Alfvén Waves in the Solar Wind

Mariner II plasma and magnetic‐field data are examined for explicit examples of low‐frequency hydromagnetic waves. From the magnetic‐field data, several sinusoidal waveforms are isolated. One of these clearly satisfies the hydromagnetic equations relating the magnetic‐field variation to the ion‐velocity perturbation for an Alfvén wave. This result is consistent with Barnes' theoretical prediction that only the Alfvén mode is not strongly damped in a plasma of moderate or high β.