Artículos Científicos
Permanent URI for this collection
Browse
Browsing Artículos Científicos by Title
Now showing 1 - 20 of 684
Results Per Page
Sort Options
Item Restricted 3D Imaging of the OH mesospheric emissive layer(Elsevier, 2010-01-15) Kouahla, M. N.; Moreels, G.; Faivre, M.; Clairemidi, J.; Meriwether, J. W.; Lehmacher, G. A.; Vidal Safor, Erick; Veliz, OscarA new and original stereo imaging method is introduced to measure the altitude of the OH nightglow layer and provide a 3D perspective map of the altitude of the layer centroid. Near-IR photographs of the OH layer are taken at two sites separated by a 645 km distance. Each photograph is processed in order to provide a satellite view of the layer. When superposed, the two views present a common diamond-shaped area. Pairs of matched points that correspond to a physical emissive point in the common area are identified in calculating a normalized cross-correlation coefficient (NCC). This method is suitable for obtaining 3D representations in the case of low-contrast objects. An observational campaign was conducted in July 2006 in Peru. The images were taken simultaneously at Cerro Cosmos (12 09 08.2 S, 75 33 49.3 W, altitude 4630 m) close to Huancayo and Cerro Verde Tellolo (16 33 17.6 S, 71 39 59.4 W, altitude 2272 m) close to Arequipa. 3D maps of the layer surface were retrieved and compared with pseudo-relief intensity maps of the same region. The mean altitude of the emission barycenter is located at 86.3 km on July 26. Comparable relief wavy features appear in the 3D and intensity maps. It is shown that the vertical amplitude of the wave system varies as exp (z/2H) within the altitude range z = 83.5–88.0 km, H being the scale height. The oscillatory kinetic energy at the altitude of the OH layer is comprised between 3 × 10−4 and 5.4 × 10−4 J/m3, which is 2–3 times smaller than the values derived from partial radio wave at 52N latitude.Item Restricted 93 years of geomagnetic data acquisition at the Huancayo Observatory(BBC SWS Regional Network, 2015-12) Ishitsuka, José K.The Department of Terrestrial Magnetism (DTM) of the Carnegie Institution of Washington (CIW) decided to establish a magnetic observatory in Peru in 1917. John A. Fleming, who belonged to the DTM, led the search for an adequate place for magnetic field observation. The Huancayo Magnetic Observatory was constructed from 1919 to 1921 and began its operation on 3 March 1922. The observatory has made important contributions to human knowledge such as the Forbush Decrease Effect and the Equatorial Electrojet. Thousands of scientists have used Huancayo's 93 years of geomagnetic data. In 1947, the DTM left the observatory to the Peruvian Government. A few years later, the Geophysical Institute of Peru (IGP) was established and becomes one of the most important scientific institutions in Peru. Details of past and present achievements of the IGP are presented.Item Restricted A case study of ionospheric storm effects during long‐lasting southward IMF Bz‐driven geomagnetic storm(American Geophysical Union, 2014-08-27) Liu, Jing; Liu, Libo; Nakamura, Takuji; Zhao, Biqiang; Ning, Baiqi; Yoshikawa, A.Multiple instrumental observations including GPS total electron content (TEC), foF2 and hmF2 from ionosondes, vertical ion drift measurements from Communication/Navigation Outage Forecasting System, magnetometer data, and far ultraviolet airglow measured by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) are used to investigate the profound ionospheric disturbances at midlatitude and low latitude during the 14–17 July 2012 geomagnetic storm event, which was featured by prolonged southward interplanetary geomagnetic field component for about 30 h below −10 nT. In the East Asian/Australian sector, latitudinal profile of TEC variations in the main phase were characterized by three bands of increments and separated by weak depressions in the equatorial ionospheric anomaly (EIA) crest regions, which were caused by the combined effects of disturbance dynamo electric fields (DDEF) and equatorward neutral winds. In the recovery phase, strong inhibition of EIA occurred and the summer crest of EIA disappeared on 16 July due to the combined effects of intrusion of neutral composition disturbance zone as shown by the TIMED/GUVI O/N2 measurements and long‐lasting daytime westward DDEF inferred from the equatorial electrojet observations. The transit time of DDEF over the dip equator from westward to eastward is around 2200 LT. In the American longitude, the salient ionospheric disturbances in the summer hemisphere were characterized by daytime periodical intrusion of negative phase for three consecutive days in the recovery phase, preceded by storm‐enhanced density plume in the initial phase. In addition, multiple short‐lived prompt penetration electric fields appeared during stable southward interplanetary magnetic field (IMF) Bz in the recovery phase and were responsible for enhanced the EIA and equatorial ionospheric uplift around sunset.Item Restricted A causal relationship between lightning and explosive spread F(American Geophysical Union, 1984-12) Woodman Pollitt, Ronald Francisco; Kudeki, ErhanExperimental evidence for a causal relationship between lightning and explosive equatorial spread F is presented. It is proposed that explosive spread F is produced by rapidly growing plasma instabilities, which are triggered by sudden increases of the F region ambient electric fields due to the penetration of lightning fields into regions of weak or marginal instability.Item Restricted A coherent integrator-decoder preprocessor for the SOUSY-VHF-Radar(American Geophysical Union, 1980-03) Woodman Pollitt, Ronald Francisco; Kugel, R.P.; Röttger, J."The 53.5-MHz radar of the Max-Planck-Institut für Aeronomie (SOUSY-VHF-Radar) was specifically designed for the study of the dynamics of the troposphere, stratosphere, and mesosphere including turbulence and other small-scale phenomena. Transmitter and receivers were designed for a pulse width and altitude resolution of the order of 75–150 m. Such high resolution corresponds to sampling rates of the order of 2 million complex samples per second. Maximum sensitivity calls for the use of coded pulse schemes (pulse compression). The digital decoding and processing requirements of such a high information rate are beyond the capabilities of a general purpose digital computer and demanded a special purpose preprocessor. A programable preprocessor was designed and built, and it is the subject of this paper. It can decode an arbitrary sequence and perform coherent integrations for as many as 1024 altitudes. The coherent integration reduces the data rate into the computer to reasonable values. Also, by performing the coherent integration before decoding (they are linear operations and can be permuted), the necessary decoding operations in the preprocessor are reduced by 2 orders of magnitude. The master controller for the device is programed by a 1024 4-bit instruction PROM. There are 16 different instructions allowing the necessary flexibility for different code and sampling schemes. The transmitter and analog-to-digital converter are also controlled by the same device. Stratospheric and mesospheric echoes obtained with the help of this device are presented for illustration. The detection of layers as thin as 150 m or less at both stratospheric and mesospheric heights justified the efforts to achieve the maximum resolution possible."Item Restricted A comparison of ground and satellite observations of F region vertical velocity near the dip equator(American Geophysical Union, 2008-01-12) Oyekola, O. S.; Ojo, Akin; Akinrimisi, J.Nighttime F region vertical electrodynamic drifts were made at the magnetic equatorial stations in Africa, Ibadan (7.4°N, 3.9°E; 6°S dip), and Ouagadougou (12°N, 1.5°W; 5.9°N dip) using ionosondes hmF2 data during 1957/1958 International Geophysical Year (IGY) and 1990 periods, respectively; for high solar flux and geomagnetic quiet time conditions. We compare the seasonal averages of vertical drifts with observations made by Incoherent Scatter Radar (ISR) at Jicamarca (11.95°S, 76.87°W; 2°N dip) and Atmospheric Explorer E (AE‐E) satellite for equatorial F layer vertical drifts reported by Fejer et al. (1995). The results indicate good accord between the three techniques at periods when convection dominates other factors (e.g., around prereversal enhancement) except for June solstice drifts. However, when the drifts are completely downward (negative) between 2000 and 0500 LT sector, the mean discrepancies between ionosondes and ISR drifts are of 8–11 m/s (December solstice), 14–17 m/s (equinoxes) but comparable during June solstice with a value of about 14 m/s. Conversely, the typical values of the differences among ionosondes and AE‐E downward velocities are 7–10 m/s (December solstice), 9–12 m/s (equinoxes), yet again similar in winter with a significantly smaller value of about 3 m/s. The evening reversal times are in excellent agreement, apart from June solstice drifts, which exhibits large fluctuations. The morning reversal times show small variations. Equinoctial prereversal enhancement velocities have amplitudes of approximately 17–35 m/s between the methods. Our data are useful for global ionospheric modeling and for the predictions of development of nighttime equatorial F region irregularities at the African region where there is paucity of data.Item Open Access A cooperative synchronous observation of winds and tides in the tropical lower stratosphere and mesosphere using VHF radars at Jicamarca and Arecibo(Society of Geomagnetism and Earth, Planetary and Space Sciences, 1986-04) Maekawa, Yasuyuki; Aso, Takehiko; Róttger, Jürgen; Czechowsky, Peter; Rüster, Rüdiger; Schmidt, Gerhard; Hirota, Isamu; Woodman Pollitt, Ronald Francisco; Kato, SusumuA cooperative synchronous observation of winds in the tropical lower stratosphere and mesosphere was carried out using VHF radars at Jicamarca (11.95°S, 76.87°W) and at Arecibo (18.35°N, 66.75°W) on 19-21 or 23 November 1981. The comparison between observations about symmetrical north and south of the equator is focussed on the mean and tidal wind characteristics. Vertical structures of the mean zonal winds at both observational stations are in fairly good agreement with the CIRA (1972) zonal circulation model at the corresponding latitudes. The nature of lower stratospheric diurnal tides is similar between Jicamarca and Arecibo. Their vertical profiles generally correspond to the theoretical predictions based on the fundamental tidal modes. The vertical scale of the tidal components including higher-order modes is shown to be longer than 10km. In the mesosphere, vertically propagating diurnal tide which is expected to be predominant at lower latitudes is partially detected in daytime at Arecibo, while it is not so well-defined at Jicamarca, suggesting a variable nature of the relevant tidal modes.Item Open Access A digital-analogue match filter for piecewise square pulses(Instituto Geofísico del Perú, 1977-02-03) Woodman Pollitt, Ronald FranciscoIt is well known that the optimum signal to noise ratio in a radar is obtained when the receiver response is matched to the transmitter pulse shape. A matched receiver has an impulse response, g(t), equal to the negative fold of the transmitter pulse shape, p(t), i .c.: g(t) = p(-t). On the other hand we have that for a given maximum peak and average power the pulse length is shortest when the transmitter pulse has a square or piecewise square shape (sequences of positive and negative pulses. In many cases the square shapes are the easiest, if not the only possible, shapes that can be obtained in a transmitter. Here we use "square" to imply pulses with fairly constant amplitude and relative short transit times. Square-wave receiver input responses can not he Obtained with simple lumped element filters. One needs either taped delay lines or digital techniques. The present note describes a hybrid analog-digital technique to achieve a matched set of piecewise "square" transmitter pulse shapes and receiver input response. The scheme works also for phase (0-180°) coded pulse sequences. The scheme is a natural use for a digital decoder for phase coded pulses. First we shall describe the operation for n single square pulse. The extension to a phase coded sequence is straightforward.Item Open Access A double demodulation algorithm for narrow spectral features(Instituto Geofísico del Perú, 1995) Ragaini, Enrico; Woodman Pollitt, Ronald FranciscoThis paper proposes a technique for processing signals in which the relevant feature consists of the shape and position of narrow doppler spectral lines. The proposed technique is the digital version of a quadrature demodulator, and its purpose is to demodulate a selected spectral line allowing to study its envelope as a slowly varying process.Item Restricted A general statistical instrument theory of atmospheric and ionospheric radars(American Geophysical Union, 1991-01) Woodman Pollitt, Ronald FranciscoSome basic functional relationships between the statistics of the signals received in a radar and the statistics of the density fluctuations of a scattering medium are derived. They vary in their degree of generality, but they are all very general in scope. They include monostatic and bistatic radars scattering from either atmospheric, ionospheric, or meteorological media. They are valid for refractive and slightly dispersive media, so they can also be used for HF ionospheric radars. They include the effects of filtering, including receiver filtering, pulse compression coding and decoding schemes, and coherent integration, or any alternative linear digital filtering scheme. Functional relationships to include cross-correlation schemes, such as Faraday rotation experiments and interferometers, are included. Some simplified expressions are derived for frequently encountered situations, where different approximations can be made. These simplified expressions cover a large number of radar techniques currently in use for atmospheric and ionospheric applications.Item Open Access A high-resolution study of mesospheric fine structure with the Jicamarca MST radar(European Geosciences Union (EGU), 2006-07-03) Sheth, R.; Kudeki, E.; Lehmacher, G.; Sarango, M.; Woodman Pollitt, Ronald Francisco; Chau Chong Shing, Jorge Luis; Guo, L.; Reyes, P.Correlation studies performed on data from recent mesospheric experiments conducted with the 50-MHz Jicamarca radar in May 2003 and July 2004 are reported. The study is based on signals detected from a combination of vertical and off-vertical beams. The nominal height resolution was 150 m and spectral estimates were obtained after ~1 min integration. Spectral widths and backscattered power generally show positive correlations at upper mesospheric heights in agreement with earlier findings (e.g., Fukao et al., 1980) that upper mesospheric echoes are dominated by isotropic Bragg scatter. In many instances in the upper mesosphere, a weakening of positive correlation away from layer centers (towards top and bottom boundaries) was observed with the aid of improved height resolution. This finding supports the idea that layer edges are dominated by anisotropic turbulence. The data also suggests that negative correlations observed at lower mesospheric heights are caused by scattering from anisotropic structures rather than reflections from sharp vertical gradients in electron density.Item Open Access A latitudinal network of GPS receivers dedicated to studies of equatorial spread F(American Geophysical Union, 2004-02) Valladares, C. E.; Sheehan, R.; Villalobos, J.Five GPS receivers have been deployed near the 74°W longitude meridian to measure the variability of total electron content (TEC) latitudinal profiles and to study the relation of this variability with the onset and evolution of spread F plasma structures. These five GPS receivers, together with two others that form part of the International GPS Service (IGS) network, three more that belong to the South Andes Project network, and an additional receiver located at Ancon, Peru, provide TEC values between 8°N and 40°S geographic latitude. In addition, all five GPS receivers managed by Boston College give the amplitude scintillation on a near-real time basis. This fact allows us to know the maximum latitude to which the irregularities extend and to infer the maximum altitude of the plasma bubbles. We have calculated TEC latitudinal profiles using the TEC values obtained by all the receivers between 1998 and 2001. We found that during the equinoxes, UHF scintillations occur when the ratio of the crest to the trough of the anomaly is 2 or larger. During the December solstice the crest is not very pronounced, but a sharp decrease of TEC at the magnetic equator precedes the onset of 1-km scale irregularities. We have also examined a longitudinal variability of scintillations by partitioning the sky in two sectors separated at the 74°W meridian. We consistently observe a greater number of GPS scintillation events at the eastern longitudes over the Amazon rain forest. This intriguing finding could well be explained by a larger population of gravity waves at longitudes east of the Andes.Item Restricted A multi-beam incoherent scatter radar technique for the estimation of ionospheric electron density and Te/Ti profiles at Jicamarca(Elsevier, 2013-12) Milla, Marco; Kudeki, Erhan; Reyes, Pablo M.; Chau Chong Shing, Jorge LuisA multi-beam incoherent scatter radar technique has been developed at the Jicamarca Radio Observatory in order to maximize the number of ionospheric parameters that can be estimated simultaneously. The technique interleaves radar observations with antenna beams pointing perpendicular and oblique to the Earth's magnetic field. For the estimation of the ionospheric parameters, we have modeled the magnetic aspect angle variations of the signal power and cross-correlation data measured in multiple directions. The data model, formulated in terms of soft-target radar equations, was built based on the theories of incoherent scattering and magneto-ionic propagation. Applying a nonlinear least-squares inversion algorithm, we have succeeded in measuring simultaneously ionospheric electron densities Ne, electronto-ion temperature ratios Te=Ti as well as vertical and zonal plasma drifts. In the past, radar users had to choose between either perpendicular-to-B or oblique modes, thus, the application of this technique extends the current capabilities of the Jicamarca radar.Item Restricted A multi-DSP signal processing and control system for MST radar(IEEE, Institute of Electrical and Electronics Engineers, 1995) Sarango, Martin F.; Woodman Pollitt, Ronald Francisco; Ragaini, Enrico; Vásquez, Edilberto H.This paper describes a signal processing and control system for atmospheric sounding rardars, with improved characteristics with respect to formerly used architectures. We refer especially to wind profiling as an important and interesting application. The system allows up to 4 receiver signals to be processed at the same time, with 60 range gates and 300 m height resolution. The use of two DSPs working in parallel allows the use of a single PC for real time data processing and system control. Some experimental results and reported.Item Restricted A multistatic HF beacon network for ionospheric specification in the Peruvian sector(American Geophysical Union, 2016-05-04) Hysell, D. L.; Milla, Marco; Vierinen, J.A small network of HF beacon transmitters and receivers has been deployed in Peru for specifying the F region ionosphere in the sector. The effort is motivated by ionospheric disturbances associated with equatorial spread F (ESF) and specifically disturbances arising under inauspicious conditions for ESF as determined by numerical simulations. The beacons operate at dual frequencies (2.72 and 3.64 MHz). They are CW but incorporate pseudorandom noise (PRN) coding so that the group delays of the links can be measured. Other observables are power, Doppler shift, bearing, and polarization. A transmitter is currently deployed at Ancon (11∘46’37’’S, 77∘09’1’’W, 51 m above sea level (asl)), while receivers are at Jicamarca (11∘57’5.8’’S, 76∘52’27’’W, 510 m asl) and Huancayo (12∘02’30’’S, 75∘19’15’’W, 3315 m asl). An algorithm for inverting the beacon data combined with electron density profiles from Jicamarca is described. Data and representative solutions from an experimental campaign held in August 2015 are presented and evaluated.Item Restricted A neural network based error correction method for radio occultation electron density retrieval(Elsevier, 2015-10-19) Pham, Viet-Cuong; Juang, Jyh-ChingAbel inversion techniques have been widely employed to retrieve electron density profiles (EDPs) from radio occultation (RO) measurements, which are available by observing Global Navigation Satellite System (GNSS) satellites from low-earth-orbit (LEO) satellites. It is well known that the ordinary Abel inversion might introduce errors in the retrieval of EDPs when the spherical symmetry assumption is violated. The error, however, is case-dependent; therefore it is desirable to associate an error index or correction coefficient with respect to each retrieved EDP. Several error indices have been proposed but they only deal with electron density at the F2 peak and suffer from some drawbacks. In this paper we propose an artificial neural network (ANN) based error correction method for EDPs obtained by the ordinary Abel inversion. The ANN is first trained to learn the relationship between vertical total electron content (TEC) measurements and retrieval errors at the F2 peak, 220 km and 110 km altitudes; correction coefficients are then estimated to correct the retrieved EDPs at these three altitudes. Experiments using the NeQuick2 model and real FORMOSAT-3/COSMIC RO geometry show that the proposed method outperforms existing ones. Real incoherent scatter radar (ISR) measurements at the Jicamarca Radio Observatory and the global TEC map provided by the International GNSS Service (IGS) are also used to valid the proposed method.Item Open Access A new approach in incoherent scatter F region E x B drift measurements at Jicamarca(American Geophysical Union, 1999-12-01) Kudeki, E.; Bhattacharyya, S.; Woodman Pollitt, Ronald FranciscoSince 1996 incoherent scatter F region plasma drift measurements at Jicamarca have been implemented using a new signal processing approach replacing the traditional pulse-to-pulse correlation method. The new method, based on Doppler spectrum estimation and nonlinear least squares fitting to model spectra obtained from incoherent scatter theory, improves the instrumental sensitivity remarkably under low signal-to-noise conditions. With the new method it has become possible to obtain very high quality drifts data at nearly all hours of the day throughout most F region heights. Altitudinal smoothing of the drifts data to reduce measurement noise is no longer necessary, and studies of the height variations of drifts can be performed with much greater certainty than before. Small-amplitude gravity wave oscillations have been detected at F region heights and a vortical circulation of the F region plasma has been observed in the post sunset period.Item Restricted A new approach to the derivation of dynamic information from ionosonde measurements(European Geosciences Union, 2003-11-30) Liu, L.; Luan, X.; Wan, W.; Ning, B.; Lei, J.A new approach is developed to derive dynamic information near the peak of the ionospheric F-layer from ionosonde measurements. This approach avoids deducing equivalent winds from the displacement of the observed peak height from a no-wind equilibrium height, so it need not determine the no-wind equilibrium height which may limit the accuracy of the deduced winds, as did the traditional servo theory. This approach is preliminarily validated with comparisons of deduced equivalent winds with the measurements from the Fabry-Perot interferometer, the Millstone Hill incoherent scatter radar and with previous works. Examples of vertical components of equivalent winds (VEWs), over Wuhan (114.4° E, 30.6° N, 45.2° dip), China in December 2000 are derived from Wuhan DGS-256 Digisonde data. The deduced VEWs show large day-to-day variations during the winter, even in low magnetic activity conditions. The diurnal pattern of average VEWs is more complicated than that predicted by the empirical Horizontal Wind Model (HWM). Using an empirical electric field model based on the observations from Jicamarca radar and satellites, we investigate the contributions to VEWs from neutral winds and from electric fields at the F-layer peak. If the electric field model is reasonable for Wuhan during this period, the neutral winds contribute mostly to the VEWs, and the contribution from the E × B drifts is insignificant.Item Open Access A new optical polarization catalog for the small magellanic cloud: the magnetic field structure(IOP Publishing, 2015-06-10) Lobo Gomes, Aiara; Magalhães, Antônio Mário; Pereyra, Antonio; Vilega Rodrigues, CláudiaWe present a new optical polarimetric catalog for the Small Magellanic Cloud (SMC). It contains a total of 7207 stars, located in the northeast (NE) and Wing sections of the SMC and part of the Magellanic Bridge. This new catalog is a significant improvement compared to previous polarimetric catalogs for the SMC. We used it to study the sky-projected interstellar magnetic field structure of the SMC. Three trends were observed for the ordered magnetic field direction at position angles (PAs) of (65◦ ± 10◦), (115◦ ± 10◦), and (150◦ ± 10◦). Our results suggest the existence of an ordered magnetic field aligned with the Magellanic Bridge direction and SMC’s Bar in the NE region, which have PAs roughly at 115◦.4 and 45◦, respectively. However, the overall magnetic field structure is fairly complex. The trends at 115◦ and 150◦ may be correlated with the SMC’s bimodal structure, observed in Cepheids’ distances and HI velocities. We derived a value of Bsky = (0.947 ± 0.079) µG for the ordered sky-projected magnetic field, and δB = (1.465±0.069) µG for the turbulent magnetic field. This estimate of Bsky is significantly larger (by a factor of ∼ 10) than the line of sight field derived from Faraday rotation observations, suggesting that most of the ordered field component is on the plane of the sky. A turbulent magnetic field stronger than the ordered field agrees with observed estimates for other irregular and spiral galaxies. For the SMC the Bsky/δB ratio is closer to what is observed for our Galaxy than other irregular dwarf galaxies.Item Restricted A new source of the midlatitude ionospheric peak density structure revealed by a new Ionosphere‐Plasmasphere model(American Geophysical Union, 2016-02-02) Maruyama, Naomi; Sun, Yang-Yi; Richards, Phillip G.; Middlecoff, Jacques; Fang, Tzu-Wei; Fuller-Rowell, Timothy J.; Akmaev, Rashid A.; Liu, Jaun-Yeng; Valladares, Cesar E.The newly developed Ionosphere‐Plasmasphere (IP) model has revealed neutral winds as a primary source of the “third‐peak” density structure in the daytime global ionosphere that has been observed by the low‐latitude ionospheric sensor network GPS total electron content measurements over South America. This third peak is located near −30° magnetic latitude and is clearly separate from the conventional twin equatorial ionization anomaly peaks. The IP model reproduces the global electron density structure as observed by the FORMOSAT‐3/COSMIC mission. The model reveals that the third peak is mainly created by the prevailing neutral meridional wind, which flows from the summer hemisphere to the winter hemisphere lifting the plasma along magnetic field lines to higher altitudes where recombination is slower. The same prevailing wind that increases the midlatitude density decreases the low‐latitude density in the summer hemisphere by counteracting the equatorial fountain flow. The longitudinal variation of the three‐peak structure is explained by the displacement between the geographic and geomagnetic equators.