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 (∼10 cm−3) and enhancements of suprathermal (∼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 ∼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. Plain Language Summary: Magnetosonic (MS) wave is one of the most common electromagnetic emissions in the magnetosphere, while quasi‐electrostatic magnetosonic (QEMS) wave is seldom observed. Here we report an interesting event detected by Van Allen Probe A on 9 January 2014, in which multi‐band QEMS emissions occurred exactly corresponding to the presence of low plasma densities (∼10 cm−3) and enhancements of suprathermal (∼10 − 100 eV) proton fluxes. Based on the observed data of three typical time intervals, we perform simulations to investigate how the plasma density and suprathermal protons affect the generation of QEMS. Our results are quite comparable with the observations in the patterns of wave intensity and frequency characteristics. We then adjust the parameters and find that QEMS growth rate increases by ∼20 times with the plasma density decreasing from 50 to 10 cm−3. Effective growth rates of higher‐band QEMS only occur when the suprathermal proton population is ≥0.75 cm−3. These results demonstrate that the low plasma density is propitious for generating distinct QEMS, and a sufficiently large suprathermal proton population is beneficial for forming the multi‐band structure of QEMS. Key Points: Multi‐band quasi‐electrostatic magnetosonic (QEMS) was observed exactly corresponding to low plasma densities and enhanced suprathermal proton fluxesSimulations indicate that QEMS growth rate can increase by about 20 times with the plasma density decreasing from 50 to 10 cm−3A large suprathermal (∼10–100 eV) proton population is favorable for extending the QEMS to higher bands [ABSTRACT FROM AUTHOR]