Multi-Case Observations of 3-second Waves at Earth's Foreshock

We presented magnetic field and plasma measurements of three case studies analyses of 3-seconds (or 3-s) waves as observed by MMS at the Earth’s ion foreshock region. We identified intervals of 3-s waves in each of the case studies with a strict frequency selection criteria close to ~0.3 Hz (or 3.3s). Our minimum variance analysis results indicate that these waves are nearly circularly right-handed polarized in the spacecraft frame, consistent with earlier studies. We also utilized multi-spacecraft techniques to determine an anti-sunward propagation direction of the 3-s wave in the plasma rest frame. We  found that, in all of the three case studies, the 3-s waves occur sequentially with steepened and compressive 30-s waves, and can modulate or scatter the backstreaming field-aligned ion beam to form complex ion distributions in the terrestrial foreshock. Both phenomena have not been previously reported. We further investigated the instability that generates the 3-s waves based on the ion distributions associated with the observations of 3-s waves using a linear dispersion solver. We concluded that the 3-s waves play an even more important role in wave-particle interactions than previously thought, with implications in the formation and growth of other wave modes in the ion foreshock.