7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Effects of heliospheric boundary in the behaviors ofgalactic and anomalous cosmic rays
S. Matsukiyo1,2, K. Yoshida3, Y. Matsumoto4, H. Washimi2, T. Hada2
1
Faculty of Engineering Sciences, Kyushu University, 2 International Space Center for Space andEnvironmental Science, Kyushu University, 3 Interdisciplinary Graduate School of EngineeringSciences, Kyushu University, 4 Institute for Advanced Academic Research, Chiba University
e-mail (speaker): matsukiy@esst.kyushu-u.ac.jp

We discuss the effects of heliospheric boundary inthe behaviors of cosmic rays entering or accelerated
in the heliosphere.
First, we focus on the entering process of galactic
cosmic rays (GCRs) into the heliosphere. GCRs arethought to be generated in our galaxy. While they
have an energy of 109-15.5 eV, most of them areprevented from entering the heliosphere due to theelectromagnetic wall of the heliospheric bounday.
Even if successfully entering the heliosphere, it is
not easy for the GCRs to reach the Earth afterward
due to the intricate electromagnetic field structuresof the heliosphere and the effects of solar windconvection. The transport of GCRs has been
extensively discussed in the context of the diffusion
convection theory. But here, we introduce ourapproach to this problem focusing on the trajectories
of a large number of GCR particles entering deep
inside the heliosphere by using numerical simulation[1-2]. First, we reproduced global electromagnetic
structures of heliosphere by using 3D MHD
simulation. Then, the trajectories of GCR test
particles initially distributed outside the heliosphere
are traced. A variety of patterns of entering particle
motion for different particle energy are reviewed in
the presentation.

Figure 1. Trajectories of test particles entering theheliosphere reproduced by an MHD simulation.
In the second part of the presentation, thetermination shock is focused. The termination shock
is believed to be the acceleration site of the so-called
anomalous cosmic rays (ACRs) which are observed
near the Earth as a cosmic ray population with highflux in the energy range of several tens to hundred
mega electron volts. The mechanism how theacceleration process is initiated has been along-standing issue. This problem is investigated byusing two-dimensional fully kinetic particle-in-cell(PIC) simulation mimicking the circumstance near
the termination shock, where a local plasma contains
pickup ions (PUIs) in addition to the backgroundsolar wind ions and electrons. Nonthermal particles
are self-consistently reproduced by tracking longtime evolution of shock with unprecedentedly large
system size in the shock normal direction. Reflected
PUIs drive upstream large amplitude waves throughresonant instabilities. Convection of the large
amplitude waves causes shock surface reformation
and alters the downstream electromagnetic structure.
A part of pickup ions are accelerated to nonthermalenergy in the time scale of ~100 times inverse ion
gyro frequency. The initial acceleration occurs
through shock surfing acceleration mechanism
followed by shock drift acceleration mechanism.
Large electrostatic potential accompanied by theupstream waves enables the shock surfing
acceleration to occur.

Figure 2. Kinetic structure of a termination shockobtained from PIC simulation.
(Top threepanels) Phase space distribution of PUIs and(Bottom three panels) magnetic field.
Reference[1] Yoshida et al., Astrophys. J., 916, 29 (2021).
[2] Yoshida et al., Acta Phys. Polon. Supp., 15,
3-A20 (2022).