ON A REMARKABLE SIMILARITY BETWEEN THE PROPAGATION OF WHISTLERS AND THE BOW WAVE OF A SHIP

It is well known that lightning-generated whistlers propagate along the Earth's magnetic field lines within a cone that at low frequencies makes an angle of 19 degrees 28' with respect to the local magnetic field. This angle turns out to be exactly the same as the half-angle of the bow wave of a ship in deep water. Both problems are complicated by the fact that the wave propagation is dispersive. In this paper we show that these two seemingly unrelated problems can be understood using the same basic approach, which is to analyze the direction of the group velocity as a function of the wave normal angle. This approach may have applications to other problems of geophysical interest, such as the bow wave generated by the interaction of a large object with a moving plasma.