The Solar Wind Electron Analyzer (SWEA), orbiting Mars on the MAVEN spacecraft, can distinguish the local electrons of the Martian ionosphere from electrons of the incoming solar wind. It does this using electric fields to bend the paths of electrons onto its detectors, identifying their distinctive energy levels. This instrument is allowing scientists to map where solar wind plasma ends and planetary plasma begins, gaining an understanding of how solar wind strips Mars of its atmosphere.
The Solar Wind Ion Analyzer (SWIA) measures the speed and density of ions in the solar wind buffeting the atmosphere of Mars….(need more info on how it works)