地球電子放射線帯内側領域における動径拡散係数

The radiation belts are the region that energetic charged particles are trapped by Earth’s magnetic field. The radiation belts have been observed by a lot of satellites and researched by theoretical approach since the discovery of it. The electron radiation belt is separated into two regions called the outer belt and the inner belt by the slot region in which the flux is very small. It is well known that the outer belt flux vary by several orders of magnitude associated with geomagnetic disturbances (e.g., Baker et al., 1994) and the exact mechanism of such variations has been a puzzle so that the outer belt flux variations have been well researched to explain this in two-decade. On the other hand, the inner belt is more stable and the flux variation in the inner belt is smaller than that of the outer belt so that the inner belt has not been attracted much attention. A radial diffusion model can reproduce an equilibrium structure of the electron radiation belts (Lyons and Thorne, 1973). Radiation belt electrons are supplied from the outer boundary of the radiation belt, and the flux is arranged by the balance of intensity of the diffusion and the loss due to pitch-angle scattering by whistler mode waves in the magnetosphere. The electron flux variations of the radiation belt can be investigated by using a radial diffusion model with time-dependent radial diffusion coefficients and a time-dependent outer boundary condition. Brautigam and Albert (2000) investigated the outer belt electron flux variation by using the time-dependent radial diffusion model. The time-dependent radial diffusion coefficients formulated by Brautigam and Albert (2000) are customarily used in the time-dependent radial diffusion model. They parameterised the amplitude of the electrostatic field variation as a linear function of Kp index, and applied them to the electrostatic coefficient derived by Cornwall (1968). They were also formulated the electromagnetic coefficient as a function of Kp index. Because these coefficients are based on the observation of the outer belt, it is not appropriate to apply them to the slot and the inner belt regions. In fact, extrapolating these diffusion coefficients to the slot and the inner belt and doing numerical simulation, the slot is not formed and the electron flux near the Earth region is extremely large. In this study, in order to find a radial diffusion coefficient that can apply the inner part of the electron radiation belt as well as the outer belt, we performed some simulations by using the time-dependent radial diffusion model assuming that several types of the amplitude and the profile of the electrostatic field variation. The results show that the amplitude of the electrostatic field variation depends on L and is smaller toward the Earth.