A statistical study of three-second foreshock ULF waves observed by the Magnetospheric Multiscale mission

We perform a statistical study of 3-s ultra-low frequency (ULF) waves using Magnetospheric Multiscale observations in the Earth's foreshock region. The average phase velocity in the plasma rest frame is determined to be anti-sunward, and the intrinsic polarization is right-handed. We further examine the linear instability conditions based on the drift-bi-Maxwellian distribution functions according to the observed plasma conditions. The resulting instability is a solution to the common dispersion equation of the ion/ion right-hand non-resonant and left-hand resonant instabilities. The predicted wave propagation is also predominantly anti-sunward. The cyclotron resonant conditions of the solar wind and backstreaming beam ions are evaluated, and we find that, in some cases, the anti-sunward propagating waves can be resonant with beam ions, which was overlooked in previous studies. The study suggests that the dispersion equation provides the 3-s ULF waves a fundamental explanation that unifies a rich variety of resonant conditions.

Automated summary

This study conducts a statistical analysis of 3-s ultra-low frequency (ULF) waves in the Earth's foreshock region using Magnetospheric Multiscale observations. The average phase velocity of the waves in the plasma rest frame is determined to be anti-sunward, with a right-handed intrinsic polarization. The study further explores linear instability conditions based on observed plasma conditions, leading to findings of ion/ion right-hand non-resonant and left-hand resonant instabilities.