On the Fine Structure of Cometary Plasma Tails

It is argued that the small-scale structures observed close to the plasma tail axis of comets may be a consequence of well-known finite-resistivity instabilities in the cometary cross-tail current sheet. While the "tearing" mode instability, which causes a breakup of the layer along the current flow lines, may be responsible for the observed small condensation or "knots" near the axis, the" ripple" mode instability may give rise to the small-scale wavy structures. Subject headings: comets plasmas