Energy transfer across magnetospheric boundary layers

Space above the Earths atmosphere is broadly filled with ionized gas, called plasma. Since the density of the space plasma is mostly small enough to neglect the viscosity that is, any collisions between ionized space plasma particles are basically negligible, the behavior of it is essentially different from neutral viscous fluids such as air and water. In such a collisionless plasma system, the boundary layer between regions with different plasma properties plays a central role in transferring energy and controlling the dynamics of the system itself. The goal of this project is to understand how the energy is transferred across the boundary layer in a collisionless plasma. Although a number of past studies have targeted this fundamental and important problem in space science, quantitative aspects of the realistic transfer process are still poorly understood. This is mainly because the energy in a collisionless plasma tends to be transferred over a broad range of spatiotemporal scales from the plasma particle (kinetic) scale to the global scale of the system, which cannot be handled only from laboratory and spacecraft measurements. Recent advances in numerical simulation enable more quantitative estimates of the transfer process, but still suffer from unrealistic assumptions. On these backgrounds, the scientific focus of this project is to quantify the energy transfer process more exactly than previous studies covering all necessary scales by effectively combining state-of-art plasma simulations and in-situ and remote plasma measurements. The uniqueness of this project is to target various types of boundary layers located in the Earths magnetosphere (the region controlled by the terrestrial magnetic field), which cover different factors and scales that control the energy transfer process across the boundary layer that is, the Earths magnetosphere acts as a great experimental station to explore the boundary layer physics.Specifically, in this project, a series of large-scale plasma particle simulations of representative boundary layers in the magnetosphere will be performed on one of the worlds largest supercomputers MareNostrum, under realistic simulation conditions obtained from real in-situ observations by recently launched high-resolution MMS (Magnetospheric Multiscale) spacecraft. The simulation results will be compared to the observation data from the MMS spacecraft, existing other in-situ spacecraft as well as ground-based observatories, which permit to treat both the local boundary layer physics and the global coupling of the local processes. Based on the project results, not only a quantitative understanding of the multi-scale boundary layer physics in the magnetosphere, but also a systematic understanding of the boundary layer physics in collisionless plasma will be obtained for the first time. These newly gained understandings will therefore be applicable to many other planetary and astrophysical objects, and will support future space exploration missions.

Space above the atmosphere is filled with ionized gas, called plasma. Their fluid behavior is mainly maintained by interactions with electro-magnetic fields and less (or mostly negligibly) by collisions as in the case for neutral gas. The region in the near-Earth space, where the plasma is controlled by the Earth's dipole field, is called the magnetosphere, which is shaped as a consequence of its interaction with the solar wind, which is a high-speed plasma flow from the sun carried by the interplanetary magnetic field. The impinging solar wind energy changes its properties at various boundary layers around and inside the magnetosphere and eventually drives the global magnetospheric dynamics and is partly deposited to the atmosphere producing aurora. This project aims to understand such energy transfer and mass transport process across boundary layers in the solar wind-magnetosphere system. Two processes at the boundary are studied extensively: (1) magnetic reconnection, which takes place in a thin current sheet, i.e. relatively high-magnetic shear condition, and (2) the Kelvin-Helmholtz (KH) instability, which takes place for boundaries with high velocity shear. The uniqueness of the project is the combination of the advanced computer simulations, including runs of super computer MareNostrum, and extensive analysis of data obtained by the four-spacecraft missions MagnetosphericMultiscale (MMS) and Cluster, as well as ground-based observations, by applying advanced multi-point data analysis methods. For the first time, a realistic computer simulation of the K-H waves with large magnetic shear case has been directly compared with MMS observations. Detailed analysis of both simulation and spacecraft data enabled to resolve internal structures of K-H waves at different spatial and temporal scales. Of particular importance is the discovery of the electron-scale current sheets at the edge of the K-H vortex that lead to magnetic reconnection. The relative role of these structures in energy conversion and plasma mixing is quantitively accessed.at the flank-magnetosphere boundary. Furthermore, our studies in the nightside magnetosphere based on extensive analysis of data from MMS and Cluster missions unveiled the dynamical process of the transient current sheets around the high-speed flows, which are ascribed to the plasma accelerated by the magnetic reconnection. These transient thin current sheets suggest a new type of secondary magnetic reconnection initiated due to the interaction between a reconnection jet and the ambient plasma. With the help of the newly modified analysis software, quantitative analysis could be performed including the geometry of the current sheet and the estimation the reconnection rate. These quantitative understanding of the solar wind-magnetosphere system has broader impacts not only on the basic magnetospheric and space plasma physics but also on more practical researches of the space weather, which are directly related to our technological infrastructures.