Upstream whistler-mode waves at planetary bow shocks: A brief review

Upstream whistler-mode waves appear to be present in front of all collisionless shocks. Because the whistler-mode group velocity exceeds its phase velocity over the frequency range in which the phase velocity increases with frequency, interesting alterations of polarization and frequency spectrum occur in the observer's reference frame. Landau resonance also plays a role in the wave properties. The source of these waves is the shock but the mechanism for wave generation is not yet understood.