Ion and electron distribution functions within regions of intense electrostatic fluctuations in the Earth’s magnetosheath

The terrestrial magnetosheath is a turbulent region characterized by large amplitude electromagnetic fluctuations. The NASA Magnetospheric Multiscale (MMS) mission has allowed investigating at high time cadence electromagnetic fields and particles in the near-Earth environment. Here, we have analyzed a well documented 5 minute MMS quasi-parallel magnetosheath crossing, focusing on the kinetic plasma properties in sub-intervals where an intense electrostatic activity at high frequencies is detected. Numerical simulations have showed that an electrostatic branch of high frequency waves with an acoustic-type dispersion relation, the so-called ion-bulk (IBk) waves, can be excited by a wave-particle instability due to the formation of a plateau in the bulk of the ion velocity distribution function (VDF). Such waves can survive against the Landau damping also for small values of electron-to-proton temperature ratios, which typically induce a strong ion-acoustic wave damping. IBk waves induce large amplitude electric field fluctuations and both ion and electron phase-space trapping, giving rise to a beam in the ion VDF and a flat-top electron VDF. Motivated by numerical results we have explored the ion and electron kinetic features when time intervals of electrostatic fluctuations are detected in the magnetosheath region. Thanks to the high time cadence of the instruments on board MMS we were able to reconstruct all the energy turbulent cascade that starts at ion scales and gives rise to IBk excitation around the electron scales. The effects of the presence of IBk waves on ion and electron VDFs have also been investigated, finding good agreement with recent numerical experiments.