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Characteristics of VHF scintillations in the Indian equatorial and low latitude stations
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.
... 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). ...
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. ...
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 ...
The amplitude scintillation information recorded by the GSV4004B GISTM (Global Ionospheric Scintillation TEC Monitor) GPS receiver at an Indian low latitude station Surat (21.160N, 72.780E) for 48 months during the years 2009, 2010, 2011 and 2012 are utilized in the present work. Multifractal Detrended Fluctuation Analysis (MF-DFA) have been carried out along with computation of q-order fluctuation function, q-order Hurst exponent, q-order mass exponent and multifractal spectrums for each monthly post-sunset S4 index time series. The non-linear dependence of mass exponent and dependence of q-order Hurst exponent on q-values reflect the existence of nonlinear interaction between different scales and multifractal structure in the system respectively. The comparison of broadness and shape of spectra with the occurrence of scintillation activities registered in the same period reveal the existence of multifractality/complexity in the turbulent ionosphere, which is influenced by the small-scale intermittency and solar flux indices. The truncation of the spectrum is the evidence of manifestation of small-scale intermittency of the turbulent ionosphere. The higher values of the Hölder exponent 0, calculated from the spectrum, imply the irregular nature of the underlying process. The present study suggests that, MF-DFA may act as an important non-linear technique for identifying the effect of large and small-scale fluctuations in complex and turbulent ionosphere.
... 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. ...
The amplitude scintillations data recorded at 244 MHz from the geostationary satellite, FLEETSAT (73°E) at a low latitude station, Waltair (17.7°N, 83.3°E) during the ten year period of high to low solar activity from 2001 to 2010 is considered to study the occurrence characteristics of the VHF scintillations. A close association between the intense scintillations on VHF signals during pre-midnight hours, associated with range type of spread-F on ionograms and a relatively weak and slow fading scintillations during post-midnight hours associated with frequency type of spread-F is observed during the relatively high sunspot years from 2001 to 2004, whereas during the low sunspot years from 2005 to 2010 the scintillation activity as well as spread-F activity are found to be minimum. During both the high and low sunspot years, it is observed that the maximum scintillation activity occurs during equinoctial months followed by winter with the minimum occurrence during summer months. The annual mean percentage occurrence of scintillations is found to be clearly associated with the variations in the annual mean sunspot number. The nocturnal variations in the occurrence of scintillations show the onset of scintillation activity starts from 19:00 h LT with maximum of occurrence around 21:00 h LT. A clear semiannual variation in the occurrence of scintillations is observed during pre-midnight hours with two peaks in equinoctial months of March/April and September/October. The number of scintillation patches observed is found to be more during pre-midnight hours compared to those during post-midnight hours. The most probable scintillation patch duration lies around 30 min. Further, it is also found that the number of scintillation patches with durations of 60 min and more decreases with the increase in the patch duration. It is also observed in general that the scintillation activity is inhibited during geomagnetic disturbed days.
Ionospheric scintillation variations are studied using GPS measurements at the low latitude station of Shenzhen (22.59°N, 113.97°E), situated under the northern crest of the equatorial anomaly region, from the Chinese Meridian Project. The results are presented for data collected during the current phase of rising solar activity (low to high solar activity) from December 2010 to April 2014. The results show that GPS scintillation events were largely a nighttime phenomenon during the whole observation period. Scintillation events mainly occurred along the inner edge of the northern crest of the equatorial anomaly in China. The occurrence of scintillations in different sectors of the sky was also investigated, and the results revealed that it is more likely for the scintillations to be observed in the west sector of the sky above Shenzhen. During the present period of study, a total number of 512 total electron content (TEC) depletions and 460 lock loss events were observed. In addition, both of these events are likely to increase during periods of high solar activity, especially because the strong scintillations are often simultaneously accompanied by TEC depletions and lock losses by GPS receivers.
The ionospheric scintillation and TEC (Total Electron Content) variations are studied using GPS (Global Positioning System) measurements at an Indian low latitude station Surat (21.160 N, 72.780 E; Geomagnetic: 12.900N, 147.350E), situated near the northern crest of the equatorial anomaly region. The results are presented for data collected during the initial phase of current rising solar activity (low to moderate solar activity) period between January 2009 and December 2011. The results show that within a total number of 656 night-time scintillation events, 340 events are observed with TEC depletions, Rate of change of TEC (ROT) fluctuations and enhancement of Rate of change of TEC Index (ROTI). A comparison of night-time scintillation events from the considered period reveal strong correlation amongst the duration of scintillation activity in S4 index, TEC depletion, ROT fluctuations and ROTI enhancement in the year 2011, followed by the year 2010 and least in 2009. The statistical analyses of scintillation activity with enhancement of ROTI also show that about 70 to 96 percent scintillation activity took place in equinox and winter months. Moreover, from a nocturnal variation in occurrence of scintillation with (S4 ≥ 0.2) and enhancement of ROTI with (ROTI ≥ 0.5), a general trend of higher occurrence in pre-midnight hours of equinox and winter seasons is observed in both indices during the year 2011 and 2010, while no significant trend is observed in the year 2009. The results suggest the presence of F-region ionospheric irregularities with scale sizes of few kilometers and few hundred meters over Surat and are found to be influenced by solar and magnetic activity.
Simultaneous observations of VHF-ionospheric scintillations at Bhopal, Varanasi and Agra, during the period (January 1991-December 1991) have been reported. Nocturnal variation of occurrence of scintillations is found to be higher at Bhopal as compared to that at Varanasi and Agra. Under the geomagnetic disturbances the scintillations are suppressed in pre-midnight period at all the three stations. Severity of suppression is found to be more at Bhopal. The storms with the D(sub st) value below -100nT, reaching in local nighttime or in very early morning hours are found to be most effective in giving rise to the scintillations in post-midnight period. The results have been interpreted in terms of (1) F-region irregularities associated with spread-F and (2) coupling of high latitude and magnetospheric current systems with equatorial electric field.
The VHF scintillation observations made at a chain of stations in India during February-March 1993 under AICPITS (third campaign), using the 244 MHz radio beacon from FLEETSAT, and analysed jointly by all participating investigators at a workshop held at Rajkot are described. The occurrence features of scintillations during the third campaign were similar to those observed during the first campaign of March-April 1991. The maximum occurrence was, however, reduced to some extent due to the lower solar activity. The latitudinal variations showed an increased occurrence in the region of about 17-18° latitude. The data during the night of 19-20 Feb. 1993 when an extensive 'Ionisation Hole Campaign' was undertaken showed scintillations marked by earlier onset and longer duration at stations Waltair and Nuzvid than at the stations close to the magnetic equator. However, based on the average variations during February-March 1993 the onset at Tiruchendur, Anantpur and Waltair was at nearly the same time. The vertical rise velocity of the plasma depletions, estimated from the time delays in the onset of scintillations at latitudes away from the dip equator, was found to range from 40 m/s to 420 m/s in the altitude region 300-1350 km.
Results are presented on VHF radio wave scintillation at 244 MHz recorded during nighttime at the Trivandrum station (India) since July 1985. These are compared with the data on scintillations at 137 MHz observed at another equatorial station, Huancayo in the American sector, for the period June 1969-May 1970. It was found that whereas in the Indian sector the seasonal variation of scintillatons exhibit maxima during the two equinoxes, the scintillations observed in the American sector exhibit only single maxima, which were observed during the December solstices. Moreover, with a decrease of solar activity, the scintillation activity decreases in the Indian sector much faster than in the American sector.
The velocity of plasma-bubble rise over the magnetic equator is calculated on the basis of simultaneous measurements of the onset times of postsunset VHF scintillations from the Japanese satellite ETS-2, obtained at a meridian array of four Indian stations during February 1980. The data and calculation results are presented in tables and graphs and discussed in detail. It is found that bubble velocities increase with altitude, varying in the ranges 128-416, 38-327, and 15-200 m/sec at altitudes 450-550, 550-1140, and 1140-1270 km, respectively. These results are shown to be in good agreement with satellite and radar measurements and with F-layer vertical drift velocities.
Over 13,000 hr of amplitude recordings of the VHF telemetry signal from ATS-3 were examined for seasonal and magnetic dependence. It was found that magnetic activity tends to increase scintillation occurrence during the June solstice and decrease the seasonal variation of scintillation occurrence in general. Scintillation during a magnetic storm was examined in detail. Comparisons of spread-F (Fs) with scintillation at stations similar in geomagnetic latitude and differing in longitude were made and a pattern similar to that reported by Lyonet al. (1960) was found.
Based on ETS-II geostationary satellite (130 deg E) 136-MHz radio beacon
amplitudes, a study of postsunset VHF scintillations observed during the
January-February 1980 solar maximum period confirms that they are
essentially controlled by the generation of equatorial F region
irregularities. This latitude is substantially beyond the 84.4 deg E
geographic meridian equatorial anomaly daytime crest. Good agreement
between the observed scintillation occurrence pattern and
characteristics, and previous results reported up to 21 deg N
(Somayajulu et al., 1984), is found, and the occurrence of 23 deg N
scintillations is found to be conditional to their prior occurrence at
lower latitudes. It is also shown that, at least during high solar
activity, the equator irregularities extend to altitudes in excess of
1300 km in the Indian sector, and consequently, cause scintillations in
a much wider latitudinal belt.
Recent studies of the physics of F-layer irregularities in the equatorial ionosphere have been concerned with the development of plumes or patches. A series of observations in the equatorial anomaly region in a year of high solar flux has been analyzed for the radio propagation effect of scintillations. The observations were made on patches in the developing, mature, and decay phases. Although irregularities develop on the west wall of the patches, the intensity of scintillation does not appear to diminish within the patch; the patches contain bursts of high level activity. Patch characteritics at microwave wavelengths match airglow depletion images when two considerations are introduced, i.e., the westward tilt of the patch as shown by optical and radar observations and the effective path length of the irregularities affecting the radio propagation path. Using optical images of depletions the effective thickness of the layer of irregularities above the peak of the F2-layer can be estimated; it is relatively short, i.e., of the order of 70 km for the gigahertz frequencies and 150 km for the 257 MHz transmissions. The total path length is 110 km for the microwave frequencies and 220 km for the lower levels of scintillation at 257 MHz. The decrease in microwave scintillations compared to meter wavelength observations in the midnight and post-midnight time period in these anomaly observations is due to the combination of decay of electron density as well as the relatively rapid decay of smaller scale irregularities, as has previously been noted in observations at the magnetic equator.
A study is made of the correlation between the occurrence of spread-F echoes at Slough, Inverness and Oslo. It is concluded that the ionospheric irregularities which cause the spreading occur in patches which have dimensions of the order of 500 km in a NS direction, and considerably greater in the EW direction. There is an indication that these “bands” of irregularities may lie along lines of magnetic latitude, and that they tend to occur in the same geographical position on successive nights. The correlation of the scintillations of the radio star in Cassiopeia with the occurrence of spread-F echoes at two places is investigated. The results are found to be consistent with the same picture of the spatial distribution of the irregularities. Some indirect arguments suggest that the irregularities causing the scintillations are at heights near 300 km.
Equatorial spread F (ESF) and VHF scintillations have only in recent years been related to large-scale upwelling plumelike irregularity structures identified on VHF backscatter radar intensity maps. The present investigation is concerned with the results of an experiment of VHF scintillations recorded at a meridian chain of stations in India extending from close to the magnetic equator to 24.8 deg N magnetic latitude. It is found that the latitudinal development of scintillation-producing irregularities during high solar activity conditions is controlled by the development of ESF irregularities over the magnetic equator. Scintillations at all the stations equatorward of 21 deg N magnetic latitude are saturated, and their onset in the postsunset hours is almost abrupt. The occurrence of scintillations at latitudes away from the equator is dependent on their prior occurrence near the magnetic equator.
Using the data from a network of low latitude field stations in India,
recording the amplitude scintillations on 244 MHz radio beacon signals
transmitted from FLEETSAT at 73 deg E longitude, dynamics of patches of
irregularities in the F-region have been estimated in E-W as well as N-S
directions. The occurrence of scintillations in the Indian sector at a
station close to the dip equator has been found to be at a maximum
during E-months, less during D-months, and least during J-months. The
scintillations are found to be greatly reduced during geomagnetic
disturbances. The signature of individual scintillation events have been
found to be remarkably identical at an E-W pair of stations seperated by
260 km.
THERE is at present conflicting evidence as to the region of the ionosphere which is responsible for the scintillation of radio stars. Thus workers in the northern hemisphere1,2 observing sources at fairly high angles of elevation, and also Mills and Thomas3 in the southern hemisphere observing the Cygnus source at an altitude of 15°, have found a marked correlation between the occurrence of scintillations and of spread F echoes. On the other hand, Bolton et al.
4 at the same southern location as Mills, but observing sources within 10° of the horizon, found an equally marked correlation with sporadic E, but not with spread F.
Using simultaneous long-term observations of ionospheric scintillation at equator and anomaly crest region in the same longitude
(Indian) zone comparative features of scintillation occurrence are brought out. The salient features are: (a) predominantly
pre-midnight occurrence of scintillation at equator during winter and equinox seasons, (b) increase of pre-midnight scintillation
occurrence with solar activity (c) shifting of occurrence peak during summer from post-midnight in low to pre-midnight in
high solar activity periods (d) similarity of scintillation behaviour at these locations during winter and equinoxes but dissimilarity
during summer. The solar activity response and magnetic effects indicate that the scintillations at the anomaly crest region
in winter and equinox, particularly during high solar activity periods, are of equatorial origin while the summer events may
be of local or mid-latitude origin.
Equatorial scintillations have been observed at Legon, Ghana for nearly 20 yr. The occurrence characteristics of the scintillations are reviewed, and the physical characteristics of the electron density irregularities summarized. A much more comprehensive summary of the seasonal variation of scintillation is given, and it is found to be remarkably similar to the variations in thermospheric temperature. Evidence for the suppression of scintillation during magnetic disturbances is given. Curves for the daily variation of the Faraday rotation angle φ are presented and their unusual post-sunset behaviour noted. It is suggested that this can be explained in terms of a theory presented by Rishbeth, in which the F region ionization moves with nearly the full velocity of the neutral atmospheric wind at night, after the E region conductivity has fallen to a relatively low value. This can account for the observed drift velocity of the irregularities. The rapid increase in the post-sunset horizontal velocity of the ionization together with the observed vertical rise, can account for the variations of φ. It is further suggested that the large gradients in the density and drift velocity of the ionization resulting from the mechanism suggested by Rishbeth give rise to the production of the observed F region irregularities in electron density which cause equatorial scintillation.
Multiantenna 50 MHz radar backscatter maps of echo power from night-time F-region equatorial irregularities obtained at Jicamarca, Peru were compared with simultaneous VHF scintillation observations from Huancayo at 137 and 254 MHz during the period 20 November–12 December 1975. Saturation of VHF scintillations in excess of 20 dB was observed at both these frequencies during times when radar maps showed large intense plume structures rising into the topside ionosphere. On nights when only thin layers of bottomside irregularities were observed moderate to weak scintillations were recorded at VHF. Preliminary values of east-west horizontal irregularity drift velocities were obtained and compared with scintillation rate observations. Using the 1.5° and 4.5° longitudinal separation between the Jicamarca radar and ionospheric observation points of the two satellites from Huancayo, information was derived regarding large-scale east-west structure during the development phase of the irregularities.