Dissertations / Theses: 'Low latitude ionosphere'

1

Wohlwend, Christian Stephen. "Modeling the Electrodynamics of the Low-Latitude Ionosphere." DigitalCommons@USU, 2008. https://digitalcommons.usu.edu/etd/11.

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The electrodynamics of the Earth's low-latitude ionosphere is dependent on the ionospheric conductivity and the thermospheric neutral density, temperature, and winds present. This two-part study focused on the gravity wave seeding mechanism of equatorial plasma depletions in the ionosphere and the associated equatorial spread F, as well as the differences between a two-dimensional flux tube integrated electrodynamics model and a three-dimensional model for the same time period. The gravity wave seeding study was based on a parameterization of a gravity wave perturbation using a background empirical thermosphere and a physics-based ionosphere for the case of 12 UT on 26 September 2002. The electrodynamics study utilized a two-dimensional flux tube integrated model in center dipole coordinates, which is derived in this work. This case study examined the relative influence of the zonal wind, meridional wind, vertical wind, temperature, and density perturbations of the gravity wave. It further looked at the angle of the wave front to the field line flux tube, the most influential height of the perturbation, and the difference between planar and thunderstorm source gravity waves with cylindrical symmetry. The results indicate that, of the five perturbation components studied, the zonal wind is the most important mechanism to seed the Rayleigh-Taylor instability needed to develop plasma plumes. It also shows that the bottomside of the F-region is the most important region to perturb, but a substantial E-region influence is also seen. Furthermore, a wave front with a small angle from the field line is necessary, but the shape of the wave front is not critical in the gravity wave is well developed before nightfall. Preliminary results from the three-dimensional model indicate that the equipotential field line assumption of the two-dimensional model is not valid below 100 km and possibly higher. Future work with this model should attempt to examine more of the differences with the two-dimensional model in the electric fields and currents produced as well as with the plasma drifts that lead to plume development.

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McDonald, Sarah E. "Day to day and longitudinal variability of the nighttime low latitude terrestrial ionosphere." Fairfax, VA : George Mason University, 2007. http://hdl.handle.net/1920/2956.

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Thesis (Ph. D.)--George Mason University, 2007.
Title from PDF t.p. (viewed Jan. 21, 2008). Thesis director: Michael E. Summers, Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computational Sciences and Informatics. Vita: p. 204. Includes bibliographical references (p.193-203). Also available in print.

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Tracy, Brian David. "Lunar Tidal Effects in the Electrodynamics of the Low-Latitude Ionosphere." DigitalCommons@USU, 2013. https://digitalcommons.usu.edu/etd/1968.

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We used extensive measurements made by the Jicamarca Unattended Long-Term Investigations of the Ionosphere and Atmosphere (JULIA) and Incoherent Scatter Radar (ISR) systems at Jicamarca, Peru during geomagnetic quiet conditions to determine the climatologies of lunar tidal effects on equatorial vertical plasma drifts. We use, for the first time, the expectation maximization (EM) algorithm to derive the amplitudes and phases of the semimonthly and monthly lunar tidal perturbations. Our results indicate, as expected, lunar tidal effects can significantly modulate the equatorial plasma drifts. The local time and seasonal dependent phase progression has been studied in much more detail than previously and has shown to have significant variations from the average value. The semimonthly drift amplitudes are largest during December solstice and smallest during June solstice during the day, and almost season independent at night. The monthly lunar tidal amplitudes are season independent during the day, while nighttime monthly amplitudes are largest and smallest in December solstice and autumnal equinox, respectively. The monthly and semimonthly amplitudes decrease from early morning to afternoon and evening to morning with moderate to large increases near dusk and dawn. We also examined these perturbation drifts during periods of sudden stratospheric warmings (SSWs). Our results show, for the first time, the enhancements of the lunar semimonthly tidal effects associated with SSWs to occur at night, as well as during the day. Our results also indicate during SSWs, monthly tidal effects are not enhanced as strongly as the semimonthly effects.

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Shim, JA Soon. "Analysis of Total Electron Content (TEC) Variations in the Low- and Middle-Latitude Ionosphere." DigitalCommons@USU, 2009. https://digitalcommons.usu.edu/etd/403.

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Detailed study of the spatial correlations of day-to-day ionospheric TEC variations on a global scale was performed for four 30-day-long periods in 2004 (January, March/April, June/July, September/October) using observations from more than 1000 ground-based GPS receivers. In order to obtain the spatial correlations, initially, the day-to-day variability was calculated by first mapping the observed slant TEC values for each 5-minute GPS ground receiver-satellite pair to the vertical and then differencing it with its corresponding value from the previous day. This resulted in more than 150 million values of day-to-day change in TEC (delta TEC). Next, statistics were performed on the delta TEC values. The study indicates strong correlationsbetween geomagnetic conjugate points, and these correlations are larger at low latitudes than at middle latitudes. Typical correlation lengths, defined as the angular separation at which the correlation coefficient drops to 0.7, were found to be larger at middle latitudes than at low latitudes. The correlation lengths are larger during daytime than during nighttime. The results indicate that the spatial correlation is largely independent of season. These spatial correlations are important for understanding the physical mechanisms that cause ionospheric weather variability and are also relevant to data assimilation. In an effort to better understand the effects of neutral wind and electric field on the TEC variability, a physics-based numerical Ionosphere/Plasmasphere Model (IPM) was used. The model solves the transport equations for the six ions, O+, NO+, O2+, N2+, H+, and He+, on convecting flux tubes that realistically follow the geomagnetic field. Two of the inputs required by the IPM are the thermospheric neutral wind and the low-latitude electric field, which can be given by existing empirical model or externally specified by the user. To study the relative importance of the neutral wind and the electric field for the TEC variations, these two model inputs were externally modified and the resulting variations in TEC were compared. Neutral wind and electric field modifications were introduced at three different local times in order to investigate the effect of different start times of the imposed perturbations on TEC. This study focused on modeled low- and middlelatitude TEC variations in the afternoon and post-sunset at three different longitude sectors for medium solar activity and low geomagnetic activity. The largest changes in TEC were found predominantly in the equatorial anomaly, and a significant longitudinal dependence was observed. The results indicate that the perturbation effect on the TEC at 2100 LT varied nonlinearly with the elapsed time after the imposed neutral wind and electric field perturbations. An important outcome of this study is that daytime neutral wind and/or electric field modifications will lead to essentially identical TEC changes in the 2100 local time sector.

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Herne, David Edwin. "The Australian Mid-latitude Continental Ionosphere with Respect to Low-frequency Radio Astronomy." Thesis, Curtin University, 2016. http://hdl.handle.net/20.500.11937/48581.

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The ionosphere above the Murchison Radio Observatory (MRO) has been characterised over half of solar cycle 24 and its response to impinging low-frequency radio waves described. The outcomes of this thesis will contribute to an operational requirement of the Murchison WideField Array (MWA) radio telescope (calibration) and delivery of the project’s scientific goals (high fidelity imaging) and shows that the MRO site is an excellent location from which to conduct low-frequency radio astronomy.

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Davila, Ricardo Cruz. "A Study of Magnetic Activity Effects on the Thermospheric Winds in the Low Latitude Ionosphere." DigitalCommons@USU, 1994. https://digitalcommons.usu.edu/etd/6808.

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The purpose of this thesis is to examine the effects of magnetic activity on the low latitude F-region thermospheric winds. The F-region (120-1600 km) is a partially ionized medium where O+ and O are the major ion and neutral species, respectively. The thermospheric winds at these altitudes are driven primarily by pressure gradient forces resulting from the solar heating during the day and cooling at night. For this study, we use measured Fabry-Perot Interferometer (FPD winds at Arequipa (16.5°S, 71.5°W) and measured FPI and Incoherent Scatter Radar (ISR) winds at Arecibo (18.6°N, 66.8°W). Previous wind studies at Arequipa and Arecibo concentrated on the climatological wind patterns highlighting solar cycle effects and seasonal variations; however, these studies did not address the effects of magnetically disturbed conditions on the seasonal averaged winds. To properly investigate storm time effects on the neutral winds, we must first investigate solar cycle effects on the seasonal averages during magnetically quiet (Kp < 3) conditions. This study will include a detailed analysis of solar cycle effects on the seasonally averaged winds for Arequipa and Arecibo. In addition to the wind averages, we used cubic splines to fit the average wind profiles and to provide better comparisons with modeled results. We also performed a study on the airglow emission heights using both Jicamarca and Arecibo electron density profiles. This established the height which we will use to compare our experimental data with the model winds. To investigate magnetic activity effects on the FPI and ISR winds, we used three magnetic activity cases which cover all storm time scenarios. These magnetic activity cases are the extended quiet, short-term disturbed, and extended disturbed conditions. The first case, the extended quiet, is the condition where the previous and short term magnetic activity is quiet (12 hour Kp ≤ 3 and the Kp ≤ 3). The short-term disturbed case is defined for the condition where the previous magnetic activity is quiet (12 hour Kp ≤ 3) and then becomes disturbed (Kp ≤ 3). Last, we considered the case where previous and short-term magnetic activity are disturbed (12 hour Kp ≤ 3 and the Kp ≤ 3). Our last objective is to use our data to validate the predictions from the Thermosphere/Ionosphere General Circulation model (TIGCM93) and the Horizontal Wind Model (HWM93). This study should further our understanding of the physical processes which produce the low latitude quiet and disturbed winds. The TIGCM93 is a first principal model and the HWM93 is an empirical model based on ground-based and satellite measurements. The main advantage of using the TIGCM93 is the ease of studying the dynamics of ionospheric phenomena by simply changing various model inputs, while the HWM93 allows us a comparison between our experimental wind data sets with the established climatology of the winds over Arequipa and Arecibo.

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Khadka, Sovit M. "Multi-diagnostic Investigations of the Equatorial and Low-latitude Ionospheric Electrodynamics and Their Impacts on Space-based Technologies." Thesis, Boston College, 2018. http://hdl.handle.net/2345/bc-ir:108001.

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Thesis advisor: Prof. Michael J. Naughton
Thesis advisor: Dr. Cesar E. Valladares
The equatorial and low-latitude ionosphere of the Earth exhibits unique features on its structuring, coupling, and electrodynamics that offer the possibility to forecast the dynamics and fluctuations of ionospheric plasma densities at later times. The scientific understanding and forecasting of ionospheric plasma are necessary for several practical applications, such as for mitigating the adverse effects of space weather on communication, navigation, power grids, space mission, and for various scientific experiments and applications. The daytime equatorial electrojet (EEJ), equatorial ionization anomaly (EIA), as well as nighttime equatorial plasma bubble (EPB) and plasma blobs are the most prominent low-latitude ionospheric phenomena. This dissertation focuses on the multi-diagnostic study of the mechanism, properties, abnormalities, and interrelationships of these phenomena to provide significant contributions to space weather communities from the ground- and space-based measurements. A strong longitudinal, seasonal, day-to-day variability and dependency between EEJ, ExB vertical plasma drift, and total electron content (TEC) in the EIA distribution are seen in the equatorial and low-latitude region. In general, the EEJ strength is stronger in the west coast of South America than in its east coast. The variability of the EEJ in the dayside ionosphere significantly affects the ionospheric electron density variation, dynamics of the peak height of F2-layer, and TEC distributions as the EEJ influences the vertical transport mechanism of the ionospheric plasma. The eastward electric field (EEF) and the neutral wind play a decisive role in controlling the actual configuration of the EIA. The trans-equatorial neutral wind profile calculated using data from the Second-generation, Optimized, Fabry-Perot Doppler Imager (SOFDI) located near the geomagnetic equator and a physics-based numerical model, LLIONS (Low-Latitude IONospheric Sector) give new perspectives on the effects of daytime meridional neutral winds on the consequent evolution of the asymmetry of the equatorial TEC anomalies during the afternoon onwards. The spatial configurations including the strength, shape, amplitude and latitudinal extension of the EIA crests are affected by the EEF associated with the EEJ under undisturbed conditions, whereas the meridional neutral winds play a significant role in the development of their asymmetric structure in the low-latitude ionosphere. Additionally, the SWARM satellite constellation and the ground-based LISN (Low-Latitude Ionospheric Sensor Network) data allow us to resolve the space-time ambiguity of past single-satellite studies and detect the drastic changes that EPBs and plasma blobs undergo on a short time scale. The coordinated quantitative analysis of a plasma density observation shows evidence of the association of plasma blobs with EPBs via an appropriate geomagnetic flux tube. Plasma blobs were initially associated with the EPBs and remained at the equatorial latitude right above the EPBs height, but later were pushed away from geomagnetic equator towards EIA latitudes by the EPB/ depleted flux tubes that grew in volume. Further, there exists a strong correlation between the noontime equatorial electrojet and the GPS-derived TEC distributions during the afternoon time period, caused by vertical E × B drift via the fountain effect. Nevertheless, only a minor correlation likely exists between the peak EEJ and the net postsunset ionospheric scintillation index (S4) greater than 0.2. This study not only searches for a mutual relationship between the midday, afternoon and nighttime ionospheric phenomena but also aims at providing a possible route to improve our space weather forecasting capability by predicting nighttime ionospheric irregularities based on midday measurements at the equatorial and low latitudes
Thesis (PhD) — Boston College, 2018
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

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8

Smith, Rasler W. "Low latitude ionospheric effects on radiowave propagation." Thesis, Monterey, California. Naval Postgraduate School, 1998. http://hdl.handle.net/10945/8638.

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This dissertation provides experimental observations and analyses that associate low-latitude transionospheric signal scintillation with transequatorial VHF radio propagation and errors in transionospheric geopositioning. The experiment observed equatorial-region ionospheric total electron content (TEC) derived from Global Positioning System (GPS) signals using receivers on Oahu, Hawaii, Christmas Island, and Rarotonga, Cook Islands. The experiment simultaneously measured VHF transequatorial propagation of VHF television signals from Hawaii to Rarotonga Analysis shows that a moving second moment of vertical-equivalent TEC strongly correlates to each VHF transequatorial radio propagation event From experimental observation analysis, the author develops models for prediction of TEP and nine-space distribution of low-latitude transionospheric scintillation. The author also develops equations that show the potential errors in nine, frequency, and angle used in geopositioning solutions. These three parameters are potentially correctable using these techniques

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9

Mohd, Ali Aiffah. "GNSS in aviation : ionospheric threats at low latitudes." Thesis, University of Bath, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.761026.

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Radio signals propagating through the ionised upper atmosphere (the ionosphere) in low latitude regions of the world can experience amplitude scintillation. This could threaten safety-critical applications of satellite navigation such as aviation. The research presented here studied the effects of amplitude scintillation on a Septentrio PolaRxS geodetic receiver and a Garmin 480 aviation receiver by means of a Spirent GNSS constellation simulator. Different types of fade profiles showed that an abrupt drop in signal strength caused a loss of lock on the signal more often than a profile with a slow, gradual fade. A performance comparison of the two receivers demonstrated that the aviation receiver was more vulnerable than the geodetic receiver. An unexpected loss of lock at a specific fade duration and depth was seen with the Garmin receiver and was not explained. A single fade with a long fade duration was more likely to cause a loss of signal lock compared to rapid multiple fades. Scintillation on signals from low elevation satellites can significantly degrade the precision and integrity of the navigation solution in an aviation receiver; especially if the satellites are within the best geometrical set. RAIM was observed to be no longer available during the critical landing approach phase of the scenario, in the case when all satellites in view were affected by the scintillation-induced signal perturbations. A technique was also developed to simulate L5 scintillation based on real scintillation events of L1, in the absence of real captured data for L5. This was done to enable future investigations on aviation receiver performance when both L1 and L5 frequencies experience scintillation. Analysis indicated that L5 signal can be more vulnerable to the scintillation compared to the L1 signal, which may have important implications for aviation safety.

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Dubazane, Makhosonke Berthwell. "Modelling Ionospheric vertical drifts over the African low latitude region." Thesis, Rhodes University, 2018. http://hdl.handle.net/10962/63356.

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Low/equatorial latitudes vertical plasma drifts and electric fields govern the formation and changes of ionospheric density structures which affect space-based systems such as communications, navigation and positioning. Dynamical and electrodynamical processes play important roles in plasma distribution at different altitudes. Because of the high variability of E × B drift in low latitude regions, coupled with various processes that sometimes originate from high latitudes especially during geomagnetic storm conditions, it is challenging to develop accurate vertical drift models. This is despite the fact that there are very few instruments dedicated to provide electric field and hence E × B drift data in low/equatorial latitude regions. To this effect, there exists no ground-based instrument for direct measurements of E×B drift data in the African sector. This study presents the first time investigation aimed at modelling the long-term variability of low latitude vertical E × B drift over the African sector using a combination of Communication and Navigation Outage Forecasting Systems (C/NOFS) and ground-based magnetometer observations/measurements during 2008-2013. Because the approach is based on the estimation of equatorial electrojet from ground-based magnetometer observations, the developed models are only valid for local daytime. Three modelling techniques have been considered. The application of Empirical Orthogonal Functions and partial least squares has been performed on vertical E × B drift modelling for the first time. The artificial neural networks that have the advantage of learning underlying changes between a set of inputs and known output were also used in vertical E × B drift modelling. Due to lack of E×B drift data over the African sector, the developed models were validated using satellite data and the climatological Scherliess-Fejer model incorporated within the International Reference Ionosphere model. Maximum correlation coefficient of ∼ 0.8 was achieved when validating the developed models with C/NOFS E × B drift observations that were not used in any model development. For most of the time, the climatological model overestimates the local daytime vertical E × B drift velocities. The methods and approach presented in this study provide a background for constructing vertical E ×B drift databases in longitude sectors that do not have radar instrumentation. This will in turn make it possible to study day-to-day variability of vertical E×B drift and hopefully lead to the development of regional and global models that will incorporate local time information in different longitude sectors.

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Maruyama, Takashi. "Experimental and Theoretical study of Ionospheric Irregularities in the F Region at Low-Latitudes." Kyoto University, 1992. http://hdl.handle.net/2433/168763.

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本文データは平成22年度国立国会図書館の学位論文(博士)のデジタル化実施により作成された画像ファイルを基にpdf変換したものである
Kyoto University (京都大学)
0048
新制・論文博士
博士(工学)
乙第7875号
論工博第2585号
新制||工||878(附属図書館)
UT51-92-K375
(主査)教授木村磐根, 教授加藤進, 教授深尾昌一郎
学位規則第4条第2項該当

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PAIVA, JOSE ANTONIO GODINHO. "EFFECT OF THE IONOSPHERE OF LOW LATITUDES IN GPS - SBAS (GLOBAL SYSTEM POSITIONING - SPACE BASED AUGMENTATION SYSTEM)." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2004. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=5863@1.

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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
A ionosfera de baixas latitudes tem características que poderiam causar problemas à operação do GPS/SBAS. Entre elas se encontra a anomalia equatorial, cuja densidade eletrônica pode apresentar intensos gradientes horizontais (e, portanto, no índice de refração do meio). Estes gradientes podem ser intensos o suficiente para introduzir erros nas previsões resultantes do GPS/SBAS. Para avaliar este problema, foi desenvolvido um programa de simulação em computador que integra modelos para: (i) a previsão das posições dos satélites da constelação GPS; (ii) a evolução temporal e espacial da densidade eletrônica da ionosfera equatorial; e (iii) uma rede de estações de referência de posições fornecidas para analisar os efeitos da anomalia equatorial sobre os erros causados pela ionosfera nos sinais dos satélites GPS recebidos pelas estações. Em cada passo da simulação, diversos procedimentos são realizados. Estes procedimentos são repetidos um grande número de vezes e, ao final da simulação, estatísticas dos erros são apresentadas. Este programa de simulação em computador foi utilizado para analisar a influência do número de estações de referência, assim como de suas localizações, nos erros de posicionamento de aeronaves.
The low-latitude ionosphere has some features that could cause problems even to the joint GPS/SBAS operation. Among them, one finds the equatorial anomaly, whose electronic density - and thus its refractive index - can present intense horizontal gradients. These gradients can be intense enough to induce errors in the predictions by the GPS/SBAS. To analyze this problem, a computer simulation program has been developed. This program integrates models for: (i) forecasting the satellite orbital positions of the GPS constellation; (ii) the temporal and spatial evolution of the electronic density of the low-latitude ionosphere; and (iii) a given network of reference stations to analyze the effects of the equatorial anomaly on the GPS satellite signals received by the stations and users. In each step of the simulation, several procedures are performed. These procedures are repeated several times and, at the end of the simulation, error statistics are presented. This computer simulation program has been used to analyze the influence of the equatorial anomaly and of the number and layout of reference stations upon the errors in aircraft positions provided by the GPS/SBAS.

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Matamba, Tshimangadzo Merline. "Long-term analysis of ionospheric response during geomagnetic storms in mid, low and equatorial latitudes." Thesis, Rhodes University, 2018. http://hdl.handle.net/10962/63991.

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Understanding changes in the ionosphere is important for High Frequency (HF) communications and navigation systems. Ionospheric storms are the disturbances in the Earth’s upper atmosphere due to solar activities such as Coronal Mass Ejections (CMEs), Corotating interaction Regions (CIRs) and solar flares. This thesis reports for the first time on an investigation of ionospheric response to great geomagnetic storms (Disturbance storm time, Dst ≤ −350 nT) that occurred during solar cycle 23. The storm periods analysed were 29 March - 02 April 2001, 27 - 31 October 2003, 18 - 23 November 2003 and 06 - 11 November 2004. Global Navigation Satellite System (GNSS), Total Electron Content (TEC) and ionosonde critical frequency of F2 layer (foF2) data over northern hemisphere (European sector) and southern hemisphere (African sector) mid-latitudes were used to study the ionospheric responses within 15E° - 40°E longitude and ±31°- ±46° geomagnetic latitude. Mid-latitude regions within the same longitude sector in both hemispheres were selected in order to assess the contribution of the low latitude changes especially the expansion of Equatorial Ionization Anomaly (EIA) also known as the dayside ionospheric super-fountain effect during these storms. In all storm periods, both negative and positive ionospheric responses were observed in both hemispheres. Negative ionospheric responses were mainly due to changes in neutral composition, while the expansion of the EIA led to pronounced positive ionospheric storm effect at mid-latitudes for some storm periods. In other cases (e.g 29 October 2003), Prompt Penetration Electric Fields (PPEF), EIA expansion and large scale Traveling Ionospheric Disturbances (TIDs) were found to be present during the positive storm effect at mid-latitudes in both hemispheres. An increase in TEC on the 28 October 2003 was because of the large solar flare with previously determined intensity of X45± 5. A further report on statistical analysis of ionospheric storm effects due to Corotating Interaction Region (CIR)- and Coronal Mass Ejection (CME)-driven storms was performed. The storm periods analyzed occurred during the period 2001 - 2015 which covers part of solar cycles 23 and 24. Dst≤ -30 nT and Kp≥ 3 indices were used to identify the storm periods considered. Ionospheric TEC derived from IGS stations that lie within 30°E - 40°E geographic longitude in mid, low and equatorial latitude over the African sector were used. The statistical analysis of ionospheric storm effects were compared over mid, low and equatorial latitudes in the African sector for the first time. Positive ionospheric storm effects were more prevalent during CME-driven and CIR-driven over all stations considered in this study. Negative ionospheric storm effects occurred only during CME-driven storms over mid-latitude stations and were more prevalent in summer. The other interesting finding is that for the stations considered over mid-, low, and equatorial latitudes, negative-positive ionospheric responses were only observed over low and equatorial latitudes. A significant number of cases where the electron density changes remained within the background variability during storm conditions were observed over the low latitude stations compared to other latitude regions.

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Aoyama, Tadashi. "A study on the origin of small-scale field-aligned currents as observed in topside ionosphere at middle and low latitudes." 京都大学 (Kyoto University), 2017. http://hdl.handle.net/2433/225408.

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Tsunomura, Satoru. "On the Contribution of Global Scale Polar-originating Ionospheric Current Systems to Geomagnetic Disturbances in Middle and Low Latitudes." 京都大学 (Kyoto University), 1999. http://hdl.handle.net/2433/182006.

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Nishioka, Michi. "Study on meso-scale ionospheric structures at low-and mid-latitudes using data of GPS receiver networks and satellites." 京都大学 (Kyoto University), 2009. http://hdl.handle.net/2433/124427.

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17

Riley, Peter. "Electrodynamics of the low-latitude ionosphere." Thesis, 1994. http://hdl.handle.net/1911/16768.

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We have undertaken a study of the low and mid latitude ionospheric electric field pattern, during both magnetospherically quiet and active periods. Our analysis can be conveniently split into two parts. i.In an effort to study the penetration of magnetospheric electric fields to low latitudes, we have compared Jicamarca F-region vertical drifts for 10 radar-observation periods with the auroral boundary index (ABI). The ABI is the latitude of the equatorward edge of the diffuse aurora at local midnight, as estimated from precipitating-electron fluxes measured from DMSP spacecraft. The periods occurred in the interval January 1984 to June 1991 inclusive and each lasted between 2 and 5 days. We focus on periods that occurred in September 1986, March 1990, and June 1991. In the post-midnight sector, where we expect the penetration to be strongest, we found many examples of correlation; specifically, associated with an ionospheric updraft (implying an eastward electric field) is a strong poleward motion of the auroral boundary. However, we also found a significant number of cases where there was little or no correlation. We conclude that there is only mediocre agreement between the observed Sudden Postmidnight Ionospheric Events (SPIEs) and the ABI. These SPIEs have also been compared with other magnetospheric parameters, namely $D\sb{\rm st}$ IMF $B\sb{z}$ and the polar cap potential. $D\sb{\rm st}$ showed significantly better correlation with the SPIEs. We summarize the proposed models for SPIEs and compare their predictions with the data, concluding that no single model can account for all events. While it is clear that some of these SPIEs can be explained in terms of direct penetration of magnetospheric electric fields, we suggest that the remainder may be due to magnetospherically-generated neutral wind effects. ii. We have constructed a model of the low- and mid-latitude potential distribution, applicable for both quiet and active times. We use the Mass-Spectrometer-Incoherent-Scatter (MSIS) model to input the number densities and temperature of the neutral species, and the International reference Ionosphere (IRI) model to input the electron/ion densities and temperatures. As our wind input we use the Horizontal Wind Model (HWM). We find that our model can reproduce the all of the main features of the low latitude ionosphere during quiet times, and supports some of our ideas about magnetospheric penetration during active periods. We use the model to probe the dependency of the low latitude penetration pattern on solar conditions and season and found that the inferred equatorial drifts are relatively insensitive to either. Thus we conclude that ionospheric pre-conditioning is unlikely to play a significant role. On the other hand, the low latitude penetration pattern is strongly dependent on the assumed poleward boundary.

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18

Yu-TingChou and 周育霆. "Study of Medium Scale Traveling Ionospheric Disturbance in Low-Latitude Ionosphere." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/9g2vre.

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Lin, Chien-hung, and 林建宏. "Low-Latitude Ionosphere Variations during Magnetic Disturbances." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/59553923513793522097.

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博士
國立中央大學
太空科學研究所
93
The low latitude ionosphere is unique in that the magnetic field is nearly horizontal, so that zonal electric fields, produced by the neutral wind dynamo during quiet geomagnetic times, can transport the plasma vertically through the E×B drift. This quiet-time vertical drift is upward during the daytime, causing plasma to drift to higher altitudes, from where it diffuses down along magnetic fields to higher latitudes creating two plasma crests on both sides of the magnetic equator. This feature is called the equatorial ionization anomaly (EIA), and the effect of transporting the plasma from the magnetic equator to higher latitudes is described as the fountain effect [Duncan 1960; Wright 1962; Hanson and Moffett 1966; Anderson 1973]. The plasma density and the peak location of the EIA can be modified by changes of: (1) the transport parallel to magnetic field lines through disturbance neutral winds and diffusion; (2) the loss process due to storm produced composition perturbations; and (3) the transport perpendicular to magnetic field lines due zonal electric field perturbations. During the magnetically quiet time, the electron density and the location of EIA peaks in both hemispheres show prominent seasonal variations. They are generally characterized by (1) in solstice, only the EIA peak in the winter hemisphere remains and a comparatively weak EIA density structure appears in the summer hemisphere, (2) in equinox, two EIA peaks are manifest and the overall electron density is larger than in solstice, (3) the offset of the magnetic equator and the geographic equator also has effects in production of the EIA asymmetry. During magnetic storms, magnetospheric energy and momentum are deposited in the ionosphere/thermosphere through auroral particle precipitation and ionospheric plasma convection driven by electric fields mapped from the magnetosphere. Intense auroral particle precipitation heats the thermosphere, ionizes the neutral gas, and increases the conductivity of the ionosphere. The increased conductivity combined with the magnetospheric electric field produces Joule heating in the ionosphere/thermosphere, which is the major energy source during storms. Heating of the thermosphere drives equatorward wind surges and causes an upwelling at high latitudes which carries heavier neutrals upward and increases the mean molecular mass. In addition to the thermospheric responses, the ionospheric electric field disturbances are observed at middle and low latitudes on different time scales. They result from both prompt penetration of time-varying magnetospheric fields from high latitudes to low latitudes and longer time lasting disturbance wind dynamo effects. In this study, the GPS derived total electron content (TEC), drift measurements from the ROCSAT-I at 600 km, and far ultraviolet airglow measured by the Global Ultraviolet Imager (GUVI) carried aboard the NASA TIMED satellite are utilized for observing the disturbance of the low latitude ionosphere during the magnetic storms. Observations from GPS-TEC often show that the equatorial ionization anomaly (EIA) expanded to much higher latitude with a great enhancement in the density during the early stage of the magnetic storm compared with quiet time. Following the expansion of the EIA, suppression of the EIA is often observed several hours after the storm onset. The derived ExB drifts measured from the Ionospheric Plasma and Electrodynamics Instrument (IPEI) onboard the ROCSAT-I show strong upward/poleward E×B drifts during the EIA expansions and downward/equatorward E×B drifts during the suppression. The [O]/[N2] inferred from the ratio of the 135.6 nm and LBH emissions from the GUVI provides information of storm-time composition perturbations which often result in negative ionospheric effect, i.e. reduced of the plasma density due to the magnetic storm. Theoretical models, the Sheffield University Plasmasphere Ionosphere Model (SUPIM) and the NCAR Thermosphere-Ionosphere Electrodynamic General Circulation Model (TIEGCM), are used to examine the relative importance of the ionospheric drivers in changing the EIA morphology during both magnetically quiet and disturbed periods. Model results show that the summer to winter meridional neutral winds produce the trans-equatorial transport of the plasma, resulting in seasonal asymmetry of the EIA peaks during the magnetically quiet period. Poleward expansion of the EIA peaks and strong increased EIA peak densities observed by the GPS TEC during the early stage of the magnetic storm are simulated and examined by the model. Simulation results show that the storm-produced equatorward meridional neutral wind plays a role in maintaining the ionospheric layer at higher altitude, where the recombination loss is smaller and the plasma is able to accumulate. Combing the upward/poleward E×B drifts with the equatorward neutral wind, poleward expansion of EIA peaks and very high EIA peak densities are simulated by the model during the early stage of the storm. Additionally, new features in the topside ionosphere, such as storm-time electron density hole and density arch are predicted by the model simulations.

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Fang, Tzu-Wei, and 方慈瑋. "Model Simulation of the Low-latitude Ionosphere." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/4dmyep.

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博士
國立中央大學
太空科學研究所
97
The Earth’s upper atmosphere, comprised of the ionosphere and thermosphere, is where neutral and charged particles interact to cause complicated physical processes. It is partly because of the complexity that the region is still actively studied by scientists. The densities of charged particles show a significant variation with altitude, latitude, longitude, universal time, season, solar cycle, and magnetic activity. The variation results from changes in solar radiation and from the coupling and feedback mechanisms between the ionosphere-thermosphere system and the mesosphere and magnetosphere. In the low latitude ionosphere, the nearly horizontal geomagnetic field lines combine with the neutral wind and electric field to create some unique phenomena. This study focuses on the daytime and post-sunset low latitude ionosphere under geomagnetic quiet conditions. Several important phenomena associated with the electrodynamic processes are investigated which include (1) the daytime equatorial electrojet (EEJ); (2) the daytime ionospheric vertical drifts; and (3) the post-sunset pre-reversal enhancement (PRE) of the vertical drifts. Two different numerical models, the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) and the Global Ionosphere and Plasmasphere Model (GIP), are utilized to examine and understand the physical processes of these phenomena. In Chapter 1, fundamental theories of ionosphere and thermosphere, their interaction and associated phenomena in the low latitude region are introduced. Structures and electrodynamic calculations of the TIE-GCM and the GIP model are described in Chapter 2. Between 100 and 120 km height at the Earth''s magnetic equator, the equatorial electrojet flows as an enhanced eastward current in the daytime E region ionosphere which induces a magnetic perturbation on the ground that is considerably larger than the perturbations at neighboring latitudes. Calculating the difference between the horizontal components of the magnetic perturbation (H) at magnetometers near the equator and about 6-9 degrees away from the equator, △H, provides us with information about the strength of the EEJ. The TIE-GCM is capable of simulating the EEJ current and its magnetic perturbation on the ground. In Chapter 3, the simulated diurnal, seasonal, and solar activity variations of △H in the Peruvian (76°W) and Philippine (121°E) sectors, and the relation of △H to the ionospheric vertical drift velocity, are presented. Results show the diurnal, seasonal and solar activity variations are captured well by the model. Agreement between simulated and observed magnitudes of △H and its linear relationship to the vertical drift is improved by modifying the original daytime E region photoionization in the TIE-GCM in order to better simulate observed E region electron densities. The model results suggest that besides the electric-field-driven EEJ, wind-driven currents can also be an important factor in changing the magnitude of △H. Therefore, we use the TIE-GCM to further examine the influence of the neutral wind. In Chapter 4, we run the model at the March equinox under moderate solar activity (F10.7 = 140 units) and modify the neutral wind and high latitude electrical potential to obtain the latitudinal distributions of H, diurnal variations of △H and daytime vertical drift in the Peruvian (75°W) longitude sector. The relationship between △H and vertical drift is also simulated and analyzed, which helps us to understand the importance of both the EEJ and the off-equatorial wind-driven currents in altering the relation. Simulations show that a height-varying wind velocity in the low latitude region is capable of modifying the ground magnetic perturbation a few degrees away from the equator, while affecting the equatorial perturbation only little. Only by combining the effects of both the EEJ and the off-equatorial wind-driven currents can the magnitude of △H and its relation with the vertical drift be accurately estimated. The pre-reversal enhancement is one of the most important phenomena controlling the nighttime ionosphere and the generation of equatorial spread F (ESF), but its causal mechanism is still not fully understood. The TIE-GCM is capable of simulating ionospheric phenomena and the pre-reversal enhancement. In Chapter 5, the model is run under moderate solar activity (F10.7=150) and geomagnetic quiet conditions to monitor the variation of ionospheric parameters when the maximum upward drift of the PRE is occurring in the Peruvian longitude (75°W). Since it is a three-dimensional model, it provides us electric potential, electric field changes, conductivity variations, ion drifts, and neutral wind at the Peruvian longitude and also at longitudes to the east (post-PRE) and west (pre-PRE). From the results, the spatial variations of the ionosphere surrounding the PRE can be illustrated and effects due to various physical parameters are evaluated. Specifically, the effects of neutral winds and conductivities separately in the ionospheric E and F regions on the PRE are examined. Results show that the F region wind and sudden decrease of E region conductivities are essential for creating the PRE. The upward propagating thermospheric tides also participate in the formation of the PRE. Through calculating the current divergence, result demonstrates that the contribution of the EEJ to the F-region upward current and the vertical drift is negligible. With the TIE-GCM simulation, we are able to diagnose the complex electrodynamic process to examine the existing theories and to further understand the morphology of nighttime low-latitude ionosphere. In Chapter 6, we investigate the annual and local time variations of the wave-4 structures in the plasma density and vertical drift in the low-latitude F region by analyzing the measurements from the first Republic of China SATellite (ROCSAT-1) and conducting simulations with the GIP model. In order to understand how the vertical drifts relate to the longitudinal structure of the topside ionosphere, we apply the equatorial vertical drifts observed from ROCSAT-1 to drive the GIP model. The model well reproduces the longitudinal structure in electron density, and the magnitudes of electron density are comparable with ROCSAT-1 measurement at 600 km. The annual and local time variations of the wave-4 component in the GIP model density also show good agreement with the vertical drift and plasma density observed by the ROCSAT-1. Finally, Chapter 7 summarizes the main conclusions of this work and identifies future research directions.

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21

Rajesh, PK, and 查傑希. "Airglow Investigations of the Low-Latitude Ionosphere." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/04728191182340097786.

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博士
國立中央大學
太空科學研究所
95
This dissertation focuses on the study of ionospheric irregularities using selected airglow emissions whose intensity is related to plasma density. All sky imaging technique is used to observe the large scale ionospheric structures, known as plasma depletions, which are often associated with Equatorial Spread-F (ESF) phenomena. The experiments conducted from Kavalur (12.5N, 78.8E; 4.6N geomagnetic), India, and Mt. Lulin (23.47N, 120.87E; 13.45N Geomagnetic), Taiwan are used in this work. The 630.0, 557.7, and 777.4 nm emissions are used for recording the images. The different emissions reveal different characteristics of depletions. The ionosonde data is used to monitor the base height of the F-layer as well as the presence or absence of Spread-F at the time of the appearance of depletions in all sky images. Several new features of plasma depletions are revealed in this study such as the new type of ‘joined’ pair of depletions where the northern (and later the southern) ends join together, producing an inverted ‘V’ shape bifurcation, which later merge together, sequences of depletions developing in the field of view of the all sky imager in the post-midnight period, depletions in 630.0 nm images entering the field of view (FOV) as dark patches from the north end in the post-midnight period, and the frequent observations of plasma depletions in 557.7 nm images. The ionospheric conditions resulting in the joining and merging of depletions, and the post-midnight development are discussed. The field aligned mapping of irregularities together with the altitude dependence of the 630.0 nm emission is proposed to be responsible for the 630.0 nm depletions to appear as dark patches at the northern ends of the FOV. A simulation is carried out to understand the important physical parameters that influence the emissions of the 557.7 nm in various local times and solar activities, and result in the appearance of depletions in the integrated images. An all sky imager has been designed and developed at the Institute of Space Science, National Central University, during the course of this dissertation. The imager was operated from Mt. Lulin, during the new moon periods since August 2006 to study the large scale ionospheric structures. The optical signatures of traveling ionospheric disturbances (TID’s) detected during this period are described. These observations are also used to understand the nocturnal variation of airglow intensity over Taiwan in different wavelengths.

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22

湯宏武. "The Low-latitude Ionosphere Response during Magnetic Storm." Thesis, 1998. http://ndltd.ncl.edu.tw/handle/75910898010635434166.

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碩士
國立中央大學
太空科學研究所
86
Low-latitude Ionosphere Tomography Network (LITN) was built in 1994. Till now, most of the research are related to theoretical study ; only a few are related to observational study. Therefore, this article try to study the variations of low-latitude ionosphere during the storm time by LITN. We also use the data of Kp and Dst index to compare with vertical total electron content(VTEC) of LITN .For the purpose of checking low-latitude ionosphere theories of VTEC as a function of latitude and time, the observational data of the Lunping station at Chungli are used . From the study , we have a better understanding of the variations of the low-latitude ionosphere layer during storm time.

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23

Jia-TingLin and 林佳廷. "Impact of stratospheric sudden warmings on the low-latitude ionosphere." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/23389523589550322270.

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碩士
國立成功大學
太空天文與電漿科學研究所
100
In this thesis, modifications of the ionospheric tidal signatures during the 2008-2010 stratospheric sudden warming (SSW) events are studied by applying atmospheric tidal analysis to ionospheric electron densities observed using radio occultation soundings of FORMOSAT-3/COSMIC. The tidal analysis indicates that the zonal mean and major migrating tidal components (DW1, SW2 and TW3) decrease around the time of the SSW, with 1.5-4 hour time shifts in the daily time of maximum. The typical ionospheric SSW signature: a semi-diurnal variation of the ionospheric electron density, featuring an earlier commencement and subsidence of EIA, can be reproduced by differencing the migrating tides before and during the SSW period. Our results also indicate that the migrating tides represent ~80% of the ionospheric tidal components at specific longitudes, suggesting that modifications of the migrating tides may be the major driver for producing ionospheric changes observed during SSW events, accounting for greater variability than the nonmigrating tides that have been the focus of previous studies.

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24

Tsai, Hong Li, and 蔡宏利. "An Investigation of the Low Latitude Ionosphere Using the Tomographic Technique." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/13993941080271903829.

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25

Liu, Chung-Cheng, and 劉仲政. "Low- and Middle- Latitude Ionosphere F-region using DE-2 Data." Thesis, 1996. http://ndltd.ncl.edu.tw/handle/73255834653106670478.

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碩士
國立中央大學
太空科學研究所
84
The Retarding Potential Analyzer (RPA) and Ion Drift Meter (IDM) onboard DE-2 satellite have provided the measurements of ion density and ion drift velocity at F-region ionosphere altitudes. Using the IDM data,we examine the average characteristics of the zonal and vertical plasma flows in the low-latitude region (< 50°MLAT) under the quiet geomagnetic conditions (Kp < 3). The data reveal that the zonal ion drifts are generally easrward at night, and westward at day in both hemispheres. The night time ion drift speeds are much larger than those of the day time. However, asymmetry in vertical drift patterns exist between the two hemispheres. Further,inspections on field-aligned components of the drift patterns,we found that the hemispheric asymmetry in vertical drift can be explained in terms of the interhemispheric transport. Analyses of the RPA ion density measurements are also carried out to study the latitudinal and local time variations of the ion density distribution. Of particular interesting observation is that the equatorial anomaly density structures are most obviously seen in the evening local time sector. The density peaks are located at ±(13 °- 21°) magnetic latitudes , consistent with the peaked density locations as indicated in the IRI density profiles. Finally, the advantage of using high resolution IDM on 35° inclination ROCSAT-1 satellite to study the low-latitude ionosphere dynamics will be discussed.

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26

Wang, Cherng Jiow, and 王承就. "An Investigation of the low Latitude Ionosphere Reconstructed using the Tomographic data." Thesis, 1996. http://ndltd.ncl.edu.tw/handle/96511250418322500783.

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27

Yu, Da-Chi, and 余大琪. "The Response of the Low-Latitude Ionosphere during the October 24, 1995 Solar Eclipse." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/11749422803611010293.

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碩士
國立中山大學
電機工程研究所
85
A solar eclipse offers a rare opportunity to observe how the sun affects the ionospheric structures. The main purpose of this paper is to study the ionospheric response to a solar eclipse that occurred in Asia on October 24, 1995. Although its path of totality passed over southern Asia, the associated region of partial eclipse covered almost all of Asia. We have collected ionosonde data from six stations in the region. Its purpose is to investigate how the ionosphere responded to this eclipse, especially because the region included the equatorial anomaly region. After processing these data, we find that geomagnetic field is a dominant force in causing the depression in Nmax following a solar eclipse in the equatorial anomaly region. The deviations in Nmax are plotted as contours using time after the first contact as the horizontal axis and geomagnetic latitude as the vertical axis. The results reveal three effects: (1) For the region with geomagnetic latitudes higher than 20 degrees the Nmax increa-sed slightly immediately following the first contact for about 30 minutes. (2) In response to the eclipse, the largest depression occurring roughly 11/2 hours after the maximum obscuration was observed at approximately 14 degrees geomagnetic latitude even though the percent obscuration of the sun at latitudes lower than 14 degrees was larger. (3) Around 6 hours after the maximum phase another secondary depression in Nmax was observed for geomagnetic latitudes lower than 14 degrees. Here we have to emphasize that these effects must cover at least two of these six stations. Physical mechanisms that may be responsible for causing these effects are proposed and examined in conclusion.

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28

Tu, Yuh-Min, and 涂裕民. "A Study of GPS Receiver Differential Code Biases and Low Latitude Area Ionosphere Model." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/22957732809891867500.

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博士
國立中興大學
土木工程學系所
101
The development of the Global Navigation Satellite Systems (GNSS), especially the widely established GPS ground stations, GPS has been widely used to investigate the ionosphere through the estimation of the Total Electron Content (TEC) and its distributions in space. One of the important factors affecting the ionosphere TEC estimation accuracy is the hardware differential code biases (DCB) inherited in both GPS satellites and receivers. This research investigates various factors affecting GPS receiver instrumental biases estimation accuracy. Through a number of designed tests, we concluded that the most important factor is the ionosphere model accuracy. Some of large daily bias variations of receiver DCB detected by other studies, such as receivers in low latitude regions, are not due to the DCB changes, but the estimation errors. The DCB estimation values can vary significantly for different ionospheric models and different size of networks. For example, the receiver DCB values obtained from the global and the station- specific models exhibit difference from -2.5 to 14.3 TECU for different stations. Different data processing methods also contribute DCB estimation errors. The results from smoothing and non-smoothing GPS observation show that the difference of DCB reaches up to 6.8 TECU for some stations. Though some ionosphere models have been established in the past research, they are most global models. For example, the global ionosphere maps are provided by IGS. But most of the IGS GPS ground network stations situated in the mid-latitude area. In the second part of this research, by selecting a proper mathematical function for the 2-D ionosphere model, we are trying to establish a 2-dimensional model fit for the some low latitude area and the model should be able to be used to predict VTEC for GPS navigation. Establishing a precise ionosphere model is one of the critical steps for satellite navigation and also for ionospheric research. This method could be provided as an exaample to improve local ionospheric delay.

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29

Chin, Jo-lan, and 金若蘭. "Discuss the responses in the mid- and low- latitude ionosphere by ROCSAT-1 during storm events." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/54151225302968614447.

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碩士
國立中央大學
太空科學研究所
98
The ROCSAT-1 satellite was launched into a circular orbit at about 600km altitude with an inclination of 35 degrees, with which the globale responses of four magnetic storm events occurred on 2000/07/15, 2001/03/31, 2003/1029, 2003/11/20, respectively, were analyzed in the mid- and low-latitude ionosphere. The data of 50 days before storm onset were taken to construct the background conditions. According to the satellite pathes of the storm events, a division for dayside and nightside can be roughly defined. The daytime penetration electric fields, eastward resulted from southward turning and westward from northward turning of the IMF Bz, enable the mid- and low- latitude ionosphere to change. The variation of ion drifts and charge particle densities with the local time and the magnetic latitudes observed by the IPEA on board ROCSAT-1 may be used to build up the distribution of the E×B drifts and plasma densities for the quiet time and storm time conditions. In the observations, we find that the increases of the daytime ionospheric plasma densities may result from the eastward penetration electric field in the storm time; the densities decrease in the nighttime. Moreover, neutral wind blowing from the high latitudes may also cause the charged particle density of ionosphere in mid- and low-latitude to rise.

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30

Chen, Jeng-Hung, and 陳政宏. "The Relationship Between the SENPOT Potential on ROCSAT-1 and the Environmental Conditions at Low Latitude Ionosphere." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/65s34g.

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碩士
國立中央大學
太空科學研究所
94
Abstract The ionospheric plasma and electrodynamics instrument (IPEI) onboard the ROCSAT-1 satellite consists of four sensors and a sensors potential circuit (SENPOT) on its aperture plane. The SENPOT is designed for minimizing the potential between the plasma and the sensor’s aperture plane to prevent from the surface potential effect on the ionized particles. The modulated potential by SENPOT varying with time is called the SENPOT potential. In this thesis, we use the SENPOT potential data to investigate the relationship between the satellite surface charging and the environmental conditions at low latitude ionosphere. The surface current of the low-orbit and low-inclination spacecraft is dominated by ambient plasma current. Theoretically, SENPOT potential is strongly related to electron temperature. Unfortunately, ROCSAT-1 IPEI did not provide the in-situ measurements of electron temperature. However, by analyzing the other IPEI data we found that the SENPOT potential is strongly varying with the ion density. During the storm days, correlated violent changes were found among the ion temperature, ion drift velocity and the SENPOT potential. In the days when there were large solar flares, the ion density enhancements were frequently observed after the sunset hours. Similar changes with local time were found in the SENPOT potentials, with the peak SENPOT potential at about 1.8 hours after the local sunset. In addition, the fading of SENPOT potential is slower than that of the normal days.

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31

Anand, Raj R. "Kalman Filter Estimation Of Ionospheric TEC And Differential Instrumental Biases Over Low Latitude Using Dual Frequency GPS Observations." Thesis, 2006. http://hdl.handle.net/2005/426.

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The low latitude tropical ionosphere has been investigated by various researchers using Global Positioning System (GPS). Presently for many civil aviation applications, the ionospheric modeling of the tropical region has gained importance, in particular for flight safety. Since ionosphere is dispersive in nature, dual frequency (L1 = 1575.42 MHz and L2 = 1227.60 MHz) GPS observations can be used to obtain Ionospheric Total Electron Content (TEC). Since TEC varies with local time and geomagnetic latitude, an Ionospheric Modeling Technique using spatial linear approximation of vertical TEC over receiver station has been implemented following Sardon et al. The effects of all the systematic errors due to the satellite plus the receiver (SPR) instrumental biases can reach upto several nanoseconds. (1 TEC is 1016 electrons/m2, 1 ns = 2.86 TEC and 1 TEC = 0.16 m). Hence, to have an accurate estimation of ionospheric TEC, the instrumental biases must also be estimated. This thesis describes a heuristic adaptive Kalman Filtering scheme developed to estimate the TEC, the constants in the linearisation scheme, as well as the above total instrumental biases. The Kalman filter implementation is basically an optimization problem of minimizing the Cost Function J based on the difference between the model output and the measurement, called as the ‘innovation’, scaled by its covariance. In order to obtain the best possible results using the Kalman Filter approach, it is essential to provide appropriate values for the initial state, process and measurement noise covariances (P0, Q and R) respectively, which in general may not be known. Usually manual tuning of the filter parameter is carried out without using the above cost function J! The filter estimates can be highly sensitive to the above chosen statistics and thus these will have to be estimated carefully. Hence, we have utilized the Adaptive Kalman Filtering procedure of Myers and Tapley extended by Gemson and Ananthasayanam. The minimization is carried out by simultaneously estimating the above statistics and the unknown parameters, which include the TEC and the instrumental bias. In addition, A Constant Gain Kalman Filter approach using Genetic Algorithm (GA) has also been developed for the above requirement. It is observed that the steady state gains in KF and AKF approaches are in good match with the constant gains obtained from Genetic Algorithm. Using the above Adaptive Kalman Filtering technique and Constant Gain Kalman Filter approach, vertical TEC values and SPR biases have been estimated from the IGS receiver observations stationed at ISTRAC/ISRO, Bangalore, India. A diurnal TEC variation over Bangalore for a period of one year for 2003 and January 2004 is estimated and reported in this thesis. This approach has also been applied to study the behaviour of the ionosphere over low latitude IGS station at Fortaleza, Brazil data during the great magnetic storm on the 15th July 2000 and the results were found to be consistent with the results of Basu et al. In addition, Using Constant Kalman filter, the TEC enhancement over Indian region has been estimated for the October 2003 Ionospheric storm, and the results were found to be consistent with the reported results in the literature.

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32

Chung, Tzu-Ping, and 鍾志斌. "Investigation of storm-time low latitude ionosphere trough and the associated plasma wave phenomena using ROCSAT-1/IPEI data." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/91707166265742853535.

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碩士
國立中央大學
太空科學研究所
92
Abstract The ROCSAT-1 satellite was launched into a circular orbit at about 600 km altitude with an inclination of 35 degrees. The onboard ionosphereic plasma and electrodynamic instrument (IPEI) takes in-situ measurements of ionospheric parameters. The ROCSAT-1 orbits cover the geographic latitude range between 35°N and 35°S (maximum magnetic latitude, about ±50°). The data used here include ionospheric ion density, temperature, ion drift velocity and ions composition measured by IPEI during the great magnetic storm on March 31 and April 1, 2001. These data are used to demonstrate the presence and the local time (LT) distribution of the ionosphere plasma density troughs whose equator-ward boundaries extended to the lower mid-latitude region during the heights of this storm. Furthermore, wave spectra of ion density, and the two cross-track velocity components at equatorward boundaries of troughs are analyzed to investigate the spatial scales of the irregularities and the associated processes. During this storm (DST is at the lowest –390 nT in 09:00 ~10:00 UT, kp > 7), the trough equatorward boundary were found at magnetic latitudes as low as -40° ~ -43°. In pre-midnight sector, the latitude of this boundary moved equator-ward as local time increasing. Inside the trough region, at latitudes which are higher than those of trough equatorward boundary, large westward ion drifts were detected (about 1000 m/s ~ 1600 m/s). In addition, significant disturbances in temperature and ion composition were observed inside the trough region. The poleward electric field observed in this region was increasing with increasing latitude in the pre-midnight sector. The results of spectral analysis indicate that there exist irregularity structures with spatial scale size from 100 m to 10 km, and electromagnetic wave signatures at the eguatorward boundary of the trough.

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Liu, Yen Hung, and 劉彥宏. "Using ROCSAT-1 In-situ Data to Study the Radiowave Scintillation Phenomenon in the Equatorial to Low-Latitude Ionosphere." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/30280497356505452744.

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博士
國立中央大學
電機工程學系
101
The global/seasonal/local-time distributions of scintillation occurrence rate have been obtained from the in-situ density measurement of ROCSAT-1 during moderate to high solar activity periods of 2000 to 2003. The scintillation was obtained with a modified procedure of the thin-phase screen model of Rino reported in Wernik et al. [2007]. The distribution of the S4 index for the weak scintillation (S4<0.3) is almost identical to that of the equatorial irregularity distribution reported in the literature. However, as the scintillation level increases (0.3< S4<0.6), the latitudinal distribution moves to the equatorial ionization anomaly (EIA) region. In addition, the distributions of the outer scale values that are valuable for the study of physical evolution of the irregularity structure are also obtained. The occurrence distribution of scintillation activity with several parameters such as dip-latitude, longitude, local time, solar activity, and geomagnetic activity during different seasons are presented and discussed in the report. In addition, a special case of coincident observation that occurred on 24 March 2000 between the irregularity structure measured by ROCSAT-1 and the scintillation experiment at the Ascension Island has been studied. The study of scintillation statistics is carried out first and the results shows that the Nakagami distribution can portray the scintillation intensities with S4 up to 1.4 for the L-band scintillation. Moreover, the departure of frequency dependence of S4 predicted by the weak scintillation is noticed due to the multiple scattering effect. The measurements between the satellite and scintillation data are then compared against each other to study the similarity in the gross feature between the characteristics of irregularity structure and the scintillation variation. The causal relationship between the fluctuation of ion density and the scintillation variation is obtained. The coincident observations are also validated by the PEM simulation result.

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34

Hsiao, Tung-yuan, and 蕭棟元. "Study of low-latitude ionospheric irregularity structure based on New LITN." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/bdk2y7.

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博士
國立中央大學
太空科學研究所
96
A radio signal traversing the ionosphere will be modified by irregularities. When received at an antenna, these signals present random temporal fluctuations in both amplitude and phase. Scintillations are particularly severe in the tropical regions around the equinoxes during the evenings. National Central University rebuilt the LITN (Low latitude Ionosphere Tomography Network) in 2006. The scintillation measurements and ionospheric tomography reconstruction of the beacon satellites in this study for three specific low-latitude locations were spaced approximately 350 km apart in the tropics, at Chung-li (25.1360N, 121.5390E), Taotsun (23.9810N, 120.6970E), Checheng (22.7250N, 120.5440E) and Itu Aba Island (10.060N, 114.3500E). The coherent beacon receiving systems receive radio signals from the LEO satellites, including 150/400MHz Navy Navigational Satellite Systems and FORMOSAT-3/COSMIC Satellites. In this thesis, we explore the low latitude scintillation near midnight, and approach correct location within the F region using a simple optical model when the radio signals traveled through them. There are 83 scintillation days from April 2006 to April 2007 (395 days) and occurred usually at 18~05 LT in the night time, especially at pre-midnight or post-midnight. The scintillations have high correlation with months when Kp < 3. The scintillations are happened at May-June and December-February less than the other months. On the contrary, scintillations are normally more frequent during the equinoctial months of August-October and March-April. Meanwhile there are 16 scintillation days happened when Kp > 3

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35

Chen, Her-Yuan, and 陳鶴元. "The simulation of low latitude ionospheric tomography by using ITS system." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/eqs26p.

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碩士
國立中央大學
太空科學研究所
94
The Low Latitude Ionospheric Tomographic Network (LITN) suitable for the study of lare scale ionosphere structure , especially in the Taiwan area , we can study the large scale electron density variation in the equatorial anomaly region . In the past, NCU had begun to set up LITN . It consisted of six ground-based receivers spanning 20∘along Taiwan meridian (121∘E) to receiver beacon signals from the US Naval Navigation Satellite System (NNSS) with receiver JMR-1 in order to carry out the large scale experiments on two-dimensional ionospheric imaging by computerized tomography until the decommission of NNSS satellites in late 1997.Since July 2003, NCU has purchased 4 ITS30 receivers from North West Research Associates, Inc. (NWRA) and set them in Pingtung , Nantou and Taoyuan in Taiwan for the reestablishment of the NEW LITN with the hope to promote the receiving stations to other countries such as Philippines , Indonesia and so on in the near future . In this thesis, We have applied STK ,IRI-model to determine the location of LEO satellite and initial electron number density . In addition, the simulation study of GPS/MET and real data inversion with the ITS receiver were accomplished. A comparison is made between our results and dynasonde data, some possible explanation were discussed. After Formosat-3, which consist of six LEO satellites and one more frequency band (1066.67MHz), entered into their orbits, we can get more data for the inversion of CIT study.

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36

"Effect of the ionosphere of low latitudes in gps - sbas (global system positioning space based augmentation system)." Tese, MAXWELL, 2004. http://www.maxwell.lambda.ele.puc-rio.br/cgi-bin/db2www/PRG_0991.D2W/SHOW?Cont=5863:pt&Mat=&Sys=&Nr=&Fun=&CdLinPrg=pt.

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37

Sun, Yang-Yi, and 孫楊軼. "Low latitude ionospheric irregularities observed by FORMOSAT-1 and GPS ground-based receivers during soloar maximum." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/31798834003405288676.

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碩士
國立中央大學
太空科學研究所
97
A ground-based receiver of the global positioning system (GPS) can easily probe the ionosphere, except desert and oceanic areas, with a high temporal resolution. By contrast, a satellite orbiting observes large area (for example, FORMOSAT-1 covers the ionosphere within +/- 35°) and, however its revisit time generally is rather long. In this study, seasonal and geographical variations as well as geomagnetic storm signatures in ionospheric irregularities within +/- 35° are examined by employing worldwide ground-based GPS receivers and FORMOSAT-1 during the solar maximum period of 1999/1/27-2004/6/17. The Hilber-Huang transform is first applied to compute the instantaneous total power of waves with period less than 25 minutes of the ion density recorded by FORMOSAT-1 and to remove middle and large scale travel ionospheric disturbances (MSTID) in the ionospheric total electron content derived from the ground-based GPS receivers. Results show that the spatial distributions of the FORMOSAT-1 total power generally agree with those of the GPS TEC phase fluctuation in various seasons. Moreover, a long-tern observation in the GPS TEC phase fluctuation indicates that magnetic storms can either trigger or suppress low latitude ionospheric irregularities.

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38

Friedel, Reiner Hans-Walter. "A study of wave induced electron precipitation at low and middle latitudes." Thesis, 1991. http://hdl.handle.net/10413/5682.

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Wave induced electron precipitation (WIEP) can modify the ionosphere above a sub-ionospherically propagating VLF signal in such a way as to perturb the amplitude and phase of the signal: The "Trimpi Event". In this thesis trimpi events are used in a study of WIEP events and in the responsible mechanism: The gyroresonant interaction. Trimpi activity at middle latitudes (SANAE, Antarctica, L = 4.02) and low latitudes (Durban, RSA , L = 1.69) together with the corresponding theory for the gyroresonant interaction is examined and compared. A newly developed computerised system for the detection and analysis of trimpi events has been developed in Durban. This system has been used to analyse tape data recorded at SANAE. Trimpi events were found on various transmitter paths to SANAE and a complete study of 1982 data has led to the establishment of trimpi characteristics as seen at SANAE: an absence of positive events and causative whistlers, a preference for short duration events (t < 25s), the occurrence of some very large events (up to 90% signal attenuation) , two minima in occurrence near 0015 and 0400 h Local Time, low occurrence and occurrence rate of events and evidence that interactions with non-ducted whistlers are of importance. The computerised sytem was then extended to collect data at Durban simultaneously from up to 20 transmitters worldwide. Examination of data from this survey showed very low occurrence rates of trimpis but yielded some daytime events for which the effectiveness of the gyroresonance interaction, which successfully explains the trimpi event at middle and low latitudes, had to be questioned. Thus a fully relativisic test particle simulation of the gyroresonant interaction was used to examine the effectiveness of gyroresonance at low L for producing trimpi events. This simulation was run for a wide range of interaction parameters and yielded the following constraints for effective pitch angle scattering (and hence precipitation) of electrons at low L: wave intensities in excess of 150 nT, wave frequencies in excess of 10 kHz and background electron densities at least one order of magnitude higher than normal. First data from the OMSKI project, a sophisticated VLF receiver operated at Durban as part of an international project, shows further evidence of low-latitude trirmpi activity. A survey of one month's continuous data is presented. In face of the evidence that trimpi events that occur at low L have the same signature as those at middle L but that the standard gyroresonance interaction is insufficient to cause them, alternate scenarios that could enhance the interaction were sought. In particular distortions in the ambient magnetic field (eg. PC-5 pulsations) were modelled using a new dipole-like background field model. This simulation showed that distortions which tend to reduce magnetic field curvature along field lines can significantly enhance the gyroresonant conditions and hence the interaction. A new set of conditions for effective gyroresonance at low L is thus established and contrasted with the more lenient conditions at middle L. A study of "frequency tracking" as a means to prolong resonance showed that natural whistlers do not posess the required frequency /time characteristics for this mechanism, and that artificial waves in a narrow range around the equatorial resonance frequency would ~ well suited for this purpose. An overview of the status of worldwide Trimpi detection networks together with the S.P.R.I. 's role in this regard is presented.
Thesis (Ph.D.)-University of Natal, Durban, 1991.

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39

Chu, Fang-Dar, and 褚芳達. "A long term observation of ionospheric irregularities at low latitudes by using GPS phase fluctuations." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/93823066607957404567.

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博士
國立中央大學
太空科學研究所
95
The electron density irregularities in the ionosphere would affect the propagation of electromagnetic wave. The spreading F layer phenomena which caused by electron density irregularities at low latitudes is an important research topic. It could have been deduced that the distribution of occurrence rates of equatorial spread F (ESF) shows obvious longitude effect by using many observation techniques (radar power map / scintillation, ionosonde spread-F, satellite measurement / scintillation, and optical imager) in the past. Nevertheless, those techniques are not only very different each other, but also with their individual limits and with different criteria to detect the existence of ionospheric irregularities. The results obtained by those techniques would only be made qualitative comparisons. Besides, those observation data are usually not sufficient long or not continuous to show the year by year variation of ESF occurrence rate during an entire solar cycle period. There are two objectives in this paper. One objective is to develop a new observation technique that can conduct a global, all weather, and long-term observation of the ionosphere by using the global position system (GPS), and moreover, can detect the existence of ionospheric irregularities with the same analysis procedure and criterion. The other objective is to investigate the distribution of ESF occurrence rates at low latitudes, and moreover, investigate the details of year by year evolution of ESF occurrence rates during an entire solar cycle period. The method was as the following. First, a new analysis approach of GPS phase fluctuation was developed by using slant total electron contents, rather than vertical total electron contents. The new analysis approach needed no instrumental biases, and therefore it could be directly applied to every GPS observation data set. The last, the new GPS phase fluctuation approach was applied to detect the ESF occurrence at global low latitudes, and moreover, the statistics of ESF occurrence were calculated and the comparisons of the occurrence rates between different longitudes were made. There were three important results. (1) The occurrence distribution of equatorial spread F did show obvious longitude effect. The occurrence rates peak at equinox and June solstice months in the Pacific sector (Kwajalein, Guam and Philippines) and Africa, nevertheless, the occurrence rates peak at equinox and December solstice months in South America sector (Brazil and Peru) and India. (2) Strong ionospheric irregularities are always positive dependent on solar activity; and similarly, moderate irregularities are usually positive dependent on solar activity, excepting that those in Brazil are independent in December solstice months, and those in Peru are independent or only weak dependent in equinox months. (3) As to the evolution of occurrence rates of irregularities with solar activity, it should be noted that irregularity occurrence rates peak in which season (month) are determined by the competition between the occurrence rates in equinox months and June or December solstice months, and therefore, would be different results in different phase of solar activity, excepting for Brazil (always December solstice maximum) and Philippines (always equinoctial maxima). The GPS phase fluctuation analysis approach developed in this study does satisfy the objective to investigate the global ionosphere with the GPS system with all weather and long term observations. This study is the first attempt to analyze long term GPS observation data for the research of equatorial spread F layer. Many interesting results have been obtained, and a more complete and reliable longitude effect on equatorial spread F layer could be portrayed.

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40

Shih, Yu-Jhong, and 施禹仲. "Global/Seasonal/Local-time Distributions of Low Latitude Ionospheric Plasma Blobs Observed by ROCSAT-1 in 1999 to 2004." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/35gm8h.

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碩士
國立中央大學
太空科學研究所
97
A plasma blob is an irregularity structure with density enhancement above the background in the nighttime ionosphere. The first report of the plasma blob observation was made with Hinotori in 1981, but the occurrence mechanism of the plasma blob is still unknown. An auto-search program is designed to find the plasma blob occurrence distribution in the ROCSAT-1 data taken during the high to moderate solar activity years of 1999 to 2004. The result of the plasma blob occurrence probability is presented in the distributions in longitude/dip-latitude/local-time/season/magnetic activity/solar activity. In the longitude distribution, although the plasma blob occurrence probability has a longitudinal variation, there is no apparent variation between ±15° dip-latitude, which is very different from the plasma bubble occurrences, and this is evident that the plasma bubble and plasma blob have different occurrence mechanisms. For the dip-latitude distribution, we find that the plasma blob has a minimum occurrence probability from -15° to 15° in dip-latitude and increases toward the higher latitudes. For the local-time variation, the plasma blob occurrence probability has a maximum around midnight, and it decreases slowly after midnight and decreases rapidly after 04 LT in most seasons. For the seasonal variation, the occurrence probability of the plasma blob is larger at solstice than at equinox. Furthermore, it has the maximum occurrence probability in both northern and southern hemispheres during the June solstice. During magnetic disturbed periods (Kp≧3), the plasma blob occurrence probability only indicates a small variation after midnight. That is, the magnetic disturbance seems to have little effect on the plasma blob occurrences. This is different to the equatorial bubbles in which the occurrence probability will increase after midnight during the magnetic disturbed time [e.g. Watanabe and Oya, 1986]. For the solar activity effect, the plasma blob seems to have more occurrences during low solar activity years which is opposite to the occurrences of the equatorial bubble [e.g. Su et al., 2006; Watanabe and Oya, 1986].

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