Study Of Solar Wind-Magnetosphere-lonosphere Coupling Under Extreme And Transient Conditions

The magnetosphere and ionosphere are significant regions for humans to explore outer space.They are also important research fields that involve complicated physical processes and have essential effects on satellite communication,radio detection and so on.Different geospace regions-the magnetopause,magnetosphere and ionosphere are interwoven by geomagnetic field lines and exchange momentum and energy with each other.Numerical simulations can be used to conduct detailed analyses of the entire ionosphere-magnetosphere region,providing an important supplement for the scarcity of observational data.Extreme solar wind events and geomagnetic activities may have important effects on high technology system and human society.By using the global solar wind-magnetosphere-ionosphere coupling model,this project studies the ionosphere-magnetosphere coupling phenomenon under extreme solar wind conditions and the associated transient physical processes.The main results and innovations are summarized as follows:(1)Simulation study of the nonlinear response of cross polar cap potential under extreme solar wind densityIt is commonly believed that the magnitude and orientation of interplanetary magnetic field(IMF)together with the solar wind(SW)velocity have the most important impact on the cross polar cap potential(φPC),so that little attention has been given to the effect of SW density,especially under northward IMF conditions.Previous studies have shown that φPC increases with SW density as a response to the changes in magnetosheath force balance,while our study shows that φPC has complicated responses to the SW density depending on the magnitude of IMF rather than a simple linear response as reported previously.The φPC may be insensitive to SW density increasing at moderate IMF Bz(cf.8 nT)and at intense Bz(20 nT)under large-density conditions.The different behavior of SW density in regulating φPC is mainly due to the competing effects originated from viscous interaction and magnetic reconnection.(2)Investigation of whether the field-aligned current will saturate under extreme interplanetary electric fieldPrevious theoretical studies have revealed that field-aligned currents(FACs)are related to the Lorentz force at magnetopause that balances solar wind ram pressure,and FACs should be limited(saturated)by the ram pressure under large interplanetary electric field(IEF).Recent statistical studies have shown that the observed FACs increase linearly with IEF without significant saturation.To address the contradiction between theory and observations,numerical experiments are conducted to explore the response of FACs under large IEF.Results show that the FACs may exhibit either linear or saturated responses depending on the Alfven Mach number,the hemispheric FAC is likely to saturate when the solar wind flow changes from super-Alfvenic to subAlfvénic.The magnetospheric origins of FACs are very different for the linear and saturated cases.In the saturated case,enhanced magnetic pressure in the sheath diverts more solar wind magnetic flux,leading to the saturation of FAC.This condition only happens occasionally in the real solar wind so the FAC usually grows linearly with IEF from the statistical perspective.(3)Simulation of magnetospheric transient bursts flows and related energy transport processesGeomagnetic activity in Earth’s outer magnetosphere stimulates intense and sometimes explosive flows of electromagnetic power propagating earthward as Alfven waves.Observations show that among its various magnetospheric sources,Alfvenic Poynting flux from the magnetotail reaches the ionosphere with the greatest intensity.Statistically,the energy flux on the nightside of the ionosphere is stronger,the flux on the dayside is weaker,and the flux on the flankside is the weakest.By using global magnetohydrodynamic(MHD)simulations,we show that these distributions can be attributed to the formation(or not)of zonally narrow,relatively uniform,low Alfven conductance channels that efficiently transmit Alfvenic power to the near-Earth space.These Alfven ducts form naturally at the heads of flow bursts in the magnetotail but not in flankside flux tubes,where the transmission of Alfvenic power is strongly attenuated by reflections at large conductance gradients along the propagation path.The results elucidate the underlying physics that control efficient transmission of electromagnetic power from the magnetotail to low altitude to power Alfvenic aurora.(4)Comparative study of Earth’s transient auroral streamer with similar aurorae on other planets.The transient energy flux released from magnetotail during geomagnetic substorms can cause particle precipitation at ionospheric height,and the auroral streamer is one of the important auroral phenomena.Auroral streamers evolve rapidly,usually last only a few minutes,and have a narrow meridional width.These features require high spatial and temporal resolution of observation and simulation.In this thesis,the characteristics of auroral streamers on Earth are reproduced using a high-resolution global MHD model.In addition to Earth,similar auroral observations have been reported on other planets,such as the nightside auroral arc observed by Cassini at Saturn,which also moves from higher to lower latitudes as it rotates,with characteristics similar to Earth’s auroral streamers.In this study,we further investigated the differences of auroral streamer generation mechanisms at different planets with Saturn as an example.It is found that the auroras on Earth are mainly caused by the reconnection of open magnetic field lines in the magnetotail,while the auroras on Saturn may be generated through several processes:plasma velocity shear,closed magnetic field line reconnection and open magnetic field line reconnection.In conclusion,this thesis focuses on the magnetosphere-ionosphere coupling and discusses several physical phenomena.φPC and FAC are both important parts of ionospheric current system in the polar region and often present similar variations.We explored the variations of φPC and FAC under different extreme SW conditions and proposed that φPC varies nonlinearly with the SW density under northward IMF,whereas the saturation(or not)of FAC under extreme IEF depends on the SW Alfven Mach number.FAC is also an important medium for energy transportation between magnetosphere and ionosphere.Therefore,we futher studied the mechanism of magnetospheric energy transfer,and found that Alfven ducts in the magnetotail can effectively transport magnetospheric energy into the ionosphere.Magnetospheric energy injection into the ionosphere promotes the formation of aurora.We further examined the formation mechanisms of aurora and compared Earth’s aurora with other planets.This study paves the way for better understanding of the SW-M-I coupling and provides a theoretical basis for space weather forecast.