Energy and pitch angle dependence of impact of interplanetary shock on ions in the inner magnetosphere

Abstract:

An interplanetary (IP) shock has a large impact on magnetosphereic ions. Cluster satellite observations have shown that, soon after arrival of the IP shock, overall intensity of trapped ions rapidly increases and multiple energy dispersion appears in an energy-time spectrogram of ions with small equatorial pitch angles [Zong et al., 2012]. We have investigated the impact on the trapped ions and its dependence on the pitch angle. We have performed test particle simulation under the electric and magnetic fields provided by the magnetohydrodynamics (MHD) simulation. The solar wind speed is increased from 372 to 500 km/s in order to reproduce the IP shock. The number density in the solar wind was set to a constant to be 5 cm-3, and the Z component of the interplanetary magnetic field (IMF) was turned from +5 to -5 nT. Just after the arrival of the IP shock, a fast mode wave propagates tailward in the magnetosphere. The amplitude of the electric field exceeds 20 mV/m. To reconstruct an energy-time spectrogram of the oxygen ions at (7, 0, 0) Re in the GSM coordinates, we started to trace trajectories of ions the backward in time starting at (7, 0, 0) Re just after arrival of the fast mode wave. Knowing initial and final positions in 6-dimensional space, we mapped phase space density f, according to Liouville’s theorem. The phase space density f’ before the arrival shock is assumed to be isotropic Maxwellian. The result shows that a multiple energy-time dispersion appears in the simulated spectrogram of the ions with small equatorial pitch angles. The multiple energy-time dispersion is not present in the spectrogram of the ions with equatorial pitch angle of 90 deg. This is consistent with the Cluster satellite observations. We will discuss the generation mechanism of the multiple dispersion in terms of gyro-betatron and drift-betatron acceleration.