A Simulation Study On The Variations Of Ionospheric Electric Fields At Low And Middle Latitudes

The coupled thermosphere-ionosphere-magnetosphere geospace system is the critical region of human space activity and is essential for communication,navigation,satellite operation,and aerospace.The electrodynamic processes play an important role in the coupling of different regions of this system.The ionospheric electric fields at low and middle latitudes are affected by the forcings from above and below of the upper atmosphere.Meanwhile,the electric field variations can modulate the thermosphere by changing ion-neutral coupling.In this work,the Thermosphere-Ionosphere-Electrodynamics General Circulation Model(TIEGCM)is used to study the spatiotemporal variations in ionospheric electric fields and the underlying physical mechanisms.The main results of this dissertation are summarized as follows:1.The Latitudinal Variations of Ionospheric Zonal Electric FieldsThe latitudinal variations of ionospheric zonal electric fields have not been well understood.We investigate the driving mechanisms for the latitudinal variations of zonal electric fields under geomagnetically quiet conditions using the TIEGCM.A series of case-controlled TIEGCM simulations were conducted to explore the effects of neutral wind and ionospheric conductivity on the latitudinal variations of zonal electric fields.It is found that the eastward electric field at noon increases with latitude,which is mainly associated with the latitudinal changes of poleward neutral winds.The prominent latitudinal variations of zonal electric fields at sunrise and sunset are attributed to the strong longitudinal(magnetic local time)gradients of the zonal winds,i.e.,the Curl-Free Mechanism.The changes of the zonal electric fields at a given location are affected by the global dynamo process,which also produces the latitudinal structure of zonal electric fields.2.The Sunrise Enhancement of Equatorial Ionospheric Zonal Electric FieldsSatellite and incoherent scatter radar observations have shown frequently a strong enhancement of zonal electric fields near sunrise,which has significant longitudinal and solar activity variations.However,the physical mechanisms responsible for the generation,longitudinal and solar activity variations of this enhancement are not well understood.In this work,three releated studies have been carried out to address this issue.The main findings of these studies are:(a)The TIEGCM reproduces the sunrise enhancement of upward vertical drifts and zonal electric fields observed at Jicamarca on 10 June 2004.The simulation results show that large eastward zonal electric fields occur around sunrise at all latitudes,but with a peak at middle latitudes.Further numerical experiments reveal that the equatorial sunrise enhancement is mainly driven by the E-region zonal wind dynamo at middle latitudes rather than by the local dynamo effect in the equatorial region.Specifically,the strength of the equatorial eastward electric field near sunrise is determined by the magnitudes of westward wind and its longitudinal gradient at middle latitudes,and the declination of the dawn terminator.(b)TIEGCM numerical experiments and diagnostic analyses of the electrodynamics equation show that the longitudinal differences of the equatorial zonal electric fields near sunrise are primarily associated with the longitudinal variations in the zonal wind dynamo,with those from the meridional wind dynamo contributing secondarily.Furthermore,the longitudinal differences of the wind dynamo near sunrise are mainly related to the longitudinal variations of U×B and conductance,which are caused primarily by the direct influence of the longitudinal structures of magnetic field declination and strength.Meanwhile,the longitudinal variations of neutral winds,which also result in moderate U×B longitudinal variations,play a secondary role in the longitudinal variations of the neutral wind dynamo,while plasma density,which has minor longitudinal differences near sunrise,contributes slightly by modifying the conductance.Overall,the sunrise enhancement in June is more significant at the longitudes where the magnetic field strength and distortion are larger or the magnetic field declination is smaller in the Northern Hemisphere.(c)The TIEGCM simulations also show that when solar activity increases,equatorial eastward electric fields at sunrise always decrease.The solar activity variations in equatorial zonal electric fields at sunrise are mainly caused by solar actvity changes in the zonal wind dynamo in the F-region at low latitudes.The solar activity variations of the E-region zonal wind dynamo are small.The changes in F-region zonal wind dynamo are mainly related to the variations of the conductivity and neutral winds in the F-region.3.Ionospheric Electrodynamics in Responses to Solar FlaresThe TIEGCM is used to analyze the responses of ionospheric electrodynamic processes and the related physical mechanisms during the solar flare events on September 6 and 10,2017.Solar flares increase global daytime currents and reduce the eastward electric fields in the sunlit hemisphere from the equator to middle latitudes.Moreover,westward electric fields and equatorial counter electrojets are induced in the early morning.These electrodynamic responses are dominated by the flare-increased E-region conductivity,as the flare responses of neutral winds and F-region conductivity are small.The flare-induced large enhancement in E-region conductivity,which reduces the ratio of the field-aligned integrated wind-driven currents to the conductance,results in the electric field reduction.4.The relationship between plasma E×B drifts and field-aligned velocity in the ionospheric F-regionIncoherent scatter radar observations of F2-region plasma drifts showed a strong anti-correlation between the temporal variations of plasma field-aligned upward velocity(Vi?)and field-perpendicular poleward E×B drift(Vi?N)over time scales from a few hours to a day at middle latitudes.The underlying physical processes remain a highly controversial issue,despite a number of speculations and qualitative inspections.Previous studies lacked especially a quantitative analysis that could lead to decisive conclusions.In this study,we provide a comprehensive modeling study to explore the physical processes relating Vi? with variations using the TIEGCM.It is found that the anti-correlation between Vi? and Vi?N has strong altitudinal and latitudinal dependences.The anti-correlation between the diurnal variations of Vi? and Vi?N is associated with the neutral wind dynamo.Poleward meridional winds result in downward Vi? and poleward Vi?N,and vice versa.The anti-correlation between short-term temporal disturbances of Vi? and Vi?N is mainly caused by ion drag,in response to high-latitude convection electric field forcing.This forcing penetrates to lower latitudes and affects poleward plasma drifts Vi?N,which drag poleward meridional winds and modulates downward Vi?·As the enhanced convection electric fields subside,the anti-correlation is mainly associated with the disturbance meridional wind dynamo.The storm-time high-latitude energy and momentum inputs change global meridional winds which modify zonal electric fields to induce Vi?N changes Furthermore,ambipolar diffusion plays a significant role in modulating the relationship between Vi? and Vi?N.