Effects of the Plasmapause on the Radial Propagation of Fast Magnetosonic Waves: An Analytical Approach

In this study, analytical approximation is used to solve the wave equations near the plasmapause boundary layer in order to examine the validity of ray tracing approach for fast magnetosonic (MS) wave propagations, and then analytical solutions for electromagnetic fields of MS waves through the plasmapause boundary layer are provided for the first time. Both theoretical calculations from the analytical expressions and observations of Van Allen Probes have indicated that electric fields of MS waves decrease rapidly but magnetic fields increase rapidly when propagating across the plasmapause boundary layer from the outside. Considering effects of the plasmapause width and wave frequency, parameter analysis has shown that when the width of the plasmapause boundary layer is narrow in comparison with the wavelength of MS waves, a significant part of waves will be reflected. In these circumstances, the WKB approximation and then ray tracing method might become invalid, and analytical approach provided in this paper could be utilized to solve the wave equation. Plain Language Summary Ray tracing approach is usually to research the propagation and retrace the excitation source of fast magnetosonic (MS) waves. However, the validity of ray tracing approach for MS wave propagations has not been examined yet, especially when it comes to sharp boundaries. In this paper, analytical expressions for electromagnetic fields of MS waves through the plasmapause boundary layer are provided for the first time. Both theoretical calculations from the analytical solution and observations of Van Allen Probes have indicated that electric (magnetic) fields of MS waves decrease (increase) rapidly when propagating across the plasmapause boundary layer from the outside. Considering effects of the plasmapause width and wave frequency, parameter analysis has shown that larger parts of waves will be reflected when the with narrower widths of the plasmapause boundary layer in comparison with the wavelength of MS waves. In these circumstances, the WKB approximation and then ray tracing method might become invalid, and analytical approach provided in this paper could be utilized to solve the wave equation. Key Points Analytical solutions for electromagnetic fields of magnetosonic (MS) waves through the plasmapause boundary layer are provided for the first time Electric (magnetic) fields of MS waves decrease (increase) rapidly when propagating across the plasmapause boundary layer from the out