ASPOC aboard NASA's MMS: 2. Development of ambient plasma data product under controlled potential condition

Short Description

Starting point / motivation

In tenuous plasmas the floating potential of a sunlit spacecraft is positively charged, reaching up to tens of Volts, which disturbs the ambient plasma measurements. The large fluxes of attracted photoelectrons can significantly reduce the lifetime of micro-channel plates on plasma instruments. By releasing positively charged Indium ions, the Active Spacecraft Potential Control (ASPOC) neutralizes the spacecraft potential.

ASPOC onboard NASA's four spacecraft Magnetospheric Multiscale (MMS) mission was built by a consortium led by the Institut für Weltraumforschung (IWF) for NASA's four spacecraft Magnetospheric Multiscale (MMS) mission.

Contents and goals

The objective of MMS is to explore and understand the fundamental plasma physics processes of magnetic reconnection, particle acceleration and turbulence, on both the micro- and mesoscales in the Earth’s magnetosphere. All eight MMS ASPOC units have been successfully operating since the start of the science phase in September 2015 and spin-resolution density data products based on the ASPOC beam currents were deduced for the nominal mission phase supported by the previous FFG project MMS-ASPOC.

The proposed project aims to develop new refined methods to obtain plasma environment parameters using spacecraft potential controlled by ASPOC instruments onboard the NASA MMS mission.

The goal of the current project is to significantly enhance the accuracy of density estimation from spacecraft potential compared to conventional methods applied in the previous MMS-ASPOC project by refining techniques which take into account additional effects. These include DC and AC electric fields, asymmetry in the sheath and its spin-modulation stemming from streaming of plasma (wake effects) and from the ambient magnetic fields.

Expected results

The results shall be obtained both from analysis of MMS data including new plasma regions covered during the extension phase as well as from comparison with charging model data. The proposed new methods in this project are expected to significantly reduce the error of the density product.

Furthermore, high time-resolution plasma data products will be obtained, including those from highly fluctuating field environment, which is important for studying electron-scale physics, i.e., one of the main objectives of the MMS mission.

The results will be disseminated by publication in scientific peer-reviewed journals and presentation in international meetings. Furthermore, newly developed refined density derivation methods together with data products will be made open to public. New knowledge is expected to further strengthen the expertise and competitiveness of IWF for ASPOC activities in future space plasma missions.

Project Partners


Austrian Academy of Sciences - Institute for Space Research

Contact Address

Austrian Academy of Sciences
Institute for Space Research
Dr. Rumi Nakamura
Schmiedlstraße 6
A-8042 Graz