Mercury upstream region as a natural laboratory of beam plasma experiments

Upstream region of the Mercury magnetosphere is of great interest in advancing our knowledge on the plasma waves and instabilities. The interplanetary magnetic field is nearly aligned with the solar wind stream at the distances of Mercury to the Sun with a Parker spiral angle of only about 20 degrees. A one-dimensional beam plasma system is likely realized ahead of or around the Mercury. The solar wind plasma streams away from the Sun and the beam ions (either shock-reflected ions or pickup ions) stream against the sola wind, forming a naturaly laboratory of head-on beam collider experiments at an energy scale of keV (through the electromagnetic interactions without binary collisions). We study the dielectric response of the beam plasma and develop various scenarios of beam instabilities relevant to the Mercury upstream waves in a systematic way including the right-hand resonant instability and the pickup ion cyclotron waves. Our wave model has the potential to serve as an analysis tool to estimate the beam velocity and the flow speed from the resonance frequency, particularly useful to in-situ magnetic field data analyses for MESSENGER and BepiColombo measurements.