Investigation Of Global MHD Simulation To The Solar Wind-Magnetosphere Interfaces

As an important part of solar-terrestrial environment,the solar wind-magnetosphere coupling involves very complicated physical processes,and is also one of the most direct and pivotal parts in solar activity affecting the Earth's space environment.A lot of evidences have proved that the burst of the Sun can lead to a disaster in the Earth's space environment,such as the damage of the spacecraft,the inaccurate of the navigation systems,the interrupt of the telecommunication,the depletion of the ozonosphere,and so on.In order to ensure the safety of the space activities,exploit the space resource,and protect the human living environment,the prediction of space weather is very necessary,especially for some disaster disturbance events.The magnetopause is the interface between the solar wind and the magnetosphere.The variations of the size and shape of the magnetopasuse not only reflect the corresponding changes of the solar wind parameters,but also influence the internal structure of the magnetic field in the magnetosphere,as well as the spatial distribution of the current system and each region.The Earth's bow shock is created by the interaction of the supersonic and superalfvenic solar wind with the magnetospheric obstacle.Before crossing the bow shock,the solar wind parameters have kept the characteristics from the Sun;when the solar wind flows through the bow shock,the parameters change obviously.For example,the velocity of the solar wind plasma decreases,while the density increases.The plasma temperature and the interplanetary magnetic field(IMF)also change correspondingly.So,when the satellites and spacecraft fly through these two interfaces,they experience significant variations in surrounding environment,and these variations may damage the spacecrafts in catastrophic space weather conditions.Therefore,the size and shape of the magnetopause and the bow shock are very important parameters in space weather.The study of the size and shape of these two interfaces can help us not only understanding the basic rules in the solar wind-magnetosphere coupling,but also forecasting the space weather disaster,and reducing the detriment as far as possible.Using a three-dimensional adaptive magnetohydrodynamic(MHD)model and the corresponding observation data,this doctoral dissertation studies the influences of the solar wind parameters,as well as the Earth's dipole tilt angle,on the size and shape of the magnetopause and the bow shock.The main research contents of this thesis are as follows:1.By using the results from global MHD simulation,we study the conversion of the pressure quantities on the Sun-Earth line for different IMF orientations,as well as its influences on the pressure balance across the magnetopause.We find that the pressure balance across the magnetopause under the different IMF orientations is mainly controlled by the magnetic reconnection.For southward IMFs the thermal pressure is dominant in the total pressure while the magnetic pressure is dominant for northward IMFs when BZ is not small(Bz>5nT).It shows that the reason for this phenomenon is the magnetic reconnection.In the northward IMF,the magnetic reconnection is difficult to occur at the subsolar point.The IMF lines can accumulate outside the subsolar magnetopause,the magnetic pressure from IMFs just outside the magnetopause is increased by the increasing Bz,and magnetic pressure near the subsolar magnetopause plays a dominant role.In the southward IMF,the magnetic reconnection can easily occur at the subsolar magnetopause,so the magnetic field at the subsolar magnetopause is very small and the magnetic pressure is close to zero.As a result,the upstream dynamic pressure in the southward IMF mainly converts to thermal pressure outside the magnetopause,that is,it is the thermal pressure that is dominant.Moreover,the total pressure just outside the magnetopause is enhanced with the increasing of Bz in both northward and southward IMF.The thermal pressure just outside the magnetopause is significantly enhanced with the increasing southward IMF.However,the enhanced thermal pressure does not play a crucial role in the earthward displacement of the Earth's magnetopause for southward IMF as opposed to the northward IMF.This effect is further enhanced by the decreasing of the magnetic pressure just inside the magnetopause which confirms the earlier theories.2.By using the global MHD results,we study the influences of the upstream solar wind parameters(including the IMF BY and BZ.solar wind dynamic pressure Pd,and IMF clock angle),as well as the subsolar magnetopause flaring angle,on the solar wind pressure coefficient.Our results show that the solar wind pressure coefficient is increased with the increasing IMF,and decreased with the IMF clock angle.Under the same other solar wind conditions,the solar wind pressure coefficient for the northward IMF is larger than that for the southward IMF.For the northward IMF the solar wind pressure coefficient decreases with the increasing solar wind dynamic pressure,while it increases with the increasing dynamic pressure for southward IMF.Moreover,for the first time,we find that:the solar wind pressure coefficient is increased with the increasing subsolar magnetopause flaring angle.3.By using both of the global MHD results and the correspongding observation data of the bow shock crossings,we investigate the influences of the dipole tilt angle on the shape and size of the bow shock.Our results show that the influences of the dipole tilt angle on the bow shock are very important,and cannot be neglected.The subsolar standoff distance and the north-south asymmetry of bow shock increase with the increasing dipole tilt angle.With the dipole tilt angle positively increasing,the flaring angle of the bow shock increases in the northern hemisphere but keeps almost unchanged in the southern hemisphere,and the rotational asymmetry slightly decreases in the northern hemisphere and rapidly decreases in the southern hemisphere.The influence of dipole tilt angle on the shape of the bow shock has a symmetric characteristic of north-south mapping.4.By using both of the global MHD results and the bow shock crossings from multiple satellites observations,we study the effect of IMF clock on the cross section of the bow shock tail.The results show that the cross section of the bow shock tail is elongated,which is similar to the magnetotail;and the elongation of the bow shock tail's cross section is closely related to the clock angle orientation of the IMFs.The cross section of the bow shock tail is elongated nearly along the orientation perpendicular to the orientation of IMF clock angle,and the elongated orientation is also slightly close to the orientation of IMF clock angle.When the northward IMF is dominate,the elongation degree of the bow shock tail's cross section is increasing with the IMF clock angle;while the elongation degree is decreasing with the increasing IMF clock angle for dominated southward IMF.The observation results also confirm the elongation phenomenon of the bow shock tail's cross section.