Early Observations with the Rosetta Langmuir Probe Instrument at the Target Comet

Rosetta provides an unprecedented opportunity to follow the evolution of the plasma environment close to a comet as activity grows and recedes from 4 AU to perihelion and out again. Like the rest of the Rosetta Plasma Consortium (RPC), the Langmuir probe instrument (RPC-LAP) has been operating from early summer 2014 to cover also the approach of Rosetta toward comet 67P/Churyumov-Gerasimenko. The instrument uses two spherical probes mounted on short (few meters) solid booms protruding from the spacecraft body. The probes can be used as classical Langmuir probes, as electric field probes with bias current and for wave observations up to 8 kHz. In the low density solar wind, probe bias sweeps are dominated by spacecraft photoelectrons, but plasma density variations can still be accessed through the spacecraft potential. As the density of the comet plasma increases with growing comet activity, direct plasma measurements become possible. We present initial data and first results on the comet plasma environment.