論文内容の要旨

論文題目 Escape of Atmospheric Ions and Creation of Large-Scale Magnetic Structures
around Mars
(火星周辺の大気イオン流出と大規模磁場構造の形成)

氏名久保田 康文This dissertation mainly discusses the physical processes of the interaction between
the solar wind and the Martian ionosphere, specifically the escape mechanisms ofionospheric ions and the structure of the Martian tail. Mars does not possess asignificant global intrinsic magnetic field, and hence the solar wind directly interacts
with the Martian ionosphere. The Phobos-2 spacecraft and recently Mars Express
spacecraft observed the Martian tail and estimated ion escape flux. These observations
indicate that heavy ions including O+, O2+ and CO2+ escape from Mars and form aplasma tail with a ray structure. Because the heavy ions such as O2+ and CO2+ are
produced in low-altitude regions of the Martian ionosphere, escape mechanisms are
required to work against gravity. In this dissertation, we try to clarify the following
problems using MHD simulations. (1) The global structure of magnetic field and plasmaaround Mars, particularly the characteristics of flows that transport planetary ionsfrom day side to night side. (2) The total amount of ion escape flux, particularly its
dependence on the solar wind total pressure and IMF.
In order to solve these problems, we construct a 3D MHD model for the Mars
environment, including all the chemical and physical processes which would be
important in the interaction processes. We discuss our simulation models and numericalmethods in Chapter 2.
In Chapter 3, we start with the discussion of the background chemical equilibrium
state of the Martian ionosphere which works as a basis for the discussion of physicalprocesses. Particularly, the vertical structure of the sub-solar ionosphere is discussed
including the photochemistry and vertical convection. We will demonstrate that thevertical convection in the dayside ionosphere is strongly dependent on the solar wind
total pressure.
In Chapter 4, we discuss the simulation results obtained in the case of a solar wind notaccompanied by IMF. The flow structure obtained for this extreme case is muchdifferent from the finite IMF case discussed later in other Chapters, and gives us hints
to interpret the mechanisms operating in the more standard case of finite IMF. Guided
by the importance of stream line topology in considering ion escape fluxes, we introduce
an evaluation method of total ion escape flux by integrating over “open” stream lines.
We will show that the solar wind total pressure controls the topology of stream lines andeventually the total escape rates. We show that O2+ and CO2+ ions escape little while thetotal O+ escape flux increases almost linearly with the density of the solar wind if thewind is not accompanied by IMF.
In Chapter 5, we discuss the results of our simulation for the standard set of solar windparameters including standard magnitude of IMF. We show that O2+ and CO2+ ionsproduced in the low-altitude regions of the day side ionosphere do escape in this case.
The reason for this is discussed and the role played by magnetic tension in the iontransport along the meridian plane is emphasized. We show that a characteristic formof central ray is produced along the x-axis within the tail by the ions transported fromthe day side by the action of magnetic tension.
In Chapter 6, we discuss the total amount of escape flux for each ion species and its
dependence on the solar wind total pressure. The total amount of escape flux is obtainedby the open-line integration method, just like in the case of zero IMF. It is found that theescape flux increases linearly with the solar wind pressure for O+, but the dependence ofO2+ and CO2+ escape rate on the solar wind pressure is more gentle than that of O+. We
interpret this result in terms of the stronger magnetic control over O2+ and CO2+ ionsthan on O+ ions. The absolute values of escape flux are consistent with the observed
values for each ion species. The detailed mechanisms of escape, in terms of source
locations and transport path, are discussed for each ion species emphasizing theimportance of relative locations of “open” stream lines and the source regions.
Finally we summarize our results in Chapter 7 and give comments on the past historyof atmospheric escape from Mars.