## Introduction

Image Credit: Com Dev Corporation

The Swarm mission is designed to make the best-ever measurements of the Earth's low-frequency electromagnetic field environment, by virtue of new-generation instrumentation and simultaneous observations on multiple platforms, allowing separation of the influence of sources within the Earth and  those in the near Earth space environment.

Two of the Swarm satellites orbit at an altitude of approximately 460 km; the third is at 510 km.  Each satellite carries identical instrumentation including an Electric Field Instrument (EF) comprising two Thermal Ion Imagers (TIIs)  and two Langmuir probes (LPs), all provided by a consortium that includes the University of Calgary,  the Swedish Institute for Space Physics (IRF), and COM DEV International.

The TII sensors use a CCD detector to record images of ion distribution functions, which are energy-angle maps from which ionospheric flows and temperatures can be derived.  Distributions functions are processes partially on board and further on the ground to produce data products at a rate of 2 s$^{-1}$.  The LPs measure spacecraft potential, plasma density, and electron temperature.  Electric fields are determined through the relation $\vec{E_\perp} = -\vec{v_i}\times\vec{B}$, which is accurate for altitudes above approximately 150 kilometers.

Full distribution images (left) are telemetered periodically to allow checks of on-board processing.

## The EFI Principle of Operation

Image Credit: Dr. Johnathan Burchill and Com Dev Corporation

The Swarm Electric Field Instruments are imaging charged-particle detectors. Positively charged ions enter a detector aperture that views in the forward-facing direction $\pm 90^\circ$. Ions are deflected by electrostatic fields that focus them onto a  micro-channel plate (MCP) detector, causing an electron cascade.  The resulting electrons strike a phosphor screen, releasing photons. This optical distribution is then imaged on a charge coupled device (CCD) chip and recorded.

Technical and scientific papers about the Swarm EFIs are available in our list of publications.