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Artykuły w czasopismach na temat "ESF irregularities"
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Tiwari, D., A. K. Patra, C. V. Devasia, R. Sridharan, N. Jyoti, K. S. Viswanathan i K. S. V. Subbarao. "Radar Observations of 8.3-m scale equatorial spread F irregularities over Trivandrum". Annales Geophysicae 22, nr 3 (19.03.2004): 911–22. http://dx.doi.org/10.5194/angeo-22-911-2004.
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Abstract. In this paper, we present observations of equatorial spread F (ESF) irregularities made using a newly installed 18MHz radar located at Trivandrum. We characterize the morphology and the spectral parameters of the 8.3-m ESF irregularities which are found to be remarkably different from that observed so extensively at the 3-m scale size. We also present statistical results of the irregularities in the form of percentage occurrence of the echoes and spectral parameters (SNR, Doppler velocity, Spectral width). The Doppler spectra are narrower, less structured and less variable in time as compared to those observed for 3-m scale size. We have never observed the ESF irregularity velocities to be supersonic here unlike those at Jicamarca, and the velocities are found to be within ±200ms–1. The spectral widths are found to be less than 150ms–1. Hence, the velocities and spectral width both are smaller than those reported for 3-m scale size. The velocities and spectral widths are further found to be much smaller than those of the American sector. These observations are compared with those reported elsewhere and discussed in the light of present understanding on the ESF irregularities at different wavelengths. Key words. Ionoshphere (equatorial ionosphere, plasma waves and instabilities; ionospheric irregularities)
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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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Chau, J. L., R. F. Woodman i L. A. Flores. "Statistical characteristics of low-latitude ionospheric field-aligned irregularities obtained with the Piura VHF radar". Annales Geophysicae 20, nr 8 (31.08.2002): 1203–12. http://dx.doi.org/10.5194/angeo-20-1203-2002.
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Abstract. We present a summary of the statistical characteristics of echoes from ionospheric (E- and F-region) field-aligned irregularities obtained with the Piura VHF radar. This radar is located at ~ 7.0° dip latitude, just outside the equatorial electrojet (EEJ) region. Our results are based on (1) intermittent observations made between 1991 and 1999 just few days a year, and (2) continuous observations made between January 2000 and June 2001. During most of the intermittent observations, simultaneous measurements of EEJ and equatorial spread F (ESF) irregularities were performed with the Jicamarca VHF radar. From the continuous measurements, we have obtained the diurnal and seasonal characteristics of a variety of parameters (percentage of occurrence, signal-to-noise ratio and/or Doppler velocities) from the lower and upper E-region irregularities and also from F-region irregularities over Piura. For example, we have found that (1) the E-region echoes are stronger and occur more frequently during local summer (i.e. between December and March); (2) between May and June, the E-region echoes are weaker and occur less frequently; moreover, during these months, a semidiurnal wave with large amplitudes is observed in the meridional wind (> 100 ms- 1); (3) there is vertical wavelength of about 20 km in the Doppler velocity, particularly after midnight; (4) the lower (upper) E-region Doppler velocities are influenced mainly by meridional winds (equatorial F-region vertical drifts). In addition, we have observed that the seasonal and daily occurrences of Piura F-region irregularities are similar to the occurrence of topside ESF irregularities over Jicamarca. The likelihood of occurrence of F-region irregularities over Piura and, therefore, topside ESF over Jicamarca is greater when there are no E-region irregularities over Piura. On the other hand, there is more probability of observing bottomtype/bottomside ESF irregularities over Jicamarca when E-region irregularities are observed over Piura.Key words. Ionosphere (ionospheric irregularities; equatorial ionosphere; instruments and techniques)
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Cueva, R. Y. C., E. R. de Paula i A. E. Kherani. "Statistical analysis of radar observed F region irregularities from three longitudinal sectors". Annales Geophysicae 31, nr 12 (3.12.2013): 2137–46. http://dx.doi.org/10.5194/angeo-31-2137-2013.
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Abstract. Equatorial Spread F (ESF) is a manifestation of ionospheric interchange instabilities in the nighttime equatorial F region. These instabilities generate plasma density irregularities with scale sizes ranging from centimetres to thousands of kilometres. The irregularities can be detected from a variety of instruments such as digisonde, coherent and incoherent scatter radars, in situ space probes, and airglow photometers. In the present study, occurrence statistics of the ESF, based on various parameters are presented using data obtained from the VHF radars located at three longitudinally separated equatorial stations: Christmas Island (2° N, 202.6° E, 2.9° N dip latitude), São Luís (2.59° S, 315.8° E, 0.5° S dip latitude) and Jicamarca (12° S, 283.1° E, 0.6° N dip latitude). The ESF parameters presented here are the onset altitude, onset time (onset refers to first appearance of signal in the radar field of view) of the bottom-type and plume, and the peak altitude of the plume. Recent studies have used these parameters to classify the spread F occurrence characteristics. The present study reveals novel features namely, the dependence of ESF parameters on the seasonal, solar flux, declination angle and longitudinal dependence from the three radar sites. In addition, we also present an empirical model to determine the nature of these ESF parameters as a function of the solar flux which may enable us to forecast (with 30 min to 1 h tolerance) the plume occurrence at any longitude located in between São Luís and Christmas Island.
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Stolle, C., H. Lühr i B. G. Fejer. "Relation between the occurrence rate of ESF and the equatorial vertical plasma drift velocity at sunset derived from global observations". Annales Geophysicae 26, nr 12 (8.12.2008): 3979–88. http://dx.doi.org/10.5194/angeo-26-3979-2008.
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Abstract. In this study, we investigate two global climatological data sets; the occurrence rate of Equatorial Spread-F (ESF), associated with equatorial plasma irregularities, at ~400 km altitude obtained from CHAMP observations, and the evening equatorial vertical plasma drift, vz, from ROCSAT-1 measurements. First, as retrieved for a solar flux level of F10.7=150, the longitudinal variation of the two independently derived quantities correlates between 84% and 93% in the seasons December solstice, equinox and June solstice. The highest correlation is found for the solstice seasons when vz is integrated over local time around the prereversal enhancement (PRE) and displaced 6° towards east. The integrated vz is a suitable estimate of the ionospheric height at the time just after the PRE and the 6° displacement is consistent with ESF eastward drift during 2 h which is assumed between creation and detection at satellite altitudes. Second, our analyses reveal a global threshold vz which is required to observe ESF at satellite altitudes. This threshold depends linearly on solar flux with correlations of 97%. Both results bring global evidence on the linear relations between ESF and the vertical plasma drift which have been proven only by local observations so far. This paper includes the first global map of the seasonal/longitudinal variation of the ESF occurrence rate over local time being valid for high solar flux years 2001–2004. The map reveals, e.g. a longitudinal dependence of the persistence of the plasma irregularities indicating that longitude dependent mechanisms other than the PRE determine the ESF lifetime.
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Sreeja, V., C. V. Devasia i R. Sridharan. "The persistence of equatorial spread F – an analysis on seasonal, solar activity and geomagnetic activity aspects". Annales Geophysicae 27, nr 2 (2.02.2009): 503–10. http://dx.doi.org/10.5194/angeo-27-503-2009.
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Abstract. The persistence (duration) of Equatorial Spread F (ESF), which has significant impact on communication systems, is addressed. Its behavior during different seasons and geomagnetic activity levels under the solar maximum (2001) and minimum (2006) conditions, is reported using the data from the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip 0.5° N) in India. The study reveals that the persistence of the irregularities can be estimated to a reasonable extent by knowing the post sunset F region vertical drift velocity (Vz) and the magnetic activity index Kp. Any sort of advance information on the possible persistence of the ionospheric irregularities responsible for ESF is important for understanding the scintillation morphology, and the results which form the first step in this direction are presented and discussed.
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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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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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Sinha, H. S. S., i S. Raizada. "First in situ measurement of electric field fluctuations during strong spread F in the Indian zone". Annales Geophysicae 18, nr 5 (31.05.2000): 523–31. http://dx.doi.org/10.1007/s00585-000-0523-y.
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Abstract. An RH-560 rocket flight was conducted from Sriharikota rocket range (SHAR) (14°N, 80°E, dip 14°N) along with other experiments, as a part of equatorial spread F (ESF) campaign, to study the nature of irregularities in electric field and electron density. The rocket was launched at 2130 local time (LT) and it attained an apogee of 348 km. Results of vertical and horizontal electric field fluctuations are presented here. Scale sizes of electric field fluctuations were measured in the vertical direction only. Strong ESF irregularities were observed in three regions, viz., 160-190 km, 210-257 km and 290-330 km. Some of the valley region vertical electric field irregularities (at 165 km and 168 km), in the intermediate-scale size range, observed during this flight, show spectral peak at kilometer scales and can be interpreted in terms of the image striation theory suggested by Vickrey et al. The irregularities at 176 km do not exhibit any peak at kilometer scales and appear to be of a new type. Scale sizes of vertical electric field fluctuations showed a decrease with increasing altitude. The most prominent scales were of the order of a few kilometers around 170 km and a few hundred meters around 310 km. Spectra of intermediate-scale vertical electric field fluctuations below the base of the F region (210-257 km) showed a tendency to become slightly flatter (spectral index n = -2.1 ± 0.7) as compared to the valley region (n = -3.6 ± 0.8) and the region below the F peak (n = -2.8 ± 0.5). Correlation analysis of the electron density and vertical electric field fluctuations suggests the presence of a sheared flow of current in 160-330 km region.Keywords: Ionosphere (Electric fields and currents; ionospheric irregularities); Radio science (ionospheric physics)
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Smith, J. M., F. S. Rodrigues i E. R. de Paula. "Radar and satellite investigations of equatorial evening vertical drifts and spread <i>F</i>". Annales Geophysicae 33, nr 11 (11.11.2015): 1403–12. http://dx.doi.org/10.5194/angeo-33-1403-2015.
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Abstract. We analyzed pre-midnight equatorial F region observations made by the 30 MHz coherent backscatter radar of São Luis, Brazil between August 2010 and February 2012. These measurements were processed, and used to create monthly maps of the echo occurrence as a function of local time and height. The maps show the inter-annual variability associated with equatorial spread F (ESF) occurrence in the Brazilian longitude sector. We also constructed monthly curves of the evening vertical drifts, for the Brazilian sector, using measurements by the ion velocity meter (IVM) onboard the C/NOFS satellite. The IVM evening drifts show a good overall agreement with the Scherliess and Fejer (1999) empirical model. Measured and model drifts show the development of the pre-reversal enhancement (PRE) of the vertical plasma drifts during ESF season. Using joint radar and satellite measurements, we found that evening (18:00–18:30 LT) mean non-negative drifts provide a necessary but not sufficient condition for the occurrence of topside ESF echoes. Evening downward (negative) drifts preceded the absence of topside ESF irregularities.
Streszczenia konferencji na temat "ESF irregularities"
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Bhattacharyya, A., i B. Kakad. "Seasonal and solar flux dependence of the growth and decay of intermediate scale ESF irregularities from VHF scintillation measurements". W 2014 XXXIth URSI General Assembly and Scientific Symposium (URSI GASS). IEEE, 2014. http://dx.doi.org/10.1109/ursigass.2014.6929722.
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Lastra, Rafael Adolfo, i Jinjiang Xiao. "Machine Learning Engine for Real-Time ESP Failure Detection and Diagnostics". W SPE Middle East Artificial Lift Conference and Exhibition. SPE, 2022. http://dx.doi.org/10.2118/206935-ms.
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Abstract Failures of electric submersible pumps (ESPs) are common occurrences in the oil industry, resulting in production disruption that amounts to thousands and in some cases millions of barrels of lost or deferred oil production annually. Timely detection of ESP developing problems will enhance the capability to timely and proactively attend and fix operational ESP issues that may cause ESP shutdowns. Early warning automated surveillance systems will significantly minimize production losses associated with ESP downtime. Traditional ESP surveillance systems provide alarms when ESP performance signals drift out of a preset range (i.e., low pressure, high current, high temperature); however, finding the root cause of the problem requires time, engineering expertise and sometimes the use of specialized tools to properly diagnose the underlying abnormal condition. After years of experience, highly trained ESP surveillance engineers are able to diagnose ESP problems by cross-referencing performance signals and finding patterns like slope increase, signal stability, unexpected step changes, etc. Pattern recognition is one of the most common applications of artificial intelligence (AI) and machine learning algorithms, which can be naturally trained to automatically detect patterns and irregularities in ESP data. This paper presents an account of the development and testing of an ESP surveillance system based on AI with the ability to automatically detect and correctly diagnose ESP problems in the early stages of development.
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Sun, Chenxi, Shenda Hong, Moxian Song, Yen-Hsiu Chou, Yongyue Sun, Derun Cai i Hongyan Li. "TE-ESN: Time Encoding Echo State Network for Prediction Based on Irregularly Sampled Time Series Data". W Thirtieth International Joint Conference on Artificial Intelligence {IJCAI-21}. California: International Joint Conferences on Artificial Intelligence Organization, 2021. http://dx.doi.org/10.24963/ijcai.2021/414.
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Prediction based on Irregularly Sampled Time Series (ISTS) is of wide concern in real-world applications. For more accurate prediction, methods had better grasp more data characteristics. Different from ordinary time series, ISTS is characterized by irregular time intervals of intra-series and different sampling rates of inter-series. However, existing methods have suboptimal predictions due to artificially introducing new dependencies in a time series and biasedly learning relations among time series when modeling these two characteristics. In this work, we propose a novel Time Encoding (TE) mechanism. TE can embed the time information as time vectors in the complex domain. It has the properties of absolute distance and relative distance under different sampling rates, which helps to represent two irregularities. Meanwhile, we create a new model named Time Encoding Echo State Network (TE-ESN). It is the first ESNs-based model that can process ISTS data. Besides, TE-ESN incorporates long short-term memories and series fusion to grasp horizontal and vertical relations. Experiments on one chaos system and three real-world datasets show that TE-ESN performs better than all baselines and has better reservoir property.