Operations

---

The Polar EFI can operate by command in either of two modes, one for measuring the potential differences between the spherical probes, and the other for measuring the ambient cold plasma density in the manner of a Langmuir probe.

A key feature of the EFI operation is the ability to store high time resolution measurements in an on-board memory. Such data are required to understand small spatial scale or fast time scale structures associated with the microsphysics of particle acceleration. AC magnetic field measurements (obtained by the plasma wave experiment PWI) can also be stored with the electric field data. This data collection is coordinated with high time resolution particle measurements stored in a burst memory that is a component of the Hydra experiment.

Electric Field Mode

In this mode of EFI operation, the spheres are current-biased until they float near the potential of the spacecraft. This state enables the most accurate measurement of the potential differences between spheres caused by the external electric field. The current bias can be between -500 nA and +500 nA.

Langmuir Probe Mode

In the EFI Langmuir probe mode the voltage on the spheres is biased instead. The current to the spheres is measured as the voltage is stepped between -40 V and +40 V. Cold plasma density is obtainable from the measured current-applied voltage relationship.

Burst Mode

Two microprocessors are included in the electronics box inside the spacecraft. One manages the normal EFI operations, which include 40,000 samples per second of the potential differences between several spheres and the spacecraft, between sphere pairs, and the power detected in one of three broadband filter outputs centered at frequencies of 32, 256 and 2048 Hz. Current or voltage bias sweeps are also recorded, as are housekeeping data including boom deployment verification flags. The other microprocessor manages the burst mode of operation and interfaces the EFI with other instruments on the Polar spacecraft, including MFE, PWI, Hydra and Tide. The burst memory can store up to 2 Megabytes of information at a ground-commanded data rate of up to 240,000 bytes/s. It can be triggered by:

a filter output above a set threshold

DC magnetic field or field standard deviations, as measured by MFE, above a set threshold

a trigger from the Hydra experiment
ground command

Last updated: March 24, 1997