Electromagnetic Characteristics of Fast Magnetosonic Waves in the Inner Magnetosphere

In evaluating the effects of fast magnetosonic (MS) waves on magnetospheric particles, their magnetic spectra are often obtained from satellite observations, while electric field components are usually derived under the cold plasma approximation. However, such an approximation has not been verified with in situ observations yet. In this study, we report the electromagnetic characteristic for MS waves in various plasma environments with observations of the Van Allen Probe A. It is found that a considerable number of observed MS waves consist of dominated electrostatic components, which also suggest the importance of inspecting the estimation algorithm for the electric field components. Moreover, the comparison between results from statistical and theoretical analysis shows that electromagnetic characteristics of the observed MS waves can be well predicted by cold plasma theory. Our result indicates the validation of cold plasma approximation to estimate the electric field components of MS waves from their magnetic counterparts in the inner magnetosphere. Plain Language Summary According to the quasilinear theory, fast magnetosonic (MS) waves can interact with magnetospheric particles via cyclotron resonances. To examine the resonance condition and calculate the diffusion rate, cold plasma approximation is often utilized, from which the wave vector and the relation between wave field components (magnetic field and electric field) can be derived. Thus, validating such an approximation becomes an essential topic, which still lacks corresponding satellite observational supports. Here, we use the observations of the Van Allen Probe A to show the electromagnetic characteristic of MS waves, and then verify the cold plasma approximation for deriving the electromagnetic characteristics of MS waves in the inner magnetosphere. Key Points Survey of the electromagnetic characteristics of fast magnetosonic (MS) waves has been made with observations of the Van Allen Probe A Satellite observations verify the validation of cold plasma approximation in deriving electromagnetic characteristics of MS waves Background magnetic field and normalized wave frequency become two key factors controlling the electromagnetic characteristics for MS waves