A Statistical Study of Quasi-Electrostatic Magnetosonic Waves

Quasi-electrostatic magnetosonic (QEMS) waves have been recently reported, referring to a distinct type of magnetosonic (MS) wave with only the electric fluctuation being detectable. Here a statistical study of QEMS waves is carried out with the Van Allen Probes data. 83% of 40,466 QEMS samples are observed with plasma density n(e) < 20 cm(-3), and intense QEMS waves tend to appear in the lower density region. QEMS waves become strong in the case of more pronounced proton rings around 10 keV and larger suprathermal proton populations. High occurrence rates and large amplitudes of QEMS waves are confined near the dayside equator (|MLAT| < 3 degrees). The wave frequencies are typically slightly below the nth harmonic of proton gyro-frequency, and the wave intensity gradually decreases with an increasing harmonic number n. Our results further demonstrate that low plasma densities and abundant suprathermal protons are beneficial for intensifying QEMS waves and contribute to the establishment of QEMS wave model.