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Diverse studies about equatorial plasma bubble structures and their relation with ionospheric scintillation have been performed in the last decades. Among many findings, the investigations usually reported dominant plasma bubble eastward velocity with a magnitude of few tens of m/s and larger amplitude scintillation for transionospheric signals aligned with these depleted structures. However, an uncommon long-lasting event with negligible average zonal drift prevailing for hundreds of minutes was registered over the Brazilian region, allowing a case study of the scintillation pattern under this particular condition. Data from ionosondes, all-sky imager (6300 A filter), geostationary satellites and Global Navigation Satellite System were used here, covering the eastern portion of the Brazilian ionosphere. The results show that the scintillation was less intense than in other nights around the event, suggesting that larger plasma density gradients and sequential bubble structures seem to cause more severe scintillation scenario than the alignment condition alone, even though the last also contributes to worsen the scintillation effects. Regarding the stagnant bubble pattern, the results suggest prevailing E region Hall conductivity and equatorward wind as potential agents.

The ionosphere is one of the main sources of errors for the GNSS (Global Navigation Satellite System) users in low-latitude regions. The ionosphere delays the signal propagation, directly influencing the pseudorange estimation, which is fundamental for positioning estimation. This study aims at evaluating empirical and theoretical models that generate profiles of Total Electron Content (TEC), with respect to their ability to represent the spatiotemporal distribution of the ionospheric plasma. For this purpose, the TEC data calculated from measurements obtained by receivers from the Brazilian Network for Continuous Monitoring of GNSS Systems (RBMC) were compared with ionospheric models that provide the TEC values. The TEC calculation procedure is presented in the manuscript. The Klobuchar model, the empirical model IRI-2016, which has the NeQuick model integrated in its calculations, and the SAMI2 model were evaluated. Data from 2 years around solar maximum period was used considering averaged values over Brazilian region. The evaluations carried out between the TEC values obtained from the models and the real TEC data showed that the models have some deviations, therefore requiring future improvements. According to the results, the deviations may reach approximately 7 m and are originated from physical peculiarities on the spatial environment over the Brazilian region.

The ionosphere over the Brazilian region has particular characteristics due to the large geomagnetic declination angle over most of the territory. Furthermore, the equatorial ionization anomaly southern crest is located over the Brazilian territory. In this region, plasma irregularities may arise in the post-sunset hours. These ionospheric irregularities develop in the form of magnetic field-aligned plasma depletions, known as equatorial plasma bubbles, which may seriously affect radio signals that propagate through them. These irregularity structures can cause amplitude and phase scintillation of the propagating signals, thereby compromising the availability, performance, and integrity of satellite-based communication and navigation systems. Additionally, the total electron content (TEC) introduces propagation delays that can contribute to range measurement errors for global positioning system (GPS) users. The ionospheric characteristics change significantly according to the time of day, season, as well as the solar and geomagnetic activities, among other factors. Indeed, the ionosphere is one of the most significant sources of errors in the positioning and navigation systems based on the GPS satellites. Due to these features, there is a strong interest by the scientific community in better understanding and characterizing the ionospheric behavior. In this context, the TEC analysis has wide applicability for space plasma studies and is a well-established tool for investigating the ionospheric behavior and its potential impact on space-based navigation systems. One of the goals of these studies is the generation of TEC maps for a geographic region based on GPS observations. In the present work, some electrodynamic processes of the low-latitude ionosphere are reviewed and the TEC estimation based on GPS measurements is revisited in detail. A methodology aimed at creating the TEC maps is presented and validated by comparison with results from other geophysical instruments, such as all-sky imagers and ionosondes. Finally, examples of the ionospheric behavior displayed by TEC maps during equatorial plasma bubble events and a geomagnetic storm are fully described and discussed.

Made available in DSpace on 2022-04-28T19:41:03Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-07-01 When plasma instabilities arise in the post-sunset time, the so-called ionospheric plasma bubbles may develop, causing amplitude fades and phase oscillations, decreasing the availability and quality of transionospheric communication systems. In the regions where these bubbles appear, deep signal fading may occasionally reach the lowest levels and with loss of signal received for a certain period. In this work, a characterization of deep power fades is performed supported by a dataset covering 5 months of high-rate monitoring data collected at four different locations in Brazil in distinct S4 scenarios for the three available frequencies. Two aspects of the deep fading events were evaluated, the deepest fading attained per minute and the fading events deeper than −15 dB. The inter-frequency analysis showed that as S4 increases, the number of −15 dB fading occurrences increases for all bands and stations. The average number of occurrences reached approximately 7 and 9 cases per minute, respectively, for Presidente Prudente and São José dos Campos. Statistical analysis for fading events deeper than −15 dB obtained probabilities of 1.61%, 2.87% and 3.97%, respectively, for L1, L2C and L5 at Presidente Prudente. Regarding the value of the deepest fading event per minute, the larger difference between L1, L2C and L5 exhibits values around −11 dB at Presidente Prudente. For this station, the average deepest fading values achieved in L2C and L5 bands were nearly twice that of the L1. The results regarding the probability of a deepest fading less than −20 dB per minute for L5 signal present a probability of 1.38% in Fortaleza, while for Presidente Prudente and São José dos Campos, these values were 5.62% and 3.34%. Instituto Tecnológico de Aeronáutica–ITA Instituto de Aeronáutica e Espaço–IAE, SP Ciência e Tecnologia de São Paulo Instituto Federal de Educação EMBRAER, SP Universidade Estadual Paulista Júlio de Mesquita Filho–UNESP Universidade Estadual Paulista Júlio de Mesquita Filho–UNESP

This study aims to evaluate the performance of phase-locked loop (PLL) under the presence of distinct scintillation patterns in the signal. Scintillation is very common in low-latitude regions due to the ionospheric dynamics. Under strong scintillation scenario the occurrence of deep fading events is usually registered and may cause severe degradation in the communication. The investigation conducted in this study uses the amplitude scintillation index S 4 , the decorrelation time τ 0 , and the bandwidth B n as main parameters. The study evaluates 54 different combinations of these parameters. The results indicate that in order to minimize the occurrence of cycle slips in the output phase of the PLL, the most appropriate tracking loop bandwidth B n depends on the values of both S 4 and τ 0 , which characterize the scintillation fading pattern. Simulations showed that as the S 4 index increases, the automatic increment in the PLL bandwidth may not be the best choice as the performance depends on the temporal characteristics of the scintillating signal. The analysis showed that, among the configurations tested, the equivalent noise bandwidth of 10 Hz achieved the best performance overall. The investigation also showed that choosing the wrong parameter regarding the PLL bandwidth may increase the probability of cycle slip by up to 10 times during fading events.

This work aims to evaluate the performance of Phase-Locked Loop under the presence of distinct scintillation patterns in the signal. Scintillation is very common in low-latitude regions due to the ionospheric dynamics. Under strong scintillation scenario the occurrence of deep fading events is usually registered and may cause severe degradation in the communication. The investigation conducted in this work uses the amplitude scintillation index S4, the decorrelation time \tau0, and the bandwidth Bn as main parameters. The study evaluates 54 different combinations of these parameters. The results indicate that in order to minimize the occurrence of cycle slips in the output phase of the Phase-Locked Loop, the most appropriate tracking loop bandwidth Bn depends on the both values of S4 and \tau0, which characterize the scintillation fading pattern. Simulations showed that as the S4 index increases, the automatic increment in the PLL bandwidth may not be the best choice as the performance depends on the temporal characteristics of the scintillating signal. The analysis showed that, among the configurations tested, the equivalent noise bandwidth of 10 Hz achieved the best performance overall. The investigation also showed that choosing the wrong parameter regarding the PLL bandwidth may increase the probability of cycle slip by up to 10 times during fading events.

The vertical component of the plasma drift, especially the evening-time pre-reversal drift, constitutes an important aspect of the nighttime electrodynamics of the equatorial ionosphere. Over the years, several studies using measurements and models have been performed to understand the characteristics of this process and its implications for the development of the instabilities leading to the plasma bubble formation and ionospheric scintillation. However, the Brazilian region presents some unique features that bring some difficulties for the vertical drift prognosis, which is required for the scintillation forecasting. These features are mainly related to the geomagnetic field lines topology that presents strong differences when compared to those of other equatorial longitudes. In this work, some of the difficulties for the pre-reversal vertical drift modeling and estimation are discussed; also, a dataset containing long-term observation (2001–2009) is compared with a widely used empirical model. The results show an intrinsic trend of underestimation by the model, which seems to be independent of latitude and seasonality thus suggesting an additional contribution arising from sources other than solely the geomagnetic topology. Also, the results indicate that the deviation can vary in the range of 0–40 m/s and the percentage error enhances with increasing values of pre-reversal vertical drift peak and reduces with increasing F10.7 values, thereby, indicating a clear possibility of meridional winds contribution which is not included in the empirical model used and may account for these differences.

The vertical coupling between the troposphere and the ionosphere presents some remarkable features. Under intense tropospheric convection, gravity waves may be generated, and once they reach the ionosphere, these waves may seed instabilities and spread F and equatorial plasma bubble events may take place. Additionally, there is a close association between severe tropospheric convection and lightning strikes. In this work an investigation covering an equinox period (September–October) during the deep solar minimum (2009) presents the relation between lightning strike activity and spread F (equatorial plasma bubble) detected over a low-latitude Brazilian region. The results show a considerable correlation between these two phenomena. The common element in the center of this conformity seems to be the gravity waves. Once gravity waves and lightning strikes share the same source (intense tropospheric convection) and the effects of such gravity waves in the ionosphere include the seeding of instabilities according to the gravity waves magnitude, the monitoring of the lightning strike activity seems to offer some information about the subsequent development of spread F over the equatorial region. Keywords. Ionosphere (equatorial ionosphere)

Equatorial plasma bubbles (EPBs), or large-scale plasma depleted regions, are one of the subjects of great interest in space weather research since such phenomena have been extensively reported to cause strong degrading effects on transionospheric radio propagation at low latitudes, especially over the Brazilian region, where satellite communication interruptions by the EPBs have been, frequently, registered. One of the most difficult tasks for this field of scientific research is the forecasting of such plasma-depleted structures. This forecasting capability would be of significant help for users of positioning/navigation systems operating in the low-latitude/equatorial region all over the world. Recently, some efforts have been made trying to assess and improve the capability of predicting the EPB events. The purpose of this paper is to present an alternative approach to EPB prediction by means of the use of mathematical numerical simulation associated with ionospheric vertical drift, obtained through Digisonde data, focusing on telling beforehand whether ionospheric plasma instability processes will evolve or not into EPB structures. Modulations in the ionospheric vertical motion induced by gravity waves prior to the prereversal enhancement occurrence were used as input in the numerical model. A comparison between the numerical results and the observed EPB phenomena through CCD all-sky image data reveals a considerable coherence and supports the hypothesis of a capability of short-term forecasting.

Made available in DSpace on 2019-10-06T15:40:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-12-01 The September 6–10, 2017 two-step magnetic storm was caused by an X9 solar flare followed by a CME. The SSC that occurred at 23:43 UT on day 06 when Sym-H reached about 50 nT, was due to a sudden increase in solar wind. The first step of the storm was caused by a Bz southward incursion on day 07. The magnetic index Kp reached 08, and the Sym-H magnetic index reached a minimum value of − 146 nT on day 08 at 01:08 UT, ending the main phase. On day 07, the solar wind intensified once again and the auroral index AE reached 2500 nT. During the recovery phase of this first storm, there was another Bz southward incursion on day 08 at 13:56 UT when Sym-H reached − 115 nT, and Kp reached a value of 08.33, marking the second step of the storm. In this work, the ionospheric irregularity over São Luís (02.5°S, 44.3°W, dip lat − 04.67°) was studied using data from the VHF, Digisonde and GPS receivers. Electron density data from the satellite SWARM-A were also analyzed for those orbits close to São Luís, and they presented large fluctuations during the storm night of 07/08. To analyze the latitudinal effects of the storm on the plasma irregularities, GPS data from 6 Novatel receivers were used. The vertical plasma drifts during daytime hours were determined using magnetometer data and during the evening using ionogram data. Compared to the ‘quiet’ days of September 2017, the VHF and GPS S4 amplitude scintillation indices increased substantially during the night of 07/08 when there was a strong intensification in the vertical plasma drift due to a prompt penetration under shielding magnetospheric electric field of eastward polarity. On the other hand, on the night of 08/09 the ionospheric scintillation was completely inhibited due to the disturbance dynamo electric field of westward polarity associated with the first and second storms. The irregularity zonal drifts measured by a VHF receiver around 24 UT (21 LT) were eastward on the nights of 05/06 and 06/07; however, during the night of 07/08, it reversed to westward. [Figure not available: see fulltext.]. Instituto Nacional de Pesquisas Espaciais, São José dos Campos Instituto Tecnológico de Aeronáutica, São José dos Campos Space Vehicles Directorate Air Force Research Laboratory Observatório Espacial do Instituto Nacional de Pesquisas Espaciais de São Luís Universidade Estadual Paulista UNESP Instituto de Aeronáutica e Espaço, São José dos Campos Universidade Estadual Paulista UNESP

Ionospheric scintillation events seriously affect the performance and availability of the GPS (Global Positioning System) based applications, especially regarding aeronautical navigation. Brazilian territory is localized in a region of intense ionospheric scintillation activity, which prevents, among other space-based applications, precision approach based on GPS service (LPV, Localizer Performance with Vertical Guidance, or LP, Localizer Performance without Vertical Guidance) which requires high integrity and availability of the GPS signal. Thereby, the knowledge of the ionospheric scintillation over the Brazilian territory is a key aspect to verify the feasibility of the use of GPS-based applications that require more precision and availability of the GPS signal. This work presents a statistical investigation of the duration of ionospheric scintillation events over Brazilian territory. The main goal is to propose models capable to estimate the chances that a user of GPS service has to be affected by ionospheric scintillation in a certain period of time. From the models it is possible to make conclusions on the effects of these irregularities and its possible threats.

This work presents an analysis of the climatology of the onset time of ionospheric scintillations at low latitude over the southern Brazilian territory near the peak of the equatorial ionization anomaly (EIA). Data from L1 frequency GPS receiver located in Cachoeira Paulista (22.4∘ S, 45.0∘ W; dip latitude 16.9∘ S), from September 1998 to November 2014, covering a period between solar cycles 23 and 24, were used in the present analysis of the scintillation onset time. The results show that the start time of the ionospheric scintillation follows a pattern, starting about 40 min earlier, in the months of November and December, when compared to January and February. The analyses presented here show that such temporal behavior seems to be associated with the ionospheric prereversal vertical drift (PRVD) magnitude and time. The influence of solar activity in the percentage of GPS links affected is also addressed together with the respective ionospheric prereversal vertical drift behavior. Based on this climatological study a set of empirical equations is proposed to be used for a GNSS alert about the scintillation prediction. The identification of this kind of pattern may support GNSS applications for aviation and oil extraction maritime stations positioning. Keywords. Ionosphere (ionospheric irregularities; modeling and forecasting) – radio science (space and satellite communication)

Made available in DSpace on 2018-12-11T17:24:17Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-10-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) The propagation paths of signals through equatorial ionospheric irregularities are analyzed by evaluating their effects on Global Navigation Satellite System (GNSS) positioning and availability. Based on observations during 32 days by a scintillation monitor at São José dos Campos, Brazil, it was noted that there is a dominance of enhanced scintillation events for Global Positioning System (GPS) ray paths aligned with the azimuth angle of 345° (geographic northwest). This azimuth corresponds to the magnetic meridian that has a large westward declination angle in the region (21.4ºW). Such results suggest that the enhanced scintillation events were associated with GPS signals that propagated through plasma bubbles aligned along the direction of the magnetic field. It will be shown that, under this alignment condition, the longer propagation path length through plasma bubbles can result in more severe scintillation cases and more losses of signal lock, as supported by proposed statistics of bit error probability and mean time between cycle slips. Additionally, large precise positioning errors are also related to these events, as demonstrated by precise point positioning experiments. Instituto de Aeronáutica e Espaço IAE/Instituto Tecnológico de Aeronáutica ITA Instituto Federal de Educação Ciência e Tecnologia de São Paulo Campus Presidente Epitácio (IFSP-PEP) Centro de Estudos em Telecomunicações Pontifícia Universidade Católica do Rio de Janeiro (CETUC/PUC-Rio), Rua Marquês de São Vicente 225 Instituto Tecnológico de Aeronáutica ITA/Instituto Nacional de Pesquisas Espaciais INPE Instituto Nacional de Pesquisas Espaciais INPE Instituto Tecnológico de Aeronáutica ITA Universidade Estadual Paulista Júlio de Mesquita Filho UNESP University of Bath Universidade Estadual Paulista Júlio de Mesquita Filho UNESP CNPq: 309013/2016-0 CNPq: 310802/2015-6 CNPq: 465648/2014-2 CAPES: 88881.134266/2016-01

Made available in DSpace on 2019-10-04T12:31:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-09-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) NSF European Commission (EC) The terrestrial ionosphere over low-latitude regions presents the unique phenomena of the equatorial ionization anomaly (characterized by global maximum in plasma concentration) and plasma-depleted regions known as equatorial plasma bubbles and associated smaller-scale plasma irregularities. Transionospheric radio signals such as those from Global Navigation Satellite Systems constellations, traveling across this ambient, may suffer severe scintillation in amplitude and phase due to these plasma structures. Presently, three civilian signals available for GPS users, at L1 (1575.42MHz), L2C (1227.60MHz), and L5 (1176.45MHz) are used to investigate the propagation effects due to these irregularities. The purpose of the present work is to evaluate statistically the distribution of severe fade events for each of these carrier frequencies based on the nonlinear ionospheric propagation effects as represented by the fading coefficients of - distribution. The results from the analyses of data sets recorded by stations at different geomagnetic latitude locations in Brazil show that regions closer to the equatorial ionization anomaly crest present higher probability of severe fade events. Additionally, the L5 signals, dedicated for safety-of-life applications, revealed more unfavorable results when compared to the L1 and L2C frequencies. The results further showed that for 0.8S(4)1.0 the probabilities of fades deeper than -10dB were between 8.0% and 6.5% depending on the station position. Considering the case of fades deeper than -20dB, the results reach values near 1%, which is quite concerning. These results show empirically the fading environment that users of the new civilian signals may experience in low-latitude region. Additionally, the fading coefficients may help in the comprehension of the distribution of amplitude scintillation and its relation with the frequency used, aiding in the future the development of signal processing algorithms capable to mitigate errors for navigation users. This work shows differences in the statistics of GPS signals at different frequencies. The results warn that new signals will be more affected by the ionosphere in regions of low latitudes. Inst Tecnol Aeronaut, Inst Aeronaut & Espaco, Sao Jose Dos Campos, Brazil Inst Fed Educ Ciencia & Tecnol Sao Paulo Campus P, Dept Informat, Presidente Epitacio, Brazil Univ Estadual Paulista, Dept Cartog, Presidente Epitacio, Brazil Pontificia Univ Catolica Rio de Janeiro, Ctr Estudos Telecomunicacoes, Rio De Janeiro, Brazil Inst Tecnol Aeronaut, Sao Jose Dos Campos, Brazil Inst Nacl Pesquisas Espaciais, Inst Tecnol Aeronaut, Sao Jose Dos Campos, Brazil Univ Texas Dallas, William B Hanson Ctr Space Sci, Richardson, TX 75083 USA EMBRAER, Sao Jose Dos Campos, Brazil Univ Estadual Paulista, Dept Cartog, Presidente Epitacio, Brazil CNPq: 465648/2014-2 FAPESP: 2017/50115-0 FAPESP: 06/04008-2 CNPq: 309013/2016-0 NSF: AST-1547048 European Commission (EC): FP7-GALILEO-2009-GSA European Commission (EC): FP7-GALILEO-2011-GSA-1a

The present work aims to evaluate the application of the κ-μ distribution as a representation of the fading effect caused by the phenomenon of scintillation on L-band transionospheric radio links. The ionospheric scintillation is a phenomenon defined as a rapid variation in the amplitude and phase of electromagnetic wave signals. This phenomenon starts in the first hours of the night, at latitudes near the geomagnetic equator. Scintillation occurs when radio signals cross ionospheric irregularities, also known as plasma bubbles. These plasma bubble structures are generated after the sunset due to instabilities in the F region of ionosphere. Distributions with non-single parameter usually present better results, however, this point requires further investigation by comparing different models. The goals of this study are: (1) the modeling of experimental data using the κ-μ distribution; (2) the κ-μ parameters characterization for empirical data and the evaluation of parameters estimation based in different approaches; (3) the comparison between the distribution proposed and other models adopted in the literature in order to verify the performance of two parameter models. The results of the analysis performed showed that the κ-μ distribution presents good fitting of the empirical scintillation data. These fitting results were calculated through the chi-square fit test under which the values reveal fair E[χ2] for κ-μ distribution in most of the cases. The evaluation of κ-μ parameters suggests that the distribution has a more conservative outcome than in the distributions traditionally used, but being a legitimate approximation due to its adjustable features in the tail region of the distribution. Typical pairs of κ-μ coefficients are presented for theoretical works. The comparison of κ-μ distribution to Rice, Nakagami-m and α-μ models showed that κ-μ is capable of describing more severe scintillation scenarios where the tail of the distribution is more raised in comparison to the other models.

[1] The collisional interchange instability (CII) is known to lead to the equatorial plasma bubble (EPB) development when the ionosphere is raised to higher altitudes by the prereversal electric field or vertical drift (PRVD). The PRVD presents considerable longitudinal variation (with scale size ∼15°) across the sunset terminator and this variation may act as a seeding perturbation as proposed by Woodman (1994) and Huang and Kelley (1996b). In the present work, we examine the efficiency of this longitudinal variation of the PRVD to act as a seeding for the CII and to give rise to EPB in the absence of any other kind of initial perturbation. To do so, we carried out the CII simulation at the equator in a plane perpendicular to the magnetic field. We consider a few simplified cases choosing a Gaussian-shaped longitudinal variation of PRVD with various possible minimum and peak values and finally consider a realistic case with a spatial-temporal configuration of PRVD obtained using SAMI2 model. Simulations with simplified cases show that the EPB develops only for minimum value greater than 20 m/s and amplitude (difference between the peak-to-minimum value) greater than 40 m/s. Simulation with a realistic case shows that the PRVD during high solar flux summer satisfies these threshold conditions and with the seeding scale ∼15°, the EPB of longitudinal size ≈2° is developed toward east of the PRVD peak.