Understanding the generation of magnetosonic waves using PIC simulations in a dipole magnetic field

Abstract:

Fast magnetosonic waves (a.k.a equatorial noise; hereafter MSWs) are known to occur most strongly within about ±10° magnetic latitude in the inner magnetosphere. The proton shell/ring distribution is the primary source of these waves. It has long been understood that the source of MSWs is located near the magnetic equator and the excited MSWs remain confined to the magnetic equator because of their quasi-perpendicular propagation. A recent two-dimensional particle-in-cell (PIC) simulation in a meridional plane using large anisotropy seemed to support the suggested generation mechanism.

In this paper, we report new results for PIC simulation of MSWs using a more realistic plasma and magnetic field configuration, with an emphasis on the generation process. Different from the recent simulation, we consider a simulation domain on a constant L-shell surface and energetic partial shell protons with a moderate anisotropy. This setup was motivated by observations that MSWs propagate preferentially in the azimuthal direction and that the anisotropy is typically moderate. Despite an extended source region in magnetic latitude due to the reduced initial anisotropy, the MSW intensity maximizes near the equator and decreases quasi-exponentially with latitude, indicating that a narrow source extent is not necessary for the observed MSW intensity distribution in magnetic 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. We also report various observable quantities from this simulation study such as power spectral densities as a function of frequency and wavenumber and the spatiotemporal evolution of the partial shell proton population.