Whistler Waves in the Foot of Quasi‐Perpendicular Supercritical Shocks

Whistler waves are thought to play an essential role in the dynamics of collisionless shocks. We use the magnetospheric multiscale spacecraft to study whistler waves around the lower hybrid frequency, upstream of 11 quasi‐perpendicular supercritical shocks. We apply the 4‐spacecraft timing method to unambiguously determine the wave vector k of whistler waves. We find that the waves are oblique to the background magnetic field with a wave‐normal angle between 20° and 42°, and a wavelength of around 100 km, which is close to the ion inertial length. We also find that k is predominantly in the same plane as the magnetic field and the normal to the shock. By combining this precise knowledge of k with high‐resolution measurements of the 3D ion velocity distribution, we show that a reflected ion beam is in resonance with the waves, opening up the possibility for wave‐particle interaction between the reflected ions and the observed whistlers. The linear stability analysis of a system mimicking the observed distribution suggests that such a system can produce the observed waves. Plain Language Summary The interaction between waves and particles is proposed to be one of the main mechanisms for energy dissipation at collisionless plasma shock waves. Of particular interest are a type of wave called whistlers, they fall in a frequency range that allows interactions with both electrons and ions, making them important for energy transfer between the two species. Their mechanism of generation is still not fully understood. We use data from the 4 magnetospheric multiscale spacecraft to unambiguously characterize whistler waves upstream of quasi‐perpendicular supercritical shocks. We find that the waves are oblique to the background magnetic field with a wavelength around the width of the shock and a frequency around the lower hybrid frequency. We also find that the shock‐reflected ions are at a velocity that allows them to exchange energy with the waves, making them a likely source. We confirm this conclusion by using a computer code to model the system and study its stability. Key Points Using the magnetospheric multiscale spacecraft, we characterize whistler waves upstream of quasi‐perpendicular supercritical shocks Shock‐reflected ions are found to be in resonance with the observed whistlers, indicating their importance for the generation of the waves Results from a dispersion solver agree with the observations supporting the kinetic cross‐field streaming instability a