Initial Investigation Of Earth, Io And Titan Magnetosphere

Among eight planets in the solar system,all have their intrinsic magnetic fields except Venus or Mars.The planets' magnetic fields interact with the solar wind,forming planets' magnetospheres.The planets' magnetospheres not only have some similar aspects,but also have their own unique magnetospheric phenomena.In recent years,the human being are more and more interested in exploring other planets besides Earth.Deep space exploration has been one of the hottest topics.Studying the planets' magnetospheres with comparison will help human to further know the planets and their evolutions and in turn to understand better the Earth's magnetosphere.This thesis makes an introduction of some phenomena in the Earth's magnetosphere,and in the magnetosphere of Jupiter and Saturn both have the larger magnetosphere in the solar system,and then investigates them theoretically through numerical simulation.The main achievements are summarized as follow:1.the coupling between North and South Hemispheres in a realistic Earth main fieldThe neutral wind will induce an electric field by dragging the ionospheric plasma across the Earth's magnetic field.The neutral wind dynamo contributes significantly to the coupling of magnetosphere-ionosphere and the ionospheric electrodynamics.By focusing on the closed part of the magnetosphere in which magnetic field lines connect from one hemisphere to the other,the coupling of magnetosphere-ionosphere can be reduced to the ionosphere dynamo if the magentosphere is treated as of plasma depleted.We investigate the effects of the seasonal variation of the winds on the ionospheric currents and electric fields.The results show that the potential and the currents in solstice seasons are much different from those in equinox seasons. The gradients of the potential are steeper in solstice seasons than in equinox seasons,and similarly the shell currents are stronger in solstice seasons,due to the stronger neutral winds,than in equinox seasons.Similar to the seasonal variation of the ionospheric currents,the Birkeland currents in solstice seasons are 2-3 times larger than those in equinox seasons,and that the Birkeland currents are much stronger in winter than in summer.We assumed the potential is constant along a magnetic line and the Birkeland currents can flow from one hemisphere to the other of the ionosphere,then the coupling of magnetosphere-ionosphere can be transformed into a Northern/Southern Hemisphere coupling,and the magnitude of the Birkeland currents reflect the strength of the coupling.Consequently,the results suggest that the coupling between North and South Hemispheres are much stronger in solstice seasons than in equinox seasons,and likewise they are stronger in winter than in summer.2.Motion of magnetic flux tube produced by reconnection in Earth's magnetotailThe high-speed flow events in a short time scale in the plasma sheet of the Earth's magnetotail are called Bursty Bulk Flows(BBFs).The current theories propose that BBFs are "bubbles"(underpopulated flux tubes) which move in the plasma sheet under the action of the interchange instability.Most of the major observed characteristics of BBFs in the plasma sheet can be interpreted naturally in terms of the bubble picture.Using a one-dimension magnetic filament in the Earth's magnetotail to represent the bubbles,we examine the motion of bubbles created by reconnection in the Earth plasma sheet.Some properties of time evolution of the filament can be obtained from the numerical simulation.The magnetic field line in the filament created by reconnection is more dipolar than the surrounding magnetic field line,and the motion of the filament displays a strong earthward flow.The results show that MHD waves such as an Alfv(?)n wave and a slow shock propagate from the equatorial plane of magnetosphere to the Earth ionosphere,then partial reflect from the ionosphere.The motion toward the equator of the ionospheric end of the filament is delayed relative to the motion toward Earth in the equatorial plane.Though the plasma pressure in the filament initially is lower than its neighbors,its plasma pressure rose to values comparable to,or sometimes greater than,its neighbors once it started moving earthward fast enough.3.The tracks of cold/hot ions in the Io plasma torusIo is the nearest satellite of Jupiter.Near its orbit there is a plasma torus which is an important source of plasma in magnetosphere of Jupiter. Io plasma torus primarily consists of high-density cold plasma.The charged particles in the torus are accelerated to enter the Jupiter's atmosphere and produce Io's wake aurora.Here we will analyze the effect of the centrifugal force to cold/hot ions through simulating the tracks of the cold/hot ions in the Io plasma torus.It is confirmed that cold ions in the Io plasma torus are impossible to enter the Jupiter's atmosphere,but only being reflected back along the magnetic field line under the centrifugal force.So let the cold ions in the Io plasma torus enter the Jupiter's atmosphere and trigger emissions of Io's wake,it should have an acceleration mechanism for cold ions to get enough energy.At present,there are two possible mechanisms:(1) The ions get energy from Alfv(?)n waves produced by the interaction between Io and Jupiter's magnetosphere;(2) Parallel electric fields(upward Birkeland electric fields) accelerate the ions.4.Asymmetry related deflection of Titan's wakeTitan is the main source of the plasma in Saturn's outer magnetosphere. Titan does not have an intrinsic magnetic field.However,it possesses an induced magnetosphere through the interaction with magnetosphere of Saturn. Both the Voyager 1 and the Cassini spacecraft had close encounters with Titan,and detected that the wake of Titan's induced magnetic field deflects from the corotation direction,and that the ions distribute asymmetrically between the Saturn-facing and the anti-Saturn facing sides of Titan's wake.To examine this phenomenon,we simulate the trajectories of ions born in Titan's exosphere through a simple model.The results show that the heavy ions have gyroradii comparable with Titan's radius but the light ions' gyroradii are not so large.Because of the finite gyroradii effects,most of the heavy ions on the side toward Saturn are absorbed by Titan's atmosphere.Therefore the densities of ions on the two sides of Titan's wake are asymmetrical.This asymmetry makes the wake unbalanced and deviate from the corotation direction. Through the force balance of the wake we find that the wake will deflect toward Saturn by about 19°from the corotation direction.This is consistent with the observation of the Voyager 1.