ALBERT

Description: Compressional Pc 5 waves are important for the energy dynamics in the terrestrial inner magnetosphere because they can accelerate the relativistic electrons (Ukhorskiy et al., 2009). One of the origins of compressional Pc 5 waves is local plasma instability, but the generation mechanism of the compressional Pc 5 waves is still controversial. Recently, drift compressional instability has been focused on because it can generate compressional waves even if plasma beta is less than unity (Rubtsov et al., 2018; Mager et al., 2019; Takahashi et al., 2022). However, the wave property and excitation mechanism of the drift compressional mode are not fully understood due to the lack of observation. This study reports the role of drift resonance in the excitation of drift compressional waves observed by the Arase satellite on 19th November 2018. From the theoretical calculations of the wave frequency (Mager et al., 2013) and pressure perturbation (Takahashi et al., 2022), we concluded that the observed wave is the drift compressional mode. We estimated the azimuthal wave number (m number) from the finite Larmor radius effect and obtained m ~ -140. This is consistent with the drift resonance with protons at 20-30 keV, whose fluxes showed the most coherent oscillations. In addition to a positive radial gradient of ion temperature, we found that an inward phase space density gradient and positive df/dW sometimes appeared, which indicates that the drift resonance gives free energy to the wave, where f and W are phase space density and energy, respectively.