Plasma sheaths formed by spacecraft affect nearby electron and ion trajectories

Because spacecraft have no connection to ground, their accumulation of charge during flight creates a plasma sheath around the vehicle, affecting scientific measurements taken in flight. A recent paper by Barrie et al. studies the deflection of particles caused by this charge accumulation. By looking at detection biases of electrons and ions, the researchers created a map of particle migration to determine from where detected particles originated.

“There are subtle error signatures present in science data, and we propose that this bias effect is one source of those error signatures,” said author Alex Barrie.

The group found the particles are deflected in both energy and direction. Both electrons and ions undergo a non-uniform deflection, which results in an asymmetry in their detection. Wire booms on the spacecraft – used to measure electric fields – are found to be the primary drivers of the observed asymmetries.

“It was interesting to see how such a thin wire could have such a large effect on measurements of the surrounding plasma,” Barrie said.

To obtain particle counts and map their trajectories, data from the Fast Plasma Investigation, a plasma instrumentation suite on NASA’s Magnetospheric Multiscale (MMS) mission, were analyzed. This on-board flight data was paired with a simulation to characterize the particle behavior through the plasma sheath.

Understanding the particle paths can help scientists properly evaluate on-board data. “The real goal of this research is to perform a correction on the science data for existing missions and to inform future spacecraft design so as to avoid these effects altogether,” said Barrie. “Future space missions that mix field measurements with particle measurements could use these techniques to minimize impacts on measured science data.”

Source: “Characterizing spacecraft potential effects on measured particle trajectories,” by A. C. Barrie, F. Cipriani, C. P. Escoubet, S. Toledo-Redondo, R. Nakamura, K. Torkar, Z. Sternovsky, S. Elkington, D. Gershman, B. Giles, and C. Schiff, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5119344 .