Observation and Fully Thermal Simulation of Quasi‐Electrostatic Magnetosonic Waves

Magnetosonic (MS) waves are generally considered as electromagnetic harmonic fluctuations below the lower hybrid frequency. Here we present a correlated observation of proton ring‐like distribution and low harmonic magnetosonic waves with undetectable magnetic components (called QEMS). We conduct fully thermal simulations (WHAMP) to investigate the wave properties. The calculated growth rates can reproduce the observed spectral characteristics and demonstrate that the QEMS waves are excited as ion Bernstein modes by proton ring‐like distribution. The ratios between the wave electric and magnetic amplitudes E/B given by WHAMP indicate that the magnetic spectral densities are below the instrument noise level, and the ratios are ∼7–80 times larger than the wave phase speeds Vph, suggesting that the QEMS waves are essentially quasi‐electrostatic, not just visually quasi‐electrostatic due to the measurement limit. This study provides further insights into the QEMS natures. Plain Language Summary Magnetosonic waves play a significant role in the magnetospheric dynamics. They usually manifest as harmonically structured electromagnetic emissions with frequencies lower than the lower hybrid frequency. Here we present a representative event of seldom reported low harmonic quasi‐electrostatic magnetosonic (QEMS) waves, which are invisible in the measured magnetic field spectra and occur together with proton ring‐like distribution. We conduct fully thermal simulations of ion Bernstein instability to investigate these waves. The calculated growth rates can well address the wave spectral characteristics and suggest that the proton ring‐like distribution is the energies source for the wave excitation. The QEMS waves may still have electromagnetic nature because the wave magnetic field may still exist but is below the instrument noise, which is true by a theoretical estimation using the measured electric field and theoretical E/B ratio. Thus, to justify the electrostatic nature, we compare E/B ratios with wave phase speed and find that E/B ratios are indeed much greater than the wave phase speed. This testifies the electrostatic nature of the QEMS waves. Our study can improve our understanding of QEMS waves. Key Points Correlated observations of low harmonic quasi‐electrostatic magnetosonic (QEMS) waves and proton ring‐like distribution are reported Fully thermal simulations indicate that the QEMS waves are excited by the proton ring‐like distribution as ion Bernstein mod