Sea Wave Fields Research Articles

The induced magnetic field of sea waves

Electric currents generated by the circular motion of seawater in the earth's magnetic field give rise to small alternating magnetic fields. These minute fields have the frequency of the ocean waves whipped up by winds. For a 100-m wave with 5-m height, a period of 8 seconds, sea state 6, the field varies from about 3 γ at the surface of the sea wave to the order of 0.1 γ at a depth of 95 m. This is a less rapid decay than the magnetic field has above the surface due to the same induced currents in the seawater. The component of the field in the direction of propagation reverses direction at a depth of approximately 8 m and reaches a negative maximum at about 24 m. The net effect is a rotating magnetic field in a vertical plane having a cyclic varying amplitude and angular velocity, with forward rotation below the reversal level and reverse rotation above that depth. For a more usual wave (36-m length, 4.8 seconds period, and 0.82-m amplitude, a moderately rough wave in the lower edge of state 4) the field varies from about 0.6 γ at the surface to 0.1 γ at a depth of 22 m. The reversal of the field component in the direction of propagation of the sea wave in this case occurs at about 3m.