Recent satellite observations, combined with instability analyses, have shown that the background plasma density variation can modulate the magnetosonic (MS) waves by controlling the wave excitation. However, the detailed modulation process needs to be identified since the MS waves propagate nearly perpendicular to the background magnetic field. In this study, we investigate the MS wave modulation by background plasma density using a 2-D general curvilinear particle-in-cell simulation in the meridian plane of a dipole magnetic field. The simulation model consists of three plasma components: the background cold electrons and protons and ring distribution protons. We find that MS waves can be locally generated by ring distribution protons in the low plasma density region, while no MS wave is generated in the high density region. These MS waves are confined near the local source region since they are damped by the background cold plasma. The background protons gain more energy than background electrons, implying the plasmaspheric protons may dominate the modulation of MS waves. Moreover, we have also investigated the generation and propagation of MS waves in the plasmapause density structure. Our simulation results demonstrate that the background plasma density variation can modulate the MS waves and may play an important role in the spatial distribution of MS waves.
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