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Student Number 93623004
Author Tzu-Wei Fang(方慈瑋)
Author's Email Address No Public.
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Department Graduate Institute of Space Science
Year 2008
Semester 2
Degree Ph.D.
Type of Document Doctoral Dissertation
Language English
Title Model Simulation of the Low-latitude Ionosphere
Date of Defense 2009-06-15
Page Count 142
Keyword
  • equatorial ionosphere
  • model simulation
  • Abstract 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.
    Table of Content 中文摘要....................................................................................................................................i
    Abstract.....................................................................................................................................iii
    Acknowledgement…………………………………………………………………………….vii
    Table of Contents…………………………………………………………………………….viii
    List of Figures………………………………………………………………………………….x
    List of Tables…………………………………………………………………………………xiv
    Chapter 1. Introduction...........................................................................................................1
    1.1 Motivation and Objective.......................................................................................1
    1.2 Thermosphere…………………………………………………………………….2
    1.3 Ionosphere………………………………………………………………………..5
    1.4 Low-latitude Ionosphere…………………………………………………….......11
    1.4.1 The Equatorial Electrojet………………………………………………………..13
    1.4.2 The Equatorial Vertical Drifts and Pre-reversal Enhancement………….………15
    1.4.3 The Relationship between Equatorial Vertical Drifts and Equatorial Electrojet..17
    Chapter 2. Theoretical Models: NCAR TIE-GCM and GIP Model……………………..20
    2.1 NCAR TIE-GCM..................................................................................................20
    2.1.1 Global-Scale Wave Model………………………………………….……….......22
    2.1.2 Modified Apex Coordinates and Quasi-Dipole Coordinates……………………29
    2.1.3 Electrodynamics (dynamo.F)………………………………….………...............31
    2.1.4 Current Density Calculation (current.F)…………………………….………......37
    2.2 Global Ionosphere and Plasmasphere Model………….…………………….......40
    2.2.1 Model Description…………………….………………………………….….......40
    2.2.2 Model Calculation……………………………………………………………….41
    2.2.3 Simulation Results……………………………………...……………………….44
    Chapter 3. Model Simulation of the Equatorial Electrojet in the Peruvian and
    Philippine Sectors………………………………………………………………53
    3.1 Introduction………………………………………………………………….......53
    3.2 Model Simulation………………………………………………………………..54
    3.3 Diurnal, Seasonal, Solar Activity Variation and Longitudinal Difference in △H
    and Vertical Drifts……………………………………………………………….56
    3.4 E Region Electron Density and Modification……………………………….......61
    3.5 Relationships between the Adjusted △H and Vertical Drift Retrieved from the
    TIE-GCM………………………………………………………………………..65
    3.6 Conclusion………………………………………………………………………69
    Chapter 4. Wind Dynamo Effects on Ground Magnetic Perturbations and Vertical
    Drifts…………………………………………………………………………….71
    4.1 Model Description and Simulations……………………………………………..72
    4.2 Latitudinal Variation of Ground Magnetic Perturbation………………………...74
    4.3 Ground Magnetic Perturbations and Vertical Drifts……………………………..79
    4.4 Linear Relations between △H and Vertical Drifts...…………………………....81
    4.5 Summary and Conclusion……………………………………………………….85
    Chapter 5. The Post-sunset Ionosphere……………………………………………………87
    5.1 The Pre-reversal Enhancement………………………………………………….87
    5.2 Influences of E- and F-region Neutral Wind and Conductivities.………………92
    5.3 Current Convergence during the Post-sunset Period……………………………99
    5.4 Tidal Effects on the Pre-reversal Enhancement………………………………..102
    5.5 Summaries……………………………………………………………………...109
    Chapter 6. Causal Link of the Wave-4 Structures in Plasma Density and Vertical Plasma
    Drift in the Low-latitude Ionosphere………………………………………...111
    6.1 Introduction…………………………………………………………………….111
    6.2 ROCSAT-1 Vertical Drift Model……………………………………………….113
    6.3 Climatology of the GIP Model Ionosphere…………………………………….116
    6.4 Annual and Local Time Variations of the Longitudinal Wave-4 Structure…….121
    6.5 Conclusions…………………………………………………………………….128
    Chapter 7. Conclusion……………………………………………………………………..129
    References……………………………………………………………………………………133
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