Browsing by Author "De Paula, E. R."
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Item Open Access Abnormal evening vertical plasma drift and effects on ESF and EIA over Brazil-South Atlantic sector during the 30 October 2003 superstorm(American Geophysical Union, 2008-11) Abdu, M. A.; De Paula, E. R.; Batista, I. S.; Reinisch, B. W.; Matsuoka, M. T.; Camargo, P. O.; Veliz, Oscar; Denardini, C. M.; Sobral, J. H. A.; Kherani, E. A.; De Siqueira, P. M.Equatorial F region vertical plasma drifts, spread F and anomaly responses, in the south American longitude sector during the superstorm of 30 October 2003, are analyzed using data from an array of instruments consisting of Digisondes, a VHF radar, GPS TEC and scintillation receivers in Brazil, and a Digisonde and a magnetometer in Jicamarca, Peru. Prompt penetrating eastward electric field of abnormally large intensity drove the F layer plasma up at a velocity 1200 ms−1 during post dusk hours in the eastern sector over Brazil. The equatorial anomaly was intensified and expanded poleward while the development of spread F/plasma bubble irregularities and GPS signal scintillations were weaker than their quiet time intensity. Significantly weaker F region response over Jicamarca presented a striking difference in the intensity of prompt penetration electric field between Peru and eastern longitudes of Brazil. The enhanced post dusk sector vertical drift over Brazil is attributed to electro-dynamics effects arising energetic particle precipitation in the South Atlantic Magnetic Anomaly (SAMA). These extraordinary results and their longitudinal differences are presented and discussed in this paper.Item Open Access Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F(American Geophysical Union, 1999-09-01) Fejer, B. G.; Scherliess, L.; De Paula, E. R.We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F. The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts. In the early night sector, the bottomside of the F layer is almost always unstable. The evolution of the unstable layer is controlled by the history of the vertical drift velocity. When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present. The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux. The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent. These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields. The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset. The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour. These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes. The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux.Item Open Access Equatorial 150 km echoes and daytime F-region vertical plasma drifts in the Brazilian longitude sector(European Geosciences Union, 2013-10-31) Rodrigues, F. S.; Shume, E. B.; De Paula, E. R.; Milla, MarcoPrevious studies showed that conventional coherent backscatter radar measurements of the Doppler velocity of the so-called 150 km echoes can provide an alternative way of estimating ionospheric vertical plasma drifts during daytime hours (Kudeki and Fawcett, 1993; Chau and Woodman, 2004). Using observations made by a small, lowpower 30 MHz coherent backscatter radar located in the equatorial site of São Luís (2.59◦ S, 44.21◦ W; −2.35◦ dip lat), we were able to detect and monitor the occurrence of 150 km echoes in the Brazilian sector. Using these measurements we estimated the local time variation of daytime vertical ionospheric drifts in the eastern American sector. Here, we present a few interesting cases of 150 km-echoes observations made by the São Luís radar and estimates of the diurnal variation of vertical drifts. These cases exemplify the variability of the vertical drifts in the Brazilian sector. Using same-day 150 km-echoes measurements made at the Jicamarca Radio Observatory in Peru, we also demonstrate the variability of the equatorial vertical drifts across the American sector. In addition to first estimates of the absolute vertical plasma drifts in the eastern American (Brazilian) sector, we also present observations of abnormal drifts detected by the São Luís radar associated with the 2009 major sudden stratospheric warming event.Item Restricted Equatorial electrojet 3-M irregularity dynamics during magnetic disturbances over Brazil: results from the new VHF radar at São Luı́s(Elsevier, 2003-10) Abdu, M. A.; Dinardini, C. M.; Sobral, J. H. A.; Batista, I. S.; Muralikrishna, P.; Iyer, K. N.; Veliz, O.; De Paula, E. R.Data collected during the first two observational campaigns, conducted in August 1998 and December 1999, using the new coherent back-scatter radar, developed at INPE, that became operational at the magnetic equatorial site at São Luı́s, (2.33S,44.2W), 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.Item Restricted Equatorial F region vertical plasma drifts during solar maxima(American Geophysical Union, 1989-09-01) Fejer, B. G.; De Paula, E. R.; Batista, I. S.; Bonelli, E.; Woodman Pollitt, Ronald FranciscoIncoherent scatter radar measurements at Jicamarca are used to study the effects of large solar fluxes and magnetic activity on the F region vertical plasma drifts. The average drifts from the two last solar maxima are almost identical except in the late afternoon-early evening sector where their variations with solar flux and magnetic activity are strongly season dependent. The average evening winter (May-August) drifts appear to remain almost constant after a certain solar flux level is reached but increase with magnetic activity. The equinoctial evening drifts increase systematically with solar-flux but decrease with magnetic activity. Very large prereversal enhancement velocities, up to about 80 m/s, were often observed during the 1978–1981 equinoctial periods when the solar flux was very high. Comparison of incoherent scatter radar drifts with vertical velocities inferred from ionosonde observations indicate that the latter technique substantially underestimates the plasma drifts during periods of large solar fluxes except during winter.Item Restricted Equatorial zonal electric fields inferred from a 3‐D electrostatic potential model and ground‐based magnetic field measurements(American Geophysical Union, 2009-06-06) Shume, E. B.; De Paula, E. R.; Maus, S.; Hysell, D. L.; Rodrigues, F. S.; Bekele, A.We present a new technique to infer quiet time zonal electric fields in the daytime equatorial ionosphere. The electric field inference technique utilizes a three‐dimensional (3‐D) electrostatic potential model of the low‐latitude ionosphere constrained by ground‐based magnetic field measurements. To test this technique, inferred zonal electric fields for the Peruvian sector in Jicamarca (11.95°S, 283.13°E, 0.6°N dip latitude) were compared with zonal electric field (vertical drift) measurements made by the Jicamarca Incoherent Scatter Radar. The comparison shows a good agreement between the inferred and measured electric fields. An example of electric field estimation for Davao (7.4°N, 125.4°E, 0.58°S dip latitude) in the Philippines sector is also presented in this report. Inferred electric fields for Davao are in good agreement with F region vertical plasma drifts measured by drift sensors onboard the AE‐E and ROCSAT‐1 satellites on that longitude sector. Our results suggest that realistic estimates of quiet time zonal electric fields for the equatorial ionosphere can be obtained from the 3‐D potential model whenever observatory magnetic field measurements are available.Item Restricted Longitudinal ionospheric effects in the South Atlantic evening sector during solar maximum(American Geophysical Union, 2002-07-10) De Paula, E. R.; Souza, J. R.; Fejer, B. G.; Bailey, G. J.; Heelis, R. A.Large-scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10S–20S, longitudes 50W–10E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450-km altitude show vertically downward ion drift velocities exceeding 120 m s−1 and depleted regions where the ion density is around 2 × 104 cm−3. It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ion density depletions appear as a result of large downward ion drifts driven by large southward winds along the magnetic meridian and by diffusion. During others seasons such behavior is not observed by the AE-E satellite, neither by SUPIM results. The roles played by the different physical processes responsible for the large downward drift velocities are investigated. The model results highlight the relationship between longitudinal variation of the ion densities and the location of the equatorial anomaly crest in the South Atlantic region.Item Open Access On the characteristics of 150-km echoes observed in the Brazilian longitude sector by the 30 MHz São Luís radar(European Geosciences Union (EGU), 2011-10-28) Rodrigues, F. S.; De Paula, E. R.; Chau Chong Shing, Jorge LuisWe present long-overdue details about the intensity and spectral characteristics of 150-km echoes observed by the Sao Lu ıs radar in Brazil. The Sao Lu ıs observations show that the echoes usually come from multiple scattering layers that descend in altitude before local noon, and ascend during afternoon hours, similar to what has been found in observations made in other longitude sectors. The layers are usually 3–5 km thick and located, mostly, between 130 and 170 km altitude. The measurements also show variations in echo intensity that are similar to observations made at other equatorial and off-equatorial sites. Analysis of observations made during 2008 shows significant (>37 %) monthly occurrence rates for every month. Reduced occurrence rates were observed around March Equinox. We associate this reduction in occurrence rate, however, to a non-geophysical factor. An increase in the daytime sky noise in the months around March Equinox causes a decrease in the signal-to-noise ratio (SNR) of the echoes, which makes them less distinguishable in our analysis. A higher occurrence of weaker echoes around March Equinox was confirmed by an statistical analysis of the seasonal variation of echo intensities. Strong, long-lasting and, therefore, more noticeable echoing layers, however, were observed between June and early September compared to other months in 2008. Spectral analyses show that most of the echoes have negative mean Doppler shifts indicating upward velocities. The echoes also have narrow spectral widths of only a few ms−1. Finally, we also found that the mean Doppler shift of the observed echoes can vary noticeably with altitude at times. Using spaced antenna measurements we show that this is caused by the wide field-ofview of the radar and the spatial distribution of the scatterers within the radar beam.Item Restricted Statistical analysis of radar observed F region irregularities from three longitudinal sectors(European Geosciences Union, 2013-12) Cueva, R. Y. C.; De Paula, E. R.; Kherani, A. E.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.Item Restricted Understanding spaced-receiver zonal velocity estimation(American Geophysical Union, 2004-10-27) Ledvina, B.M.; Kintner, P. M.; De Paula, E. R.Spaced-receiver analysis can be used to estimate the ionospheric zonal irregularity velocity. This involves first measuring the scintillation pattern velocity and then estimating the zonal irregularity velocity. A straightforward technique involving projection planes has been used to derive a relationship between the satellite position and velocity, the irregularities' position and velocity, and the scintillation pattern velocity. This relationship relies on the alignment of the irregularities along the field lines but is independent of the irregularity spectrum. When simplified, this relationship can be used to estimate the zonal irregularity velocity using either geostationary satellites or Global Positioning System (GPS) satellites. This estimation is straightforward but must be performed in a careful manner. An example of zonal irregularity velocity estimation using GPS satellites is presented. The average zonal irregularity velocity is estimated over a 3-week period in March 2002 at São Luís, Brazil (−3.59 dip angle). This hourly average velocity has been compared with the climatological radar observations from Jicamarca Observatory. The spaced-receiver zonal velocity agrees well (within +-8 m/s) with the radar's observations during postsunset hours.Item Restricted Variabilities of the equatorial electrojet in Brazil and Perú(American Geophysical Union, 2010-06-16) Shume, E. B.; Denardini, C. M.; De Paula, E. R.; Trivedi, N. B.This report presents seasonal and longitudinal variabilities of the equatorial electrojet in the east (Brazil, São Luís: 2.3° S; 315.8° E; 0.5° S dip latitude) and west (Jicamarca, Perú: 11.95° S; 283.13° E; 0.6° N dip latitude) coasts of the continent of South America. Ground‐based magnetic field perturbation measurements ΔH for solar maximum (2001/2002) and solar minimum (2006/2007) conditions from the two equatorial stations (São Luís and Jicamarca) have been used for the study. The ΔH signal which is a measure of the strength of the equatorial electrojet is spectrally analyzed using wavelet analysis. The results of our analysis show that (1) the equatorial electrojet has maxima around equinoxes in Jicamarca, Perú but it has a prominent maximum during Southern Hemisphere summer (centered about December/January) in São Luís, Brazil. The observed seasonal behavior of the equatorial electrojet in São Luís is highly likely due to the large magnetic declination angle (about 20° west) there. (2) The equatorial electrojet is stronger in the west coast (Jicamarca) compared to the east coast (São Luís), irrespective of solar activity condition. (3) The magnitude of the equatorial electrojet is more variable with season and solar cycle over São Luís than over Jicamarca.