Browsing by Author "Valladares, Cesar E."
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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.Item Restricted An effective TEC data detrending method for the study of equatorial plasma bubbles and traveling ionospheric disturbances(American Geophysical Union, 2015-11-18) Pradipta, Rezy; Valladares, Cesar E.; Doherty, Patricia H.Using a mechanical analogy of rolling a cylindrical barrel on a rough uneven surface, we developed a special method for detrending the GPS‐derived total electron content (TEC) data. This method is specifically designed to recognize the presence of depletions in the TEC time series data and handle them differently from wavelike features. We also demonstrate a potential application of this technique to map the detailed geographic profile of TEC depletions over the equatorial region, using the South American sector as an example.Item Restricted Interhemispheric propagation and interactions of auroral traveling ionospheric disturbances near the equator(American Geophysical Union, 2016-02-11) Pradipta, Rezy; Valladares, Cesar E.; Carter, Brett A.; Doherty, Patricia H.We present the results of our GPS total electron content and ionosonde observations of large‐scale traveling ionospheric disturbances (LSTIDs) during the 26 September 2011 geomagnetic storm. We analyzed the propagation characteristics of these LSTIDs from the auroral zones all the way to the equatorial region and studied how the auroral LSTIDs from opposite hemispheres interact/interfere near the geomagnetic equator. We found an overall propagation speed of ∼700 m/s for these LSTIDs and that the resultant amplitude of the LSTID interference pattern actually far exceeded the sum of individual amplitudes of the incoming LSTIDs from the immediate vicinity of the interference zone. We suspect that this peculiar intensification of auroral LSTIDs around the geomagnetic equator is facilitated by the significantly higher ceiling/canopy of the ionospheric plasma layer there. Normally, acoustic‐gravity waves (AGWs) that leak upward (and thus increase in amplitude) would find a negligible level of plasma density at the topside ionosphere. However, the tip of the equatorial fountain at the geomagnetic equator constitutes a significant amount of plasma at a topside‐equivalent altitude. The combination of increased AGW amplitudes and a higher plasma density at such altitude would therefore result in higher‐amplitude LSTIDs in this particular region, as demonstrated in our observations and analysis.Item Restricted Ionosonde observations of ionospheric disturbances due to the 15 February 2013 Chelyabinsk meteor explosion(American Geophysical Union, 2015-10-29) Pradipta, Rezy; Valladares, Cesar E.; Doherty, Patricia H.We report the results of our investigations on the potential ionospheric effects caused by the 15 February 2013 Chelyabinsk meteor explosion. We used the data from a number of digisonde stations located in Europe and Russia to detect the traveling ionospheric disturbances (TIDs) likely to have been caused by the meteor explosion. We found that certain characteristic signatures of the TIDs can be identified in individual ionogram records, mostly in the form of Y‐forking/splitting of the ionogram traces. Based on the arrival times of the disturbances, we have inferred the overall propagation speed of the TIDs from Chelyabinsk to be 171 ± 14 m/s.Item Restricted Latitudinal and Local Time Variation of Ionospheric Turbulence Parameters during the Conjugate Point Equatorial Experiment in Brazil(Hindawi, 2012-03-05) Carrano, Charles S.; Valladares, Cesar E.; Groves, Keith M.Previous authors have reported on the morphology of GPS scintillations and irregularity zonal drift during the 2002 Conjugate Point Equatorial Experiment (COPEX) in Brazil. In this paper, we characterize the turbulent ionospheric medium that produced these scintillations. Using 10 Hz GPS carrier-to-noise measurements at Boa Vista (2.9°N, 60.7°W), Alta Floresta (9.9°S, 56.1°W), and Campo Grande (20.5°S, 54.7°W), we report on the variation of turbulent intensity, phase spectral index, and irregularity zonal drift as a function of latitude and local time for the evening of 1-2 November 2002. The method of analysis is new and, unlike analytical theories of scintillation based on the Born or Rytov approximations, it is valid when the scintillation index saturates due to multiple-scatter effects. Our principal findings are that (1) the strength of turbulence tended to be largest near the crests of the equatorial anomaly and at early postsunset local times, (2) the turbulent intensity was generally stronger and lasted two hours longer at Campo Grande than at Boa Vista, (3) the phase spectral index was similar at the three stations but increased from 2.5 to 4.5 with local time, and (4) our estimates of zonal irregularity drift are consistent with those provided by the spaced-receiver technique.Item Open Access Lunar atmospheric tidal effects in the plasma drifts observed by the Low‐Latitude Ionospheric Sensor Network(American Geophysical Union, 2011-07-21) Eccles, Vince; Rice, Donald D.; Sojka, Jan J.; Valladares, Cesar E.; Bullet, Terence; Chau Chong Shing, Jorge LuisData from the Low‐Latitude Ionospheric Sensor Network are used to examine ionospheric electrodynamics during quiet, low solar conditions from September to November 2009. The ground‐based magnetometers and the Jicamarca Vertical Incidence Pulsed Ionospheric Radar ionosonde in the Peruvian Sector are used to identify the neutral winds and plasma drifts that control the large‐scale plasma structure of the ionosphere. It is observed that the solar‐ and lunar‐driven semidiurnal tides have a significant influence on the background electrodynamics during this period of extreme solar minimum. The lunar tidal influence of the ionosphere electrodynamics is a large component of the variation of the vertical drift during the geophysically quiet study period. A significant portion, though not all, of the variation through the lunar month can be attributed to the lunar semidiurnal tide.Item Restricted Measurement of the characteristics of TIDs using small and regional networks of GPS receivers during the campaign of 17-30 July of 2008(Hindawi, 2012-02-17) Valladares, Cesar E.; Hei, Matthew A.This scientific report presents the results of a dedicated experiment that was conducted within the framework of the Low-latitude ionospheric Sensor Network (LISN) observatory to measure the characteristics of medium-scale (hundreds of km) Traveling Ionospheric Disturbances (TIDs) as they transit through the low-latitude ionosphere. A small array of 3 GPS receivers separated by 4-5 km placed in a triangular configuration was installed near Huancayo in Peru possessing several characteristics of a radio-interferometer. During the campaign days, 17–30 July 2008, TIDs were observed daily. On July 20, 2008 between 22 and 24 UT several TIDs moved across the small array of GPS receivers with a velocity near 130 m/s, were directed northward and had wavelengths close to 450 km. Other GPS receivers that were operating hundreds of km away from Huancayo show also similar TEC traces and provide a phase velocity equal to 150 m/s. This value was measured using the GPS at Piura, Cuzco and Huancayo. Based on this positive result, we conclude that small and/or regional arrays of GPS receivers can be used at low latitudes to study the role that gravity waves may have on seeding plasma bubbles.Item Restricted Observations of TIDs over South and Central America(American Geophysical Union, 2016-11-18) Valladares, Cesar E.; Sheehan, Robert; Pacheco, Edgardo E.TEC values measured by GPS receivers that belong to the low‐latitude ionosphere sensor network (LISN) and several other networks that operate in South and Central America were used to study the characteristics and origin of traveling ionospheric disturbances (TID) in these regions. The TEC perturbations associated with these TIDs show a high degree of spatial coherence over distances > 1000 km allowing us to use measurements from receivers spaced by hundreds of km to calculate the TIDs' travel velocities, propagation direction, and scale size. We first applied the TID analysis to TEC measurements corresponding to 4 July 2011. This processing method is then used to study the characteristics of TIDs for 20 and 21 August 2011, a period when a tropical storm was active in the Caribbean region. A pronounced increase in TID activity was observed in South and Central America at 16 UT on 20 August 2011 lasting until the end of 21 August 2011. The TID velocities show a very variable pattern that depends upon their local time and location. Counter‐streaming TIDs were observed over the western part of South America on 21 August 2011. Regional maps of tropospheric temperature brightness, measured by the GOES‐12 satellite, are used to identify and follow the development of the tropical storm (TS) Irene and several deep convective plumes. TIDs were observed propagating away from TS Irene. This storm moved into the Caribbean region and intensified earlier on 20 August spawning a train of atmospheric gravity waves (AGW). The small scale size, the velocity less than 150 m/s, and the close location of several TIDs with respect to TS Irene indicate that these TIDs may be the result of primary AGWs that reached the F‐region bottomside. These results open the possibility of using TEC values measured by networks of GPS receivers to construct regional, and probably global, maps of TIDs, identify their origin, and study in detail the characteristics of TIDs corresponding to primary and secondary AGWs.Item Open Access Radio-tomographic images of postmidnight equatorial plasma depletions(American Geophysical Union, 2014-01-15) Hei, Matthew A.; Bernhardt, Paul A.; Siefring, Carl L.; Wilkens, Matthew R.; Huba, Joseph D.; Krall, Jonathan F.; Valladares, Cesar E.; Heelis, Roderick A.; Hairston, Marc R.; Coley, W. Robin; Chau Chong Shing, Jorge Luis; De la Jara, CésarFor the first time, equatorial plasma depletions (EPDs) have been imaged in the longitude-altitude plane using radiotomography. High-resolution (~10 km) reconstructions of electron density were derived from total electron content (TEC) measurements provided by a receiver array in Peru. TEC data were obtained from VHF/UHF signals transmitted by the C/NOFS CERTO beacon. EPDs generated pre-midnight were observed near dawn. On one night, the bubble densities were highly reduced, 100-1000 km wide, and embedded within a layerlike ionosphere. Three nights later, the EPDs exhibited similar features, but were embedded in a locally uplifted ionosphere. The C/NOFS in-situ instruments detected a dawn depletion where the reconstruction showed lifted EPDs, implying that the postmidnight electric fields raised sections of ionosphere to altitudes where embedded/reactivated fossil-EPDs were detected as dawn depletions. Satellites flying under domelike distortions of the ionosphere may observe these distortions as Broad Plasma Decreases (BPDs).Item Restricted The November 2004 superstorm: Comparison of low-latitude TEC observations with LLIONS model results(Elsevier, 2009-04-07) Hei, Matthew A.; Valladares, Cesar E.We investigate the effects of penetration electric fields, meridional thermospheric neutral winds, and composition perturbation zones (CPZs) on the distribution of low-latitude plasma during the 7–11 November 2004 geomagnetic superstorm. The impact on low-latitude plasma was assessed using total electron content (TEC) measurements from a latitudinally distributed array of ground-based GPS receivers in South America. Jicamarca Radio Observatory incoherent scatter radar measurements of vertical E×B drift are used in combination with the Low-Latitude IONospheric Sector (LLIONS) model to examine how penetration electric fields and meridional neutral winds shape low-latitude TEC. It is found that superfountain conditions pertain between ∼1900 and 2100 UT on 9 November, creating enhanced equatorial ionization anomaly (EIA) crests at ±20° geomagnetic latitude. Large-amplitude and/or long-duration changes in the electric field were found to produce significant changes in EIA plasma density and latitudinal location, with a delay time of ∼2–2.5 h. Superfountain drifts were primarily responsible for EIA TEC levels; meridional winds were needed only to create hemispherical crest TEC asymmetries. The [O/N2] density ratio (derived from the GUVI instrument, flown on the TIMED satellite) and measurements of total atmospheric density (from the GRACE satellites), combined with TEC measurements, yield information regarding a likely CPZ that appeared on 10 November, suppressing TEC for over 16 h.