Simulation study on impact of interplanetary shock on trapped particles in the inner magnetosphere

Abstract:

When an interplanetary shock arrives at the magnetosphere, the particles trapped in the inner magnetosphere are promptly redistributed. Observations have shown that the response of the particles to the shock is not simple, and is dependent on energy, pitch angle, mass and location. The purpose of this study is to provide a unified view of the impact of the interplanetary shock on the trapped particles in the inner magnetosphere by using a global magnetohydrodynamics (MHD) simulation, a bounce-averaged drift advection simulation, and test particle simulation. When the solar wind speed is abruptly increased, a strong electric field propagates tailward; negative Ey comes first, followed by positive Ey. The electric field is strong enough to transport particles with energy from eV to MeV inward and outward. The differential flux can increase and decrease, depending on initial distribution of it. Particles with relatively low energy (<several tens of keV) experience inward and outward motion because the drift period of the particles is much longer than the time scale of the pulse of the electric and magnetic fields. Particles with relatively high energy (>100 keV) also experience the inward motion, but they drift rapidly, so that they do not experience the outward motion effectively. This results in multiple energy-time dispersions in an energy-time spectrum of the PSD, which is known as ‘drift echo.’ As the wave front propagates tailward, particles are accelerated at off-equator. When the velocity of particles parallel to the magnetic field is close to that of the wave front, the particles are efficiently accelerated. This can result in multiple energy-time dispersions associated with ‘bounce echo.' The bounce echo tends to appear in ions with energy less than tens of keV. The wave front also modifies gyrophase of particles. Because the travelling time of the particles depending on energy and length between an observer and an acceleration point, the PSD, or flux appears to vary periodically in the energy direction because of 'gyro echo' or gyrophase bunching. We discuss the variation of the PSD, or flux of the particles depending on energy, pitch angle, mass as well as position.