A Numerical Study Of The Ion-Neutral Coupling In The Ionosphere-Thermosphere System

The Earth's ionosphere-thermosphere(I-T)system is an important regime of the geospace environment and is critical to radio propagation and communication,navigation systems,and human's space activities.The I-T system is affected by the magnetosphere via energy and electric field input,the lower atmosphere through exchange of momentum and energy via large-scale waves,in addition to complicated internal photochemical,dynamic and electrodynamic processes.Currently,the complex ion-neutral coupling processes between the ionosphere and thermosphere have not been fully understood.Numerical simulation is an effective tool to study the coupled I-T system.Based on the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model(TIEGCM),this thesis explores the physical mechanisms driving a number of the ionospheric and thermospheric phenomena,such as the ionospheric equatorial anomaly and ionospehric annual asymmetry,which have not been well understood so far.Furthermore,a high-resolution version of the TIEGCM,which has a horizontal resolution of 0.625° ×O.625°in a geographic latitude-longitude grid and a vertical resolution of 1/4 scale height,has been developed to simulate mesoscale processes in the I-T system.New findings in the IT responses to the magnetic storm and solar eclipse are obtained in this thesis on the basis of the high-resolution TIEGCM simulations.The main results of this dissertation are summarized as follows:1.The physical mechanisms of the ionospheric and thermospheric phenomena(1)The causes of the interhemispheric asymmetry of the equatorial ionization anomaly in solsticeThe daytime equatorial ionosphere exhibits a trough at the magnetic dip equator and two crests at about ±15° geomagnetic latitude,which is recognized as the equatorial ionization anomaly(EIA).An interesting feature of the EIA is its interhemispheric asymmetry,which refers to the fact that the crest is stronger in one hemisphere and weaker in the opposite hemisphere.The mechanisms of the interhemispheric asymmetry of the EIA in the June solstice during solar minimum have been investigated through a series of simulations using the TIEGCM.The results indicate that the transequatorial neutral wind is the main cause of the interhemispheric asymmetry.The transequatorial wind transports plasma from the summer hemisphere to the winter hemisphere,leading to an enhancement in the winter EIA crest.Meanwhile,the ion production and loss are also important factors contributing to the EIA asymmetry through photochemical processes.The longitudinal variations of the EIA interhemispheric asymmetry are also explored through investigating the longitudinal variation of each term of the ion continuity equation.Our term analysis results suggest that the effects of neutral wind variation dominate the longitudinal patterns of the interhemispheric asymmetry,while E×B drifts and photochemical processes contribute less.(2)The driving mechanisms of the Sun-Earth distance effect on the ionospheric annual asymmetryAn interesting feature of the ionosphere is its annual asymmetry,which is characterized by the electron density being over 20%larger in the December solstice than it is in the June solstice when it is averaged globally.It has been suggested that the difference of the Sun-Earth distance between the December and June solstices has a great impact on the ionospheric annual asymmetry.In this study,the physical mechanisms of the Sun-Earth distance effects on the ionospheric annual asymmetry are investigated using TIEGCM simulations.The simulation results indicate that the Sun-Earth distance affects the ionospheric annual asymmetry mainly through photochemical processes.During the daytime,this photochemical process results from the combined effect of ionization rate of atomic oxygen and the recombination of the electrons with neutral species.The solar irradiation variation between December and June directly leads to about 6%December-June electron density difference via the ionization of atomic oxygen,whereas the recombination with neutral composition contributes to 12%-15%December-June electron density difference.Moreover,in the region of large plasma fountain effect(between 200°and 400°magnetic latitude),ambipolar diffusion can also be modulated by the Sun-Earth distance effect and contributes to the ionospheric annual asymmetry.In addition,during the nighttime,the Sun-Earth distance effect impacts the annual asymmetry by changing thermospheric composition and ionospheric diffusion.2.The development of the high-resolution TIEGCM and studies of the mesoscale processes in the I-T system(1)Development of the high-resolution TIEGCMThe mesoscale processes,which are defined as having a characteristic scale between about 100 km and 1000 km,have critical importance in understanding the physics and chemistry in the IT system and space weather.However,these mesoscale processes are not well simulated by the existing global models since their grid sizes are close to greater than the characteristic scales of these processes.In this study,a conservative averaging-reconstruction method(Ring Average)is used for the finite difference solvers in the TIEGCM.The Ring Average technique is implemented as a run-time step thus requires no changes to the existing data structure,grid definition and numerical methods in the original solvers.By using the Ring Average technique,a high resolution version of the TIEGCM is developed,with a horizontal resolution of 0.625°x0.625° in a geographic latitude-longitude grid.The high-resolution TIEGCM can better catch mesoscale structures in the I-T system and thus be used to explore the dynamic evolution of these strcutures that have not been able to before.(2)Double Tongue of Ionization structures during strong geomagnetic stormDuring geomagnetic disturbances,the ionospheric plasma in the polar cap region often flows as a two-cell convection pattern.The high-latitude enhanced convection electric field during storm time transports the dense plasma from the daytime mid-latitudes into the polar cap region and then causes a tongue of ionization(TOI),which is characterized by a longitudinally narrow region of enhanced plasma density from middle to high latitudes in the noon-midnight direction.In this study,we use the high-resolution TIEGCM to study the polar ionosphere during the 2013 St.Patrick's Day storm.Two TOI structures occur simultaneously;one comes from the morning convection cell and the other one associated with the evening convection cell.This unusual double TOIs behavior is caused by both the unusual geometry of the lower latitude source region of ionization and the temporal change in the polar convection pattern.This is the first time that double TOI structures are simulated with a detailed description of their evlution with changes in the convection pattern.(3)The global response of the I-T system to the 2017 Great American EclipseA solar eclipse occurs when the Moon passes between the Sun and the Earth.It is commonly believed that solar eclipses have a great impact on the I-T system within the eclipse shadow and this impact is short lived,but little attention has been paid to the global response to these events.In this thesis,we investigate the global upper atmospheric responses to the recent Great American Solar Eclipse that occurred on 21 August 2017 using the high-resolution TIEGCM.The simulation results show that the ionosphere and thermosphere response to the eclipse is not just local,but global and long lasting.Large-scale travelling atmospheric disturbances(TADs),seen in the thermospheric temperature and winds,were triggered from the eclipse region and propagated in a southeast direction when the eclipse ended.A large total electron content(TEC)enhancement occurred over South America after the eclipse was over.The TEC enhancement was primarily the result of transport by the themospheric wind perturbations associated with the eclipse-induced TADs.The perturbations of TEC,neutral temperature and winds exhibited asymmetric distributions with respect to the totality path during the solar eclipse.Furthermore,ionospheric electrodynamic processes also play an important role in the global responses of the I-T system to the solar eclipse.Unlike the case of large-scale TADs propagating from the eclipse region to other locations in the globe,the ionospheric electric fields and plasma drifts began to show significant perturbations even during the local pre-eclipse period when local wind and temperature had not been perturbed.This is related to instantaneous global response of the ionospheric current system to changes in the ionospheric conductivity and winds in the eclipse region.(4)The polar ionospheric behavior during the 2017 Great American EclipseIt has long been recognized that during solar eclipses,the ionosphere-thermosphere system changes greatly within the eclipse shadow,due to the rapid reduction of solar irradiation.However,the concept that a solar eclipse impacts the behavior and dynamics of the polar ionosphere as well as magnetosphere-ionosphere coupling has not been appreciated.In this study,we investigate the potential impact of the 21 August 2017 solar eclipse on the polar TOI using the high-resolution TIEGCM.The reduction of electron densities by the eclipse in the middle latitude TOI source region led to a suppressed TOI in the polar region.The TOI suppression occurred when the solar eclipse moved into the afternoon sector.The Global Positioning System(GPS)Total Electron Content(TEC)observations show similar tendency of polar region total electron content suppression.This study,for the first time,reveals that a solar eclipse occurring at middle latitudes may have significant influences on the polar ionosphere and magnetosphere-ionosphere coupling.