The Electric Field Instrument (EFI) measures the vector electric field and thermal electron density along the Polar orbit. The electric fields may be related to the motion of magnetic fields that are carried past the spacecraft with the ambient plasma (convection electric fields), or they may be related to structures or waves associated with the aurora. They can be used in conjunction with other particle, plasma wave, and magnetic field experiments on Polar and to study the origin and nature of auroral zone processes such as energetic electron precipitation into the atmosphere or the generation of auroral electromagnetic noise emission. They also play a key role in the deposition of solar wind energy into the magnetosphere, ionosphere, and upper atmosphere through their ability to accelerate particles either directly (in the case of charged particles) or through collisions (between accelerated charged particles and ambient neutral particles).

The high latitude magnetospheric regions to be explored by the Polar spacecraft are known to be rich in electric field structure. There are underlying large-scale (hundreds to thousands of km) potential structures that result from the circulation caused by magnetospheric coupling to the solar wind, and so change in response to interplanetary parameters. There are also many imbedded small scale (<1 km) structures and waves that are related in some cases to the large scale structure, and in some cases to the physics occurring in distant regions to which they map along magnetic field lines. The manner in which these different scale structures are related is a focus of ongoing research.

It is also known that the high latitude magnetosphere contains sometimes narrow regions which are practically devoid of cold plasma. These density depletions have been found to be associated with both the small scale electric field structures and with the phenomenon known as Auroral Kilometric Radiation. Whether their role in the overall physics of auroral arc formation is more cause or effect is unknown.

Two aspects of these problems are being addressed by NASA missions. The FAST satellite (for further information, see here and also here) is investigating the microphysics of the high latitude magnetosphere between 300 km and 3500 km altitude, while the Polar spacecraft, carrying the EFI, will participate in studies of the global behavior of the magnetosphere together with the other ISTP spacecraft . The Polar EFI is expected to provide measurements along the 2 Re X 9 Re orbit of DC to 20 kHz vector electric fields in the amplitude range of .02 to 1000 mV/m, and cold electron densities in the range between 0.1 and 100 cm-3.

The EFI includes three sets of double spherical probes, two pairs of which extend to separations of 100 m or more on wire booms in the spacecraft spin plane, and the other pair are spaced along the spacecraft spin axis on rigid booms that keep them 14 m apart. The configuration of EFI on the Polar spacecraft can be seen by clicking here.