On the size of the cometary tail magnetic field

Ershkovich has recently criticized earlier arguments by Mendis for substantial magnetic fields (B"" 1001) in the cometary plasma tail. It is shown that these criticisms are unjustified. Contrary to Ershkovich's claim, it is shown that the folding rate of the tail rays is indeed indicative of a substantial tail magnetic field. Much of the disagreement stems from Ershkovich's idealization of the plasma tail of the comet as a uniform plasma cylinder immersed in the solar wind and separated from it by a well-defmed discontinuity surface across which pressure balance is maintained. The observations are indicative of a dynamical situation, not one of static equilibrium. The rapidly folding tail ray morphology is analogous to that of the terrestrial magnetotail, wherein the 'cometary tail magnetic field' is simply the vector sum of the interplanetary magnetic field and the fields associated with the current system in the plasma tail. Ershkovich's arguments for small magnetic fields based on his interpretation that the amplitude of the helical waves observed in comet tails is unconvincing since it is based on his highly oversimplified and inaccurate model. These helical waves are likely to be associated with field-aligned current discharges from the tail.