Dependence of Quasi-Electrostatic Magnetosonic Wave Generation on Plasma Density and Suprathermal Protons

Magnetosonic (MS) wave is one of the most common electromagnetic emissions in the magnetosphere, while quasi-electrostatic magnetosonic (QEMS) waves are seldom observed. Here we present an interesting QEMS event detected by Van Allen Probe A on 9 January 2014, in which the presence of multi-band QEMS emissions shows evident correlations with relatively low plasma densities (similar to 10 cm(-3)) and enhancements of suprathermal (similar to 10 - 100 eV) proton fluxes. Based on the observed proton distributions, simulations demonstrate very similar profiles to the observed QEMS in the wave spectral characteristics. Parametric studies indicate that the QEMS growth rate can increase by similar to 20 times with the plasma density decreasing from 50 to 10 cm(-3). Effective growth rates of higher-band QEMS occur when the suprathermal proton population is sufficiently large. This study reveals that low densities and large suprathermal proton populations are favorable for generating distinct multi-band QEMS in the magnetosphere.