The first 3D ionospheric imager in orbit, the Electric Field Instrument (EFI) determines the ion density, the ion drift velocity, and the electric field at the front panel of the Swarm satellites.

The instrument consists of three major components: the Langmuir Probe, the Thermal Ion Imager, and the Electronics Assembly. It measures drift and acceleration in high resolution to characterise the electric field around Earth and plasma density mapping in conjunction with GPS. The probe provides measurements of electron density, electron temperature and the electric potential of the satellite.