Estimations of the intensity of short-period perturbations of the horizontal drift velocity at 80 - 110 km altitude are made using data from the regular low-frequency D1 ionospheric reflection observations at Collm, Germany (52° N, 15° E) for the period 1983 - 1999. A simple half-hourly-difference numerical filter is used to extract perturbations with time scales between 0.7 and 3 hours. The results are compared with the mean drift analyses in order to study the interaction between short-period perturbations and the mean circulation. The average monthly variances of short-period perturbations of the zonal velocity near 80 km altitude show a main maximum in summer, a smaller maximum in winter, and minimum values at the equinoxes. At higher altitudes the summer maximum is shifted towards spring, and another maximum of perturbation variances in autumn appears at altitudes near and above 100 km. The seasonal changes of variances of the meridional velocity show maximum values in spring and summer, also some indications for an increase of the summer maximum at altitudes larger than 100 km are found. The observed altitude changes of the seasonal variations of drift perturbation variances are consistent with some numerical calculations of the height structure of a spectrum of internal gravity waves in the middle and upper atmosphere. / Die Intensität kurzperiodischer Störungen der horizontalen Driftgeschwindigkeit im Höhenbereich zwischen 80 und 110 km wurde anhand der regulären D1 Langwellenreflexionsmessungen in Collm (52° N, 15° E), bestimmt. Verwendet wurden Daten der Jahre 1983 - 1999. Ein einfache numerische Filter basierend auf den Unterschieden aufeinanderfolgender halbstündiger Windmittelwerte wurde verwendet, um Störungen im Zeitbereich von 0.7 - 3 Stunden zu ermitteln. Die Ergebnisse wurden mit Analysen der mittleren Drift verglichen, um die Wechselwirkungen zwischen kurzperiodischen Störungen und der mittleren Zirkulation zu untersuchen. Die mittlere monatliche Varianz der kurzperiodischen Störungen der Zonalgeschwindigkeit bei etwa 80 km zeigt ein Hauptmaximum im Sommer und ein schwächeres Maximum im Winter, wobei die Minima während der Aquinoktien auftreten. In grösseren Höhen verschiebt sich das Sommermaximum zum Frühjahr hin, und in Höhen über 100 km erscheint im Herbst ein weiteres Maximum. Der Jahresgang der meridionalen Windstörungen zeigt maximale Werte in Frühjahr/Sommer, und es sind auch Hinweise auf eine Verstärkung des Sommermaximums oberhalb von 100 km zu finden. Die gemessenen Höhenänderungen im Jahresgang der kurzperiodischen Driftschwankungen entsprechen numerischen Ergebnissen der Höhenabhängigkeit interner Schwerewellen in der mittleren und oberen Atmosphäre.

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.

This dissertation focuses on data analyzed from the Auroral Current and Electrodynamics Structure (ACES) sounding rocket mission. ACES consisted of two payloads launched nearly simultaneously in 2009 into a dynamic multiple-arc aurora. The mission was designed to observe the three-dimensional current system of an auroral arc system. To constrain the spatial-temporal ambiguity, the payloads were flown along nearly conjugate magnetic field footpoints, at various altitudes with small temporal separation. The high altitude payload took in situ measurements of the plasma parameters above the current closure region to measure the input signature into the lower ionosphere. The low-altitude payload took similar observations within the current closure region, where perpendicular cross-field currents can flow. A detailed description of the experimental configuration is presented, including operational details of the fields and plasma instruments flown on both payloads. The methods used to process data from the electrostatic particle detectors and the fluxgate magnetometer on both payloads are presented. Data from the all-sky imager details the auroral configuration at the time of launch. In situ data are presented detailing observations of the electric fields, magnetic fields, and the electron differential energy flux, as the payloads crossed nearly conjugate magnetic field lines. Field-aligned currents were calculated from magnetometer observations on the high altitude payload. These data were combined with electron flux data to show that the high altitude payload traversed regions of upward and downward field-aligned current. The low altitude payload observed signatures in the residual magnetic field components consistent with perpendicular closure current. Ionospheric collisionality is investigated to determine if it is a significant mechanism to explain observed differences in the low energy electron flux between the high altitude and low altitude payload. As a result of increased ionospheric collisionality, the ionospheric conductivity is investigated to interpret the in situ electric field observations. A model of auroral electrodynamics, that is under development, is discussed in the context of interpreting magnetometer data from the low altitude payload. The evolution of precipitating electron flux into the ionosphere and the effect this precipitation has on generating ionization is presented. The electron spectrum produced by the model were fit to the electron flux data observed by the low altitude payload. The height ionization profile, equilibrium electron density, and Hall and Pedersen conductivities were determined from the model electron spectrum incident to the ionosphere. It was shown that the low altitude payload flew just above the peak Hall and Pedersen conductivities, suggesting that the low altitude payload flew directly in the region where perpendicular closure currents were most significant.

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.

Ionospheric disturbance electric fields of magnetospheric origin play an important role in determining the global morphology and dynamics of the ionosphere of the Earth. In this work, we present a number of numerical simulations of the transient electric fields in the middle and inner magnetosphere and the ionosphere equatorward of the auroral zone caused by idealized changes in the magnetospheric driving parameters. For these studies, we u se the Rice Convection Model (RCM), a large computer code of the magnetosphere-ionosphere coupling which consistently computes the electric fields, currents, and plasma densities in the magnetosphere and the electric field and currents in the ionosphere in the quasi-static slow-flow approximation. We made substantial upgrades to the code, which include a module computing realistic solar EDY-produced ionospheric conductances and a new potential solver. Our upgraded version of the RCM also includes a time - varying magnetospheric magnetic field and a self-consistently estimated auroral zone. We first discuss numerical problems encountered in modeling electrodynamics of convection with a time-varying magnetic field, realistic ionospheric conductances, and a self-consistent auroral zone, and our solutions to those difficulties. We then present a number of "computer experiments" with the new version of the RCM with idealized changes in the magnetospheric parameters such as sudden changes in the cross polar cap potential drop, magnetic field reconfiguration corresponding to the overall changes in the high-latitude convection, as well as rotations of the electric field on the polar cap boundary. Prompt penetration ionospheric electric fields simulated with the upgraded RCM are shown to be consistent with the previous simulations. The new simulations and their results are discussed in the context of (1) possible contribution to the variability of the ionospheric electric fields, and (2) role of time-varying magnetic field on the characteristic lifetimes of prompt penetration electric fields at subauroral, middle, and low latitudes.

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.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 197-204).

In this thesis we describe a coupled model of Earth's magnetosphere that consists of the Lyon-Fedder-Mobarry (LFM) global magnetohydrodynamics (MHD) simulation, the MIX ionosphere solver and the Rice Convection Model (RCM). We report some results of the coupled model using idealized inputs and model parameters. The algorithmic and physical components of the model are described, including the transfer of magnetic field information and plasma boundary conditions to the RCM and the return of ring current plasma properties to the LFM. Crucial aspects of the coupling include the restriction of RCM to regions where field-line averaged plasma-beta <=1, the use of a plasmasphere model, and the MIX ionosphere model. Compared to stand-alone MHD, the coupled model produces a substantial increase in ring current pressure and reduction of the magnetic field near the Earth. In the ionosphere, stronger region-1 and region-2 Birkeland currents are seen in the coupled model but with no significant change in the cross polar cap potential drop, while the region-2 currents shielded the low-latitude convection potential. In addition, oscillations in the magnetic field are produced at geosynchronous orbit with the coupled code. The diagnostics of entropy and mass content indicate that these oscillations are associated with low-entropy flow channels moving in from the tail and may be related to bursty bulk flows and bubbles seen in observations. As with most complex numerical models, there is the ongoing challenge of untangling numerical artifacts and physics, and we find that while there is still much room for improvement, the results presented here are encouraging. Finally, we introduce several new methods for magnetospheric visualization and analysis, including a fluid-spatial volume for RCM and a field-aligned analysis mesh for the LFM. The latter allows us to construct novel visualizations of flux tubes, drift surfaces, topological boundaries, and bursty-bulk flows.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis (M. Sc.)--University of Hong Kong, 1972. / Mimeographed.