Browsing by Author "Valladares, C. E."
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Item Open Access A latitudinal network of GPS receivers dedicated to studies of equatorial spread F(American Geophysical Union, 2004-02) Valladares, C. E.; Sheehan, R.; Villalobos, J.Five GPS receivers have been deployed near the 74°W longitude meridian to measure the variability of total electron content (TEC) latitudinal profiles and to study the relation of this variability with the onset and evolution of spread F plasma structures. These five GPS receivers, together with two others that form part of the International GPS Service (IGS) network, three more that belong to the South Andes Project network, and an additional receiver located at Ancon, Peru, provide TEC values between 8°N and 40°S geographic latitude. In addition, all five GPS receivers managed by Boston College give the amplitude scintillation on a near-real time basis. This fact allows us to know the maximum latitude to which the irregularities extend and to infer the maximum altitude of the plasma bubbles. We have calculated TEC latitudinal profiles using the TEC values obtained by all the receivers between 1998 and 2001. We found that during the equinoxes, UHF scintillations occur when the ratio of the crest to the trough of the anomaly is 2 or larger. During the December solstice the crest is not very pronounced, but a sharp decrease of TEC at the magnetic equator precedes the onset of 1-km scale irregularities. We have also examined a longitudinal variability of scintillations by partitioning the sky in two sectors separated at the 74°W meridian. We consistently observe a greater number of GPS scintillation events at the eastern longitudes over the Amazon rain forest. This intriguing finding could well be explained by a larger population of gravity waves at longitudes east of the Andes.Item Restricted C/NOFS satellite observations of equatorial ionospheric plasma structures supported by multiple ground‐based diagnostics in October 2008(American Geophysical Union, 2011-10-28) Nishioka, M.; Basu, Su.; Basu, S.; Valladares, C. E.; Sheehan, R. E.; Roddy, P. A.; Groves, K. M.In early October 2008, the C/NOFS satellite orbited near the magnetic equator at its perigee altitude of ∼400 km at dusk in the Peruvian sector. This provided an ideal opportunity for a comparison, under the current very low solar flux condition, of equatorial ionospheric disturbances observed with the Communication/Navigation Outage Forecasting System (C/NOFS) in situ measurements and ground‐based observations available near Jicamarca Observatory. The primary objective was the comparison of plasma density disturbances measured by a Planar Langmuir Probe (PLP) instrument on the C/NOFS satellite with VHF scintillation activity at Ancon near Jicamarca for this period. Here we discuss in detail two extreme cases: one in which severe in situ disturbances were accompanied by mild scintillation on a particular day, namely, 10 October while there was little in situ disturbance with strong scintillation on 5 October. This apparent contradiction was diagnosed further by a latitudinal ground‐based GPS network at Peruvian longitudes, a Digisonde, and the incoherent scatter radar (ISR) at Jicamarca. The crucial distinction was provided by the behavior of the equatorial ionization anomaly (EIA). The EIA was well‐developed on the day having severe in situ disturbances (10 Oct). This led to lower equatorial plasma density and total electron content (TEC) at the equator and consequently reduced the scintillations detected at Ancon. On the other hand, on the day with severe scintillations (5 Oct), the EIA was not so well developed as on 10 October, leading to relatively higher equatorial plasma density and TEC. Consequently the severe scintillations at Ancon were likely caused by ionospheric structure located below the altitude of C/NOFS. The NRL SAMI2 model was utilized to gain a greater understanding of the role of neutral winds and electric fields in reproducing the TEC as a function of latitude for both classes of irregularities. Spectral studies with high resolution in situ PLP data were also performed. The power law spectra within the plasma bubbles showed two slopes: the low frequency slope being ∼−5/3 and the high frequency ∼−5 with a break around λ = 70 m. This particular type of two‐slope spectra may be related to the extremely low solar activity and its impact on ion composition and temperature.Item Restricted Comparison of CHAMP and Digisonde plasma frequencies at Jicamarca, Peru(American Geophysical Union, 2007-03-13) McNamara, L. F.; Cooke, D. L.; Valladares, C. E.; Reinisch, B. W.Ionospheric plasma frequencies at the altitude of the CHAMP satellite have been deduced from ionosonde true‐height profiles for Jicamarca, Peru, and have been compared with the in situ measurements made by CHAMP. The differences between the plasma frequencies have been found to be well within the uncertainties associated with the ionosonde profiles, confirming the validity of the CHAMP measurements. For satellite‐ionosonde separations of less than 250 km and for satellite altitudes below the peak of the F2 layer, the average discrepancy between the two plasma frequencies is 0.25 MHz or 4%. For the most reliable ionosonde measurements, the average discrepancies reduce to 0.18 MHz (or 1.7%), with a standard deviation of 0.16 MHz (or 1.5%). Given the validity of the CHAMP plasma frequencies, corresponding ionosonde and CHAMP observations have been used to support the practice of extending the ionosonde profile above hmF2 by assuming a Chapman layer with a constant scale height equal to that of the lower side of the F2 layer peak. The average discrepancy for CHAMP passing above the peak of the F2 layer is 0.22 MHz (or 2.6%), and the standard deviation is 0.8 MHz (or 13.3%).Item Open Access Comparison of equatorial GPS-TEC observations over an African station and an American station during the minimum and ascending phases of solar cycle 24(European Geosciences Union (EGU), 2013-11) Akala, A. O.; Seemala, G. K.; Doherty, P. H.; Valladares, C. E.; Carrano, C. S.; Espinoza, Jhan Carlo; Oluyo, S.GPS-TEC data were observed at the same local time at two equatorial stations on both longitudes: Lagos (6.52° N, 3.4° E, 3.04° S magnetic latitude), Nigeria; and Pucallpa (8.38° S, 74.57° W, 4.25° N magnetic latitude), Peru during the minimum (2009, 2010) and ascending (2011) phases of solar cycle 24. These data were grouped into daily, seasonal and solar activity sets. The day-to-day variations in vertical TEC (VTEC) recorded the maximum during 14:00-16:00 LT and minimum during 04:00-06:00 LT at both longitudes. Seasonally, during solar minimum, maximum VTEC values were observed during March equinox and minimum during solstices. However, during the ascending phase of the solar activity, the maximum values were recorded during the December solstice and minimum during the June solstice. VTEC also increased with solar activity at both longitudes. On longitude by longitude comparison, the African GPS station generally recorded higher VTEC values than the American GPS station. Furthermore, harmonic analysis technique was used to extract the annual and semi-annual components of the amplitudes of the TEC series at both stations. The semi-annual variations dominated the TEC series over the African equatorial station, while the annual variations dominated those over the American equatorial station. The GPS-TEC-derived averages for non-storm days were compared with the corresponding values derived by the IRI-2007 with the NeQuick topside option. The NeQuick option of IRI-2007 showed better performance at the American sector than the African sector, but generally underestimating TEC during the early morning hours at both longitudes.Item Restricted Comparison of storm time equatorial ionospheric electrodynamics in the African and American sectors(Elsevier, 2010-08-12) Yizengaw, E.; Moldwin, M. B.; Mebrahtu, A.; Damtie, B.; Zesta, E.; Valladares, C. E.; Doherty, P.The characteristics of storm time (corotating interaction regions (CIR)-driven storm that happened on 9 August 2008) equatorial electrojet (EEJ) phenomena and their effect on the ionospheric density structure at two different longitudinal sectors are presented. Equatorial magnetometer data, occultation density profiles from COSMIC and CHAMP LEO satellites, and ground-based GPS TEC are used. We find unusual density reduction around local noon at the same time when we observe the reversal of electrojet current and thus counter-equatorial electrojet (CEJ) signatures. The continuous energy deposition in to high latitudes due to the CIR-driven storm that triggers the E-region dynamo and the penetrating magnetospheric origin electric field is suggested to be responsible for the reversal of equatorial electrojet current flows. We also compare the magnitude and direction of the driving force (E×B drift) in the American and African sectors for the first time. It was found that at the same local time the E×B drift in the American sector is stronger than that of the African sector. Previously, the uneven distribution of ground-based instruments hindered our ability to obtain a global understanding of the dynamics and structure of the ionosphere. The newly deployed ground-based instruments, primarily in the African sector, provide the opportunity to observe the governing equatorial electrodynamics simultaneously with the ionospheric density structures detected by the instrument onboard low-Earth-orbit (LEO) satellites. To our knowledge this is the first simultaneous observation performed in the African sector. This case study may provide additional input that could be used to explain the unique density irregularities that are often seen from in situ satellite observation in the African sector, a region that has been devoid of ground-based instrumentations.Item Open Access Correlative study of neutral winds and scintillation drifts measured near the magnetic equator(American Geophysical Union, 2002-07-18) Valladares, C. E.; Meriwether, J. W.; Sheehan, R.; Biondi, M. A.Measurements of the thermospheric neutral wind at Arequipa, Peru, and observations of the drift of the irregularities at Ancon, Peru, are used to study the coupling that exists between ions and neutrals at equatorial latitudes and the variability of this coupling as a function of the occurrence of scintillations. This study is based on data collected at the Arequipa and Ancon stations between 1996 and 1998. Our comparative analysis indicates that the relative wind-drift values vary depending on season and the solar flux level. We found that during the equinoxes and low solar flux values, the averaged zonal drift is larger than the wind by 15 m s−1, but for solar flux values above 130 units, the average wind exceeds the drift values by 10 to 20 m s−1. We suggest that the occurrence of larger equinoctial drifts can be explained by the existence of altitude gradients in the zonal wind during that season. During the June solstice the zonal wind seems to exceed the irregularity drift by ∼10–20 m s−1 independent of the solar flux. We also find that the meridional wind shows a modest dependence on the scintillation activity during the June solstice. During scintillation events and between 2000 and 2400 LT the averaged meridional wind observed to the south and north of Arequipa exceeds their corresponding no-scintillation values by 20 m s−1. We present likely explanations of this effect.Item Restricted Early morning equatorial ionization anomaly from GOLD observations(American Geophysical Union, 2020-07) Laskar, F. I.; Eastes, R. W.; Martinis, C. R.; Daniell, R. E.; Pedatella, N. M.; Burns, A. G.; McClintock, W.; Goncharenko, L. P.; Coster, A.; Milla, Marco; Wang, W.; Valladares, C. E.; Codrescu, M. V.During geomagnetically quiet and solar minimum conditions, spatial variations of the early morning thermosphere‐ionosphere (TI) system are expected to be mainly governed by wave dynamics. To study the postmidnight dynamical coupling, we investigated the early morning equatorial ionization anomaly (EIA) using Global‐scale Observations of the Limb and Disk (GOLD) measurements of OI‐135.6 nm nightglow emission and global navigation satellite system (GNSS)‐based total electron content (TEC) maps. The EIA structures in the OI‐135.6 nm emission over the American landmass resemble, spatially and temporally, those observed in the GNSS‐TEC maps. The early morning EIA (EM‐EIA) crests are well separated in latitude and mostly located over the middle of South America during October–November. In February–April the crests are less separated in latitude and predominantly located over the west coast sector of South America. Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCMX) simulations with constant solar minimum and quiet‐geomagnetic conditions show that EM‐EIA can occur globally and shows properties similar to longitudinal Wave 4 pattern. Thus, we propose that EM‐EIA is driven by dynamical changes associated with the lower atmospheric waves.Item Restricted Effect of magnetic activity on the dynamics of equatorial F region irregularities(American Geophysical Union, 2002) Bhattacharyya, A.; Basu, S.; Groves, K. M.; Valladares, C. E.; Sheehan, R.Two different aspects of the effect of magnetic activity on the dynamics of equatorial spread F (ESF) irregularities are studied here using spaced receiver scintillation observations. The first one deals with the question of how magnetic activity affects the generation of ESF irregularities. For this, a parameter designated the “random velocity,” which is a measure of random changes in the irregularity drift velocity, is evaluated from the data. In past studies, this parameter has been found to have large values in the early phase of evolution of ESF irregularities during the postsunset period, with a steep decline to a low value by 22 LT. This behavior is attributed to the decline in the height of the F region. Therefore, a sudden increase in the “random velocity” in the postmidnight period is attributed to an increase in the height of the F region due to the ionospheric zonal electric field turning from westward to eastward due to the effect of magnetic activity, which may also generate fresh irregularities that produce the observed scintillations. This idea has been used to suggest that for two of the magnetically active days considered in the present study the irregularities may be freshly generated in the postmidnight period. The second aspect is the identification of geomagnetically disturbed plasma drifts, which is generally possible only after 22 LT, when the estimated irregularity drift velocities are close to that of the background plasma. The pattern of the estimated drift after 22 LT (3 UT) is found to be well defined for magnetically quiet days with scintillations during a period of a month. This allows the identification of a superimposed westward perturbation in the drift, produced by a disturbance dynamo due to magnetic activity, for all the three events studied here. On 19 February and 1 March 1999, the eastward drift velocities show an identical decrease of about 50 m/s from the undisturbed drift at 0440 UT. On 1 March, the decay phase of the storm sets in later, and the eastward velocity continues to decrease until 0530 UT, turning westward with a maximum decrease of about 80 m/s from the undisturbed drift. On 22 October 1999, which was more disturbed than these two days, the westward perturbation was larger, causing the drift velocity to turn westward around 5 UT and a decrease of nearly 150 m/s from the quiet time drift at 8 UT. The results are in broad agreement with some of the recent empirical models of the evolution, with storm time, of equatorial disturbance dynamo electric fields.Item Restricted Equatorial ionospheric zonal drift model and vertical drift statistics from UHF scintillation measurements in South America(European Geosciences Union, 2004-09) Sheehan, R. E.; Valladares, C. E.UHF scintillation measurements of zonal ionospheric drifts have been conducted at Ancon, Peru since 1994 using antennas spaced in the magnetic east-west direction to cross-correlate geo-synchronous satellite signals. An empirical model of average drift over a wide range of Kp and solar flux conditions was constructed from successive two-dimensional fits of drift vs. the parameters and day of year. The model exhibits the typical local time trend of maximum eastward velocity in the early evening with a gradual decrease and reversal in the early morning hours. As expected, velocities at all hours increase with the solar flux and decrease with Kp activity. It was also found that vertical drifts could contribute to the variability of drift measurements to the east of Ancon at a low elevation angle. The vertical drift at the ionospheric intersection to the east can be estimated when combined with nearly overhead observations at Ancon or a similar spaced-antenna site at Antofagasta, Chile. Comparisons on five days with nearly simultaneous measurements of vertical drift by the Julia radar at Jicamarca, Peru show varying agreement with the spaced-antenna estimates. Statistical results from 1997 to 2001 generally agree with radar and satellite studies.Item Restricted Equatorial plasma bubbles and L-band scintillations in Africa during solar minimum(European Geosciences Union (EGU), 2012-04-16) Paznukhov, V. V.; Carrano, C. S.; Doherty, P. H.; Groves, K. M.; Caton, R. G.; Valladares, C. E.; Seemala, G. K.; Bridgwood, C. T.; Adeniyi, J.; Amaeshi, L. L. N.; Damtie, B.; D’Ujanga Mutonyi, F.; Ndeda, J. O. H.; Baki, P.; Obrou, O. K.; Okere, B.; Tsidu, G. M.We report on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa. The measurements were made in 2010, as a first step toward establishing the climatology of ionospheric irregularities over Africa. The scintillation intensity is obtained by measuring the standard deviation of normalized GPS signal power. The EPBs are detected using an automated technique, where spectral analysis is used to extract and identify EPB events from the GPS TEC measurements. Overall, the observed seasonal climatology of the EPBs as well as GPS scintillations in equatorial Africa is adequately explained by geometric arguments, i.e., by the alignment of the solar terminator and local geomagnetic field, or STBA hypothesis (Tsunoda, 1985, 2010a). While plasma bubbles and scintillations are primarily observed during equinoctial periods, there are longitudinal differences in their seasonal occurrence statistics. The Atlantic sector has the most intense, longest lasting, and highest scintillation occurrence rate in-season. There is also a pronounced increase in the EPB occurrence rate during the June solstice moving west to east. In Africa, the seasonal occurrence shifts towards boreal summer solstice, with fewer occurrences and shorter durations in equinox seasons. Our results also suggest that the occurrence of plasma bubbles and GPS scintillations over Africa are well correlated, with scintillation intensity depending on depletion depth. A question remains about the possible physical mechanisms responsible for the difference in the occurrence phenomenology of EPBs and GPS scintillations between different regions in equatorial Africa.Item Open Access Equatorial scintillation and systems support(American Geophysical Union, 1997-09) Groves, K. M.; Basu, S.; Weber, E. J.; Smitham, M.; Kuenzler, H.; Valladares, C. E.; Sheehan, R.; MacKenzie, E.; Secan, J. A.; Ning, P.; McNeill, W. J.; Moonan, D. W.; Kendra, M. J.The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short-term forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.Item Restricted Evolution of equatorial ionospheric plasma bubbles and formation of broad plasma depletions measured by the C/NOFS satellite during deep solar minimum(American Geophysical Union, 2011-03-08) Huang, Chao‐Song; de la Beaujardiére, O.; Roddy, P. A.; Hunton, D. E.; Pfaff, R. F.; Valladares, C. E.; Ballenthin, J. O.An unexpected feature revealed by the measurements of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite is the presence of broad plasma depletions in the midnight–dawn sector during deep solar minimum. It has not been well understood what causes the broad plasma depletions and how equatorial plasma bubbles are related to the broad depletions. In this paper we present the C/NOFS measurements of equatorial plasma bubbles and broad depletions in a few cases. The ion density perturbations and enhanced ion vertical velocity are first identified in the topside F region at ∼2200 LT, suggesting that the plasma bubbles start to form earlier at lower altitudes. The observations show that the plasma bubbles observed in the midnight–dawn sector may originate in the evening sector. The plasma bubbles continue growing for more than 3.3 h, and the decay time of the bubbles is also longer than 3.3 h. The continuous growth of the plasma bubbles in the evening sector and the slow decay after midnight determine that most plasma bubbles become fully developed and are easily detected in the midnight–dawn sector. The plasma flow inside the bubbles remains strongly upward throughout the entire nighttime. We propose the following mechanism for the generation of wide plasma bubbles and broad depletions. A series of plasma bubbles is generated through the Rayleigh‐Taylor instability process over a large longitudinal range. These plasma bubbles grow and merge to form a wide bubble (width of ∼700 km as observed), and multiple regular and/or wide bubbles can further merge to form broad plasma depletions (thousands of kilometers in longitude). The ion vertical drift inside each plasma bubble is driven by the polarization electric field and remains large after the bubbles have merged. This mechanism provides a reasonable interpretation of the large upward ion drift velocity inside the broad depletion region.Item Restricted Flux tube analysis of L‐band ionospheric scintillation(American Geophysical Union, 2013-04-23) Shume, E. B.; Mannucci, A. J.; Butala, M. D.; Pi, X.; Valladares, C. E.This manuscript presents magnetic flux tube analysis of L‐band signal scintillation in the nighttime equatorial and low‐latitude ionosphere. Residues of the scintillation index urn:x wiley:jgra:media:jgra50285:jgra50285-math-0001 estimated from the L‐band signals received from Geostationary Earth Orbit (GEO) satellites are employed in the analysis. The urn:x wiley:jgra:media:jgra50285:jgra50285-math-0002 estimates have been shown to be associated with simultaneous GPS VTEC variations derived from JPL's GIPSY‐GIM package. We have applied the wavelet decomposition technique simultaneously on the urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0003 time series in a flux tube over the equatorial and low‐latitude regions. The technique decomposes the urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0004 signal to identify the dominant mode of variabilities and the temporal variations of scintillation‐producing irregularities in the context of a flux tube. Statistically significant regions of the wavelet power spectra considered in our study have mainly shown that (a) dominant plasma irregularities associated with urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0005 variabilities in a flux tube have periods of about 4 to 15 minutes (horizontal irregularity scales of about 24 to 90 km). These periods match short period gravity waves, (b) scintillation‐producing irregularities are anisotropic along the flux tube and in the east‐west direction, and (c) the occurrences of scintillation‐producing irregularities along the flux tube indicate that the entire flux tube became unstable. However, plasma instability occurrences were not simultaneous in most cases along the flux tube, there were time delays of various orders. Understanding the attributes of L‐band scintillation‐producing irregularities could be important for developing measures to mitigate L‐band signal degradation.Item Restricted GPS observation of continent‐size traveling TEC pulsations at the start of geomagnetic storms(American Geophysical Union, 2014-08-11) Pradipta, R.; Valladares, C. E.; Doherty, P. H.We report our experimental observation of continent‐size traveling plasma disturbances using GPS measurements of total electron content (TEC) over the North American sector. These plasma disturbances occurred at the beginning of geomagnetic storms, immediately after the shock arrived, and prior to the appearance of large‐scale traveling ionospheric disturbances (LSTIDs) from the auroral region. Specifically, these supersize TEC perturbations were observed when the interplanetary magnetic field Bz was oscillating between northward and southward directions. They were found to propagate zonally with a propagation speed of 2–3 km/s. We interpret these TEC pulsations as ion drift waves in the magnetosphere/plasmasphere that propagate azimuthally inside the GPS orbit.Item Restricted Impact of sudden stratospheric warmings on equatorial ionization anomaly(American Geophysical Union, 2010-10-07) Goncharenko, L. P.; Coster, A. J.; Chau Chong Shing, Jorge Luis; Valladares, C. E.We investigate the ionospheric response to several stratospheric sudden warming events which occurred in Northern Hemisphere winters of 2008 and 2009 during solar minimum conditions. We use GPS total electron content data in a broad latitudinal region at ±40° geographic latitude and a single longitude, 75°W. In all cases, we find a strong daytime ionospheric response to stratospheric sudden warmings. This response is characterized by a semidiurnal character, large amplitude, and persistence of perturbations for up to 3 weeks after the peak in high‐latitude stratospheric temperatures. The ionospheric perturbations at the lower latitudes usually begin a few days after the peak in stratospheric temperature and are observed as an enhancement of the equatorial ionization anomaly (EIA) in the morning sector and a suppression of the EIA in the afternoon sector. There is also evidence of a secondary enhancement in the postsunset hours. Once observed in the low latitudes, the phase of semidiurnal perturbations progressively shifts to later local times in subsequent days. This progressive shift occurs at a different rate for different stratospheric warming events. The large magnitude and persistence of ionospheric perturbations, together with the predictability of stratospheric sudden warmings several days in advance, present an opportunity to investigate these phenomena in a systematic manner which may eventually lead to a multiday forecast of low‐latitude ionosphere conditions.Item Restricted Impacts of ionospheric scintillations on GPS receivers intended for equatorial aviation applications(American Geophysical Union, 2012-07-25) Akala, A. O.; Doherty, P. H.; Carrano, C. S.; Valladares, C. E.; Groves, K. M.This study examines the impacts of ionospheric scintillations on GPS receivers that are intended for equatorial or transequatorial aviation applications. We analyzed GPS data that were acquired at Ascension Island during the Air Force Research Laboratory (AFRL) campaign of the solar maximum year of 2002. Strong scintillations impacted the receiver‐satellite geometry, leading to poor dilution of precisions and positioning accuracy. In addition, deep signal fades (>20 dB‐Hz), leading to navigation outages were observed during most of the nights of the campaign. Under quiescent conditions, the C/No of satellites fluctuated slowly between 50 dB‐Hz and 35 dB‐Hz baselines for both L1 (1.5754 GHz) and L2 (1.2276 GHz) signals, depending on the satellite's elevation angle. The satellite's elevation angle and the effective scan velocity of the satellite's ionospheric penetration point (IPP) with respect to the magnetic field and plasma drift influenced the rate of fading of satellite signals.Item Open Access Instrumentation and science at Jicamarca and LISN for CAWSES-II TG4(Instituto Geofísico del Perú, 2010) Chau Chong Shing, Jorge Luis; Hysell, D. L.; Valladares, C. E.; Meriwether, J. W.Diapositivas presentadas en el 2010 CEDAR Workshop, University of Colorado, Boulder, CO, 20-25 June 2010.Item Open Access Ionospheric effects of recent stratospheric sudden warmings(Instituto Geofísico del Perú, 2011) Goncharenko, L.; Coster, A.; Chau Chong Shing, Jorge Luis; Valladares, C. E.Recent studies have shown large variations in low-latitude ionospheric parameters occurring after stratospheric sudden warming events. We use observations of vertical ion drift from Jicamarca ISR and GPS total electron content data in the Western Hemisphere for winters of 2008-2009 and 2009-2010 to illustrate main features of ionospheric changes related to stratospheric sudden warmings. The common feature in all events is the increase in the electron density during the morning hours and the decrease in the afternoon, related to amplification of 12-hour signature in low-latitude vertical ion drifts. This feature persists for several days after the peak in stratospheric temperature. The observed phenomena is related to quasistationary planetary waves, which have a high amplitude level prior to the stratospheric warmings. Non-linear interaction of planetary waves with tides leading to increase in tidal amplitudes in the low latitude lower thermosphere and modulation of E-region electric field with subsequent mapping to the F-region is thought to be the primary mechanism responsible for the observed ionospheric response. We investigate the characteristics of ionospheric oscillations with planetary wave periods between 2 and 30 days and in a wide range of latitudes in context of variations in stratospheric parameters.Item Open Access Ionospheric variations during January 2009 stratospheric sudden warming(Instituto Geofísico del Perú, 2009) Goncharenko, L.; Coster, A.; Rideout, W.; Chau Chong Shing, Jorge Luis; Liu, H. -L.; Valladares, C. E.The stratospheric sudden warming peaking in January 2009 was the strongest and most prolonged on record. We report significant ionospheric variations is association with this event, which are especially pronounced at low latitudes. Large increase in the vertical drifts is observed at Jicamarca, displaying 12-hour signature with upward drifts in the morning hours and downward drifts in the afternoon hours, with pattern persisting for several days. Analysis of GPS TEC data indicates that variations in electron density are observed in a large range of longitudes and latitudes. The entire daytime ionosphere is affected, with morning increase in low-latitude TEC exceeding 100% of the mean value, and afternoon decrease in TEC approaching ~50% of the mean value. These variations are consistent with ionospheric disturbances observed during other stratospheric warming events. We suggest the observed phenomena is related to planetary waves, which have a high amplitude level prior to the stratospheric warmings. Interaction of planetary waves with tides and modulation of tides can lead to changes in the low-latitude electric field through the wind dynamo process, which in turn is responsible for a largescale redistribution of ionospheric electron density.Item Restricted Latitudinal extension of low-latitude scintillations measured with a network of GPS receivers(European Geosciences Union, 2004-09) Valladares, C. E.; Villalobos, J.; Sheehan, R.; Hagan, M. P.A latitudinal-distributed network of GPS receivers has been operating within Colombia, Peru and Chile with sufficient latitudinal span to measure the absolute total electron content (TEC) at both crests of the equatorial anomaly. The network also provides the latitudinal extension of GPS scintillations and TEC depletions. The GPS-based information has been supplemented with density profiles collected with the Jicamarca digisonde and JULIA power maps to investigate the background conditions of the nighttime ionosphere that prevail during the formation and the persistence of plasma depletions. This paper presents case-study events in which the latitudinal extension of GPS scintillations, the maximum latitude of TEC depletion detections, and the altitude extension of radar plumes are correlated with the location and extension of the equatorial anomaly. Then it shows the combined statistics of GPS scintillations, TEC depletions, TEC latitudinal profiles, and bottomside density profiles collected between September 2001 and June 2002. It is demonstrated that multiple sights of TEC depletions from different stations can be used to estimate the drift of the background plasma, the tilt of the plasma plumes, and in some cases even the approximate time and location of the depletion onset. This study corroborates the fact that TEC depletions and radar plumes coincide with intense levels of GPS scintillations. Bottomside radar traces do not seem to be associated with GPS scintillations. It is demonstrated that scintillations/depletions can occur when the TEC latitude profiles are symmetric, asymmetric or highly asymmetric; this is during the absence of one crest. Comparison of the location of the northern crest of the equatorial anomaly and the maximum latitude of scintillations reveals that for 90% of the days, scintillations are confined within the boundaries of the 50% decay limit of the anomaly crests. The crests of the anomaly are the regions where the most intense GPS scintillations and the deepest TEC depletions are encountered. In accord with early results, we observe that GPS scintillations/TEC depletions mainly occur when the altitude of the magnetic equator F-region is above 500km. Nevertheless, in many instances GPS scintillations and TEC depletions are observed to exist when the F-layer is well below 500km or to persist when the F-layer undergoes its typical nighttime descent. Close inspection of the TEC profiles during scintillations/depletions events that occur when the equatorial F-layer peak is below 500km altitude reveals that on these occasions the ratio of the crest-to-equator TEC is above 2, and the crests are displaced 10° or more from the magnetic equator. When the equatorial F-layer is above 500km, neither of the two requirements is needed, as the flux tube seems to be inherently unstable. We discuss these findings in terms of the Rayleigh-Taylor instability (RTI) mechanism for flux-tube integrated quantities. We advance the idea that the seeming control that the reverse fountain effect exerts on inhibiting or suppressing GPS scintillations may be related to the redistribution of the density and plasma transport from the crests of the anomaly toward the equatorial region and then to much lower altitudes, and the simultaneous decrease of the F-region altitude. These two effects originate a decrease in the crest/trough ratio and a reduction of the crests separation, making the whole flux tube more stable to the RTI. The correspondence between crest separation, altitude of the equatorial F-region, the onset of depletions, and the altitude (latitude) extension of plumes (GPS scintillations) can be used to track the fate of the density structures.