present work with the earlier reported statistical observations of seasonal solar activity dependence of maximum electron density of F- region (NmF2), occurrence, and duration of range and frequency type of ESF. Furthermore, the present finding of ionospheric scintillations events have also been linked with the coincidence of numerous precursors of equatorial ionospheric parameters like rising of F-region height due to abrupt increase of upward vertical drift resulting in enhancement in eastward electric field as well as eastward drift after the sunset time over equator with experimentally observed by other researchers. The combined effect of equatorial low ionospheric irregularities phenomena such as development of equatorial ionization anomaly strength over off side of equatorial region is also discussed in view of active precursors to produce the scintillations occurrence in patchy nature away from the equatorial side from the reported studies.
Page(s): 191-202
CC License:  CC Attribution-Noncommercial-No Derivative Works 2.5 India
ISSN: 0975-105X (Online); 0367-8393 (Print)
Source:IJRSP Vol.40(4) [August 2011]

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