Frequency Characteristics of Magnetosonic Waves Driven by Hot Ring-Beam Protons

Abstract Magnetosonic (MS) waves can be generated by energetic ring-beam protons, appearing as harmonics near the local proton cyclotron frequency, in the planetary ionospheres/magnetospheres. A theoretical dispersion relation is derived using kinetic theory to study the growth and frequency characteristics of these locally generated MS waves in a background of cold plasma consisting of electrons, protons, and heavier ions such as atomic and molecular oxygen ions. The real frequencies and linear growth rates are calculated numerically by choosing parameters relevant to the Martian dayside upper ionosphere, where ring-beam generated MS waves have been previously reported. The study reveals that the ring component plays a dominant role in determining the growth or damping rates of the waves, while the beam component shifts the growth peaks of each harmonic farther away from integer multiples of the local proton cyclotron frequency. This frequency shift is influenced by the harmonic number, beam energy, and propagation angle. An increase in harmonic number and beam energy increases the shift, which is further enhanced as the propagation angle deviates from 90°. While the numerical results are specific to Martian parameters, the theoretical framework presented here provides a generalized description of MS waves driven by hot ring-beam protons, relevant to diverse space plasma environments.