Magnetometer and Electron Reflectometer (MAG/ER)

Description

The magnetometer/electron reflectometer (MAG/ER) experiment was designed to: (1) establish the nature of the magnetic field of Mars; (2) develop appropriate models for its representation; and, (3) map the Martian crustal remanent field to a resolution consistent with the Mars Observer orbital altitude and ground track separation. The experiment consisted of two separate subsystems: the magnetometer (MAG) and the electron reflectometer (ER).

The magnetometer subsystem consisted of two triaxial fluxgate magnetometers, one placed at the end of a 6 m boom, the other placed about 2/3 of the way between the outer sensor and the main body of the spacecraft. The dual magnetometer technique, similar to instruments developed for other missions (e.g., Voyager and Magsat), allowed the real-time estimation of the spacecraft-generated magnetic field and provided redundancy for the in situ magnetic field measurements. The MAG subsystem was capable of operating over a large dynamic range of field intensities, from +/- 4 nT to +/- 65,536 nT, in eight selected ranges and three orthogonal directions.

The electron reflectometer consisted of a 360 degree x 12 degree field-of-view (FOV) hemispherical electrostatic analyzer, 12.0 cm in diameter. The ER was mounted ~3 m out on the boom. Particles entering the analyzer were focused onto an imaging detector, providing fine-angular resolution (1.4 x 14 degrees) in the plane of the FOV. Since the analyzer covered a complete 360 degrees in a plane, this allowed particle pitch angles in the range of 90+/-D degrees to be sampled, where D is the angle of the magnetic field to the plane of the analyzer's FOV. During the mission, the FOV would have been oriented to include the nadir and be perpendicular to the direction of the orbital velocity of the spacecraft. This would permit the measurement of electrons with pitch angles in a range no less than 65--115 degrees. The analyzer was stepped through 32 energy ranges every second. Particle energies between 0--20 keV were measured.

Because of the volume of data this measurement technique created, a primary task of the data processing unit for the MAG/ER was to reduce the ER data to meaningful parameters (e.g., loss cone angles, energy spectra, plasma parameters, and averaged distributions), perform averages and fits to the data, and format the information for transmission. Calibrations (to remove the effects of any spacecraft-generated field and establish instrument offsets) were to be performed by periodically performing a spacecraft roll maneuver about the nadir-pointed axis a few times during the mapping mission.

Contact with Mars Observer was lost for unknown reasons on August 21, 1993, three days before scheduled orbit insertion, so only magnetometer cruise mode data were returned for this investigation. This experiment has been re-scheduled to fly on the Mars Global Surveyor.