Numerical simulations on the dynamics of charged particles in the inner magnetosphere associated with a magnetic storm Numerical Simulations on the dynamics of charged particles in the inner magnetosphere associated with a magnetic storm

博士論文

This dissertation is concerned with the study of the dynamic behavior of the energetic charged particles trapped by the earth's magnetic field in the inner magnetosphere associated with magnetic storms by using a newly developed numerical simulation scheme.<br /> The earth's magnetosphere where the earth's magnetic field governs is filled with many charged particles having various energies and pitch angles; they are mainly composed of electrons and light tons, H +, He+ and O+.　 At certain times more particles than usual are injected from the magnetotail into the near earth region by the large-scale convection field; the particles become trapped by the earth's dipolar magnetic field. Since the transported particles have a sufficient energy, they can contribute to the formation of the large-scale current surrounding the earth at altitudes of ～20000～30000 km as if a current flows in a ring; this current is called a ring current. The growth and decay of the ring current can be observed by magnetograms on the earth's surface. The resultant current depresses the horizontal-component of equatorial earth's magnetic field. The depression usually lasts several hours or a few days and reaches its minimum field of about 1 % of the earth's internal field. Such strong depressions of the earth's field have been noticed in magnetograms; this is called magnetic storms.<br /> A time-dependent three-dimensional plasmaspheric model was developed to evaluate the Coulomb collision loss of the ring current tons. The Coulomb collision loss is one of significant loss processes of ring current tons, together with the charge exchange loss with neutral hydrogen. The results of the model calculation were compared with the direct satellite observation of EXOS-B; they are in good agreement with the observation. Using this plasmaspheric model, the loss effects of the ring current tons due to the charge exchange and the Coulomb collision were examined; the Coulomb collision loss hardly affects the bulk of storm-time ring current decay because the core region of the plasmasphere shrank within 3 earth radii at midnight during the storm.<br /> A temporal enhancement of the differential flux associated with a substorm was examined. The source distribution function that causes the flux enhancement observed by Explorer 45 in the inner region could be deduced from the particle simulation including an additional electric field associated with a substorm. Temporal developments of the plasma pressure and the current density in the equatorial plane responding to the injection are also calculated. The intensity of the calculated magnetic disturbance at the center of the earth is approximately 2 nT. Next, following the concept that a storm is composed of substorms, the ring current buildup is examined by superposing many substorm injections. The result suggests that the curve of calculated Dst* is unnatural for this particular weak storm and that an intense or a large storm are hardly explained this concept. Therefore other parameters' dependent sources as well as other mechanisms of particle flux enhancements may play an important role for the storm time ring current buildup.<br /> A quantitative ring current model depending on the solar wind and IMF was developed. After investigating the relation between the solar wind density and the plasma sheet density as a direct source of the ring current tons, it is found that the plasma sheet density at L=10 is also well responding to the solar wind density. Using the solar wind and IMF dependent conditions, the temporal variation of the differential flux in the magnetosphere was calculated. The time-dependent three-dimensional plasma pressure, the current density and the magnetic disturbance induced by the current density were also calculated and the magnetic disturbance at the center of the earth was compared with the ground observation (Dst) during successive storms in April 1997. The following items were found: (1) The major variation of corrected Dst is mainly due to the convection field and the plasma sheet density as well. (2) The plasma sheet temperature is insensitive to the ring current buildup. (3) The tons with energies of around 15-40 keV in the dusk region mostly contribute to the storm-time ring current. (4) The distorted magnetic field in the equatorial plane due to the ring current is calculated, The ▽B drift trajectory under the distorted magnetic field is highly changed by the distortion.

総研大甲第400号