According to Biermann’s theory the repulsive force in comet tails is due to a corpuscular radiation from the sun. It is shown that some of the difficulties of this theory can be overcome if the assumed radiation consists of beams with a frozen-in magnetic field of the same type as required in the electric field theory of magnetic storms and aurorae. The interaction between such a beam and the head of the comet produces an amplified magnetic field which determines the shape of the tail. The high accelerations, which have been observed in the tails may be due to electromagnetic forces.