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The MAG instrument comprises a fluxgate magnetometer (FGM) and a vector helium magnetometer capable of operating in both vector and scalar mode (V/SHM). The instrument is intended to measure small changes in fields spanning four orders of magnitude with extremely high sensitivity. This goal is achieved in part by mounting the sensors on an 11-metre spacecraft boom; the V/SHM at the end of the boom, the FGM halfway along. The magnetometer boom distances the sensors from the magnetic field associated with the spacecraft and its subsystems, and especially from spacecraft-generated temporal field variations. Spacing the sensors at different distances along the boom allows the spacecraft fields to be better characterised and removed from the observations.

Both magnetometers are capable of measuring the magnetic-field vector at rates from 0 Hz up to 10 Hz (VHM) or at least 30 Hz (FGM). The VHM optimises low- frequency vector measurements in weak fields. The FGM is best suited to high- frequency measurements and can operate over an extremely wide dynamic range, from very weak fields up to strong fields. The twin-sensor configuration contributes to overall instrument reliability; if one sensor fails, field measurements can be made with the other sensor, with sufficient performance to achieve many of the major objectives of the investigation.

The magnetometer Key Parameter data set includes calibrated vector magnetic field measurements averaged over one minute intervals. Magnetic field vectors are provided in several different coordinate systems (KSM, KSO, KG, SC and RTN). RTN coordinates are provided during the cruise phase of the mission (before DOY 160, 2004) while coordinates that are more useful for study in the magnetosphere (KSM,KSO,KG) are are provided only after DOY 145, 2004. Data in spacecraft (SC) coordinates are provided throughout the mission.