资源环境科技发展态势分析平台

The electromagnetic ion cyclotron (EMIC) wave usually observed in the Earth's magnetosheath is thought to be generated through the ion temperature anisotropy instability. This paper presents an observation of a long-lasting large-amplitude EMIC wave event in the dawnside flank of the magnetosheath by the Magnetospheric Multiscale mission, lasting from 06:33:00 to 12:35:00 UT on 16 April 2018. The wave amplitude is around 0.2 nT as compared to the ambient magnetic field similar to 12 nT. The characteristic frequency and scale size are around 0.2 Hz and 1,028 km, respectively. Accompanying EMIC waves are density fluctuations, which exhibit both positive and negative correlations with the longitudinal magnetic field. Using the fitted parameters for the ion and electron phase space densities, plasma kinetic theory predicts local excitations of both EMIC and mirror instabilities, which provide the energy of these observed EMIC waves and density fluctuations.

Plain Language Summary The Earth's Magnetosheath is a region slowing down high-speed particles originating from the solar wind, protecting the Earth from the hazards of high-energy charged particles. A considerable fraction of the energy of energetic particles is transferred into plasma waves. This paper presents an observational event of large-amplitude low-frequency electromagnetic waves lasting nearly 6 hr, propagating from the outer magnetosheath to the inner magnetosheath. Density fluctuations are found to coexist with these electromagnetic waves. Applying the measurements of charged particles to modeling, we show the observed waves are likely excited by free energy provided by ions with an anisotropic temperature distribution, consistent with wave theory.