Dissertations / Theses on the topic 'Ionospheric variations'

The wavefronts of high frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial non-linearities and temporal variations which limit the performance of large aperture receiving arrays. The objective of this investigation was to measure the phase and amplitude of ionospherically propagated signals in order to relate these parameters to the reflection process. This thesis describes the design and construction of a large aperture multi-element array and its implementation for wavefrot investigations. The hardware and software developed to control the equipment and to record the measurements are described. The procedures required to verify the performance of the experimental system are discussed and results are presented which demonstrate the accuracy of the measurements. The array was utilised for studies of signals received from several transmitters situated throughout Western Europe. The results obtained demonstrate the widely different behaviour of signals received over the various propagation paths and these have been related to the modal content of the received signals. Limited periods existed during which a single ionospheric mode was received and data corresponding to this condition have been compared with those which would be expected if the signal consisted of both a specular component and a cone of diffracted rays. This model is unable to explain the experimental results. Numerical models of the received signal were therefore developed. Results of these and comparisons with experimental results suggest that the measured parameters can be explained by the existence of a specular component with a varying direction of arrival (DOA), plus some contribution from random components. The experimental results indicate that the random or diffracted components normally contribute less than 10% of the received power in a single moded signal.

Kyoto University (京都大学)
0048
新制・課程博士
博士(理学)
甲第14895号
理博第3464号
新制||理||1507(附属図書館)
27333
UT51-2009-M809
京都大学大学院理学研究科地球惑星科学専攻
(主査)教授 町田 忍, 教授 家森 俊彦, 教授 里村 雄彦
学位規則第4条第1項該当

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.

The ionospheric response to solar EUV variability during 2011 - 2014 is shown by an EUV proxy based on primary ionization calculations using combined solar spectra from SDO/EVE and SolACES on board the ISS. The daily proxies are compared with global mean TEC analyses. At time scales of the solar rotation and longer, there is a time lag between EUV and TEC variability of about one to two days, indicating dynamical processes in the thermosphere/ionosphere systems. This lag is not seen at shorter time scales. When taking this delay into account the TEC variance at the seasonal and short-term time scale explained by EUV variations increases from 71% to 76%.
Die ionosphärische Antwort auf Variationen des solaren EUV im Zeitraum 2011-2014 wird anhand eines Proxys dargestellt, welcher die primäre Ionisation auf der Basis gemessener solare EUV-Spektren beinhaltet. Die täglichen Werte werden mit Analysen des global gemittelten Gesamtelektronengehalts verglichen. Auf Zeitskalen der solaren Rotation und länger findet sich eine Zeitverzögerung zwischen der EUV-Variation und des derjenigen des Gesamtelektronengehalts von ein bis 2 Tagen, welche auf dynamische Prozesse im System Thermosphäre/Ionosphäre hinweist. Die Verzögerung ist auf kurzen Zeitskalen nicht zu sehen. Wenn diese Verzögerung berücksichtigt wird, erhöht sich die durch EUV-Variationen erklärte Varianz des Elektronengehalts von 71% auf 76%.

The plasma drifts or electric fields and their structures in the ionosphere affect the accuracy of the present-day space-based systems. For the first time, we have used ionospheric plasma drift data from Jicamarca radar measurements to study the climatology of altitudinal variations of vertical and zonal plasma drifts in low latitudes during daytime. We used data from 1998 to 2014 to derive these climatological values in bimonthly bins from 150 km to 600 km. For the vertical plasma drifts, we observed the drifts increasing with altitudes in the morning and slowly changing to drifts decreasing with altitude in the afternoon hours. The drifts change mostly linearly from E- to F-region altitudes except in the morning hours of May-June when the gradients are very small. The zonal drifts show a highly nonlinear increase in the westward drifts at the lower altitudes and then increase slowly at the higher altitudes. We see a break in the slopes at lower altitudes during the morning hours of March-April and May-June. The E-region zonal drifts, unlike vertical drifts, show a very large variability compared to F-region drifts. We also explored the altitudinal profiles of vertical drifts during late afternoon and evening hours when the electrodynamic properties in the ionosphere change rapidly. For the first time using drifts up to 2000 km, we have shown the drifts increase and decrease below and above the F-region peak before becoming height independent. These structures arise to satisfy the curl-free condition of electric fields in low latitudes. The altitudinal gradients of vertical drifts are balanced by a time derivative of the zonal drifts to satisfy the curl-free condition of electric fields. We have shown how these structures evolve with local time around the dusk sector and change with solar flux. During solar minimum, the peak region can go well below 200 km. The present-day electric field models do not incorporate these gradients, particularly in the evening sectors when they change very rapidly. Very often their results do not match with the observations. Including these gradients along with proper magnetic field models will improve the model results and accuracy of our navigation, communication, and positioning systems.

The Global Positioning System (GPS) has been widely used for precise positioning applications throughout the world. However, there are still some limiting factors that affect the performance of satellite-based positioning techniques, including the ionosphere. The GPS Network-RTK (NRTK) concept has been developed in an attempt to remove the ionospheric bias from user observations within the network. This technique involves the establishment of a series of GNSS reference stations, spread over a wide geographical region. Real time data from each reference station is collected and transferred to a computing facility where the various spatial and temporal errors affecting the GNSS satellite observations are estimated. These corrections are then transmitted to users observations in the field. As part of a Victorian state government initiative to implement a cm-level real time positioning service state-wide, GPSnet is undergoing extensive infrastructure upgrades to meet high user demand. Due to the sparse (+100km) configuration of GPSnet's reference stations, the precise modelling of Victoria's ionosphere will play a key role in providing this service. This thesis aims is to develop a temporal model for the ionospheric bias within a Victorian NRTK scenario. This research has analysed the temporal variability of the ionosphere over Victoria. It is important to quantify the variability of the ionosphere as it is essential that NRTK corrections are delivered sufficiently often with a small enough latency so that they adequately model variations in the ionospheric bias. This will promote the efficient transmission of correctional data to the rover whilst still achieving cm-level accuracy. Temporal analysis of the ionosphere revealed that, during stable ionospheric conditions, Victoria's double differenced ionospheric (DDI) bias remains correlated to within +5cm out to approximately two minutes over baselines of approximately 100km. However, the data revealed that during more disturbed ionospheric conditions this may decrease to one minute. As a preliminary investigation, four global empirical ionospheric models were tested to assess their ability to estimate the DDI bias. Further, three temporal predictive modelling schemes were tested to assess their suitability for providing ionospheric corrections in a NRTK environment. The analysis took place over four seasonal periods during the previous solar maximum in 2001 and 2002. It was found that due to the global nature of their coefficients, the four global empirical models were unable to provide ionospheric corrections to a level sufficient for precise ambiguity resolution within a NRTK environment. Three temporal ionospheric predictive schemes were developed and tested. These included a moving average model, a linear model and an ARIMA (Auto-Regressive Integrated Moving Average) time series analysis. The moving average and ARIMA approaches gave similar performance and out-performed the linear modelling scheme. Both of these approaches were able to predict the DDI to +5cm within a 99% confidence interval, out to an average of approximately two minutes, on average 90% of the time when compared to the actual decorrelation rates of the ionosphere. These results suggest that the moving average scheme, could enhance the implementation of next generation NRTK systems by predicting the DDI bias to latencies that would enable cm-level positioning.

The statistical analysis of the variations of the dayly-mean frequency of the maximum ionospheric electron density foF2 is performed in connection with the occurrence of (more than 60) earthquakes with magnitudes M > 6.0, depths h < 80 km and distances from the vertical sounding station R < 1000 km. For the study, data of the Tokyo sounding station are used, which were registered every hour in the years 1957-1990. It is shown that, on the average, foF2 decreases before the earthquakes. One day before the shock the decrease amounts to about 5 %. The statistical reliability of this phenomenon is obtained to be better than 0.95. Further, the variations of the occurrence probability of the turbulization of the F-layer (F spread) are investigated for (more than 260) earthquakes with M > 5.5, h < 80 km, R < 1000 km. For the analysis, data of the Japanese station Akita from 1969-1990 are used, which were obtained every hour. It is found that before the earthquakes the occurrence probability of F spread decreases. In the week before the event, the decrease has values of more than 10 %. The statistical reliability of this phenomenon is also larger than 0.95. Examining the seismo-ionospheric effects, here periods of time with weak heliogeomagnetic disturbances are considered, the Wolf number is less than 100 and the index ∑ Kp is smaller than 30.

The ionospheric response to solar extreme ultraviolet (EUV) variability during 2011–2014 is shown by simple proxies based on Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment solar EUV spectra. The daily proxies are compared with global mean total electron content (TEC) computed from global TEC maps derived from Global Navigation Satellite System dual frequency measurements. They describe about 74% of the intra-seasonal TEC variability. At time scales of the solar rotation up to about 40 days there is a time lag between EUV and TEC variability of about one day, with a tendency to increase for longer time scales.

Includes bibliography.
In this study we have demonstrated that a seasonal and diurnal correlation exists between occurrence frequencies of wave-like structures in the form of Traveling Ionospheric Disturbances (TID) in the ionosphere and tropospheric lightning in the mid-latitude region over South Africa. Lightning induced changes in total electron content (TEC) are strongest between September and March, with the more-pronounced effects occurring 12:00 - 22:00 UT, but from April through August there is a low probability of having significant lightning-induced TID occurrence. The strongest oscillations in the total electron content of the ionosphere have dominant periods of range 0.6 to 0.8 and 1.2to 2.5 hours, typical periods for medium scale TIDs and large scale TIDs respectively. Since ionospheric scintillation is caused by irregularities in electron density which act as wave scatterers, it is feasible that lightning-induced TIDs may provide the mechanism for causing the concomitant and co-located changes in ionospheric total electron con-tent that was observed. Both the lightning and the ionospheric irregularity have spatial dependence over South Africa dominating around Bloemfontein. We have also found a strong seasonal and diurnal correlation between occurrence frequencies of the high rate of change of TEC index (ROTI _ 0.8 TECU/min) as a proxy for amplitude scintillationS4 index and lighting stroke rate. The correlation coefficient linking diurnal lightning stroke rate and high ROTI is found to be about 86%. While the seasonal correlation between the monthly average ROTI and average stroke rate is about 70%, the seasonal average ROTI and average stroke rate correlation is found to be about 84%. This there-fore implies that the presence of lightning is a likely cause of the generation of TIDs and subsequent irregularities in the ionosphere.

碩士
國立中央大學
太空科學研究所
96
Pre-seismo-ionospheric signatures are studied by comparing variations in the electron temperature during the occurrence of large earthquake (M≧6.0). The ionospheric electron temperature was measured by electron temperature probe (ETP) on board Hinotori satellite during the period form March 1981 to June 1982. Eleven earthquakes in the vicinity of Taiwan and Philippines area are isolated and examined. Four of them show that anomalous deviations between observed Te and the associated empirical model around the dusk period or afternoon overshoot period(1500LT – 2000LT) appear from about 5 days before to 5 days after the earthquakes. A westward electric field possibly generated during the earthquake preparation period due to the change of the charge distribution. We also discuss the ELF(Extremely Low Frequency) emission observed by DE2 satellite.

碩士
國立中央大學
太空科學研究所
102
This thesis focuses on the studies of variations of neutral densities at a height of 410 Km in response to solar activity. It use solar radiation indices F10.7 and EUV to linearly fit neutral density measured by the Challenging Minisatellite Payload (CHAMP) satelliteduring the period 2003 - 2008. Aclearly phase delay in solar radiation and neutral density have been found when we were doing data analysis. It's also pointed out that neutral density variation has a time delay with solar radiation in the previous papers. In this study, we could get a conclusion that the time delay is one day during 2003 to 2008. In doing the linear regression,the result from multiple parameters is almost the same as that from single parameter. And, because of the more completeness of the observations of F10.7 than EUV, we decide to use F10.7 index to be the main parameter in the following studies. To compare original data of neutral density with the fitted data has a significant difference in the period of low solar activity. The difference has high relation with Kp index. Additional linear regression is required for these differences with Kp index.The results indicate that solar radiation is a dominate factor in the variations of neutral densities in the period of high solar activity, and geomagnetic activity produced by the solar wind becomes important in the low solar activity. The difference between original data of neutral density with the fitted data has a period of 130 days, which can be attributed to the satellite orbit.Removement of this component can increase the correlation coefficient of the original data the fitted data up to 0.95. But this effect is only important in the period near solar minimum.

博士
國立中央大學
太空科學研究所
94
Abstract Examining the parameters foF2、 h'F、 foEs and h'Es recorded by ionosonde to study the diurnal、 seasonal、latitudinal and solar activity variations in heights and plasma frequencies. The results show that foF2 yields maximum values at the two equatorial anomaly regions. The minimum values in the northern and southern anomaly regions appear in summer and winter, respectively. In general, the foF2 value is in proportion to the sunspot number. It is found that there are always exist ionospheric height lift in pre-sunrise and post sunset periods at higt attitude and h'F lifts in its attitude during pre-sunrise and post sunset periods. For the latitudinal distribution the pre-sunrise covers a greater range in southern hemisphere than in northern. Moreover, the pre-sunrise lifts in the northern and southern reach their maximum va1ues in winter and spring, respectively. The lifts of the post sunset appear mainly in stations HU、VA and TT of the southern hemisphere. The most clear signatures usually found in summer. The lift in altitude is also in proportion to the sunspot number. The foEs value in northern hemisphere is possibly greater than 5MHz. No obvious relationship can be found between foEs value and sunspot. h'Es lifts its altitude simply during morning period in winter and early spring and during both early morning and afternoon periods during late spring-fall. The lifts increase while the latitude decreases in the northern hemisphere. However, there is no latitudinal and solar activity effect found in the southern hemisphere. Finally, the lifts signatures in h'F and h'Es indicate that the E × B vertical drift and Zonal/meridional wind plays an important role.

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.

碩士
國立中央大學
太空科學研究所
98
On July 22 2009, the total solar eclipse was occured, which partially covered about 80% Chung-Li VHF radar. We used the Chung-Li 52MHz VHF radar to observe the 3-m field aligned irregularities(FAI) of sporadic E from July 20 to July 24, 2009 to study the solar eclipse effect on the FAIs. Interferometry technique was used to locate the positions of the FAIs and analyze the mean Doppler velocity, spectral width, and total power of radar returns from the irregularities. The results showed that there are some fluctuations about the mean Doppler velocity during five days. It also showed that the spectral width and total power in the eclipse time are much larger than those in the same time of other days. On the other hand, in this research we also analyze the ionograms recorded by the Chung—Li ionosonde station. Due to the effect of strong magnetic storm occurred in the afternoon on eclipse day, we found that the behavior of the foEs and foF2 are severe perturbed and will discuss and interpret this phenomenon in this thesis. We also compare the parameters data of Chung-Li VHF radar data and ionosonde, including spectral width, the thickness of Es, SNR and foEs, and discuss there differences quantitively and qualitatively.

The high power, broadband very low frequency (VLF, 3--30 kHz) and extremely low frequency (ELF, 3--3000 Hz) electromagnetic waves generated by lightning discharges and propagating in the Earth-ionosphere waveguide can be used to measure the average electron density profile of the lower ionosphere (D region) across the wave propagation path due to several reflections by the upper boundary (lower ionosphere) of the waveguide. This capability makes it possible to frequently and even continuously monitor the D region electron density profile variations over geographically large regions, which are measurements that are essentially impossible by other means. These guided waves, usually called atmospherics (or sferics for short), are recorded by our sensors located near Duke University. The purpose of this work is to develop and implement algorithms to derive the variations of D region electron density profile which is modeled by two parameters (one is height and another is sharpness), by comparing the recorded sferic spectra to a series of model simulated sferic spectra from using a finite difference time domain (FDTD) code.

In order to understand the time scales, magnitudes and sources for the midlatitude nighttime D region variations, we analyzed the sferic data of July and August 2005, and extracted both the height and sharpness of the D region electron density profile. The heights show large temporal variations of several kilometers on some nights and the relatively stable behavior on others. Statistical calculations indicate that the hourly average heights during the two months range between 82.0 km and 87.2 km with a mean value of 84.9 km and a standard deviation of 1.1 km. We also observed spatial variations of height as large as 2.0 km over 5 degrees latitudes on some nights, and no spatial variation on others. In addition, the measured height variations exhibited close correlations with local lightning occurrence rate on some nights but no correlation with local lightning or displaced lightning on others. The nighttime profile sharpness during 2.5 hours in two different nights was calculated, and the results were compared to the equivalent sharpness derived from International Reference Ionosphere (IRI) models. Both the absolute values and variation trends in IRI models are different from those in broadband measurements.

Based on sferic data similar to those for nighttime, we also measured the daytime D region electron density profile variations in July and August 2005 near Duke University. As expected, the solar radiation is the dominant but not the only determinant source for the daytime D region profile height temporal variations. The observed quiet time heights showed close correlations with solar zenith angle changes but unexpected spatial variations not linked to the solar zenith angle were also observed on some days, with 15% of days exhibiting regional differences larger than 0.5 km. During the solar flare, the induced height change was approximately proportional to the logarithm of the X-ray fluxes. During the rising and decaying phases of the solar flare, the height changes correlated more consistently with the short (wavelength 0.5-4 Å), rather than the long (wavelength 1-8 Å) X-ray flux changes. The daytime profile sharpness during morning, noontime and afternoon periods in three different days and for the solar zenith angle range 20 to 75 degrees was calculated. These broadband measured results were compared to narrowband VLF measurements, IRI models and Faraday rotation base IRI models (called FIRI). The estimated sharpness from all these sources was more consistent when the solar zenith angle was small than when it was large.

By applying the nighttime and daytime measurement techniques, we also derived the D region variations during sunrise and sunset periods. The measurements showed that both the electron density profile height and sharpness decrease during the sunrise period while increase during the sunset period.

Dissertation

The Sun ionizes a small fraction of Earth's atmosphere above roughly 60 km, producing the plasma that constitutes the ionosphere. Radio signals passing through the ionosphere scatter off of plasma density structures created by the Farley-Buneman instability (FBI). While numerous studies have characterized the FBI's intrinsic nature, its evolution within the broader context of the surrounding plasma remains enigmatic. This dissertation answers two fundamental questions about the FBI: How does it interact with density gradients? How does its non-linear evolution depend on the background plasma? The fourth chapter examines the combined development of the FBI and the gradient drift instability (GDI) using a 2-D simulation of the equatorial ionosphere. A half-kilometer wave perturbs a plasma layer perpendicular to the ambient magnetic field, causing the perturbed layer to develop GDI waves along the gradient aligned with the ambient electric field, as well as FBI waves in a region where the total electric field exceeds a certain threshold. Early radar observations suggested that these two instabilities were distinct phenomena; the reported results illustrate their coupled nature. The fifth chapter presents 2-D simulations in which a one-kilometer plasma wave develops an electric field large enough to trigger meter-scale waves. Such large-scale waves arise via the GDI within the daytime ionospheric gradient around 100-110 km. Typical ionospheric radars only observe meter-scale irregularities but observations show meter-scale waves tracing out larger structures. Simulated meter-scale FBI in the troughs and crests of kilometer-scale GDI matches radar observations of the daytime equatorial ionosphere, answers a question about electric-field saturation raised by rocket observations in the 1980s, and predicts an anomalous cross-field conductivity important to magnetosphere-ionosphere (M-I) coupling. The sixth chapter of this dissertation presents 3-D simulations of the FBI at a range of altitudes and driving electric fields appropriate to the auroral ionosphere, where it plays a role in M-I coupling. Research has thoroughly established the linear theory of FBI but rigorous analysis of radar measurements requires an understanding of the turbulent stage. These simulations explain the change in instability flow direction with altitude, with regard to the direction of background plasma flow.

碩士
國立中央大學
太空科學研究所
98
A network of Ionosphere Tomography System(ITS) have been employed to derive the total electron content (TEC) and/or electron density of the ionosphere above Taiwan and the east Asia region. The TaiWan Ionospheric numerical Model (TWIM) is a numerical model of global ionospheric electron density ,which provides critical frequency , peak density height, and scale height globally for layers D , E , and F . In 2009, there are five earthquakes (M≧6.0) occurred in Taiwan. An analysis has revealed that the VTEC difference between ITS and TWIM data surrounding the epicenters of which events decreased or increased abnormally before the earthquakes.