Two-dimensional particle-in-cell simulations of magnetosonic waves in
the dipole magnetic field: On a constant L-shell
Abstract
Two-dimensional particle-in-cell (PIC) simulations are carried out on a
constant L-shell (or drift shell) surface of the dipole magnetic field
to investigate the generation process of near-equatorial fast
magnetosonic waves (a.k.a equatorial noise; MSWs hereafter) in the inner
magnetosphere. Unlike the simulation domain in a meridional plane used
in a recent study, the present simulation box allows wave propagation
and growth in the azimuthal direction, to which MSWs were shown to
propagate and grow in the source region. Furthermore, the equatorial
ring-like proton distribution used to drive MSWs in the present study is
(realistically) weakly anisotropic. Consequently, the ring-like velocity
distribution projected along the field line by Liouville’s theorem
extends to rather high latitude, and linear instability analysis using
the local plasma conditions predicts substantial MSW growth up to ±27°
latitude. In the PIC simulations, however, the MSW intensity maximizes
near the equator and decreases quasi-exponentially with latitude.
Further analysis reveals that the stronger equatorward refraction at
higher latitude due to the larger gradient of the dipole magnetic field
strength prevents off-equatorial MSWs from growing continuously, whereas
MSWs of equatorial origin experience little refraction and can fully
grow. Furthermore, the simulated MSWs exhibit a rather complex wave
field structure varying with latitude, and the scattering of energetic
ring-like protons in response to MSW excitation occurs faster than the
bounce period of those protons so that they do not necessarily follow
Liouville’s theorem during MSW excitation.