Formation of Standing Whistler Waves in Earth's Magnetosheath

The dispersion of shock is universal in various media, and in plasmas, standing whistler waves represent the dispersion of collisionless shocks. However, at present, our understanding of the plasma behavior and electric field properties within these waves remains limited. Using conjoint THEMIS and Magnetospheric Multiscale (MMS) observations, we report the first observations of standing whistler waves upstream of fast shocks in Earth’s magnetosheath resulting from the interaction between solar wind tangential discontinuities and the bow shock. High-resolution MMS measurements provide unprecedented insights into these waves, characterizing their circular polarization, near-parallel propagation to the shock normal, and fixed phase relative to the shock ramp. Moreover, generated ion acoustic waves and wave‒particle interactions are observed in these waves. These findings highlight that the magnetosheath is a compelling region for investigating standing whistler wave properties.