Three component plasma electron distribution in the intermediate ionized coma of comet Giacobini‐Zinner

On 11 September, 1985, in the interval from 1056 to 1110 UT surrounding closest approach (i.e. within ∼1.2 × 10⁴ km), ICE encountered a region characterized by an unusually low electron temperature, a high density, a bulk flow speed V < 30 km/s, and a relatively low level of plasma turbulence. We call this region the intermediate ionized coma, IIC. Electron velocity distribution functions in the IIC are composed of three distinct components: cold, mid, and hot. We believe that the cold population represents electrons produced close to the comet nucleus by ionization of cometary matter and subsequent cooling by Coulomb collisions. The mid distribution also appears to be composed of electrons produced by photoionization of cometary neutrals, but sufficiently far from the comet nucleus that the distributions are largely unaffected by Coulomb interactions. Thus, the mid population is not in thermal equilibrium with the cold distribution. The temperature of the mid distribution compares well with the average energy of electrons resulting from the ionization of H₂O, which leads us to conclude that the composition of the ion population at this distance is consistent with the interpretation that H₂O is the dominant species. The hot component is most likely a population of electrons of solar wind origin. Throughout the IIC the electrostatic potential of the spacecraft was very low (< 0.8 eV), implying ICE generated very little impact‐produced plasma during its passage.