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MAG - Magnetometer

MISSION OBJECTIVES

• Map the Jovian magnetosphere and analyze its dynamics.

• Measure fluctuations in the ambient magnetic field.

• Investigate magnetopheric-ionospheric coupling.

• Determine the whether the Galilean satellites have intrinsic magnetic fields.

• Investigate the nature of the magnetosphere's interaction with the satellites.

SUMMARY

The magnetometer (MAG) uses two sets of three sensors. The three sensors allow the three orthogonal components of the magnetic field section to be measured. One set is located at the end of the magnetometer boom and, in this position, is about 11 meters from the spin axis of the spacecraft. The second set, designed to detect stronger fields, is 6.7 meters from the spin axis. The boom is used to remove the MAG from the immediate vicinity of the spacecraft to minimize magnetic effects from the spacecraft. However, not all these effects can be eliminated by distancing the instrument. The rotation of the spacecraft is used to separate natural magnetic fields from engineering-induced fields.

Another source of potential error in measurement comes from bending and twisting of the long magnetometer boom. To account for these motions, a calibration coil is mounted rigidly on the spacecraft and puts out a reference magnetic field during calibrations.

The strength of a magnetic field is measured in units of "tesla." The magnetic field at the surface of the Earth has a strength of about 50,000 nT. (The letter "n" stands for the prefix "nano," which indicates one thousand millionths of a tesla, or, in scientific notation, 10?-9 tesla.) At Jupiter, the outboard (11-meter) set of sensors can measure magnetic field strengths in the range from +/- 32 to +/- 512 nT while the inboard (6.7-meter) set is active in the range from +/- 512 to +/- 16,384 nT.

The MAG experiment weighs 7 kilograms and uses 3.9 watts of power.

MAG DESCRIPTION

• The Magnetometer consists of an electronics box and two sets of ring core triaxial fluxgate sensors. The inboard sensor is mounted on the magnetometer boom at 6.87 m, and the outboard sensor is on the end of the boom at 11.03 m from the spacecraft spin axis.

• Each triaxial fluxgate sensor is composed of three orthogonal fluxgates mounted on a flipping mechanism similar in design to those on the Pioneer 9 spacecraft. The flipping mechanism rotates the sensor shaft by 90º and back via electrically heated bi-metallic springs and a series of levers.

• Each sensor has a fluxgate closely aligned along the spacecraft spin axis and the other two in the spacecraft spin plan, orthogonal to each other.

DESIGN DETAILS

• The main electronics package is mounted in the spacecraft body near the base of the magnetometer boom and contains the sensor drive and sense electronics, analog-to- digital converters, microprocessor, and power conditioning and control circuitry.

• Each of the fluxgates contain drive and sense coils surrounding a magnetically permeable core. The drive coils drive the permeable cores into saturation in a symmetrical way (positive and negative) twice during each drive cycle. An external field breaks the symmetry of the periodic core saturation and produces harmonics of the drive frequency. The second harmonic amplitude, which is proportional to the component of the external magnetic field along the axis of the sense coil, is measured and a feedback current is generated in a coil around the sensor to drive the amplitude to zero. A digitized voltage measurement proportional to the feedback current is transmitted to Earth as the magnetic field measurement.

• The outboard sensors have dynamic ranges of +/-32 nT and +/- 512 nT. The inboard sensors have dynamic ranges of +/-512 nT and +/-16384 nT.

INSTRUMENT PARAMETERS

• Instrument Mass: 7.2 Kg
• Power Consumption:
  • Instrument - 3.9 W
  • Flippers - 3.45 W
• Microprocessor: 1802 Type
• ROM/RAM: 4 Kb/4 Kb
• Size: 41.8 x 17.8 x 14.6 cm
• Thermal Range:
  • Operating (GLL3-210)
    • Mag. Sensors: -15 to 110 Deg. C
  • Non-operating (GLL2-310)
    • Mag. Sensors: -20 to 110 Deg. C
• Instrument Modes:
  • Off
  • On
  • POR
  • Snapshot
  • Internal Cal.
  • Inboard Sensor On/Off
  • Outboard Sensor On/Off
  • Inbd/Outbd Sensor select
  • Flip right/left
  • Flip (a toggle command)
  • Hi/Low range
  • Despin select
  • Optimal averaging
  • Flipper power On/Off