South American Sector Research Articles

Ionospheric GPS-TEC responses from equatorial region to the EIA crest in the South American sector under intense space weather conditions

We present and discuss the ionospheric F-region observations from equator to the equatorial ionization anomaly (EIA) regions over the South American sector during an intense space weather event occurred between 27 and May 29, 2017. During this geomagnetic storm, the symmetric-H (SYM-H) reached a minimum of −142 nT at ∼0700 UT on May 28, 2017. For this investigation, we analyze the vertical total electron content (VTEC) observations from a chain of nearly 120 Global Positioning System (GPS) stations. Magnetometer measurements obtained at two stations in the low latitude regions are also presented. The observations do not indicate prompt penetration electric field (PPEF) effects in the VTEC variations. Magnetometer's observations over Cuiabá (CBA) and Cachoeira Paulista (CXP) in central west and south parts of Brazil, respectively, have shown a strong cross-correlation with SYM-H in the period between 3 and 48 h. The results also show positive ionospheric storm phase during the recovery phase on May 28, 2017. Positive effect during the recovery phase of the geomagnetic storm is possibly associated with effects of disturbances winds. During the recovery phase, a strong intensification of the EIA took place, possibly related to an additional ionization effect. The VTEC values show differences between the west and east sectors. This indicates that the EIA crest is stronger in the east sector than in the west sector, possibly due to the combination of disturbance wind effects and geomagnetic field geometry where in the east sector the field lines are more inclined.

A Study of Magnetic Conjugate Property in a Large Density Irregularity Structure Using Hilbert‐Huang Transform

AbstractA large density irregularity structure in latitudinal extent was observed by the sun‐synchronous orbiting FORMOSAT‐5 satellite at 720 km topside ionosphere in the South American sector on November 24, 2018 during a magnetically quiet time. The observed density irregularity structure has a latitudinal extent of ∼30° across the dip equator, and a longitudinal width of ∼6°. Unlike previously studied equatorial plasma bubble structures with the airglow images that indicate hemispheric magnetic conjugate similarity, the gross feature of this large density irregularity structure indicates a north‐south hemispheric asymmetry in density variation. We use the Hilbert‐Huang Transform analysis to study the irregularity structure in different scales of density variations. It is found that the amplitude of density variations in the scale sizes from 22.5 to 450 km all indicate the hemispheric asymmetry. However, the spectral property of density variations in the two hemispheres is almost identical to each other. This implies that the density irregularity structure which originates from the same Rayleigh‐Taylor (RT) instability process has preserved the spectral characteristics inside the magnetic flux tube. Therefore, the cause that fails to preserve the magnetic conjugate property in the density irregularity structure comes from the fact that the background ionospheric density distribution has a north‐south hemispherical asymmetry in the beginning of the RT instability process. This hemispheric asymmetrical density distribution is caused by the seasonal variation of density distributions in the two hemispheres and the prevailing inter‐hemispheric neutral wind during solstices in the South American sector.

Influence of the uncertainties in the Solar Quiet Reference Field (SQRF) model for deriving geomagnetic indices over South America

In this work, we present the uncertainties of the empirical model developed to predict the solar quiet regular daily field variation (Sq) over the Brazilian sector called the Solar Quiet Reference Field (SQRF) model. The SQRF is based on the magnetic station records from the Embrace Magnetometer Network (Embrace MagNet) during the solar cycle 24. Although the prediction and the magnetic field records have a good agreement, we observe that each magnetic station shows a random error behavior, propagating small errors in the model. Also, some physical aspects, as tidal winds and ionospheric conductivity, may have influenced our predictions. Thus, these uncertainties in the Sq field can propagate errors and influence geomagnetic indices' calculation by erroneously scaling the geomagnetic disturbances. In this context, we introduce a new approach to derive a ‘real-time’ geomagnetic index based on the SQRF model for the Sq-H field over the South American sector. We analyze one geomagnetic storm event to show that the results have a good representation of the geomagnetic indices. Finally, this analysis yields a new approach to derive the geomagnetic index for space weather applications and studies that requires an Sq field prediction.

Ionospheric Response Over Brazil to the August 2018 Geomagnetic Storm as Probed by CSES‐01 and Swarm Satellites and by Local Ground‐Based Observations

AbstractThe geomagnetic storm that occurred on 25 August 25 2018, that is, during the minimum of solar cycle 24, is currently the strongest ever probed by the first China Seismo‐Electromagnetic Satellite (CSES‐01). By integrating the in situ measurements provided by CSES‐01 (orbiting at altitude of 507 km) and by Swarm A satellite (orbiting at ca., 460 km) with ground‐based observations (ionosondes, magnetometers, and Global Navigation Satellite System receivers), we investigate the ionospheric response at lower‐ and mid‐latitudes over Brazil. Specifically, we investigate the electrodynamic disturbances driven by solar wind changes, by focusing on the disturbances driving modifications of the equatorial electrojet (EEJ). Our proposed multisensor technique analysis mainly highlights the variations in the topside and bottomside ionosphere, and the interplay between prompt penetrating electric fields and disturbance dynamo electric fields resulting in EEJ variations. Thanks to this approach and leveraging on the newly available CSES‐01 data, we complement and extend what recently investigated in the Western South American sector, by highlighting the significant longitudinal differences, which mainly come from the occurrence of a daytime counter‐EEJ during both 25 and 26 August at Braziliian longitudes and during part of 26 August only in the Peruvian sector. In addition, the increased thermospheric circulation driven by the storm has an impact on the EEJ during the recovery phase of the storm. The observations at the CSES‐01/Swarm altitudes integrated with the ground‐based observation recorded signatures of equatorial ionospheric anomaly crests formation and modification during daytime coupled with the positive ionospheric storm effects at midlatitude.

Comparison of EEJ Longitudinal Variation from Satellite and Ground Measurements over Different Solar Activity Levels

The longitudinal variability and local time of equatorial electrojet (EEJ) current using simultaneous data recorded by ground and satellite magnetometers at different levels of solar activity were investigated. In this study, we used data from the CHAMP and Swarm satellites to obtain EEJ current measurements around the globe. The ground data were provided by the MAGDAS, INTERMAGNET, and IIG networks. The ground observation was carried out by analyzing magnetometer data in four different sectors: the South American, Indian, African, and Southeast Asian sectors. These ground data were normalized to the dip equator to overcome the latitudinal variation of each station. The analysis for both measurements was performed using quiet day data. Both the ground and satellite data were categorized according to solar activity level; low, moderate, and high. The results revealed that, during the low solar activity, there was a good agreement between the longitudinal profiles of the EEJ measured using the satellite and the ground data. In general, strong correlations were obtained in most of the sectors where ground data were available between 11 and 13 local time (LT). Besides that, our analysis revealed that the different times of maximum EEJ appearances were seasonally dependent only at certain longitude sectors.

Comparison of Performance of the IRI 2016, IRI‐Plas 2017, and NeQuick 2 Models During Different Solar Activity (2013–2018) Years Over South American Sector

AbstractThis study mainly focuses on the prediction of the variability of the vertical total electron content (VTEC) and validation of the new versions of the International Reference Ionosphere (IRI 2016), IRI extended to the plasmasphere (IRI‐Plas 2017), and NeQuick 2 models over the equatorial anomaly region during 2013–2018. The Global Positioning System (GPS)‐derived VTEC data inferred from the South American equatorial region during the relatively high (2013–2014), descending (2015–2016), and low (2017–2018) solar activity years have been considered as measurements to validate the models. The modeled (IRI 2016, IRI‐Plas 2017, and NeQuick 2) VTEC values are generally larger than the GPS‐derived VTEC values, with the highest overestimation being illustrated by the IRI‐Plas 2017 model. Small underestimations are also observed in employing the IRI 2016 and NeQuick 2 models, especially when the solar activity increases. The highest root‐mean‐square Deviations (RMSDs) reaching up to 10 TEC units (TECU) is observed in some hours while using the IRI‐Plas 2017 model during the high solar activity years. RMSDs less than 5 TECU are also observed on most of the hours while utilizing the model during all solar activity years. However, RMSD values less than 2 TECU (while using the NeQuick 2 model) and less than 1 TECU (while using the IRI 2016 model) are seen on most of the hours during 2013–2018. This shows that the performance of the IRI 2016 and NeQuick 2 is better than the IRI‐Plas 2017 with the IRI 2016 model consistently revealing the best in all solar activity years.

Ionospheric disturbances in a large area of the terrestrial globe by two strong solar flares of September 6, 2017, the strongest space weather events in the last decade

On September 6, 2017, the solar active region AR 2673 emitted two solar flares: the first at 08:57 UT (X2.2) and the second at 11:53 UT (X9.3); both were powerful enough to black-out high and low frequency radio waves (where UT is universal time). The X9.3 was the strongest solar flare event in the past decade. In this study, we took the advantage of these two extreme flare events to investigate corresponding effects on the ionosphere using multi-instrument observations from magnetometers, Global Positioning System – Total Electron content (GPS-TEC) receivers, ionosondes and Swarm satellites over a large geographical extent covering South American, African and European sectors. During the X2.2 flare, European and African sectors were sunlit and during X9.3 European, African, and South American sectors were sunlit and exposed to the solar flare radiation. During the X2.2 flare, there was an ionosonde blackout for a duration of about 45 min, while during the X9.3 flare this blackout lasted for 1 h and 30 min. The blackout are seen over a large global extent which demonstrates the severity of solar flare events in disrupting the radio communication. The horizontal component of Earth’s geomagnetic field has shown ripples and enhancements during these flare events. The ionospheric Vertical Total Electron Content (VTEC) showed a positive phase along with an intensification of the Equatorial Ionization Anomaly (EIA) over the South American and African sectors. The dynamical and physical processes associated with the TEC and EIA variabilities due to solar flare are discussed.

Morphological Features of Ionospheric Scintillations During High Solar Activity Using GPS Observations Over the South American Sector

AbstractThe main objective of this study is to investigate the ionospheric irregularities observed by Global Positioning System‐total electron content (GPS‐TEC) receivers during the high solar activity years of 2013 and 2014 at different stations in the equatorial and low‐latitude regions in the South American sector. The ionospheric parameters used in this investigation are the TEC, the rate of change of the TEC index (ROTI), and the amplitude scintillation index (S4). In the South American sector, the ROTI and S4 indices showed that the ionospheric irregularities have an annual variation with maximum occurrence from September to April, between 20:00 LT and 02:00 LT, and no occurrence from May to August. Also, strong phase fluctuations (ROTI >1) are observed over South America at 19 LT in October and November. Morlet wavelet analysis of ROTI and S4 showed that planetary wave‐scale periods ranging from 2 to 8 days are predominant during September–March at 20–02 LT in South America. In addition, using a keogram it was possible to evaluate the distance between adjacent ionospheric plasma depletions, and this result is presented and discussed. The longitudinal distances between adjacent bubbles vary around ~600–1000 km, which is larger than values reported in most previous studies.

The role of the Atlantic Multidecadal Oscillation precondition in the teleconnection of different El Niño‐Southern Oscillation types and impacts on the 15°N–15°S South American sector precipitation

AbstractThe Eastern Pacific (EP) and Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) types and their impacts on the tropical North Atlantic (TNA) SST variability and 15°N–15°S South American precipitation during the warm and cold phases of the Atlantic Multidecadal Oscillation (WAMO and CAMO) were evaluated during the 1901–2012 period. The results show more frequent ENSO events during the CAMO. The El Niño (EN) (La Niña [LN]) events, regardless of type (EP or CP), during the WAMO (CAMO) were accompanied by a warming (cooling) in the TNA after its mature phases. In these cases, extratropical teleconnection patterns are established through variations in the Pacific/North America (PNA) teleconnection pattern and are accompanied by variations in the Walker circulation. For the EN (LN) in the CAMO (WAMO), the tropical teleconnections occur predominant, through the Walker cell and the zonal inter‐basin gradient, which is intensified due to the SST gradient between the eastern equatorial Pacific (non‐neutral anomalies) and the equatorial Atlantic (neutral anomalies). These circulation pattern changes affect the precipitation patterns in the 15°N–15°S South American sector during December–January–February (D(0)JF(+1)) and March–April–May (MAM(+1)). The EP EN (EPEN) events are associated with the intensification of the negative precipitation anomalies in northeastern Brazil (NEB) during the WAMO and in the central part of the Amazon during the CAMO. In the case of CP EN (CPEN) events, the greatest differences between the AMO phases occur during MAM(+1), with reverse sign anomalies over northwestern South America. In the case of LN events, the largest differences occur in NEB, with reduced rainfall in the WAMO, regardless of type EP or CP. The results presented here highlight the role of low frequency oscillations in defining the teleconnection patterns between tropical Pacific and Atlantic Oceans, not discussed previously.

Effects on the equatorial and low latitude thermosphere and ionosphere during the 19–22 December 2015 geomagnetic storm period

In this paper, the critical frequency of F2 layer (foF2) and their peak height (hmF2) recorded at equatorial and low latitude stations of the South American sector were analyzed during the 19–22 December 2015 geomagnetic storm period. We have observed initially decreases of foF2 at equatorial and low latitudes over the North crest of the equatorial ionization anomaly (EIA) and almost no effect over the South crest. These initial negative storm effects can be fully explained by the penetration of a westward electric field to low and equatorial latitudes. The positive storm effects that replaced the negative storm effects during the development of the main phase may have occurred possibly because of an intensification of the “fountain effect” due to an increase of the upward vertical drift at equatorial latitudes. Changes in the O/N2 ratio (increases or decreases) may be the most probable cause for the positive and negative storm effects observed during the recovery.

Interhemispheric Asymmetry in Response of Low‐Latitude Ionosphere to Perturbation Electric Fields in the Main Phase of Geomagnetic Storms

AbstractStructures of sudden enhancements/depressions and associated interhemispheric asymmetry in low‐latitude total electron content (TEC) during the main phase (MP) of geomagnetic storms have remained unpredictable majorly due to oscillating equatorial vertical E×B drifts and resultant redistribution of plasma in low latitudes in a given seasonal background. Robust analysis of 7 major and 30 moderate ionospheric storms during the years 2000–2018 is performed with comprehensive literature review encompassing various sources of asymmetry in magnetosphere‐ionosphere coupling. Taking advantage of simultaneous long‐term observations of E×B drift from Jicamarca, H component from magnetometers, and global ionospheric map vertical TEC (VTEC) and TEC observations across the dip equator from the South American sector, simultaneous formation of peaks and valleys in VTEC and associated asymmetry are studied. Additionally, a three‐layer neural network‐based E×B drift model is developed using delta‐H observations that provide drift estimates in the absence of Jicamarca drifts. The main results establish simultaneous high‐magnitude short‐lived (1–2 hr) enhancements and depression in VTEC during the MP in daytime in both hemispheres with varying differences of −30 to 100 TECU with respect to quiet time mean and along with prominent existence of interhemispheric asymmetry in TEC during the MP regardless of seasons. Maximum VTEC in the northern and southern low latitudes is found to occur at different times during storms. Large difference of VTEC is found ranging between 10 and 30 TECU between the near conjugate locations of the hemispheres. Coincident global episodic peaks marked by steep VTEC falls show dominance of episodic eastward and westward penetration electric fields in the low‐latitude daytime ionosphere.

A global analysis of distribution and endemism within Late Llandovery (Telychian) brachiopods

Cocks, L.R.M. & Jiayu, R. 10 July 2019. A global analysis of distribution and endemism within Late Llandovery (Telychian) brachiopods. Alcheringa 43, 406–422. ISSN 0311–5518The genera of brachiopods of early Silurian (late Llandovery: Telychian) age have been critically reviewed and are listed from the major continental areas: South China, Avalonia-Baltica, Laurentia, Siberia and adjacent areas, and Gondwana (including the adjacent Kazakh terranes, Southwest Tien Shan and Iran). All those continents lay within tropical latitudes, apart from the South American sector of Gondwana, which hosted the Clarkeia Fauna, the earliest constituent of the largely high-latitude Devonian Malvinokaffric Province in the southern hemisphere. Additionally, the then northern (today’s southern) part of the Siberian continent, which included parts of Mongolia and North China, was at temperate latitudes in the northern hemisphere so that it hosted the Tuvaella Fauna, which was also dominated by endemic brachiopod genera. Of the 202 genera listed, 50 are endemic to one of the six regions, and a further eight must have lived during Telychian times since they are known from both the underlying Aeronian and the overlying Sheinwoodian.L. Robin M. Cocks [rcocks@nhmacuk], Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK; Rong Jiayu [jyrong@nigpasaccn], State Key Laboratory, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China.

Equatorial and low-latitude positive ionospheric phases due to moderate geomagnetic storm during high solar activity in January 2013

The day-to-day variability of the equatorial and low-latitude ionosphere during quiet and disturbed periods is one of the ionospheric highlighted Space Weather research topics, particularly the ionospheric electrodynamics during geomagnetic storms. This study investigates the response of ionospheric F-region from the equatorial region to beyond the Equatorial Ionization Anomaly (EIA) crest during moderate geomagnetic storm (minimum Dst = −53 nT) that took place on January 17 to 18, 2013, during the high solar activity period of solar cycle 24. The Total Electron Content (TEC) obtained through a network of 82 dual frequency GPS receivers, spanning over an area of 30° × 30° in latitude and longitude are used. Also the F-layer virtual height (h’F) and critical frequency (foF2) observations from 3 ionosondes, in the South American sector are used. Specifically, these GPS-TEC receivers and ionosondes are used to investigate how the F-layer was disturbed by two positive ionospheric phases occurred during the aforementioned disturbed period. The first positive ionospheric phase was probably due to a travelling ionospheric disturbance (TID). When this TID reached the Brazilian coast at low-latitude, the EIA crest was in the growth phase and makes it challenging to separate the spatial-temporal evolution of both phenomena. The second positive ionospheric phase was caused by an anomalous nighttime equatorial positive ionospheric cloud travelling from the east sector towards the west sector. In addition, how the EIA was disturbed by these two positive ionospheric phases in the eastern and western Brazilian sectors is also investigated.

Response of Ionospheric Profile Parameters to Equatorial Electrojet Over Peruvian Station

AbstractThe response of the ionospheric bottomside electron density profile parameters of the F2 layer, namely, the maximum electron density (NmF2), the maximum height of F2 layer (hmF2), and bottomside thickness (B0) parameter to the equatorial electrojet (EEJ) current is examined for a Peruvian location at the Jicamarca station (12 °S, 76.9 °W) in the South American sector. The results of the analysis show that both hmF2 and B0 increase for ~2 h before sunrise and exhibit a postsunset peak during the equinoctial and summer months. The increase in the peak height, hmF2, is observed to terminate before midday, while B0 continued to increase throughout the daytime. The apparent midday and postnoon peaks in NmF2 occur in all the seasons under study. It was demonstrated that a relationship exists between EEJ and the profile parameters hmF2 and B0 during low and moderate solar conditions. Conversely, the correlation coefficient between EEJ and NmF2 is statistically significant only during solar minimum conditions but correlates poorly, if at all, with EEJ during moderate solar activity.

Ionospheric disturbances at low and mid-low latitudes of the South American sector during the March 2015 great storm

This paper analyzes the response of the near equatorial and low latitude ionosphere of the South American sector to the geomagnetic storm occurred on 17 March 2015. Ionosonde data from Ramey (18.5° N, 292.9° E), Jicamarca (12.0° S, 283.2° E), Boa Vista (2.8° N, 299.3° E), Sao Luis (2.6° S, 315.8° E), Fortaleza (3.9° S, 321.6° E) and Cachoeira Paulista (22.7° S, 315.0° E) are used for the study. The results show negative disturbances in foF2 at low latitudes during the main phase of the storm, which were attributed to prompt penetration electric fields. Thus, the Equatorial Anomaly (EA) started to reduce their structure in this sector since on 17 March. During the recovery phase (on 18 March), positive disturbances were observed at low, mid-low latitudes (in the post-midnight – predawn hours), which can be mainly attributed to enhanced storm-time neutral winds and composition changes (i.e., increase in the O/N2 ratio). Disturbance dynamo electric fields would also contribute in modulating the electron density of the EA during this storm period.

Statistics of Durations and Spacings of Equatorial Plasma Depletions Detected by the C/NOFS Planar Langmuir Probe

AbstractThe Planar Langmuir Probe onboard the Communication/Navigation Outage Forecasting System satellite monitored ionospheric plasma densities and their irregularities with high resolution almost seamlessly for approximately five years since May 2008. Planar Langmuir Probe data from two different years (1 October 2008 to 30 September 2009 and 5 June 2012 to 4 June 2013) were selected for analysis. The first data set corresponds to solar minimum conditions, and the second one is as close to solar maximum conditions of solar cycle 24 as possible, with basis on their continuous availability. The results from a level‐crossing analysis applied to detrended evening‐time data from a fixed altitude range show how depletion durations and spacings between consecutive depletions are statistically distributed as functions of solar activity and longitude sector. In quantitative terms, they show that (i) the fewer depletions observed in the solar‐minimum year are also shorter, shallower, and closer together than those detected in the more active one and (ii) great similarities exist between structures of depletion durations and spacings between consecutive depletions observed in the South American and African longitude sectors, which are more noticeable than those associated with in the Indian‐Asian longitude sector.

Long‐Term Geochemical and Geodynamic Segmentation of the Paleo‐Pacific Margin of Gondwana: Insight From the Antarctic and Adjacent Sectors

AbstractCombined zircon geochemistry and geochronology of Mesozoic volcaniclastic sediments of the central Transantarctic Mountains, Antarctica, yield a comprehensive record of both the timing and geochemical evolution of the magmatic arc along the Antarctic sector of the paleo‐Pacific margin of Gondwana. Zircon age populations at 266–183 Ma, 367–328 Ma, and 550–490 Ma correspond to episodic arc activity from the Ediacaran to the Jurassic. Zircon trace element geochemistry indicates a temporal shift from granitoid‐dominated source(s) during Ediacaran to Early Ordovician times to mafic sources in the Devonian through Early Jurassic. Zircon initial εHf shifts to more radiogenic Hf isotope compositions following the Ross Orogeny and is inferred to reflect juvenile crustal growth within an extensional arc system during progressive slab rollback. These new ages and Hf isotopic record are similar to those from the Australian sector, indicating that these regions constituted an ~3,000 km laterally continuous extensional arc from at least the Carboniferous to the Permian. Conversely, the South American sector records enriched zircon Hf isotopic compositions and compressional/advancing arc tectonics during the same time period. Our new data constrain the location of this profound along‐arc geochemical and geodynamic “switch” to the vicinity of the Thurston Island block of West Antarctica.

Bottom side profiles for two close stations at the southern crest of the EIA: Differences and comparison with IRI-2012 and NeQuick2 for low and high solar activity

Bottom side electron density profiles for two stations at the southern crest of the Equatorial Ionization Anomaly (EIA), São José dos Campos (23.1°S, 314.5°E, dip latitude 19.8°S; Brazil) and Tucumán (26.9°S, 294.6°E, dip latitude 14.0°S; Argentina), located at similar latitude and separated by only 20° in longitude, have been compared during equinoctial, winter and summer months under low (year 2008, minimum of the solar cycle 23/24) and high solar activity (years 2013–2014, maximum of the solar cycle 24) conditions. An analysis of parameters describing the bottom side part of the electron density profile, namely the peak electron density NmF2, the height hmF2 at which it is reached, the thickness parameter B0 and the shape parameter B1, is carried out. Further, a comparison of bottom side profiles and F-layer parameters with the corresponding outputs of IRI-2012 and NeQuick2 models is also reported. The variations of NmF2 at both stations reveal the absence of semi-annual anomaly for low solar activity (LSA), evidencing the anomalous activity of the last solar minimum, while those related to hmF2 show an uplift of the ionosphere for high solar activity (HSA). As expected, the EIA is particularly visible at both stations during equinox for HSA, when its strength is at maximum in the South American sector. Despite the similar latitude of the two stations upon the southern crest of the EIA, the anomaly effect is more pronounced at Tucumán than at São José dos Campos. The differences encountered between these very close stations suggest that in this sector relevant longitudinal-dependent variations could occur, with the longitudinal gradient of the Equatorial Electrojet that plays a key role to explain such differences together with the 5.8° separation in dip latitude between the two ionosondes. Furthermore at Tucumán, the daily peak value of NmF2 around 21:00 LT during equinox for HSA is in temporal coincidence with an impulsive enhancement of hmF2, showing a kind of “elastic rebound” under the action of the EIA. IRI-2012 and NeQuick2 bottom side profiles show significant deviations from ionosonde observations. In particular, both models provide a clear underestimation of the EIA strength at both stations, with more pronounced differences for Tucumán. Large discrepancies are obtained for the parameter hmF2 for HSA during daytime at São José dos Campos, where clear underestimations made by both models are observed. The shape parameter B0 is quite well described by the IRI-2012 model, with very good agreement in particular during equinox for both stations for both LSA and HSA. On the contrary, the two models show poor agreements with ionosonde data concerning the shape parameter B1.

NeQuick 2 and IRI Plas VTEC predictions for low latitude and South American sector

Using vertical total electron content (VTEC) measurements obtained from GPS satellite signals the capability of the NeQuick 2 and IRI Plas models to predict VTEC over the low latitude and South American sector is analyzed. In the present work both models were used to calculate VTEC up to the height of GPS satellites. Also, comparisons between the performance of IRI Plas and IRI 2007 have been done. The data correspond to June solstice and September equinox 1999 (high solar activity) and they were obtained at nine stations. The considered latitude range extends from 18.4°N to −64.7°N and the longitude ranges from 281.3°E to 295.9°E in the South American sector. The greatest discrepancies among model predictions and the measured VTEC are obtained at low latitudes stations placed in the equatorial anomaly region. Underestimations as strong as 40 TECU [1 TECU = 1016 m−2] can be observed at BOGT station for September equinox, when NeQuick2 model is used. The obtained results also show that: (a) for June solstice, in general the performance of IRI Plas for low latitude stations is better than that of NeQuick2 and, vice versa, for highest latitudes the performance of NeQuick2 is better than that of IRI Plas. For the stations TUCU and SANT both models have good performance; (b) for September equinox the performances of the models do not follow a clearly defined pattern as in the other season. However, it can be seen that for the region placed between the Northern peak and the valley of the equatorial anomaly, in general, the performance of IRI Plas is better than that of NeQuick2 for hours of maximum ionization. From TUCU to the South, the best TEC predictions are given by NeQuick2.The source of the observed deviations of the models has been explored in terms of CCIR foF2 determination in the available ionosonde stations in the region. Discrepancies can be also related to an unrealistic shape of the vertical electron density profile and or an erroneous prediction of the plasmaspheric contribution to the vertical total electron content. Moreover, the results of this study could be suggesting that in the case of NeQuick, the underestimation trend could be due to the lack of a proper plasmaspheric model in its topside representation. In contrast, the plasmaspheric model included in IRI, leads to clear overestimations of GPS derived TEC.

Specialization and Exports in South America after Trade Agreements

A vast body of literature recognizes that economic integration brings asymmetric benefits for the actors carrying out the process. Regional integration agreements influence the level of industrial activity and its location, preventing some countries from fully participating as beneficiaries. This study aims to examine the changes in production and export structures of South American manufacturing sector after the signing of trade agreements. We performed unit root tests with endogenous breaks, cointegration tests, and stochastic frontier models on production and export for the period 1985~2008. Our results show that after the signing of trade agreements, changes in the structure of production and export have been weak, and thus trade agreements have not boosted structural changes in specialization and export intensity in South American countries.