Ion acceleration in cometary plasma tails

The curious bursts of hot ions in the inner coma of comet Halley detected by plasma instruments on the Giotto probe remain unexplained. In the present paper we discuss the possible contribution from current sheet acceleration in the near‐tail region of this comet. If the transformation of the tear‐drop shape of the magnetic field free boundary of the ionospheric contact surface to an elongated current sheet is accompanied by the formation of a magnetic neutral line, the introduction of a cross‐tail electric field could be effective in generating keV ions near the neutral line and the current sheet. The beaming of these accelerated ions towards the inner coma will likely result in the production of a secondary hot ion component with degraded energies from collisional thermalization. Such an “auroral” activity could play an important role in the energetics, ionization and dynamics of cometary ionospheres.