Cosmic rays are high energy charged particles observed in space as well as on the ground. The energy distribution of cosmic rays shows power law nature in wide range of energy. The reason why such energy distribution is formed has not been well understood. We study acceleration/transport of charged particles by using theory, modeling, and numerical simulations.

Roles of the heliosphere in acceleration/transport of cosmic rays

Heliosphere is regarded as an ideal laboratory for verifying the theory of cosmic ray acceleration and transport. A termination shock of the solar wind is formed in the heliospheric boundary region. The anomalous cosmic rays (10^7-8 eV) are thought to be generated there. However, the Voyager spacecraft did not observe the expected amount of cosmic rays when they crossed the termination shock. The acceleration mechanism of the anomalous cosmic rays has been still unresolved. We discuss the detailed structures of the boundary region causing particle acceleration by using numerical simulations.

The heliosphere also plays a role of the barrier which prevents galactic cosmic rays from entering the heliosphere. Very high energy cosmic rays are present outside the heliosphere. However, most of them cannot invade deep inside the heliosphere because of complex magnetic structures of  the heliospheric boundary and the convection effect due to the solar wind. These are known as the solar modulation of cosmic rays. The information of cosmic rays observed on the ground contains the solar modulation. We need to remove the effects of solar modulation in order to obtain precise information of galactic cosmic rays, although it is not easy. The studies of solar modulation have been dedicated for long time by using the theory based on Parker's transport model. We study this problem by using a new approach combining global MHD simulation and test particle simulation.

(left) Energy distribution of ions near a pickup ion mediated shock reproduced by numerical simulation, (right) Test particle simulation of cosmic rays invading into the heliosphere
 

Acceleration and transport of cosmic rays via MHD waves

Cosmic rays are accelerated through the scattering by MHD waves. The classical scattering theory has been established under the assumptions that wave amplitudes are small and wave phases are random.  However, the waves observed in space are often coherent and large amplitude. We study acceleration and transport processes of charged particles by taking those characteristics explicitly into account. 

The diffusion characteristics of charged particles in coherent and large amplitude waves are quite different from the classical diffusion such as the Brownian motion. For instance, if large amplitude magnetic solitary waves propagate as in the figure, a charged particle is multiply reflected by the approaching two solitary waves. In this case the behavior of the particle totally differs from the Brownian motion so that the particle is efficiently accelerated. We study non-classical particle acceleration/transport processes by using theory, modeling, and numerical simulations.

An orbit of a charged particle accelerated in large amplitude magnetic solitary waves