The electronegative plasma of the Enceladus Plume

The Langmuir probe data during the five Enceladus close flybys, during which the Cassini spacecraft crossed the plume over the south pole at an altitude of ~100 km, are being re-analyzed. The LP sweep-mode data revealed the plasma properties that consist of the positive and negative ions in the plasma, the so-called electronegative plasma. The density of the free electrons must negligible and the current by the ions dominates the observed current in both positive and negative bias voltages. Therefore, in this electronegative plasma region, the LP continuous high time resolution mode shows a local current enhancement due to the negative ion. Some current-voltage (IV) curves of the LP sweep also show the additional current likely due to doubly charged molecules. The electronegative plasma region coincides with negatively charged nanograins observed by the electron spectrometer (CAPS/ELS) and is mapped to the tiger stripes where the water vapor jet and ice grains originate. Outside the electronegative plasma, the positive ion density is still enhanced but the number of the free electrons increases, although their density is still depleted below the positive ion density and the surrounding magnetospheric plasma density. The plasma state here is dusty, with the electrons attached to the dust grains, similar to the dusty plasma observed during high altitude flybys. The charge number density of the negative ions found above the Tiger Stripes is close to the electron deficiency found in the dusty plume during distant flybys. Given the different characteristic dust sizes at different altitudes, the evolution of the dust size must take place in the plume. The low ionization and the presence of dust near the plume favor the electronegative plasma and enhance particle coagulation by mutual electrostatic attraction.