The Electric Field Instrument (EFI) aboard the Swarm satellites is the first 3D ionospheric imager in space. The instrument measures plasma density, drift and velocity in high resolution to characterise the electric field around Earth. The instrument was developed by COM DEV in Canada and was funded by the Canadian Space Agency, giving it the alternate name CEFI (Canadian Electric Field Instrument). The instrument consists of the thermal ion imager which was designed by the University of Calgary, and the Langmuir probe, which was designed by the Swedish Institute of Space Physics.

The Electric Field Instrument sensor is based on SII (Suprathermal Ion Imager) - a Canadian particle detector design - to gather precise measurements of ion winds. EFI is used to characterise the electric field about the Earth by measuring the plasma density, drift, and acceleration at high resolution; also for plasma density mapping in conjunction with GPS. EFI derives its heritage from the CPA (Cold Plasma Analyzer) instrument on Freja, the Nozomi TPA instrument and the CUSP, JOULE and GEODESIC sounding rocket missions.

The EFI instrument is comprised of three main parts: the SII (Suprathermal Ion Imager) sensors, the LP (Langmuir Probe) sensors, and the Electronics Assembly. The electronics assembly contains all of the electronics necessary to support power supply, sensor data acquisition, instrument control and communications with the spacecraft bus.

The EFI instrument consists of aluminium alloy used for its low mass, high strength and good thermal conductivity. The instrument is attached to the spacecraft via six attachment points. The SII sensors are directly installed on the radiator for heat rejection in order to limit dark current on the CCD. The EFI sensors are positioned on the Ram Side of the Swarm spacecraft. The ram side of a spacecraft is the side facing the direction of the satellite’s motion. It rams/impacts the fluid that the satellite moves through, in Swarm's case the particle environment of Earth. The Langmuir Probes are located ram-nadir.