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Theory and Modeling of the Solar-terrestrial Space Environments Home > Research
Operation of every space mission is either directly or indirectly affected by solar activities and the response of the Earth’s magnetosphere to them. Particularly, the radiations of both solar and magnetospheric origins can endanger astronauts’ health and life and cause malfunction of instruments onboard the spacecraft. It is thus an essential part of space science to understand the mechanism of solar eruption, trace the path of solar ejecta and radiations in the interplanetary space, and predict the impacts of the solar-originated disturbances on the Earth’s magnetosphere. In parallel to the development of payloads and analysis of satellite data, we perform theoretical studies of diverse solar-terrestrial processes and their numerical modeling. Our theoretical study includes, but is not limited to, the following research topics.

1. Construction of Coronal Magnetic Fields and Modeling of Solar Eruptive Phenomena
All solar activities are rooted in solar magnetism. The magnetic fields in the solar corona, where most geoeffective solar activities take place, can hardly be measured directly by the current technology. One can only construct the coronal magnetic field based on the magnetic field measurement in the photosphere. We are developing a novel method of coronal field construction with robustness and efficiency. Once fully developed, the computational code will be an essential part of the space environment modeling system.

Numerical simulation studies of solar eruptive phenomena are being performed with the MHD codes developed by Prof. Choe in our team. This study is not simply purposed to reproduce observational features, but to find critical observable parameters for eruption. The results of simulations are also to be used as backgrounds of test particle simulations aiming at understanding of particle acceleration in solar eruptions.

<Fig 1. Erupting flux rope generated by merging of small scale flux ropes (simulation by G. S. Choe)>

2. Generation and Propagation of Solar-originated High Energy Particles
Sporadically, protons and electrons of keV to GeV radiated from the Sun are detected in a near-earth orbit. This phenomenon is called a solar energetic particle event (SEP). It is generally accepted that SEPs are caused by solar flares and/or coronal mass ejections (CMEs). In relation to CMEs, SEP particles are believed to be accelerated in a CME front shock. To investigate the particle acceleration processes, we will perform test particle simulations in time-dependent background fields obtained from MHD simulations of respective eruptive phenomena. This simulation study is intended to (1) identify the major particle acceleration region, (2) illuminate the detailed acceleration mechanism, and (3) find the resulting energy and velocity distributions of particles.

When our payloads are in lunar orbit, they will measure high energy particle distribution functions and this measurement can be used to make feedback to our prediction model and improve it.

3. Structures of Solar Plasma Ejecta and Their Interactions with the Earth’s Magnetosphere
Solar eruptive phenomena generally expel plasma ejecta (also called ICME, plasmoid or driver gas) into the interplanetary space and some of them can reach the Earth’s orbit and generate sudden change in space weather such as geomagnetic storms. Although it is generally believed that the magnetic field of the ejecta is a helical flux rope, it is quite uncertain whether field lines are connected to the Sun or to the IMF (Gosling, 1995). The global field line connectivity is very important in understanding the motion of the ejecta away from the Sun. We will perform numerical simulation studies on dynamics of ejecta with different field connectivities in the interplanetary space. Particularly, our interest lies in the directions of magnetic and velocity fields and their variation along the lunar orbit, because simulation case studies will be used to interpret the satellite observation data.

<Fig 2. Possible field line connectivities of solar-originated flux ropes (Gosling et al., 1995)>

4. Magnetotail Dynamics and Radiation Particle Dynamics
A geomagnetic substorm is a fundamental process of releasing magnetic energy, which has been deposited in the magnetotail by the solar wind. High energy particles generated in a substorm are transported to the plasmasphere and are fed into the radiation belt enhancing the ring current. Due to its large spatial scale, the computational study of magnetotail dynamics should resort to MHD simulations or multi-fluid simulations. Then the acceleration of particles and their transport to the inner magnetosphere can be handled by test particle simulations. We will construct a global numerical model of the Earth’s magnetosphere with a multi-level modular approach. In the coarsest level, a global MHD simulation will be employed and a multi-fluid model, an electron fluid-ion particle hybrid model, and a particle-in-cell model will be sequentially developed to handle finer scale dynamics. For global scale non-fluid phenomena, a test particle simulation will be used. When fully developed, this modular numerical modeling system will constitute an essential part of the future space weather forecasting system.

<Fig 3. Sketch of substorm phases and corresponding auroral observations from space>

5. Distribution of Heavy Ions in the Magnetotail near the Lunar orbit
The lunar orbit is located at a nearly constant distance of about 60 Earth radii (RE) and thus is an ideal location for studying the distant tail. In order to study the distribution of heavy ions in this region, we will adopt our recent theoretical method. Unlike single ion cases, multi-ion plasmas have their own resonances, which are composed of in-phase or anti-phase motions among the ions. Since the period of these resonances is determined by the heavy ion population, electromagnetic waves strongly excited at the resonance enable us to estimate the local population of heavy ions. As this feature is well confirmed in the magnetosphere (Lee et al., 2008), we will extend the same technique to the distant magnetotail region including the lunar orbit as well as to a region closer to the lunar surface wherever such EM wave data are available. The supra-thermal ion measurement planned in our proposal will also be compared with our estimates on the relatively cold background ion composition.

6. Interaction of Plasmas with the Moon
The behavior of collisionless plasmas at transition from kinetic (particle) to fluid scales is a fundamental topic in plasma physics. The Moon has numerous regions with magnetic field of different sizes ranging from kinetic to fluid scales. On the other hand, the lunar passage through the solar wind, magnetosheath, and magnetotail provides a wide range of different plasma conditions. The near-lunar plasma environment can thus serve as a laboratory for plasma physics studies at the kinetic-fluid interface. In our study, we will first investigate how the time-dependent impact of MHD discontinuities such as shocks disturbs the near-lunar background plasma parameters. This study is purposed to predict how the lunar environment evolves when either the solar wind or the magnetotail has sudden variations caused by solar activities or substorms. Secondly, we will investigate small scale phenomena near the boundaries of the solar wind/magnetosphere and the Moon. By particle simulations and kinetic theory, we will try to identify electromagnetic variations and particle dynamics before real data are obtained from the payload experiments in the planned lunar mission.