Clezio De Nardin | Instituto Nacional de Pesquisas Espaciais (original) (raw)

Papers by Clezio De Nardin

Annales Geophysicae, 2018

Brazilian Journal of Geophysics

In this work, we present a study of the Total Electronic Content (TEC) spatiotemporal variability... more In this work, we present a study of the Total Electronic Content (TEC) spatiotemporal variability during geomagnetically disturbed periods over South America. For this, we evaluated the Disturbance Ionosphere indeX (DIX) scaling factor (β coefficient) for different magnetic dip angles and seasons. In this context, the β values are given in TEC units (TECU) and correspond to a latitudinal-dependent factor used to normalize the DIX output into a scale from 0 to 5. Thus, we selected thirteen magnetic storm events between 2013 and 2017, subdivided into seven moderate, four intense, and two extreme magnetic storms. The results showed that the β coefficient latitudinal variation is characterized by two asymmetric crests concerning the magnetic equator. This suggests the presence of transequatorial thermospheric wind's effects on the ionosphere's plasma distribution. Additionally, we have observed that the crests occur close to the magnetic equator during the winter and autumn and ...

Revista Brasileira de Geofísica, 1999

Journal of Atmospheric and Solar-Terrestrial Physics, 2018

Advances in Space Research, 2018

Journal of Geophysical Research: Space Physics, 2017

Journal of Atmospheric and Solar-Terrestrial Physics, 2017

Journal of Geophysical Research: Space Physics, 2015

Annales Geophysicae, 2018

Brazilian Journal of Geophysics

Revista Brasileira de Geofísica, 1999

Journal of Atmospheric and Solar-Terrestrial Physics, 2018

Advances in Space Research, 2018

Journal of Geophysical Research: Space Physics, 2017

Journal of Atmospheric and Solar-Terrestrial Physics, 2017

Journal of Geophysical Research: Space Physics, 2015

A new 50 MHz coherent back-scatter radar has recently become operational at the Equatorial Space ... more A new 50 MHz coherent back-scatter radar has recently become operational at the Equatorial Space Observatory of INPE at Sao Luis (2.33degreesS, 44.2degreesW, dip: -0.5degrees), in Brazil, located close to the center of the equatorial electrojet. Preliminary results from a 12-day campaign in December 1999 involving this radar and a nearby digisonde are presented in this paper. The range-time-intensity (RTI) maps of electrojet irregularities show short-period (similar to5 min or larger) fluctuations and significant day-to-day variability. A notable characteristic is the midday descent in the altitude of the RTI pattern observed on some days when the dominant Doppler spectra are of type-2. This feature is similar to the diurnal variation of the base height of the q-type sporadic-E-layer (h'Es-q) simultaneously observed by the digisonde. Quasi-periodic fluctuations in the RTI and type-1 and type-2 spectral intensities seem to indicate electric field modulation from winds associated with typical gravity wave periods. Large surges in the westward irregularity drift velocity, presenting a higher velocity tail to the Doppler spectral distribution, is observed at sunset concurrent with the evening F-layer vertical uplift as observed by the digisonde. Some implications of these results on the E- and F-region electrodynamics are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.

Data collected during the first two observational campaigns, conducted in August 1998 and Decembe... more Data collected during the first two observational campaigns, conducted in August 1998 and December 1999, using the new 50 MHz coherent back-scatter radar, developed at INPE, that became operational at the magnetic equatorial site at Sao Luis, (2.33degreesS, 44.2degreesW), Brazil, are analyzed in this paper. The spatial and temporal distribution of 3-m irregularity power in the form of range-time-intensity maps and spectral distribution in the form of spectrograms are analyzed for 'quiet' conditions and during geomagnetic storm disturbances. The analysis has brought out some new findings, besides confirming some of the already known storm response features of the EEJ and its plasma instabilities. Among the highlights of the results are: the electrical coupling between the equatorial and auroral electrojets is important even on a 'quiet' day, and gets very strong during magnetic storm disturbances; disturbance prompt penetration electric field, and the delayed electric field from disturbance dynamo, control the 3-m plasma wave development and inhibition in different degrees during the storm main phase and recovery phase; the amplitudes of the disturbance are larger during the morning hours than in the afternoon, in agreement with theoretical models; the height dependence of the relative dominance of the type-1 and type-2 waves generated by disturbance electric field is different from that of quiet conditions, the relative power of the type-2 getting enhanced at higher levels in the former case. A few other results are also discussed in this paper. (C) 2003 Elsevier Ltd. All rights reserved.

In this paper we present the first steps given towards the development of the South American K in... more In this paper we present the first steps given towards the development of the South American K index (Ksa) based on the measurements made by the Embrace Magnetometer Network. We present: (a) a description of the magnetometer and of the network; (b) the procedure used to calibrate the network equipments; (c) the procedure to obtain each station K scale and its corresponding upper limits of ranges for the three-hour-range station K index (thereafter referred as K9 threshold); and (d) some particularities regarding the Quiet Day Curve (QDC) deviation and its long term variation.

In this paper we present field-line-integrated local ionospheric conductivity model for the Brazi... more In this paper we present field-line-integrated local ionospheric conductivity model for the Brazilian equatorial region. It was developed aiming to calculate the E-region electric fields that drive the 3-meter plasma irregularity of the equatorial electrojet. This model was based on the neutral atmosphere and electron densities, on the ion composition, and on E-region critical frequency foE measured by digisonde. Due to the large magnetic declination angle in the Brazilian sector we approximate the geomagnetic field model with a dipole which is not located at the centre of the Earth. We have also considered the eccentric dipole having an inclination of 20° with respect to the Earth rotation axis. The local conductivities resulting from our model are compared to the local conductivities obtained from the conductivity model of the Kyoto University.

This paper presents results from a new model of field-line-integrated ionospheric conductivity fo... more This paper presents results from a new model of field-line-integrated ionospheric conductivity for the Brazilian equatorial region. It was developed aiming to calculate zonal electric fields at E-region heights In the equatorial region. The present model is based on a constant neutral atmosphere model and on an empirical electron densities model (which also gives the ion composition) adjusted by E-region electron density measured by digisonde. It is also based on a geomagnetic field model that we approximate with a dipole which is not located at the centre of the Earth due to the large magnetic declination angle In the Brazilian sector. We have also considered the eccentric dipole having an inclination of 20° with respect to the Earth rotation axis. The conductivities are calculated for the year 2002 and the results from the present model are compared to those obtained from the conductivity model of the Kyoto University. {\textcopyright} 2007 Sociedade Brasileira de Geof{\'{i}}sica.

We have being studying the zonal and vertical E region electric field components inferred from th... more We have being studying the zonal and vertical E region electric field components inferred from the Doppler shifts of type 2 echoes (gradient drift irregularities) detected with the 50 MHz backscatter coherent radar set at S{\~{a}}o Luis, Brazil (SLZ, 2.3°S, 44.2°W) during the solar cycle 24. In this report we present the dependence of the vertical and zonal components of this electric field with the solar activity, based on the solar flux F10.7. For this study we consider the geomagnetically quiet days only (Kp ≤ 3+). A magnetic field-aligned-integrated conductivity model was developed for proving the conductivities, using the IRI-2007, the MISIS-2000, and the IGRF-11 models as input parameters for ionosphere, neutral atmosphere, and Earth magnetic field, respectively. The ion-neutron collision frequencies of all the species are combined through the momentum transfer collision frequency equation. The mean zonal component of the electric field, which normally ranged from 0.19 to 0.35 mV/m between the 8 and 18 h (LT) in the Brazilian sector, show a small dependency with the solar activity. Whereas the mean vertical component of the electric field, which normally ranges from 4.65 to 10.12 mV/m, highlights the more pronounced dependency of the solar flux.

\textcopyright} 2016 COSPAR. The effects of high-energy particle precipitation on mesospheric and... more \textcopyright} 2016 COSPAR. The effects of high-energy particle precipitation on mesospheric and stratospheric ozone have been investigated during the last decades. However, while these effects have been widely discussed for the auroral region, little is known about the role of the high-energy particle precipitation on the stratospheric composition and thermal structure in the tropical/subtropical region. Here we show that the spatial distribution of both the stratospheric ozone and temperature in the southern hemisphere matches the pattern of the Southern Hemisphere Magnetic Anomaly. We found that during the austral winter and spring, in the subtropical region (below 30°S), the reduction of the stratospheric ozone and temperature occurs systematically in the magnetic anomaly area. The differences between the temperatures inside the magnetic anomaly (60°W) and outside the anomaly (150°E) for 42.5°S from June to November are higher than 2. K. The maximum difference at this latitude is approximately 5.9. K and occurs in October during the austral spring.

The present work is the third of a three-part review of space weather in Latin America, specifica... more The present work is the third of a three-part review of space weather in Latin America, specifically observing its evolution in three countries (Argentina, Brazil and Mexico). This work presents the decision process for the spinning off of space weather prediction centers from space science groups with our interpretation of the reasons/opportunities that lead to this. Lastly, the constraints for the progress in space weather monitoring, research, and forecast are listed with recommendations to overcome them, which we believe will lead to the access of key variables for the monitoring and forecasting space weather, which will allow these centers to better monitor space weather and issue warnings, watches and alerts.

Equatorial electrojet (EEJ) observations using VHF radars show backscattered echoes from two type... more Equatorial electrojet (EEJ) observations using VHF radars show backscattered echoes from two types of electron density irregularities explained by the modified two-stream (Type I) and the gradient drift (Type II) instabilities. From the Type II irregularity velocities obtained by a radar installed at S{\~{a}}o Luiz Space Observatory (2.58°S, 44.23°W, magnetic latitude 0.23°S) we have inferred the vertical electric fields (E z). The Ez inference was based on geomagnetic field and atmospheric models. A harmonic analysis of these electric fields shows the presence of gravity waves (GW) induced oscillations in the EEJ. We calculated the ratio between GW-related electric fields and the total Ez, which is an indicator of the efficiency in the production of an additional electric field due to a gravity wave neutral wind. In the present paper we describe the methodology of analysis and discuss some characteristics of the gravity waves that could modify the equatorial ionospheric electric fields. Copyright 2009 by the American Geophysical Union.

Daytime E-region electric fields play a crucial role in the ionospheric dynamics at the geomagnet... more Daytime E-region electric fields play a crucial role in the ionospheric dynamics at the geomagnetic dip latitudes. Due to their importance, there is an interest in accurately measuring and modeling the electric fields for both climatological and near real-time studies. In this work, we present the daytime vertical (Ez) and eastward (Ey) electric fields for a reference quiet day (February 7, 2001) at the S{\{}{\~{a}}{\}}o Lu{\{}$\backslash$'i{\}}s Space Observatory, Brazil (SLZ, 2.31{\{}$\backslash$textdegree{\}}S, 44.16{\{}$\backslash$textdegree{\}}W). The component Ez is inferred from Doppler shifts of type II echoes (gradient drift instability) and the anisotropic factor, which is computed from ion and electron gyro frequencies as well as ion and electron collision frequencies with neutral molecules. The component Ey depends on the ratio of Hall and Pedersen conductivities and Ez. A magnetic field-line-integrated conductivity model is used to obtain the anisotropic factor for calculating Ez and the ionospheric conductivities for calculating Ey. This model uses the NRLMSISE-00, IRI-2007, and IGRF-11 empirical models as input parameters for neutral atmosphere, ionosphere, and geomagnetic field, respectively. Consequently, it is worth determining the uncertainties (or errors) in Ey and Ez associated with these empirical model outputs in order to precisely define the confidence limit for the estimated electric field components. For this purpose, errors of {\{}$\backslash$textpm{\}}10 {\{}{\%}{\}} were artificially introduced in the magnitude of each empirical model output before estimating Ey and Ez. The corresponding uncertainties in the ionospheric conductivity and electric field are evaluated considering the individual and cumulative contribution of the artificial errors. The results show that the neutral densities and temperature may be responsible for the largest changes in Ey and Ez, followed by changes in the geomagnetic field intensity and electron and ions compositions.

We have routinely monitored the total frequency (ftEs) and the blanketing frequency (fbEs) of spo... more We have routinely monitored the total frequency (ftEs) and the blanketing frequency (fbEs) of sporadic E layers with the digital sounder under the magnetic equator in the Brazilian sector. Sporadic layers appear in the equatorial region (Esq) at heights between 90 and 130 km, mainly due to irregularities in the equatorial electrojet current. However, during the recovery phase of the October 2003 superstorm, an anomalous intensification of the ionospheric density that exceeded the normal ambient background values for local time and location was observed. The parameter fbEs rose to almost 7.5 MHz during this event, due to a type “c” blanketing sporadic layer (Esc), which is driven by wind shear. This result is discussed in terms of the atmosphere dynamics based on magnetic signature of the equatorial electrojet current using magnetometer data. Also, using data measured by sensors onboard the Geostationary Operational Environmental Satellite (GOES) 10 we analyze the possible influence of the solar flare-associated X-ray flux as an additional source of ionization.

Abstract In this paper, the effects of a minor sudden stratospheric warming (SSW) event of 2012 i... more Abstract In this paper, the effects of a minor sudden stratospheric warming (SSW) event of 2012 in the ionosphere over South American and African sectors have been studied using C/NOFS satellite data and GPS observations. Also, the magnetometer measurements obtained at two stations in the equatorial and low-latitude regions in the South American sector are presented. There were significant Vertical Total Electron Content (VTEC) depletions in the afternoon in both South American and African sectors during the 2012 minor SSW event. A novel feature of the present study is the reduction of the quasi 16-day oscillation (periods ranging from 11 to 20 days) in the daily averaged VTEC in the Brazilian and African sector during an SSW event. Also, the results for the Brazilian sector show an amplification of the {\~{}}2–6day period in the daily averaged VTEC at equatorial and low-latitude regions, after the SSW temperature peak. This investigation shows that a minor SSW can affect the irregularities at ionospheric heights in the Brazilian and African sectors. Ground-based magnetometer measurements in the American sector shows strongly enhanced equatorial electrojet (EEJ) after the SSW temperature peak.

We carried out an equatorial electrojet campaign in December 1999, using the RESCO coherent backs... more We carried out an equatorial electrojet campaign in December 1999, using the RESCO coherent backscatter radar at S{\~{a}}o Lu{\'{i}}s, at the geomagnetic equator. In this paper we selected the period from December 11 through December 19, which covers both relatively disturbed and quiet days. For this period 16 spectrograms were obtained for each day of observation with each spectrogram corresponding to one range gate. The first gate corresponds to 97.4-km height while the last gate corresponds to 115.6-km and they were sampled at 2.6-km intervals. Daily Range Time Intensity maps were generated for this period. Through the analysis of the spectrograms and considering the enhancements in the equatorial electrojet observed in the evening of some days, it was possible to observe three distinct regions as identified earlier by Pfaff et al. (1987) and Kudeki et al. (1987). Indeed, our observations agree with the Condor Equatorial Electrojet Campaign results. We have used the auroral activity indexes AU and AL as well as the equatorial ring current index Dst to evaluate the response of the equatorial electrojet to magnetic disturbances.

In the present work we show new results regarding equatorial counter electrojet (CEJ) events in t... more In the present work we show new results regarding equatorial counter electrojet (CEJ) events in the Brazilian sector, based on the RESCO radar, two set of fluxgate magnetometer systems and a digital sounder. RESCO radar is a 50 MHz backscatter coherent radar installed in 1998 at Sao Luis (SLZ, 2.33 degrees S, 44.60 degrees W), an equatorial site. The Digital sounder routinely monitors the electron density profile at the radar site. The magnetometer systems are fluxgate-type installed at SLZ and Eusebio (EUS, 03.89 degrees S, 38.44 degrees W). From the difference between the horizontal component of magnetic field at SLZ station and the same component at EUS (EEJ ground strength) several cases of westward morning electrojet and its normal inversion to the eastward equatorial electrojet (EEJ) have been observed. Also, the EEJ ground strength has shown some cases of CEJ events, which been detected with the RESCO radar too. Detection of these events were investigated with respect to their time and height of occurrence, correlation with sporadic E (Es) layers at the same time, and their spectral characteristics as well as the radar echo power intensity.

Zonal and vertical electric fields were estimated at E region heights in the Brazilian sector. Zo... more Zonal and vertical electric fields were estimated at E region heights in the Brazilian sector. Zonal electric fields are obtained from the vertical electric fields based on their relation through the Hall-to-Pedersen ionospheric conductivities ratio. The technique for obtaining the vertical electric field is based on its proportionality to the Doppler velocities of type 2 irregularities as detected by coherent radars. The 50 MHz backscatter coherent (RESCO) radar was used to estimate the Doppler velocities of the type 2 irregularities embedded in the equatorial electrojet. A magnetic field-line integrated conductivity model was developed to provide the conductivities. It considers a multi-species ionosphere and a multi-species neutral atmosphere, and uses the IRI 2007, the MISTS 2000 and the IGRF 10 models as input parameters for ionosphere, neutral atmosphere and Earth's magnetic field, respectively. The ion-neutral collision frequencies of all the species are combined through the momentum transfer collision frequency equation, and different percentages of electron-neutral collisions were artificially included for studying the implication of such increase in the zonal electric field, which resulted ranging from 0.13 to 0.49 mV/m between the 8 and 18 h (LT), under quiet magnetic conditions. (C) 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.

We analyze the daytime efficiency of the interplanetary electric field (IEF) penetration to the e... more We analyze the daytime efficiency of the interplanetary electric field (IEF) penetration to the equatorial ionosphere based on a correlation analysis carried out between different levels of decomposition applied to IEF intensity measured at the ACE spacecraft and ionospheric electric field intensity inferred from ground based magnetometers located in the equatorial region in Brazil. We compare the time variations of those two electric field intensities by means of a scale-by-scale decomposed time series through wavelet multi-resolution analysis. Efficiency is here defined as the fraction of the variation of the IEF intensity that has penetrated into the equatorial region, and it increases with increasing fraction. Two cases of prompt penetration electric fields (PPEF) are analyzed: one occurring on March 31, 2001, and other on April 17, 2002. Our results show that the penetration effect with time scale ranging around 1 h is maximized in relation to other scales. (C) 2010 Elsevier Ltd. All rights reserved.

The Instituto Nacional de Pesquisas Espaciais (National Institute for Space Research) is installi... more The Instituto Nacional de Pesquisas Espaciais (National Institute for Space Research) is installing in S{\~{a}}o Lu{\'{i}}s do Maranh{\~{a}}o a coherent VHF pulsed Doppler radar system for studying the electrodynamics of equatorial electrojet in Brazil. The system consists of 8 transmitters, 15 KW each one, an array of 16 antennas north-south aligned with 48 dipoles for each, and a data acquisition system (Janardhanan, 1983). The set of transmitters generate a pulsed signal at 50 MHz which feeds the antennas array and is transmitted to be scattered by ionospheric irregularities at E and F layers height. When the signal finds irregularities in about half wavelength, it is backscattered in the receiver system, the result is stored as a digital code. S{\~{a}}o Lu{\'{i}}s do Maranh{\~{a}}o was chosen by its geographical placement (2.6 {\textordmasculine} S, 44.2{\textordmasculine} W) on the geomagnetic equator. At present, the radar is partially operated collecting dta on equatorial electrojet. However some improvements are being done for the complete system operation. Several researches have been done concerning irregularities on geomagnetic equator in Brazil (Abdu et al., 1982; Abdu, 1983; Sobral et al., 1985; Sobral and Abdu, 1991; Batista et al., 1996) and the coherent radar will be another contribution for the important equatorial electrojet study.

Using the RESCO 50 MHz backscatter radar (2.33° S, 44.2° W, DIP: –0.5), at S{\~{a}}o Lu{\'{i}}s, ... more Using the RESCO 50 MHz backscatter radar (2.33° S, 44.2° W, DIP: –0.5), at S{\~{a}}o Lu{\'{i}}s, Brazil, we obtained Range Time Intensity (RTI) maps covering the equatorial electrojet heights during daytime and evening. These maps revealed a scattering region at an altitude of about 108 km during the sunset period. The type of 3-m irregularity region we present here has not been reported before in the literature, to our knowledge. It was mainly observed around the Southern Hemisphere summer-solstice period, under quiet magnetic activity condition. The occurrence of this echo region coincides in local time with the maximum intensity of an evening pre-reversal eastward electric field of the ionospheric F-region. A tentative explanation is proposed here in terms of the theory of the divergence of the equatorial electrojet (EEJ) current in the evening ionosphere presented by Haerendel and Eccles (1992), to explain the partial contribution of the divergence to the development of the pre-reversal electric field. The theory predicts an enhanced zonal electric field and hence a vertical electric field below 300 km as a consequence of the EEJ divergence in the evening. The experimental results of the enhanced echoes from the higher heights of the EEJ region seem to provide evidence that the divergence of the EEJ current can indeed be the driver of the observed scattering region.

We have analyzed the data collected during an observational campaign of the equatorial electrojet... more We have analyzed the data collected during an observational campaign of the equatorial electrojet (EEJ) plasma irregularities using 50MHz coherent back-scatter radar operated at Sao Luis, Brazil. The 7-day campaign were characterized by a disturbed 3-day period in the beginning followed by 'quiet' days. A correlative analysis of the auroral electrojet (AE) indices and radar and magnetometer data are carried out to investigate the diverse characteristics of the responses of the EEJ and its instabilities processes to the disturbance zonal electric fields that penetrate to equatorial latitudes during auroral disturbance. Large degrees of day-to-day variability in the EEJ current intensity, 3-m echo powers and spectral features were observed even under absence of any auroral disturbances. Among the outstanding results of this analysis are the following findings: (a) the observed EEJ current and 3-m irregularity response phases to the intensification/decay phases of an auroral sub-storm event agree with the prediction by the existing models; (b) the phase relationship of the EEJ response versus AE intensification is altered/inverted under the presence of a ring current (D-st) development; (c) the amplitude of the EEJ response show clear local time dependence being maximum around noon; and (d) significant increase in the type-1 (two-stream wave) Doppler velocity exceeding the nominal ion-acoustic speed is observed under magnetically disturbed conditions. These results are presented and discussed. (C) 2004 Elsevier Ltd. All rights reserved.

Equatorial ionosphere observations using VHF radars show backscattered echoes from electron densi... more Equatorial ionosphere observations using VHF radars show backscattered echoes from electron density irregularities in the equatorial electrojet (EEJ). The studies of the echoes have revealed distinct spectral signatures of two types of irregularities, named type 1 and type 2, explained by the modified two-stream and the gradient drift instabilities, respectively. Type 1 echo spectra normally presents a sharp peak centered at around 120 Hz for 50 MHz radars, which corresponds to a Doppler shift of about 360 m/s (close to the ion-acoustic speed). Several experiments have been done to investigate the EEJ irregularities in order to characterize its phenomenology using RESCO 50 MHz radar at the Brazilian sector. In the present work we summarize some characteristics of type 1 echoes based on power spectra of backscattered signals from 3-m EEJ plasma irregularities. We also show statistics of its occurrence in simultaneous height and time, including a velocity distribution analysis.

Counter electrojet events have been detected in Brazil with the RESCO radar and two set of fluxga... more Counter electrojet events have been detected in Brazil with the RESCO radar and two set of fluxgate magnetometer. RESCO radar is a 50 MHz backscatter coherent radar installed in 1998 at S{\~{a}}o Lu{\'{i}}s (SLZ, 2.33° S, 44.60° W), an equatorial site. The magnetometers are fluxgate-type installed at SLZ and Eus{\'{e}}bio (EUS, 03.89° S, 38.44{\textordmasculine} W). Several cases of westward morning electrojet and its inversion to the normal eastward equatorial electrojet (EEJ) have been observed as seen in magnetometers signatures of the EEJ strength obtained from the difference between the horizontal component of magnetic field at SLZ station and the same component at the low latitude magnetic station, EUS. In addition, some cases of counter electrojet (CEJ) have been detected. In the present work, we show some characteristics of normal EEJ inversion in the morning hours and a CEJ event observed in Brazil with the magnetometers as well as with the RESCO radar. The spectral characteristics and power intensity of the backscattered echoes are examined.

A VHF pulsed Doppler coherent back-scatter radar system developed at the Instituto Nacional de Pe... more A VHF pulsed Doppler coherent back-scatter radar system developed at the Instituto Nacional de Pesquisas Espaciais -INPE has recently been put into operation at S{\~{a}}o Lu{\'{i}}s (2.33° S, 44.2° W. dip: -0.5°), a magnetic equatorial location. in Brazil. The modular system consists of 8 transmitters. each one having 15 kW peak power, a co-co array of 16 antennas composed by 48 dipoles each one aligned north-south producing a beam of3° north-south and 5° east-west and a data acquisition system. Progressive phase shifts applied to the 8 transmitters allow operation with oblique beam tilted in east-west plane. The pulsed 50 MHz signal is transmitted at PPS of 1024 and the echoes back-scattered by ionospheric irregularities are received by the same antenna array using a T/R switch system. Although high precision to Doppler shift measurement demands continuous wave we have opted for a pulsed system because it is easier to get signal reflection height and Doppler shift of the scattered echoes. Improvements are being made to the system. During the first successful operational campaign conducted in August 1998 the radar was operated with its partial capability (40 kW) which permitted data collection from equatorial electrojet irregularities. We have carried out some prelimin{\'{a}}ry analyses of the data taken during the last campaign. Few days of the arialyzed data covered the period of the moderate to intense magnetic storm of August 26 (Dst {\textless} -140). The equatorial electrojet intensity was generally very weak and showed large fluctuations in response to the magnetic storms effects. as could be confirmed from magnetometer data ftom S{\~{a}}o Luis and by auroral activity index (AE). AIso, inhibitions ofthe electrojet irregularlty processes were observed during the disturbed periods. Height of the maximum intensity ofthe EEJ irregularities showed lowering (rising) before (after) midday. This feature is in agreement with the observations of daytime 150 km echoes observed over Jicamarca observatory.

Wavelet analysis is used to derive solar tide on the Earth's magnetic field from magnetometer ins... more Wavelet analysis is used to derive solar tide on the Earth's magnetic field from magnetometer installed in the American sector. A latitudinal analysis over the whole American sector are shown and discussed. New results regarding the diurnal and semidiurnal are obtained and revealed a different domination of the atmospheric tidal modes with respect to previous published works. We have also obtained an evidence of a coupling process between the solar tide and the induced magnetic field at the ground level. In addition, we have find out and interesting dependence between the spectral density and the magnetic activity. Finally, the study of tides on the geomagnetic field revealed itself to be very important for a better understanding of the local space weather, since it plays a major role in the ionosphere electric fields during geomagnetic quiet days.

We have applied a wavelet analysis technique on the quiet-time equatorial magnetic field perturba... more We have applied a wavelet analysis technique on the quiet-time equatorial magnetic field perturbation measurements (signature of the equatorial electrojet) from S{\~{a}}o Lu{\'{i}}s (Brazil) and Jicamarca (Peru), to study longitudinal, seasonal, and solar cycle variabilities of the equatorial electrojet over the East and West coasts of South America.

$Research paper thumbnail of Resultados preliminares de estudo do comportamento da camada F ionosf{\'{e}}rica sob o equador magn{\'{e}}tico a partir de dados de digissonda$

The pre-reversal enhancement is a phenomenon of the evening equatorial ionosphere that has been p... more The pre-reversal enhancement is a phenomenon of the evening equatorial ionosphere that has been pointed as decisive factor in the formation of the plasma bubbles of the F region. This enhancement can be observed through the data analysis of several equipments, among them: radars and digisondes. A digisonde 256 of INPE is installed and collecting data in S{\~{a}}o Lu{\'{i}}s (2.33° S, 44.2° W, dip: -0.5°). In the present work we have used the peak height of the F2 layer (hmF2) and the virtual height of the F layer (h'F) from some selected days in 2003 to study of speed of the layer F ascent in the equatorial zone during the pre-reversal period. The peak heights of the F2 layer obtained from the data analysis are compared with the same parameter obtained from simulations of electron density profiles. For the simulations we have used the International Reference Ionosphere model - IRI. The speed of vertical drift is calculated from the data collected by the digisonde. The data set selected for this analysis covers some days from the periods around summer solstice and around autumn equinox in 2003. According the level of magnetic activity, the days of data acquisitions were classified between calm and disturbed. The results are presented and discussed in terms of the season and in terms of magnetic disturbance level.

We have analyzed the data collected during an observational campaign of the equatorial electrojet... more We have analyzed the data collected during an observational campaign of the equatorial electrojet (EEJ) plasma irregularities using 50 MHz coherent back-scatter radar operated at S{\~{a}}o Lu{\'{i}}s, Brazil. We have selected a disturbed 3-day period for the present analysis. A correlative analysis of the auroral electrojet indices and radar and magnetometer data are carried out to investigate the diverse characteristics of the responses of the EEJ and its instabilities processes to the disturbance zonal electric fields that penetrate to equatorial latitudes during auroral disturbance. Large degree of day-to-day variability in the EEJ current intensity, 3-meter echo powers were observed. Among the outstanding results of this analysis are the following findings: (a) the observed EEJ current and 3-meter irregularity response phases to the intensification/decay phases of an auroral sub-storm event agree with the prediction by the existing models; (b) the phase relationship of the EEJ response versus AE intensification is altered/inverted under the presence of a ring current (Dst) development; and (c) the amplitude of the EEJ response show clear local time dependence being maximum around noon. These results are presented and discussed.

In the present paper we present a new feature of the equatorial electrojet observed in the Brazil... more In the present paper we present a new feature of the equatorial electrojet observed in the Brazilian sector based on the RESCO coherent radar observations. The center height of the scattering region of the equatorial electrojet is observed to systematically decrease in altitude along the years. It has been observed through VHF radar soundings at S{\~{a}}o Lu{\'{i}}s (2.3{\textordmasculine} S, 44.2{\textordmasculine} W). This feature is discussed in terms of the displacement of the dip equator from the radar site due to the long term variations in the Earth magnetic field, which can be detected by measurements using ground magnetometers at the radar site. The math tolls used to infer the center height is presented and all different aspects of the equatorial electrojet are present and discussed in terms of established concepts and theories.

In this paper we present the new feature of the fieldaligned-integrated ionospheric conductivity ... more In this paper we present the new feature of the fieldaligned-integrated ionospheric conductivity model developed for the Brazilian equatorial region. A brief historical of model development is remembered and original features are summarized to allow a straight comparison with the new features introduced in the 2009 version. Some special aspects related to the changes in the neutral atmosphere model are discussed in terms of the impact in the collision frequency.

$Research paper thumbnail of Efeitos da altera{\c{c}}{\~{a}}o nas taxas de colis{\~{o}}es no modelo de condutividade ionosf{\'{e}}rica para a regi{\~{a}}o E brasileira$

In the present paper we show some new results related to the evolution in the set of equations us... more In the present paper we show some new results related to the evolution in the set of equations used to calculate the electron-neutron and ion-neutron collision rates in the Field Aligned Integrated Conductivity Model developed for the Brazilian Equatorial E-Region and its implications on the Hall and Pedersen conductivities and Hall-to-Pedersen conductivities ratio. The results are shown in a comparative way between the current version and its old version, and have reveled an increase of the region of high Hall-to-Pedersen conductivities ratio.

In the present paper we present a discussion about some aspects of the solar-terrestrial system c... more In the present paper we present a discussion about some aspects of the solar-terrestrial system coupling with regard to space weather effects based on some recent studies of energetic particles precipitation at the South American Magnetic Anomaly and the cosmic noise absorption seem by image riometers, and abnormal enhancement of the equatorial sporadic E-layers density during the recovery phase of magnetic storm.

Equatorial electrojet (EEJ) observations using VHF radars show backscattered echoes from two type... more Equatorial electrojet (EEJ) observations using VHF radars show backscattered echoes from two types of electron density irregularities, Type 1 and Type 2. In this paper, we present a statistics of occurrence irregularities in the ionospheric plasma at the eregion hight, observed by a 50 MHz coherent backscatter radar installed in Brazil and the radar installed in Jicamarca Radio Observatory. These radars detect small-scale irregularities (3 meters) between approximately 90 to 130 km heights in the equatorial ionosphere.

The spectral analysis of the received echoes from equatorial electrojet irregularities may show t... more The spectral analysis of the received echoes from equatorial electrojet irregularities may show the signatures of two different plasma irregularities: Type 1 and Type 2. Each Type is related to a distinct process of plasma instabilities. Each irregularity presents itself as one gaussian-like shape in the frequency domain and the characteristics of the curve may be extracted using nonlinear curve fitting methods. Some techniques are applied to the irregularities spectra before the curve fitting so that the variance between the data and the fitting is minimum. One of these techniques is called incoherent integration and consists of averaging subsequent spectra. We simulated equatorial electrojet radar power spectra of Type 1 irregularities to elucidate the effects. The advantages and disadvantages of applying such technique are analyzed in terms of the variance between the radar spectra and the fitted curve.

During 3 - 7 July, 2003, long-lasting and large-amplitude interplanetary magnetic field perturbat... more During 3 - 7 July, 2003, long-lasting and large-amplitude interplanetary magnetic field perturbation event occurred. This event is classified as high-intensity long-duration continuous AE activity (HILDCAA) which occurs outside the main phase of magnetic storms. The responses of the equatorial ionosphere to HILDCAA events have been poorly known so far. In this work, we investigate the effects of HILDCAA events on the equatorial ionosphere using ionosonde data (hmF2) observed at S{\~{a}}o Lu{\'{i}}s, Brazil and the ACE satellite data. The results indicate presence of short-time prompt penetration electric field disturbances and long-lived positive charge accumulation at the magnetic equator around the midnight sector due to the disturbance dynamo effect.

Equatorial electrojet (EEJ) observations using VHF radars show backscattered echoes from two type... more Equatorial electrojet (EEJ) observations using VHF radars show backscattered echoes from two types of electron density irregularities, Type 1 and Type 2. In this paper, we present an statistics of occurrence irregularities in the ionospheric plasma at the eregion hight, observed by a 50 MHz coherent backscatter radar installed in Brazil. This radar detects small-scale irregularities (3 meters) between approximately 90 to 130 km heights in the equatorial ionosphere.

In the current paper we present the study about the seasonal variation of the Earth's magnetic fi... more In the current paper we present the study about the seasonal variation of the Earth's magnetic field based on magnetic measurements from the Embrace Magnetic Network at three different latitudes in Brazil (SLZ, EUS and CXP). For this study, we used data covering the period from September 2010 to December 2012. Among the results we show clear seasonal modulation in all components, irrespective the latitude. The H component analysis revealed to have a seasonal dependence in both aspects: the duration of positive excursion along the day and the maximum amplitude.

$Research paper thumbnail of Estudo sobre a presen{\c{c}}a de camadas ionosf{\'{e}}ricas espor{\'{a}}dicas em per{\'{i}}odos calmos e perturbados em S{\~{a}}o Lu{\'{i}}s (MA): Um caso estudado$

related to the study on the presence of sporadic ionospheric layers at S{\~{a}}o Lu{\'{i}}s (2{\t... more related to the study on the presence of sporadic ionospheric layers at S{\~{a}}o Lu{\'{i}}s (2{\textordmasculine}31´ S, 44{\textordmasculine}16´ W), on the equatorial region, during quiet and disturbed periods before, during and after the magnetic storm that occurred in October 2003. Ionospheric profiles obtained from digital sounders were used in order to verify comparatively the height of reflection and typical frequencies related to the E and Es layers. It was made with the intention to identify in the ionospheric parameters possible evidences of the reaction of the Magnetosphere-Ionosphere system to the strong magnetic storm. To characterize the level of magnetic disturbances the Dst index has been chosen. The achieved results evidenced that the E-Region can behave in a particular way that shows the influence of the magnetic storm depending upon the magnetic activity level and its latitudinal location.

$Research paper thumbnail of Estudo da varia{\c{c}}{\~{a}}o do {\^{a}}ngulo de inclina{\c{c}}{\~{a}}o magn{\'{e}}tica em S{\~{a}}o Lu{\'{i}}s (MA): modelo IGRF vs magnet{\^{o}}metro$

In the present paper we show some new results related to a comparative study on variation of the ... more In the present paper we show some new results related to a comparative study on variation of the dip angle at S{\~{a}}o Lu{\'{i}}s (2{\textordmasculine}31´ S, 44{\textordmasculine}16´ W), on the equatorial region, calculated from magnetometers data and from IGRF simulations. We have investigated the secular variation of the Earth magnetic field using both magnetometers and IGRF method. The altitudinal dependence of the dip equator location was investigated from IGRF simulations alone. Short time scale (less then a day) magnetic variations of the dip angle were also investigated from magnetometers measurements using a spectral analysis. Among the conclusion, we have obtained an evidence of a coupling processes between the neutral atmosphere (thought the Sq system) and its effects on the induced magnetic field at the ground level. The analysis have reveled a significance order for the atmospheric tides. The diurnal, semidiurnal and terdiurnal tides driven by solar heating appeared to dominate. And the semidiurnal, diurnal and terdiurnal lunar gravitational tides appeared next in strength.

$Research paper thumbnail of Resultados de Observa{\c{c}}{\~{o}}es do Eletrojato Equatorial Utilizando um Radar de Espalhamento Coerente de 50 MHz no Brasil$

Um radar de espalhamento coerente desenvolvido no INPE est{\'{a}} operando parcialmente no equado... more Um radar de espalhamento coerente desenvolvido no INPE est{\'{a}} operando parcialmente no equador magn{\'{e}}tico, em S{\~{a}}o Lu{\'{i}}s (2,33° S; 44,2° O; dip: -0,5°), no Brasil. O radar tem freq{\"{u}}{\^{e}}ncia de opera{\c{c}}{\~{a}}o de 50 MHz, e foi projetado para transmitir pulsos com pico de pot{\^{e}}ncia de 120 kW. Entretanto, o radar foi operado somente com capacidade parcial de {\~{}}40 kW nas primeiras medidas da din{\^{a}}mica das irregularidades do eletrojato, conduzidas durante uma semana de agosto de 1998. Algumas caracter{\'{i}}sticas do radar s{\~{a}}o discutidas nesse trabalho, e s{\~{a}}o comentados os primeiros resultados obtidos pela an{\'{a}}lise dos dados j{\'{a}} reduzidos. Alguns dias de observa{\c{c}}{\~{a}}o cobriram um intervalo perturbado, relacionado com a forte tempestade magn{\'{e}}tica do dia 27 de agosto de 1998 (Dst{\textgreater}140). A intensidade do eletrojato mostrou grande flutua{\c{c}}{\~{a}}o em resposta a penetra{\c{c}}{\~{a}}o imediata de campos el{\'{e}}tricos magnetosf{\'{e}}ricos at{\'{e}} latitudes equatoriais, indicado por flutua{\c{c}}{\~{o}}es simult{\^{a}}neas no {\'{i}}ndice de atividade auroral, e nos magnetogramas das esta{\c{c}}{\~{o}}es de S{\~{a}}o Lu{\'{i}}s e Valentia (51,9° N; 10,2° O). Longos per{\'{i}}odos de total inibi{\c{c}}{\~{a}}o dos processos de instabilidades de plasma tamb{\'{e}}m foram observados, os quais sugerem que dist{\'{u}}rbios dos campos do d{\'{i}}namo podem estar relacionados com a tempestade magn{\'{e}}tica.

The RESCO 50 MHz coherent back-scatter radar has been operated since 1998 at the INPE/MCT's S{\~{... more The RESCO 50 MHz coherent back-scatter radar has been operated since 1998 at the INPE/MCT's S{\~{a}}o Lu{\'{i}}s Space Observatory (2.33° S, 44.2° W, DIP: -0.5°), Brazil, on the dip equator to study the equatorial electrojet dynamics. Spectral analysis of the received echo from equatorial electrojet irregularities allow us to identify the dominant type of plasma irregularities in the electrojet bulk observed by the radar. Using curve fitting methods on the resulting power spectra it is also possible to obtain other characteristics from the echoes, such as: center of frequency distribution, spectral width and power. The usual approach in this analysis is through a Gaussian fitting based on the method of Least Square Error to parameters estimation. Before fitting the power spectra it is usual to smooth it in order to reduce the noise level and define better the center of frequency distribution. From the center of frequency distribution, we are able to deduce the Doppler shift of the irregularities in relation to the radar, which is close related to the electric fields that drives the plasma instabilities. In this work, we have simulated echoes signals from 3-meter type 1 plasma instabilities, and analyzed it in order to recover the center of frequency distribution used to simulate the data generated. As a smoothing method we have used incoherent integration. We have applied three distinct levels of smoothing in order to evaluate the response of the fitting to this technique. The advantages and disadvantages of applying different levels of incoherent integration over power spectra of back-scatter echoes from type 1 irregularities in the parameter estimation are presented and discussed.

$Research paper thumbnail of {\'{I}}ndice dos Dados Coletados com o Radar de Espalhamento Coerente de 50 MHz (RESCO) no Ano de 2002$

Este material {\'{e}} de propriedade ao Grupo de Pesquisas IONO da Divis{\~{a}}o de Aer... more Este material {\'{e}} de propriedade ao Grupo de Pesquisas IONO da Divis{\~{a}}o de Aeronomia (DEA) das Ci{\^{e}}ncias Espaciais e Atmosf{\'{e}}rica (CEA) do Instituto Nacional de Pesquisas Espaciais (INPE). Ele cont{\'{e}}m informa{\c{c}}{\~{o}}es sobre os dados cient{\'{i}}ficos de espalhamento coerente nas irregularidades de 3-metros da regi{\~{a}}o da ionosfera equatorial brasileira.

$Research paper thumbnail of Estima{\c{c}}{\~{a}}o de Par{\^{a}}metros de Dados F{\'{i}}sicos$

This dissertation will present details of the development and construction of the phase correctio... more This dissertation will present details of the development and construction of the phase correction unit, which was successfully implemented, with the help of the participating technical staff and collaboration with other scientific groups. It is needed for efficient functioning of the transmitting system it is required to achieve phase stabilization and correct phase relationship of the signals transmitted by the different antennas. The unit was tested with the radar at "S{\~{a}}o Lu{\'{i}}s" when its successful operation was conducted during an operational and observational campaign in August 1998. During this campaign the first data on the dynamics of electrojet irregularities were registered with a resolution of 6s in time and 2.6 km in height resolution. The data collected during a few days of the campaign are now being analyzed. Preliminary results of analysis of some data are presented in this thesis. The analysis software utilizes Fast Fourier (FFT) technique to extract the Doppler frequency of the electrojet irregularity echoes, and hence their velocities, at a large number of height bins that can be adjusted to cover height regions of specific interest, for electrojet or plasma bubble investigations. It also provides the integrated spectral power at each height, which permits plots of RTI (Range Time Intensity) maps as a function of local time. Analysis done for 31 August, which was a magnetically disturbed day, shows large variation in electrojet intensity. Such variations are found to be associated with fluctuations in the Auroral Electrojet (AE), Equatorial Electrojet (EEJ) magnetic field intensities and in the Dst indices. Also the height of the electrojet irregularities on this day appears to show a minimum near local midday. Detailed analyses are under way.

$Research paper thumbnail of Estudo da eletrodin{\^{a}}mica da ionosfera equatorial durante o per{\'{i}}odo de m{\'{a}}xima atividade solar (1999-2002)$

This thesis presents the results from the study of the equatorial ionosphere electrodynamics duri... more This thesis presents the results from the study of the equatorial ionosphere electrodynamics during the solar maximum period (1999-2002). For this study a new approach in the analysis of the 50 MHz coherent back-scatter radar data based on Gaussians fitting of the Doppler spectra was developed. The technique is presented, demonstrated and validated with test and experimental data. The variability of the 3-meter irregularities of the equatorial electrojet is studied during quiet and disturbed auroral activity to demonstrate the diversity of the equatorial electrojet phenomenology and its variability in the Brazilian sector. This study had given us a good idea of some important aspects of the equatorial electrojet during quiet and disturbed conditions. An analysis of intensity and format of the equatorial electrojet based on its parameterization was carried on in two steps. First step of analysis comprehended the data set covering the period from January 23 to February 15, 2001. This step confirmed the initial expectations and was used to improve the technique of parameterizations. Second step was an extension of the preliminary study using the 2002 data set covering the whole year. In this step, the data set was separated according to the magnetic activity and season. The second step analysis confirmed the presence of an East-West power asymmetry in the back-scatter radar signal in the Brazilian sector, as seen in the first step. Besides, some daily and seasonal characteristics of this power asymmetry are presented. The appearance of a scattering region after sunset during magnetically quiet conditions around the summer solstice is also shown. The Gaussian fitting applied to the spectrograms yielded the Doppler frequencies of the plasma irregularities that were used to calculate the polarization and the zonal electric fields at various electrojet heights. This study included an analysis of the variability of these fields in height, season and according the geometry of the sounding. Moreover, the results from our calculations were compared to the others values of electric fields from the equatorial electrojet available in the scientific literature. From this study we could confirm the presence of an asymmetry in the frequencies/velocities type 2, similar to the power asymmetry observed in the earlier studies.