How spacecraft measure space plasma up close: a review of instruments and future needs
This paper is a review of how we measure plasmas in space directly where they occur. Space plasmas include the solar wind and the charged-particle environment around Earth. “In situ” measurements—taken at the location of the plasma—let scientists watch key processes that are hard or impossible to reproduce in Earth laboratories. Modern instruments can record particle motions and electromagnetic fields with high time and energy detail while only lightly disturbing the plasma being observed.
The authors explain modern diagnostic methods for detecting plasma particles. A central example is the top-hat electrostatic analyser, an instrument that uses electric fields to sort incoming charged particles by their energy‑per‑charge and then counts them as a function of angle. By sweeping energies and directions, these analysers build the particle velocity distribution function, which shows how many particles travel at each speed and in each direction. Spacecraft also carry sensors for electric and magnetic fields so that particle measurements can be linked to the forces acting on the plasma.
The review highlights recent scientific work made possible by these instruments on heliospheric missions such as Parker Solar Probe and Solar Orbiter. Data from those spacecraft have produced new findings about particle behavior close to the Sun and in the solar wind. The authors use these examples to show what modern space plasma instruments can do in practice: measure detailed particle distributions and fields at small scales and high time resolution.
Looking ahead, the paper discusses upcoming and proposed missions that will push the field further. These include the operational space‑weather mission Vigil, the multi‑spacecraft mission HelioSwarm, the Mars mission M‑MATISSE, and the proposed electron‑astrophysics mission Debye. Meeting the science goals of these projects will require better diagnostics: higher resolution, more simultaneous measurements at different locations, and instruments that remain non‑intrusive in a wider range of plasma conditions.