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Artykuły w czasopismach na temat „VHF scintillation”

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Data publikacji: 6 września 2023

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1

Pathak, K. N., R. D. Jivrajani, H. P. Joshi i K. N. Iyer. "Characteristics of VHF scintillations in the equatorial anomaly crest region in India". Annales Geophysicae 13, nr 7 (31.07.1995): 730–39. http://dx.doi.org/10.1007/s00585-995-0730-7.

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Abstract. The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h'F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported.

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Chatterjee, S., i S. K. Chakraborty. "Variability of ionospheric scintillation near the equatorial anomaly crest of the Indian zone". Annales Geophysicae 31, nr 4 (19.04.2013): 697–711. http://dx.doi.org/10.5194/angeo-31-697-2013.

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Abstract. Multistation observations of ionosphere scintillation at VHF (250 MHz) and GNSS L1 frequency from three locations – (i) Bokkhali (BOK) (geographic 21.6° N, 88.2° E, dip 31.48°, (ii) Raja Peary Mohan College Centre (RPMC) (geographic 22.66° N, 88.4° E, dip 33.5°) and (iii) Krishnath College Centre (KNC), Berhampore (geographic 24.1° N, 88.3° E, dip 35.9°) – at ~ 1° latitudinal separations near the northern crest of the equatorial ionization anomaly (EIA) of the Indian longitude sector are investigated in conjunction with total electron content (TEC) data and available ionosonde data near the magnetic equator to study fine structure in spatial and temporal variability patterns of scintillation occurrences. The observations are carried out in the autumnal equinoctial months of a high solar activity year (2011). In spite of smaller latitudinal/spatial separation among the observing stations, conspicuous differences are reflected in the onset time, duration, fade rate and fade depth of VHF scintillations as well as in spectral features. Scintillations are mostly associated with depletion in TEC around the anomaly crest and occurrence of ESF near the magnetic equator at an earlier time. Not only the strength of EIA, but also the locations of observing stations with respect to the post-sunset resurgence peak of EIA seem to play dominant role in dictating the severity of scintillation activity. A secondary enhancement in diurnal TEC in the post-sunset period seems to accentuate the irregularity activities near the anomaly crest, and a threshold value of the same may fruitfully be utilized for the prediction of scintillation around the locations. An idea regarding latitudinal extent of scintillation is developed by considering observations at L1 frequency from the GPS and GLONASS constellation of satellites. A critical value of h'F near the magnetic equator for the occurrence of simultaneous scintillation at the three centres is suggested. The observations are discussed considering electrodynamical aspect of equatorial irregularities.

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3

Shume, E. B., A. J. Mannucci i R. Caton. "Phase and coherence analysis of VHF scintillation over Christmas Island". Annales Geophysicae 32, nr 3 (28.03.2014): 293–300. http://dx.doi.org/10.5194/angeo-32-293-2014.

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Abstract. This short paper presents phase and coherence data from the cross-wavelet transform applied on longitudinally separated very high frequency (VHF) equatorial ionospheric scintillation observations over Christmas Island. The phase and coherence analyses were employed on a pair of scintillation observations, namely, the east-looking and west-looking VHF scintillation monitors at Christmas Island. Our analysis includes 3 years of peak season scintillation data from 2008, 2009 (low solar activity), and 2011 (moderate solar activity). In statistically significant and high spectral coherence regions of the cross-wavelet transform, scintillation observations from the east-looking monitor lead those from the west-looking monitor by about 20 to 60 (40 ± 20) min (most frequent lead times). Using several years (seasons and solar cycle) of lead (or lag) and coherence information of the cross-wavelet transform, we envisage construction of a probability model for forecasting scintillation in the nighttime equatorial ionosphere.

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4

Vijayakumar, P. N., i P. K. Pasricha. "Parametrization of spectra of plasma bubble induced VHF satellite scintillations and its geophysical significance". Annales Geophysicae 15, nr 3 (31.03.1997): 345–54. http://dx.doi.org/10.1007/s00585-997-0345-2.

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Abstract. An important component of ionospheric plasma irregularity studies in the Indian low latitudes involves the study of the plasma bubbles which produce intense scintillations of the transionospheric satellite signals. Many such plasma bubble induced (PBI) scintillation events were identified while recording 244 MHz signal from the geostationary satellite Fleetsat (73°E) at Delhi (28.6°N, 77.2°E) during March-April 1991. This type of scintillations represents changes in plasma processes. These scintillations are spectrally analyzed using an autoregressive (AR) scheme, which is equivalent to maximum entropy method of spectrum analysis, amenable to extracting optimum spectral content from short data lengths (20 – 40 s). Each spectrum is assigned a level of detectability using the final prediction error (FPE) derived from the optimum filter order required to resolve the spectrum. Lower detectability together with a higher order filter indicate a higher level of coherence for the plasma irregularities (discrete structures). Consistent patterns for these scintillations emerge from the present analysis as follows: (1) the initial and final phases of a scintillation patch display quasiperiodic oscillations. Their corresponding spectra show dominant (Gaussian shaped) spectral features with detectability levels of –6 dB to –12 dB and requiring a higher order (>6) AR filter for their spectral resolution. These are most likely associated with discrete "filament-like" or "sheet-like" plasma structures that exist near the bubble walls. (2) Two main features of the scintillation spectra could be positively associated with the well-developed plasma bubble stage: (a) spectra displaying a power-law process with a single component spectral slope between 1.6 to 3.0. Generally such spectra are resolved with a 2nd order filter and have a 1 dB to 6 dB of detectability. (b) Spectra displaying a double slope, indicating an inner and an outer scale regime for the power-law irregularities. These spectra are resolved with higher order filters (>3 but <7) and possess detectability levels of –1 dB to 3 dB. These spectra display finer spectral changes, perhaps indicative of the nature of continuously evolving plasma irregularities. As an example, an analysis of a single scintillation patch is presented to highlight the geophysical significance of the present approach. Some important parameters used in the AR scheme of spectral analysis are given in the Appendix.

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5

Rama Rao, P. V. S., P. T. Jayachandran, P. Sri Ram, B. V. Ramana Rao, D. S. V. V. D. Prasad i K. K. Bose. "Characteristics of VHF radiowave scintillations over a solar cycle (1983−1993) at a low-latitude station: Waltair (17.7°N, 83.3°E)". Annales Geophysicae 15, nr 6 (30.06.1997): 729–33. http://dx.doi.org/10.1007/s00585-997-0729-3.

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Abstract. The characteristics of VHF radiowave scintillations at 244 MHz (FLEETSAT) during a complete solar cycle (1983–93) at a low-latitude station, Waltair (17.7°N, 83.3°E), are presented. The occurrence of night-time scintillations shows equinoctial maxima and summer minima in all the epochs of solar activity, and follows the solar activity. The daytime scintillation occurrence is negatively correlated with the solar activity and shows maximum occurrence during the summer months in a period of low solar activity. The occurrence of night-time scintillations is inhibited during disturbed days of high solar activity and enhanced during low solar activity.

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6

Singh, R. P., R. P. Patel i A. K. Singh. "Effect of solar and magnetic activity on VHF scintillations near the equatorial anomaly crest". Annales Geophysicae 22, nr 8 (7.09.2004): 2849–60. http://dx.doi.org/10.5194/angeo-22-2849-2004.

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Abstract. The VHF amplitude scintillation recorded during the period January 1991 to December 1993 in the declining phase of a solar cycle and April 1998 to December 1999 in the ascending phase of the next solar cycle at Varanasi (geogr. lat.=25.3°, long.=83.0°, dip=37°N) have been analyzed to study the behavior of ionospheric irregularities during active solar periods and magnetic storms. It is shown that irregularities occur at arbitrary times and may last for <30min. A rise in solar activity increases scintillations during winter (November-February) and near equinoxes (March-April; September-October), whereas it depresses the scintillations during the summer (May-July). In general, the role of magnetic activity is to suppress scintillations in the pre-midnight period and to increase it in the post-midnight period during equinox and winter seasons, whilst during summer months the effect is reversed. The pre-midnight scintillation is sometimes observed when the main phase of Dst corresponds to the pre-midnight period. The annual variation shows suppression of scintillations on disturbed days, both during pre-midnight and post-midnight period, which becomes more effective during years of high solar activity. It is observed that for magnetic storms for which the recovery phase starts post-midnight, the probability of occurrence of irregularities is enhanced during this time. If the magnetic storm occurred during daytime, then the probability of occurrence of scintillations during the night hours is decreased. The penetration of magnetospheric electric fields to the magnetic equator affects the evolution of low-latitude irregularities. A delayed disturbance dynamo electric field also affects the development of irregularities.

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7

Tiwari, D., B. Engavale, A. Bhattacharyya, C. V. Devasia, T. K. Pant i R. Sridharan. "Simultaneous radar and spaced receiver VHF scintillation observations of ESF irregularities". Annales Geophysicae 24, nr 5 (3.07.2006): 1419–27. http://dx.doi.org/10.5194/angeo-24-1419-2006.

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Abstract. Simultaneous observations of equatorial spread F (ESF) irregularities made on 10 nights during March-April 1998 and 1999, using an 18-MHz radar at Trivandrum (77° E, 8.5° N, dip 0.5° N) and two spaced receivers recording scintillations on a 251-MHz signal at Tirunelveli (77.8° E, 8.7° N, dip 0.4° N), have been used to study the evolution of Equatorial Spread F (ESF) irregularities. Case studies have been carried out on the day-to-day variability in ESF structure and dynamics, as observed by 18-MHz radar, and with spaced receiver measurements of average zonal drift Vo of the 251-MHz radio wave diffraction pattern on the ground, random velocity Vc, which is a measure of random changes in the characteristics of scintillation-producing irregularities, and maximum cross-correlation CI of the spaced receivers signals. Results show that in the initial phase of plasma bubble development, the greater the maximum height of ESF irregularities responsible for the radar backscatter, the greater the decorrelation is of the spaced receiver scintillation signals, indicating greater turbulence. The relationship of the maximum spectral width derived from the radar observations and CI also supports this result.

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8

Gupta, J. K., Lakha Singh i R. S. Dabas. "Faraday polarization fluctuations and their dependence on post sunset secondary maximum and amplitude scintillations at Delhi". Annales Geophysicae 20, nr 2 (28.02.2002): 185–90. http://dx.doi.org/10.5194/angeo-20-185-2002.

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Abstract. VHF Faraday rotation (FR) and amplitude scintillation data recorded simultaneously during May 1978–December 1980 at Delhi (28.63° N, 77.22° E; Dip 42.44° N) is analyzed in order to study the Faraday polarization fluctuations (FPFs) and their dependence on the occurrence of post sunset secondary maximum (PSSM) and amplitude scintillations. It is noted that FPFs are observed only when both PSSM and scintillations also occur simultaneously. FPFs are observed only during winter and the equinoctial months of high sunspot years. FPFs events are associated with intense scintillation activity, which is characterized by sudden onsets and abrupt endings, and are observed one to three hours after the local sunset. When FPFs and scintillation data from Delhi is compared with the corresponding data from a still lower latitude station, Hyderabad (17.35° N, 78.45° E), it is found that the occurrence of FPFs and scintillations at Delhi is conditional to their prior occurrence at Hyderabad, which indicates their production by a plasma bubble and the as-sociated irregularities generated initially over the magnetic equator. In addition, FPFs and scintillation data for October 1979, when their occurrence was maximum, is also examined in relation to daytime (11:00 LT) electrojet strength (EEj) values and evening hour h’F from an equatorial location, Kodaikanal (10.3° N, 77.5° E). It is interesting to note that FPFs and scintillations are most likely observed when the EEj was 100 nT or more and h’F reaches around 500 km. These results show that EEj and evening hours h’F values over the magnetic equator are important parameters for predicting FPFs and scintillation activity at locations such as Delhi, where scintillation activity is much more intense as compared to the equatorial region due to the enhanced back-ground ionization due to the occurrence of PSSM.Key words. Ionosphere (equatorial ionosphere; ionospheric irregularities) – Radio science (ionospheric physics)

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9

Rama Rao, P. V. S., P. Sri Ram, P. T. Jayachandran i D. S. V. V. D. Prasad. "Multistation VHF scintillation studies at low latitudes". Planetary and Space Science 44, nr 10 (październik 1996): 1209–17. http://dx.doi.org/10.1016/s0032-0633(96)00014-1.

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10

Engavale, B., K. Jeeva, K. U. Nair i A. Bhattacharyya. "Solar flux dependence of coherence scales in scintillation patterns produced by ESF irregularities". Annales Geophysicae 23, nr 10 (30.11.2005): 3261–66. http://dx.doi.org/10.5194/angeo-23-3261-2005.

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Abstract. The coherence scale length, defined as the 50% decorrelation scale length along the magnetic east-west direction, in the ground scintillation pattern obtained at a dip equatorial location, due to scattering of VHF radio waves by equatorial spread F (ESF) irregularities, is calculated, using amplitude scintillation data recorded by two spaced receivers. The average east-west drift of the ground scintillation pattern, during the pre- and post-midnight periods, also calculated from the same observations, shows an almost linear increase with 10.7-cm solar flux. In the present paper the variability of the drift is automatically taken into account in the calculation of the coherence scale length of the ground scintillation pattern. For weak scintillations, the coherence scale depends on the Fresnel scale, which varies with the height of the irregularity layer, and also on the spectral index of the irregularity power spectrum. It is found that for weak scintillations, the coherence scales are much better organized according to the 10.7-cm solar flux, during the pre-midnight period, than during the post-midnight period, with a general trend of coherence scale length increasing with 10.7-cm solar flux except for cases with F 10.7-cm solar flux <100. This indicates that, during the initial phase of ESF irregularity development, the irregularity spectrum does not have much variability while further evolution of the spatial structure in ESF irregularities is controlled by factors other than the solar flux.

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11

Singh, S. B., K. Patel, R. P. Patel, A. K. Singh i R. P. Singh. "Modeling of VHF scintillation observed at low latitude". Journal of Physics: Conference Series 208 (1.02.2010): 012065. http://dx.doi.org/10.1088/1742-6596/208/1/012065.

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12

Kumar, Sushil, V. Ramachandran i Amol Kishore. "Geomagnetic storms and associated scintillation on VHF signals". South Pacific Journal of Natural and Applied Sciences 22, nr 1 (2004): 75. http://dx.doi.org/10.1071/sp04014.

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13

Huang, Chao-Song. "Occurrence Characteristics of VHF Scintillation and Equatorial Spread F over Kwajalein during Moderate Solar Activity in 2012". Atmosphere 14, nr 5 (19.05.2023): 889. http://dx.doi.org/10.3390/atmos14050889.

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The occurrence probability of equatorial plasma bubbles and the associated spread F (ESF) irregularities have been derived from ground-based and space-borne measurements. In general, ESF occurrence depends on season and longitude and is high in equinoctial months and low around June solstice. In the West Pacific sector, previous statistical results show that the ESF occurrence probability increases gradually and continuously from March to August. In this study, we use trans-ionospheric VHF data received at Kwajalein Atoll in 2012 to derive the occurrence characteristics of scintillation. It is found that the occurrence probability of strong scintillation had two maxima in June and September and a minimum in July in the evening and midnight sector but only one maximum in June in the post-midnight sector. The monthly variations of scintillation occurrence at Kwajalein are different from almost all previous studies on ESF and scintillation at or near this longitude. To identify the cause for the June peak and the July minimum of scintillation, the ion density and velocity data measured by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite in 2011–2012 are used to derive the ESF occurrence and the post-sunset vertical ion drift near Kwajalein. The ESF occurrence probability and the ion drift measured by the C/NOFS satellite showed two maxima in May/June and August/September and a minimum in July, verifying that the June peak and the July minimum of the VHF scintillation are realistic and caused by the similar variations in the ionospheric ion drift and density.

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Rama Rao, P. V. S., S. Tulasi Ram, K. Niranjan, D. S. V. V. D. Prasad, S. Gopi Krishna i N. K. M. Lakshmi. "VHF and L-band scintillation characteristics over an Indian low latitude station, Waltair (17.7° N, 83.3° E)". Annales Geophysicae 23, nr 7 (14.10.2005): 2457–64. http://dx.doi.org/10.5194/angeo-23-2457-2005.

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Abstract. Characteristics of simultaneous VHF (244 MHz) and L-band (1.5 GHz) scintillations recorded at a low-latitude station, Waltair (17.7° N, 83.3° E), during the low sunspot activity year of March 2004 to March 2005, suggest that the occurrence of scintillations is mainly due to two types, namely the Plasma Bubble Induced (PBI), which maximizes during the post sunset hours of winter and equinoctial months, and the Bottom Side Sinusoidal (BSS) type, which maximizes during the post-midnight hours of the summer solstice months. A detailed study on the spectral characteristics of the scintillations at both the frequencies show that the post-sunset scintillations are strong with fast fading (≈40 fad/min) and are multiple in nature in scattering, giving rise to steep spectral slopes, whereas the post-midnight scintillations, which occur mostly on the VHF signal with low fading rate (≈4 fad/min), are of the BSS type, often showing typical Fresnel oscillations with reduced roll off spectral slopes, indicating that the type of irregularity resembles a thin screen structure giving rise to weak scattering. Using the onset times of several similar scintillation patches across the two satellite (FLEETSAT 73° E, INMARSAT 65° E) ray paths (sub-ionospheric points are separated by 82 km), the East ward movement of the irregularity patches is found to vary from 150 to 250 m/s during the post sunset hours and decrease slowly during the post midnight hours. Further, the east-west extent of the PBI type of irregularities is found to vary from 100 to 500 km, while that of the BSS type extend up to a few thousand kilometers. Keywords. Ionosphere (Ionospheric irregularities; Auroral ionosphere; Electric fields and currents)

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15

Sripathi, S., S. Bose, A. K. Patra, T. K. Pant, B. Kakad i A. Bhattacharyya. "Simultaneous observations of ESF irregularities over Indian region using radar and GPS". Annales Geophysicae 26, nr 11 (21.10.2008): 3197–213. http://dx.doi.org/10.5194/angeo-26-3197-2008.

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Abstract. In this paper, we present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E, Dip latitude 6.3° N), a low latitude station, made simultaneous observations. In particular, the latitudinal and longitudinal extent of TEC and L-band scintillations was studied in the Indian region for different types of ESF structures observed using the MST radar during the low solar activity period of 2004 and 2005. Simultaneous radar and GPS observations during severe ESF events in the pre-midnight hour reveal that significant GPS L band scintillations, depletions in TEC, and the double derivative of the TEC index (DROTI), which is a measure of fluctuations in TEC, obtained at low latitudes coincide with the appearance of radar echoes at Gadanki. As expected, when the irregularities reach higher altitudes as seen in the radar map during pre-midnight periods, strong scintillations on an L-band signal are observed at higher latitudes. Conversely, when radar echoes are confined to only lower altitudes, weak scintillations are found and their latitudinal extent is small. During magnetically quiet periods, we have recorded plume type radar echoes during a post-midnight period that is devoid of L-band scintillations. Using spectral slopes and cross-correlation index of the VHF scintillation observations, we suggest that these irregularities could be "dead" or "fossil" bubbles which are just drifting in from west. This scenario is consistent with the observations where suppression of pre-reversal enhancement (PRE) in the eastward electric field is indicated by ionosonde observations of the height of equatorial F layer and also occurrence of low spectral width in the radar observations relative to pre-midnight period. However, absence of L-band scintillations during post-midnight event, when radar observed plume like structures and scintillations were recorded on VHF band, raises questions about the process of evolution of the irregularities. A possible explanation is that whereas small scale (~3 m) irregularities are generated through secondary waves that grow on the walls of km scale size irregularities, in this case evolution of the Rayleigh-Taylor instability itself did not extend to irregularities of scale sizes of a few hundred meters that produce scintillation on a L-band signal.

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Knepp, Dennis L., i Eric L. Mokole. "Space-based radar coherent processing during scintillation: VHF throughLband". Radio Science 27, nr 1 (styczeń 1992): 47–61. http://dx.doi.org/10.1029/91rs02456.

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Hajkowicz, L. A., i H. Minakoshi. "Mid-latitude ionospheric scintillation anomaly in the Far East". Annales Geophysicae 21, nr 2 (28.02.2003): 577–81. http://dx.doi.org/10.5194/angeo-21-577-2003.

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Abstract. A long-term (over 3 years) study has been undertaken to obtain a comprehensive evaluation of VHF ionospheric scintillation morphology in East Asia (at Kokobunji in Japan), using amplitude records from Transit satellites. It is now evident that summer day and night scintillation enhancement in this mid-latitude region is a long-term evidence of a well-known Asian ionospheric disturbance anomaly. The scintillation activity is particularly strong during summer nights (21:00–24:00 LT) and on occasion, all satellite passes recorded on consecutive days are associated with pronounced scintillation activity. A second sub-maximum is observed in the summer pre-noon period (09:00–12:00 LT). The scintillation regions extend latitudinally for a distance of 400–600 km in the F-region and 100–200 km in the E-region, mostly equatorwards of Kokobunji. For comparison similar scintillation data obtained for one year at the same longitudinal sector but in southern mid-latitudes (Brisbane in Australia) were compared with the simultaneous northern scintillation data. The scintillation activity at Brisbane was much less pronounced in the southern summer but was of the same low level during other seasons as that for Kokobunji. This consistent scintillation anomaly, as yet, has not been included in the global scintillation models, which are essential for radio-satellite communications.Key words. Ionosphere (mid-latitude ionosphere; ionospheric irregularities)

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Tereshchenko, E. D., N. Yu Romanova i A. V. Koustov. "VHF scintillations, orientation of the anisotropy of F-region irregularities and direction of plasma convection in the polar cap". Annales Geophysicae 26, nr 7 (18.06.2008): 1725–30. http://dx.doi.org/10.5194/angeo-26-1725-2008.

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Abstract. Scintillation data recorded at the polar cap station Barentsburg are shown to occasionally exhibit two or more peaks in the latitudinal profiles of the amplitude dispersion. Comparison with concurrent SuperDARN radar convection maps indicates that multiple peaks occur when Barentsburg is located within the area of strong changes in the plasma flow direction. When parameters of the ionospheric irregularities are inferred from the scintillation data, the orientation of the irregularity anisotropy in a plane perpendicular to the magnetic field is found to coincide well with the E×B flow direction, individually for each peak of the scintillation data. The differences were found to be mostly less than 20° for a data set comprised of 104 events. The conclusion is made that analysis of scintillation data allows one to infer the direction of plasma flow with a certain degree of detail.

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Kersley, L., S. E. Pryse i P. A. Bradley. "Comparison of boundaries for HF auroral backscatter and VHF scintillation". Journal of Atmospheric and Terrestrial Physics 57, nr 2 (luty 1995): 169–76. http://dx.doi.org/10.1016/0021-9169(93)e0052-b.

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Yadav, V., B. Kakad, T. K. Pant, A. Bhattacharyya i D. S. V. V. D. Prasad. "Study of equatorialEregion irregularities using rare daytime VHF scintillation observations". Journal of Geophysical Research: Space Physics 120, nr 10 (październik 2015): 9074–86. http://dx.doi.org/10.1002/2015ja021320.

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Kumar, S., A. K. Gwal, B. M. Pathan i D. R. K. Rao. "Zonal drifts of ionospheric irregularities at temperate latitude in the Indian region". Annales Geophysicae 13, nr 7 (31.07.1995): 724–29. http://dx.doi.org/10.1007/s00585-995-0724-5.

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Abstract. The systematic time differences observed in the onset of postsunset VHF scintillations recorded simultaneously at Ujjain (Geogr. lat. 23.2°N, Geogr. long. 75.6°E) and Bhopal (Geogr. lat. 23.2°N, Geogr. long. 77.6°E), situated at the peak of the anomaly crest in the Indian region, have been analysed to determine the zonal drifts of scintillation-producing irregularities. The method is based on the assumption that the horizontal movement of irregularities does not change while crossing the F-region cross-over points of these stations. The calculated velocities of irregularities indicate an eastward drift decreasing from about 180 m s–1 to 55 m s–1 during the course of night. In the premidnight period, the drifts are reduced under the magnetically disturbed conditions. The average east-west extension of irregularites is found to be in the range of 200–500 km.

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Joshi, L. M., L. ‐C Tsai, S. ‐Y Su, Ronald G. Caton, C. ‐H Lu i K. M. Groves. "VHF Scintillation and Drift Studied Using Spaced Receivers in Southern Taiwan". Radio Science 54, nr 5 (maj 2019): 455–67. http://dx.doi.org/10.1029/2018rs006722.

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Forte, Biagio, Richard A. Fallows, Mario M. Bisi, Jinge Zhang, Andrzej Krankowski, Bartosz Dabrowski, Hanna Rothkaehl i Christian Vocks. "Interpretation of Radio Wave Scintillation Observed through LOFAR Radio Telescopes". Astrophysical Journal Supplement Series 263, nr 2 (1.12.2022): 36. http://dx.doi.org/10.3847/1538-4365/ac6deb.

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Abstract Radio waves propagating through a medium containing irregularities in the spatial distribution of the electron density develop fluctuations in their intensities and phases. In the case of radio waves emitted from astronomical objects, they propagate through electron density irregularities in the interstellar medium, the interplanetary medium, and Earth’s ionosphere. The LOFAR radio telescope, with stations across Europe, can measure intensity across the VHF radio band and thus intensity scintillation on the signals received from compact astronomical objects. Modeling intensity scintillation allows the estimate of various parameters of the propagation medium, for example, its drift velocity and its turbulent power spectrum. However, these estimates are based on the assumptions of ergodicity of the observed intensity fluctuations and, typically, of weak scattering. A case study of single-station LOFAR observations of the strong astronomical source Cassiopeia A in the VHF range is utilized to illustrate deviations from ergodicity, as well as the presence of both weak and strong scattering. Here it is demonstrated how these aspects can lead to misleading estimates of the propagation medium properties, for example, in the solar wind. This analysis provides a method to model errors in these estimates, which can be used in the characterization of both the interplanetary medium and Earth’s ionosphere. Although the discussion is limited to the case of the interplanetary medium and Earth’s ionosphere, its ideas are also applicable to the case of the interstellar medium.

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24

Rodrigues, F. S., E. R. de Paula, M. A. Abdu, A. C. Jardim, K. N. Iyer, P. M. Kintner i D. L. Hysell. "Equatorial spreadFirregularity characteristics over São Luís, Brazil, using VHF radar and GPS scintillation techniques". Radio Science 39, nr 1 (luty 2004): n/a. http://dx.doi.org/10.1029/2002rs002826.

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25

Kumagai, Hiroshi, i Tadahiko Ogawa. "Behavior of mid-latitude F-region irregularities deduced from spaced-receiver VHF scintillation measurements". Journal of Atmospheric and Terrestrial Physics 48, nr 3 (marzec 1986): 221–30. http://dx.doi.org/10.1016/0021-9169(86)90097-8.

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26

Dabas, R. S., D. R. Lakshmi i B. M. Reddy. "Effect of geomagnetic disturbances on the VHF nighttime scintillation activity at equatorial and low latitudes". Radio Science 24, nr 4 (lipiec 1989): 563–73. http://dx.doi.org/10.1029/rs024i004p00563.

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27

Franke, S. J., i C. H. Liu. "Analysis of a long period fading component in saturated VHF amplitude scintillation observed at Ascension Island". Journal of Atmospheric and Terrestrial Physics 49, nr 5 (maj 1987): 421–31. http://dx.doi.org/10.1016/0021-9169(87)90036-5.

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28

Chen, Bo, Yi Liu, Jian Feng, Yuqiang Zhang, Yufeng Zhou, Chen Zhou i Zhengyu Zhao. "High-Resolution Observation of Ionospheric E-Layer Irregularities Using Multi-Frequency Range Imaging Technology". Remote Sensing 15, nr 1 (3.01.2023): 285. http://dx.doi.org/10.3390/rs15010285.

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E-region field-aligned irregularities (FAIs) are a hot topic in space research, since electromagnetic signal propagation through ionospheric irregularities can undergo sporadic enhancements and fading known as ionospheric scintillation, which could severely affect communication, navigation, and radar systems. However, the range resolution of very-high-frequency (VHF) radars, which is widely used to observe E-region FAIs, is limited due to its bandwidth. As a technology that is widely used in atmosphere radars to improve the range resolution of pulsed radars by transmitting multiple frequencies, this paper employed the multifrequency radar imaging (RIM) technique in a Wuhan VHF radar. The results showed that the range resolution of E-region FAIs greatly improved when compared with the results in traditional single-frequency mode, and that finer structures of E-region FAIs can be obtained. Specifically, the imaging results in multifrequency mode show that E-region FAIs demonstrate an overall descending trend at night, and it could be related to the tides or gravity waves due to their downward phase velocities or even driven by downwind shear. In addition, typical quasi-periodic (QP) echoes with a time period of around 10 min could be clearly seen using the RIM technique, and the features of the echoes suggest that they could be modulated by gravity waves. Furthermore, the RIM technique can be used to obtain the fine structure of irregularities within a short time period, and the hierarchical structure of E-region FAIs can be easily found. Therefore, the multifrequency imaging RIM technique is suitable for observing E-region FAIs and their evolution, as well as for identifying the different layers of E-region FAIs. Combined with the RIM technique, a VHF radar provides an effective and promising way to observe the structure of E-region FAIs in more detail to study the physical mechanism behind the formation and evolution of ionospheric E-region irregularities.

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29

Pathan, B. M., i D. R. K. Rao. "Seasonal and solar cycle association of zonal drifts of ionospheric plasma irregularities in the Indian equatorial region". Annales Geophysicae 14, nr 3 (31.03.1996): 297–303. http://dx.doi.org/10.1007/s00585-996-0297-y.

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Abstract. Long series of simultaneous VHF scintillation observations at two stations situated in near magnetic east-west direction in the vicinity of the dip equator in the Indian region have been employed to investigate the night-time ionospheric plasma zonal drifts. The drifts are found to be predominantly easterly. On comparing the magnitudes of the drifts with those results derived earlier by HF fading technique, monitoring signals from two satellites at a station and spaced receiver experiment, their associations with the season and the degree of solar activity are discussed. On a broader scale, the annual mean sunspot number is shown to have a direct control on the derived drift, the positive relationship even on day to day basis with the solar flux is established. However, the relationship, as understood by the slope of the best fit line, in the Indian region (0.27) is found to be weaker when compared with the similar slope (0.45) in the American sector. There appears to be no geomagnetic activity control on the estimated drifts.

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30

Olwendo, O. J., T. Baluku, P. Baki, P. J. Cilliers, C. Mito i P. Doherty. "Low latitude ionospheric scintillation and zonal irregularity drifts observed with GPS-SCINDA system and closely spaced VHF receivers in Kenya". Advances in Space Research 51, nr 9 (maj 2013): 1715–26. http://dx.doi.org/10.1016/j.asr.2012.12.017.

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31

Chakraborty, S. K., A. DasGupta, S. Ray i S. Banerjee. "Long-term observations of VHF scintillation and total electron content near the crest of the equatorial anomaly in the Indian longitude zone". Radio Science 34, nr 1 (styczeń 1999): 241–55. http://dx.doi.org/10.1029/98rs02576.

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32

Tilahun, Samson, i Yekoye Asmare Tariku. "Verification of ionospheric perturbation induced L-band frequency scintillation using HF/VHF bands over the African equatorial and low latitude region, Ethiopia". Journal of Atmospheric and Solar-Terrestrial Physics 195 (listopad 2019): 105135. http://dx.doi.org/10.1016/j.jastp.2019.105135.

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33

Koparkar, P. V., i R. G. Rastogi. "VHF radio scintillations at Bombay". Journal of Atmospheric and Terrestrial Physics 47, nr 8-10 (sierpień 1985): 907–10. http://dx.doi.org/10.1016/0021-9169(85)90066-2.

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34

Tulasi Ram, S., P. V. S. Rama Rao, K. Niranjan, D. S. V. V. D. Prasad, R. Sridharan, C. V. Devasia i S. Ravindran. "The role of post-sunset vertical drifts at the equator in predicting the onset of VHF scintillations during high and low sunspot activity years". Annales Geophysicae 24, nr 6 (3.07.2006): 1609–16. http://dx.doi.org/10.5194/angeo-24-1609-2006.

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Abstract. The day-to-day variability in the occurrence of ionospheric scintillations, which are of serious concern in the trans-ionospheric communications, makes their prediction still a challenging problem. This paper reports on a systematic study in quantitatively identifying the precursors responsible, such as pre-reversal E×B drift velocity, geo-magnetic activity index (Kp) and the Equatorial Ionization Anomaly (EIA) gradient, for the onset of VHF scintillations over a low-latitude station, Waltair (20° N dip), during high (2001) and low (2004) sunspot activity years. The percentage of occurrences of VHF scintillations over Waltair show a good correlation with the monthly mean post-sunset vertical drift velocities at the equator, during both the high and low sunspot activity years. During the days on which intense (>10 dB) scintillations occur, the ionization anomaly gradient (dN/dL), measured from ionosonde data of an equatorial (Trivandrum, 0.9° N dip) and an off-equatorial station (Waltair, 20° N dip) shows an enhancement in the gradient prior to the onset of the scintillations. However, this enhancement is not seen on days when the scintillations are weak (<10 dB) or absent. The day-to-day post sunset enhancement in the E×B drift is found to decrease with increasing Kp-index and this decrease is more prominent in the equinoxes, less in winter and insignificant in the summer months. On a day-to-day basis, it is found that the value of the upward drift velocity at the equator should be ≥30 m/s for the onset of strong scintillations over Waltair for magnetically quiet days with average Kp≤2 (6 h prior to the local sunset) during the high sunspot year, 2001. This threshold value of the upward drift reduces to 20 m/s with the decrease in the sunspot activity during 2004. Further, these conditions for the onset of intense scintillations is well defined in equinoxes, less in winter and least in the summer solstices.

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35

RASTOGI, R. G., P. V. KOPARKAR, A. PATIL i B. M. PATHAN. "Daytime VHF Radio Wave Scintillations at Equatorial Latitudes." Journal of geomagnetism and geoelectricity 43, nr 7 (1991): 549–61. http://dx.doi.org/10.5636/jgg.43.549.

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36

Mathew, B., K. N. Iyer i B. M. Pathan. "Patchy occurrence of VHF scintillations at tropical latitudes". Journal of Atmospheric and Terrestrial Physics 54, nr 7-8 (lipiec 1992): 963–68. http://dx.doi.org/10.1016/0021-9169(92)90062-p.

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37

de Paula, E. R., K. N. Iyer, D. L. Hysell, F. S. Rodrigues, E. A. Kherani, A. C. Jardim, L. F. C. Rezende, S. G. Dutra i N. B. Trivedi. "Multi-technique investigations of storm-time ionospheric irregularities over the São Luís equatorial station in Brazil". Annales Geophysicae 22, nr 10 (3.11.2004): 3513–22. http://dx.doi.org/10.5194/angeo-22-3513-2004.

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Abstract. On 11 April 2001, a large magnetic storm occurred with SSC at 13:43 UT, and Dst reached below -200nT after two southward Bz excursions. The Kp index during this storm reached 8 and remained high (>4) for about 21h, and the São Luís magnetometer H component presented simultaneous oscillations and decreased substantially relative to the previous magnetically quiet days. This storm triggered strong ionospheric irregularities, as observed by a recently installed 30MHz coherent scatter radar, a digisonde, and a GPS scintillation receiver, all operating at the São Luís equatorial station (2.33° S, 44° W, dip latitude 1.3° S). The ionospheric conditions and the characteristics of the ionospheric irregularities observed by these instruments are presented and discussed. The VHF radar RTI (Range Time Intensity) echoes and their power spectra and spectral width for the storm night 11-12 April 2001, were used to analyse the nature and dynamics of the plasma irregularities and revealed the coexistence of many structures in the altitudinal range of 400-1200km, some locally generated and others that drifted from other longitudinal sectors. The radar data also revealed that the plumes had periodic eastward and westward zonal velocities after 22:20 UT, when well-developed quiet-time plumes typically drift eastward. Another interesting new observation is that the F-layer remained anomalously high throughout the 11-12 April 2001 storm night (21:00 UT to 09:00 UT next day) (the LT at São Luís is UT -3h), as indicated by the digisonde parameters hmF2 and h'F, which is a condition favourable for spread F generation and maintenance. The AE auroral index showed enhancements (followed by decreases) that are indicative of magnetospheric convection enhancements at about 15:00 UT, 20:00 UT and 22:00 UT on 11 April 2001 and at 00:20 UT (small amplitude) on 12 April 2001, associated with many Bz fluctuations, including clear two southward incursions that gave rise to large and long lasting Kp values and large negative Dst values. This intense auroral activity generated disturbance dynamo and prompt penetration electric fields that were responsible for the maintenance of the F-layer at a high altitude along the night of 11-12 April 2001. The short-lived F-region height rise seen between 16:00 to 18:00 UT on 11 April 2001 is probably due to the prompt penetration eastward electric fields of magnetospheric origin during the first IMF Bz turning to south around 15:00 UT.

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38

Rama Murthy, B., U. Eranna, K. Bhanu Prasad i R. Manjula. "POWER SPECTRAL STUDIES OF VHF SCINTILLATIONS OVER NEAR EQUATORIAL STATION ANANTAPUR". International Journal on Intelligent Electronic Systems 3, nr 1 (2009): 50–61. http://dx.doi.org/10.18000/ijies.30044.

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39

Vasilyev, Roman, Mariia Globa, Dmitry Kushnarev, Andrey Medvedev i Konstantin Ratovsky. "Spectral characteristics of ionospheric scintillations of VHF radiosignal near magnetic zenith". Journal of Atmospheric and Solar-Terrestrial Physics 160 (lipiec 2017): 48–55. http://dx.doi.org/10.1016/j.jastp.2017.05.016.

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40

Ahmad, Altaf, M. M. Ahmad i B. M. Pathan. "VHF scintillations as a diagnostic tool for the study of ionospheric irregularities". Earth, Moon, and Planets 65, nr 3 (1994): 247–68. http://dx.doi.org/10.1007/bf00579536.

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41

McNamara, L. F., R. G. Caton, R. T. Parris, T. R. Pedersen, D. C. Thompson, K. C. Wiens i K. M. Groves. "Signatures of equatorial plasma bubbles in VHF satellite scintillations and equatorial ionograms". Radio Science 48, nr 2 (marzec 2013): 89–101. http://dx.doi.org/10.1002/rds.20025.

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42

Hajkowicz, L. A. "Spatial Characteristics of Mid-Latitude Ionospheric Scintillations in VHF Radio-Satellite Transmissions". Acta Geodaetica et Geophysica Hungarica 33, nr 1 (marzec 1998): 41–51. http://dx.doi.org/10.1007/bf03325521.

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Rao, P. V. S. Rama, D. S. V. V. D. Prasad, K. Niranjan, G. Uma, S. Gopi Krishna i K. Venkateswarlu. "Multi-station Studies on Spread-F and VHF Scintillations in the Indian Sector". Terrestrial, Atmospheric and Oceanic Sciences 15, nr 4 (2004): 667. http://dx.doi.org/10.3319/tao.2004.15.4.667(a).

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44

Kumar, Sushil, i A. K. Gwal. "VHF ionospheric scintillations near the equatorial anomaly crest: solar and magnetic activity effects". Journal of Atmospheric and Solar-Terrestrial Physics 62, nr 3 (luty 2000): 157–67. http://dx.doi.org/10.1016/s1364-6826(99)00090-5.

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Kumar, Sushil, P. K. Purohit i A. K. Gwal. "Solar and Magnetic Activity Control on the VHF Ionospheric Scintillations at Low Latitude". Acta Geodaetica et Geophysica Hungarica 33, nr 1 (marzec 1998): 9–17. http://dx.doi.org/10.1007/bf03325518.

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Kelley, M. C., i R. R. Ilma. "Generation of a severe convective ionospheric storm under stable Rayleigh–Taylor conditions: triggering by meteors?" Annales Geophysicae 34, nr 2 (3.02.2016): 165–70. http://dx.doi.org/10.5194/angeo-34-165-2016.

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Abstract. Here we report on four events detected using the Jicamarca Radio Observatory (JRO) over an 18-year period, in which huge convective ionospheric storms (CISs) occur in a stable ionosphere. We argue that these rare events could be initiated by meteor-induced electric fields. The meteor-induced electric fields map to the bottomside of the F region, causing radar echoes and a localized CIS. If and when a localized disturbance reaches 500 km, we argue that it becomes two-dimensionally turbulent and cascades structure to both large and small scales. This leads to long-lasting structure and, almost certainly, to scintillations over a huge range of latitudes some ±15° wide and to 3 m irregularities, which backscatter the VHF radar waves. These structures located at high altitudes are supported by vortices shed by the upwelling bubble in a vortex street.

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47

Lakshmi, D. R., R. S. Dabas, B. C. N. Rao i B. M. Reddy. "A study of low-latitude VHF scintillations in relation to electric fields during magnetic storms". Radio Science 28, nr 3 (maj 1993): 389–400. http://dx.doi.org/10.1029/93rs00280.

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Dabas, R. S., i B. M. Reddy. "Nighttime VHF scintillations at 23°N magnetic latitude and their association with equatorial F region irregularities". Radio Science 21, nr 3 (maj 1986): 453–62. http://dx.doi.org/10.1029/rs021i003p00453.

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49

Singh, S. B., V. S. Rathore, Ashutosh K. Singh i A. K. Singh. "Ionospheric irregularities at low latitude using VHF scintillations during extreme low solar activity period (2008–2010)". Acta Geodaetica et Geophysica 52, nr 1 (16.03.2016): 35–51. http://dx.doi.org/10.1007/s40328-016-0168-2.

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50

Iyer, K. N., K. N. Pathak, H. P. Joshi i R. D. Jivrajani. "Characteristics of VHF scintillations in the equatorial anomaly crest region in India and comparison with model". Advances in Space Research 18, nr 6 (styczeń 1996): 107–10. http://dx.doi.org/10.1016/0273-1177(95)00909-4.

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