||Theory and Modeling of the Solar-terrestrial
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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.
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.
|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
|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>
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.