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Found 10 entries in the Bibliography.
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2017 |
Total electron content responses to HILDCAAs and geomagnetic storms over South America Total electron content (TEC) is extensively used to monitor the ionospheric behavior under geomagnetically quiet and disturbed conditions. This subject is of greatest importance for space weather applications. Under disturbed conditions the two main sources of electric fields, which are responsible for changes in the plasma drifts and for current perturbations, are the short-lived prompt penetration electric fields (PPEFs) and the longer-lasting ionospheric disturbance dynamo (DD) electric fields. Both mechanisms modulate the TEC around the globe and the equatorial ionization anomaly (EIA) at low latitudes. In this work we computed vertical absolute TEC over the low latitude of South America. The analysis was performed considering HILDCAA (high-intensity, long-duration, continuous auroral electrojet (AE) activity) events and geomagnetic storms. The characteristics of storm-time TEC and HILDCAA-associated TEC will be presented and discussed. For both case studies presented in this work (March and August\ 2013) the HILDCAA event follows a geomagnetic storm, and then a global scenario of geomagnetic disturbances will be discussed. Solar wind parameters, geomagnetic indices, O / N2 ratios retrieved by GUVI instrument onboard the TIMED satellite and TEC observations will be analyzed and discussed. Data from the RBMC/IBGE (Brazil) and IGS GNSS networks were used to calculate TEC over South America. We show that a HILDCAA event may generate larger TEC differences compared to the TEC observed during the main phase of the precedent geomagnetic storm; thus, a HILDCAA event may be more effective for ionospheric response in comparison to moderate geomagnetic storms, considering the seasonal conditions. During the August HILDCAA event, TEC enhancements from \~ 25 to 80 \% (compared to quiet time) were observed. These enhancements are much higher than the quiet-time variability observed in the ionosphere. We show that ionosphere is quite sensitive to solar wind forcing and considering the events studied here, this was the most important source of ionospheric responses. Furthermore, the most important source of TEC changes were the long-lasting PPEFs observed on August\ 2013, during the HILDCAA event. The importance of this study relies on the peculiarity of the region analyzed characterized by high declination angle and ionospheric gradients which are responsible for creating a complex response during disturbed periods. Negreti, Patricia; de Paula, Eurico; Candido, Claudia; Published by: Annales Geophysicae Published on: 06/2017 YEAR: 2017   DOI: 10.5194/angeo-35-1309-2017 |
2014 |
GPS L1-Frequency Observations of Equatorial Scintillations and Irregularity Zonal Velocities In this work, the climatology of ionospheric scintillations at global positioning system (GPS) L-band frequency and the zonal drift velocities of scintillation-producing irregularities were depicted for the equatorial observatory of S\~ao Luis (2.33\textdegreeS; 44.21\textdegreeW; dip latitude 1.3\textdegreeS), Brazil. This is the first time that the hourly, monthly, and seasonal variations of scintillations and irregularity zonal drifts at S\~ao Luis were characterized during periods of different solar activity levels (from December 1998 to February 2007). The percentage occurrence of scintillations at different sectors of the sky was also investigated, and the results revealed that the scintillations are more probable to be observed in the west sector of the sky above S\~ao Luis, whereas the north\textendashsouth asymmetries are possibly related to asymmetries in the plasma density distribution at off-equatorial latitudes. The scintillations on GPS signals occurred more frequently around solar maximum years, but it is also clear from the results of a strong variability in the scintillation activity in the years with moderate solar flux during the descending phase of the solar cycle. The equatorial scintillations occur predominantly during pre-midnight hours with a broad maximum near the December solstice months. In general, weak level of scintillations (S 4 index between 0.2 and 0.4) dominated at all seasons; however, during the winter months around solar maximum years (although the scintillation occurrence is extremely low), stronger levels of scintillations (S 4\ \>\ 0.6) may occur at comparable rate with the weak scintillations. The irregularity zonal velocities, as estimated from the GPS spaced-receiver technique, presented a different scenario for the two seasons analyzed; during the equinoxes, the magnitude of the zonal velocities appeared not to change with the solar activity, whereas during the December solstice months, the larger magnitudes were observed around solar maximum years. Other relevant aspects of the observations are highlighted and discussed. Muella, Marcio; de Paula, Eurico; Jonah, Olusegun; Published by: Surveys in Geophysics Published on: 08/2014 YEAR: 2014   DOI: 10.1007/s10712-013-9252-0 GPS; Ionospheric drifts; ionospheric irregularities; Ionospheric scintillation |
2010 |
Muella, M.; Kherani, E.; de Paula, E.; Cerruti, A.; Kintner, P.; Kantor, I.; Mitchell, C.; Batista, I.; Abdu, M.; Published by: Journal of Geophysical Research Published on: Jan-01-2010 YEAR: 2010   DOI: 10.1029/2009JA014788 |
2009 |
The data from ground based experiments conducted during the 2005 SpreadFEx campaign in Brazil are used, with the help of theoretical model calculations, to investigate the precursor conditions, and especially, the role of gravity waves, in the instability initiation leading to equatorial spread F development. Data from a digisonde and a 30 MHz coherent back-scatter radar operated at an equatorial site, Sao Luis (dip angle: 2.7\textdegree) and from a digisonde operated at another equatorial site (dip angle: -11.5\textdegree) are analyzed during selected days representative of differing precursor conditions of the evening prereversal vertical drift, F layer bottom-side density gradients and density perturbations due to gravity waves. It is found that radar irregularity plumes indicative of topside bubbles, can be generated for precursor vertical drift velocities exceeding 30 m/s even when the precursor GW induced density oscillations are marginally detectable by the digisonde. For drift velocities <=20 m/s the presence of precursor gravity waves of detectable intensity is found to be a necessary condition for spread F instability initiation. Theoretical model calculations show that the zonal polarization electric field in an instability development, even as judged from its linear growth phase, can be significantly enhanced under the action of perturbation winds from gravity waves. Comparison of the observational results with the theoretical model calculations provides evidence for gravity wave seeding of equatorial spread F. Abdu, M.; Kherani, Alam; Batista, I.; de Paula, E.; Fritts, D.; Sobral, J.; Published by: Annales Geophysicae Published on: Jan-01-2009 YEAR: 2009   DOI: 10.5194/angeo-27-2607-2009 |
Overview and summary of the Spread F Experiment (SpreadFEx) We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite.\ Fritts, D.; Abdu, M.; Batista, B.; Batista, I.; Batista, P.; Buriti, R.; Clemesha, B.; Dautermann, T.; de Paula, E.; Fechine, B.; Fejer, B.; Gobbi, D.; Haase, J.; Kamalabadi, F.; Kherani, E.; Laughman, B.; Lima, P.; Liu, H.-L.; Medeiros, A.; Pautet, P.-D.; Riggin, D.; Rodrigues, F.; Sabbas, F.; Sobral, J.; Stamus, P.; Takahashi, H.; Taylor, M.; Vadas, S.; Vargas, F.; Wrasse, C.; Published by: Annales Geophysicae Published on: Jan-01-2009 YEAR: 2009   DOI: 10.5194/angeo-27-2141-2009 |
Takahashi, H.; Taylor, M.; Pautet, P.-D.; Medeiros, A.; Gobbi, D.; Wrasse, C.; Fechine, J.; Abdu, M.; Batista, I.; Paula, E.; Sobral, J.; Arruda, D.; Vadas, S.; Sabbas, F.; Fritts, D.; Published by: Annales Geophysicae Published on: Jan-01-2009 YEAR: 2009   DOI: 10.5194/angeo-27-1477-2009 |
The Spread F Experiment (SpreadFEx): Program overview and first results Fritts, D.; Abdu, M.; Batista, B.; Batista, I.; Batista, P.; Buriti, R.; Clemesha, B.; Dautermann, T.; de Paula, E.; Fechine, B.; Fejer, B.; Gobbi, D.; Haase, J.; Kamalabadi, F.; Kherani, E.; Laughman, B.; Lima, J.; Liu, H.-L.; Medeiros, A.; Pautet, P.-D.; Riggin, D.; Rodrigues, F.; Sabbas, Sao; Sobral, J.; Stamus, P.; Takahasi, H.; Taylor, M.; Vadas, S.; Vargas, F.; Wrasse, C.; Published by: Earth Planets Space Published on: |
2008 |
Muella, Marcio; de Paula, Eurico; Kintner, Paul; Kantor, Ivan; Cerruti, Alessandro; Mitchell, Cathryn; Crowley, Geoff; Smorigo, Paulo; Batista, Inez; Published by: 37th COSPAR Scientific Assembly Published on: |
Rodrigues, FS; Hysell, DL; de Paula, ER; Published by: Published on: |
2007 |
During the SpreadFEx campaign from September 22 to November 8, 2005, two airglow CCD imagers, located at near Brasilia (14.8S, 47.6W, Mag. 10S) and at Cariri (7.4S, 36.5W, Mag. 9S) were operated simultaneously and measured the equatorial ionospheric bubble structures and their time evolution by monitoring the OI 6300 emission. From the 10 nights of coincident data, we observed that on some nights the bubbles was formed at the west of Cariri, but not seen from the Brasilia site. This suggests that the bubble formation and development started near the Cariri observation site. Identification of a longitudinal zone where the SpF is seeding is very important in order to find the mechanism of formation. The present paper will discuss SpF seeding mechanisms and possible contribution of the mesospheric gravity wave activity. Takahashi*, H.; Pautet, P.-D.; Fechine, J.; Abdu, M.; Batista, I.; Paula, E.; Sobral, J.H.A.; Gobbi, D.; Arruda, D.; Batista, P.; Sabba, F.; Taylor, M.; Medeiros, A.; Buriti, R.; Wrasse, C.; Fritts, D.; Published by: Published on: YEAR: 2007   DOI: 10.1190/sbgf2007-404 |
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