In tenuous plasma regions the floating potential of a sunlit spacecraft is positively charged, reaching up to tens of Volts. The corresponding electric field disturbs the ambient plasma measurements obtained from electron and ion sensors and the large fluxes of attracted photo–electrons can significantly reduce the lifetime of the micro–channel plate. The electric field measurements can be also contaminated by the high spacecraft potential values. The Active Spacecraft Potential Control (ASPOC) neutralizes the spacecraft potential by releasing positively charged Indium ions.

ASPOC was built by a consortium led by the Institut für Weltraumforschung (IWF) for NASA's four spacecraft Magnetospheric Multiscale (MMS) mission. Each ASPOC instrument unit contains four ion emitters, whereby one emitter per instrument is planned to be operated at a time. Compared to the previous missions, MMS ASPOC includes new developments in the design of emitters and the electronics and is equipped with a more capable control software.

The main goals of this project are:

• ASPOC operations in space to control the spacecraft potential in order to ensure the maximum science output. This includes anticipatory planning for commanding the instruments depending on region traversed, as well as regular data monitoring to verify the emitter performance.

• Analysis of inflight data and comparison of those with data from numerical simulations to guarantee a constant quality of the beam and its positive effects on the plasma and field measurements under controlled potential within the regions of interest.

• Derivation of a new density data product that better characterizes the plasma environment using ASPOC inflight data and spacecraft potential.

Detailed knowledge about the operation of ASPOC, its operational status, and of potential anomalies, is critical for the interpretation of the measurements affected by ASPOC. Short–time variations of the ion beam emitted by ASPOC and a newly derived density parameter information using the beam current are important for a full understanding of plasma, electric field and spacecraft potential data, and are expected to make significant contributions for fulfilling the scientific goals of MMS.