Article

Characteristics of VHF scintillations in the Indian equatorial and low latitude stations

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Abstract

The VHF scintillation data from three different locations, namely, Pondicherry (11°N, 78°E, dip 7.6°N), a near-equatorial station, and Waltair (17.7°N, 83.3°E, dip 20°N) and Mumbai (19°N, 73°E, dip 25°N), the two anomaly crest region stations, are considered for the study of some typical characteristic features and geophysical effects on the occurrence of VHF scintillations both on short and long term basis. The data pertaining to the period of increasing sunspot years of 1998 and 1999 from the three different locations are considered for these studies. The percentage occurrences of scintillations revealed that the occurrence is more at the equatorial station, Pondicherry, compared to those at the two off-equatorial stations, Waltair and Mumbai. The onset of scintillations, in most cases, is found to occur first at the equatorial station, and later at the two off-equatorial stations. However, there are some occasions when the occurrence of scintillations is first seen at Waltair and Mumbai without their prior occurrence at the equatorial station, Pondicherry. These cases are believed to be associated with the generation of irregularities at the anomaly crest regions. Further, it is found that the scintillation occurrence increases as the sunspot number increases at all the three stations with increasing latitude, suggesting the widening of the scintillation activity belt with the increase of solar activity. The suppression effects of magnetic activity on the occurrence of scintillations seem to be more evident during the pre-midnight hours than during post-midnight hours.

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... The scintillation activity during disturbed days was maximum (14%) in 2013. The low scintillation occurrence percentage during disturbed periods compared to quiet days is corroborated with earlier results reported from this station (Prasad et al. 2004) and from other low (Bhopal) and moderately mid-latitude stations in India, which include, Varanasi and Agra (Kumar et al. 1993;Kumar and Gwal 2000). ...
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This research reports on recently recorded 250 MHz amplitude scintillations at Waltair (17.7°N, 83.3°E), a low-latitude station in India, using the signals radiated from a geostationary satellite (FLEETSAT, 73°E) during a six-year period (2008 - 2013), which covers extremely low and higher solar activity years (2008 and 2013). The morphological features in terms of local time, month and season during different geophysical conditions are presented. The scintillation patches (segregated based on their occurrence durations) have shown an increasing trend with the increasing sunspot activity. The scintillation patches with 30- minute duration show increasing trends with increasing sunspot activity, and their occurrence frequencies also show increasing trends with increasing sunspot activity. The scintillation activity during disturbed epochs (Kp index lies between 3+ and 9) is found to be less compared to its quiet day counterparts. The plausible mechanisms for these observational results are discussed. In addition, power spectral characteristics, including Fresnel frequency, upper role of frequency and spectral slope of scintillations are calculated and the salient results are presented. © 2016 Terrestrial, Atmospheric and Oceanic Sciences (TAO). All rights reserved.
... The effects of geomagnetic disturbances on equatorial as well as low latitude ionospheric scintillations have been reported by several workers Rastogi et al. 1981;Vyas and Ayanandan 2011;Singh et al. 2010) who explained that with increase in the magnetic activity, the probability of occurrence of scintillation increases during the post-midnight periods in all longitude sectors, while the pre-midnight phenomenon depends on the season as well as on longitude. By comparing the percentage occurrence of scintillations during quiet and disturbed days, Rama Rao et al. (1996Rao et al. ( , 1997 and Prasad et al. (2004Prasad et al. ( , 2012 have reported that the occurrence of nighttime scintillation is inhibited during disturbed days of higher solar activity period. Kumar et al. (1993) and Kumar and Gwal (2000) studied the effects of geomagnetic disturbances on scintillations at low latitudes and reported that the geomagnetic disturbances suppresses the scintillations throughout the night at Bhopal (23.2N, 77.6E), but at Varanasi (25.3N, 83E) the scintillations were inhibited in pre-midnight period and enhanced in the post-midnight period. ...
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In the present study, we have used 250 MHz radio signal radiated by geostationary satellite UFO-02 to study the occurrence characteristics of very high frequency (VHF) scintillations associated with ionospheric irregularities during recent extreme low solar activity period from 2008 to 2010 at low latitude Indian station Varanasi (Geomag. latitude =14o 55/ N, long. = 1540 E, Dip angle = 37.3o, Sub-ionospheric dip = 340). The impact of this recent extreme low solar activity period on ionosphere is investigated. It is observed that the scintillation occurrence is low having maximum percentage occurrence during pre-midnight periods. With increasing interest in understanding the behavior of ionospheric irregularities, an effort has been made to examine also the influence of solar and magnetic activity over the occurrence of scintillations. During the extreme low solar activity years the scintillation occurrences do not vary linearly with the sunspot number. The inhibition and generation of irregularities during enhanced magnetic activity period are explained by considering changes in the electric field. The spectral analysis provide spectral index for irregularities which varied between -1.5 and -8 and characteristic length of irregularities varied between 400 m to 1200 m which confirms that 250 MHz scintillations observed over Varanasi were associated with intermediate scale irregularities.
... (1) Data for only equatorial/low latitude scintillation-post-sunset hours (18:30 LT-06:30 LT) were used (Aarons, 1982;Basu et al., 1988Basu et al., , 2002Prasad et al., 2004;Tanna et al., 2013) (2) In order to minimize the effects of the multipath on the observations, measurements with a satellite's elevation angle greater than 30 0 are taken into account and ...
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... During the disturbed days the percentage occurrence of scintillations is maximum around 21:00 h LT during equinoctial and summer months while during winter months, it is mostly equal between 20:00 and 02:00 h LT. By making a comparison between the percentage occurrence of scintillations during quiet and disturbed days, Rama Rao et al. (1996Rao et al. ( , 1997 and Prasad et al. (2004) reported that the occurrence of night-time scintillations is inhibited during disturbed days of the high solar activity. Alex and Rastogi (1987) also reported an inhibition in the occurrence of scintillations at low latitude on geomagnetic disturbed days in the pre-midnight period. ...
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