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The noise mechanism in a well-designed magnetometer is shown to be due to small-scale magnetic moments which are probably small volumes of the core material that are not oriented by the drive field. Design developments include the sense coil radius and the development of a drive waveform which yields considerably improved performance over the commonly used sine wave drive.
The OGO-5 fluxgate magnetometer experiment (E-14) was designed to measure the vector magnetic field over the full range of the OGO-5 orbit. Thus, it had a dynamic range of + or - 64,000 gamma yet it maintained a precision of + or - 1/16 gamma at all times. This enabled a broad spectrum of problems to be attached. Studies of the magnetospheric waves, currents, waves-particle interactions, pitch angle distributions and wave normal directions were made. The structure of the magnetopause, the magnetotail, and bow shock were probed, waves and discontinuities in the solar wind were examined and the various phases of substorms were examined in depth.
The work performed by the magnetic fields investigation team during the mission definition phases of the Outer Planets Grand Tour (OPGT) and the Mariner Jupiter Saturn (MJS) Missions is reported. This work involved three tasks: (1) defining the objectives of the magnetic fields investigations, (2) defining the magnetometer systems required to meet these objectives, and (3) developing and testing hardware elements in certain mission-specific areas.
The Apollo 16 lunar surface magnetometer (LSM) activation completed the network installation of magnetic observatories on the lunar surface and initiated simultaneous measurements of the global response of the moon to large-scale solar and terrestrial magnetic fields. Fossil remanent magnetic fields have been measured at nine locations on the lunar surface, including the Apollo 16 LSM site in the Descartes highlands area. This fossil record indicates the possible existence of an ancient lunar dynamo or a solar or terrestrial field much stronger than exists at present. The experimental technique and operation of the LSM are described and the results obtained are discussed.
The purpose of the Apollo 16 lunar portable magnetometer (LPM) experiment is to measure the permanent magnetic field at different geological sites on the lunar surface. The LPM field measurements are a vector sum of the steady remanent field from the lunar crust and of the time-varying ambient fields. The remanent magnetic fields measured in the Descartes region are the largest extraterrestrial fields yet measured in situ. These measurements show for the first time that the Descartes highlands have a stronger remanent magnetization than do the mare regions of the previous Apollo landing sites. The experimental technique used in the LPM experiment is described and the preliminary results obtained are discussed.
Apollo 12 and 15 lunar surface magnetometer data with simultaneous lunar orbiting Explorer 35 data are used to plot hysteresis curves for the whole moon. From these curves a whole-moon permeability mu = 1.029 + 0.024 or - 0.019 is calculated. This result implies that the moon is not composed entirely of paramagnetic material, but that ferromagnetic material such as free iron exists in sufficient amounts to dominate the bulk lunar susceptibility. From the magnetic data the ferromagnetic free iron abundance is calculated. Then for assumed compositional models of the moon the additional paramagnetic iron is determined, yielding total lunar iron content. The calculated abundances are as follows: ferromagnetic free iron = 5 + or - 4 wt. percent, and total iron in the moon = 9 + or - 4 wt. percent.
The High Field Fluxgate Magnetometer Experiment flow aboard the Pioneer 11 spacecraft to investigate Jupiter's magnetic field is described. The instrument extends the spacecraft's upper limit measurement capability by more than an order of magnitude to 17.3 gauss with minimum power and volume requirements.
Apollo 12 and 15 lunar surface magnetometer data with simultaneous lunar orbiting Explorer 35 data are used to plot hysteresis curves for the whole moon. From these curves a whole-moon permeability of 1.029 (+0.024 or -0.019) is calculated. This result implies that the moon is not composed entirely of paramagnetic material, but that ferromagnetic material such as free iron exists in sufficient amounts to dominate the bulk lunar susceptibility. From the magnetic data the ferromagnetic free iron abundance is calculated. Then for assumed compositional models of the moon the additional paramagnetic iron is determined, yielding total lunar iron content. The calculated abundances are as follows: ferromagnetic free iron, 5 plus or minus 4 wt %; total iron in the moon, 9 plus or minus 4 wt %.
The exploration of the interplanetary medium and the magnetospheres of the outer planets requires the implementation of magnetic field measuring instrumentation with wide dynamic range, high stability, and reliability. The fluxgate magnetometers developed for the Pioneer 11 and Mariner-Jupiter-Saturn missions are presented. These instruments cover the range of .01 nT to 2 million nT with optimum performance characteristics and low power consumption.
The main magnetic field of Jupiter was measured by the Fluxgate Magnetometer on Pioneer 11 and analysis reveals it to be relatively more complex than expected. In a centered spherical harmonic representation with a maximum order of n = 3 (designated GSFC model 04), the dipole term (with opposite polarity to the Earth's) has a moment of 4.28 Gauss x (Jupiter radius cubed), tilted by 9.6 deg towards a system 111 longitude of 232. The quadrupole and octupole moments are significant, 24% and 21% of the dipole moment respectively, and this leads to deviations of the planetary magnetic field from a simple offset tilted dipole for distances smaller than three Jupiter radii. The GSFC model shows a north polar field strength of 14 Gauss and a south polar field strength of 10.4 Gauss. Enhanced absorption effects in the radiation belts may be predicted as a result of field distortion.
A unique deployable boom developed for accurately positioning magnetometers 6 meters (19.7 feet) from a spacecraft is described. Weight, mounting system, magnetic cleanliness, thermal dimensional stability, and natural frequency were critical constraints that were met. The boom was flown on Mariner 10 and deployed flawlessly. The design, development, and testing of the boom and optical alignment of the sensors are described. Design trades and problem solutions are discussed.
Lunar analysis of data from a total of nine lunar magnetometers is described. Results obtained concerning electromagnetic, compositional, and structural properties of the lunar interior are discussed. Specific topics covered include: lunar magnetic permeability and iron abundance; limits on a highly conducting lunar core; lunar electrical conductivity; and internal structure inferred from conductivity and permeability results.
A survey of potential users of low altitude satellite magnetic measurements for solid-earth and geological studies was conducted. The principal objectives of this survey were to: document the utility and application of the data and resultant products obtained from such a satellite mission, and establish a users committee for the proposed low altitude vector magnetometer satellite.
A summary of the design of the University of California at Los Angeles' fluxgate magnetometer is presented. Instrument noise in the bandwidth 0.001 to 1.0 Hz is of order 85 m gamma. The DC field of the spacecraft transverse to the earth-pointing axis is 1.0 + or - 21 gamma in the X direction and -2.4 + or - 1.3 gamma in the Y direction. The spacecraft field parallel to this axis is less than 5 gamma. The small spacecraft field has made possible studies of the macroscopic field not previously possible at synchronous orbit. At the 96 W longitude of Applications Technology Satellite-6 (ATS-6), the earth's field is typically inclined 30 deg to the dipole axis at local noon. Most perturbations of the field are due to substorms. These consist of a rotation in the meridian to a more radial field followed by a subsequent rotation back. The rotation back is normally accompanied by transient variations in the azimuthal field. The exact timing of these perturbations is a function of satellite location and the details of substorm development.
The electrical, mechanical, and environmental interfaces between the boom-mounted flux gate magnetometer and the AMPS/Spacelab are described.
Procedures are described for the calibration of a vector magnetometer of high absolute accuracy. It is assumed that the calibration will be performed in the magnetic test facility of Goddard Space Flight Center (GSFC). The first main section of the report describes the test equipment and facility calibrations required. The second presents procedures for calibrating individual sensors. The third discusses the calibration of the sensor assembly. In a final section recommendations are made to GSFC for modification of the test facility required to carry out the calibration procedures.
Tests performed to prove the critical elements of the triaxial digital fluxgate magnetometer design were described. A method for improving the linearity of the analog to digital converter portion of the instrument was studied in detail. A sawtooth waveform was added to the signal being measured before the A/D conversion, and averaging the digital readings over one cycle of the sawtooth. It was intended to reduce bit error nonlinearities present in the A/D converter which could be expected to be as much as 16 gamma if not reduced. No such nonlinearities were detected in the output of the instrument which included the feature designed to reduce these nonlinearities. However, a small scale nonlinearity of plus or minus 2 gamma with a 64 gamma repetition rate was observed in the unit tested. A design improvement intended to eliminate this small scale nonlinearity was examined.
A two-position (0 deg and 180 deg) actuating mechanism (flipper) driven by alternately-heated wax motors (pellets) used to rotate the low field triaxial fluxgate magnetometer experiment on the 1977 Mariner Jupiter-Saturn spacecraft to its 0 deg and 180 deg positions is described. The magnetic field, power requirements, weight and volume of this device are discussed. The problems encountered in design and development of this mechanism are presented.