Inverse Doppler effect

The Doppler effect, as any physics student knows, causes light from a receding object to shift to the red and light from an approaching object to shift to the blue. But astrophysicists have known for more than 40 years that in strongly magnetized plasmas, the interaction of electromagnetic waves with the plasma magnetic field can create strange effects. In a narrow range of frequencies above the gyro-frequency (the frequency at which electrons circle around magnetic field lines), the Doppler effect may reverse and cause approaching objects to be red-shifted rather than blue-shifted.