The purpose of the lunar portable magnetometer (LPM is to measure the permanent magnetic field at different geological sites on the lunar surface. These measurements can be used to determine the present magnetic and structural properties of the local region and to explain magnetic aspects of the history of the Moon.

Remnant magnetic fields were measured at the landing sites of Apollo 12, 13, 14, 15, and 16 missions. During the Apollo 16 mission to the lunar highlands, the magnetic field of 313  γ measured in the North Ray area proved to be the highest ever measured on another body of planetary size. Other field measurements obtained by the commander and the lunar module pilot at different sites along the three surface traverses varied from 121 to 313 γ. In addition, an in situ magnetic field measurement was made to determind the total remnant magnetization acquired in the native lunar environment of the sample.

The discovery of fossil magnetic fields on the luanr surface has caused modification of existing ideas concerning internal lunar structure and reexamination of theories concerning the origin and early evolution of the Moon. The high magnetic fields measured in the Descartes region should have a strong effect on the accretion of solar wind particles and the reimplantation of outgassed ions into the lunar regolith (reference 12-1). This will possibly appear as a measureable effect in future angular distribution measurements of the solar wind foil experiment (reference 12-2) and in lunar sample analyses. The surface fields also provide reference values for extrapolation of subsatellite magnetometer measurements (reference 12-3) to the lunar surface. Further analysis should yield information on the geological nature and the origin of the lunar remnant field, including the possibility of an ancient lunar dynamo (references 12-4 and 12-5), a shock-induced magnetization, or another mechanism to account for the strong magnetization found in lunar surface samples.

The magnetic field measured by the LPM is a vector quantity associated with every point in the region, and its space and time variations are described by Maxwell's equations. The total magnetic field measured by the LPM at the Descartes site is primarily the steady remnant field intrinsic to lunar material; secondary fields include external solar fields, time-dependent lunar-induced fields, and fields caused by interaction of the solar wind with the remnant lunar field (reference 12-6).

By the principle of superposition, the total magnetic field B_A measured on the lunar surface can be expressed vectorially as

Where B_S is the steady remnant field at the surface site;B_E is the total external (solar or terrestrial) driving magnetic field independently measured by the Explorer 35 magnetometer and the Apollo 15 and 16 subsatellite lunar-orbiting magnetometers while outside the antisolar lunar cavity; Bμ is the magnetization field induced in permeable lunar material; B_P is the poloidal field caused by eddy currents induced in the lunar interior by changing external fields; B_T is the toroidal field corresponding to unipolar electrical currents driven through the moon by V X B_E electric field (where V is the velocity of the Moon with respect to the solar wind); B_D is the field associated with the diamagnetic lunar cavity; and B_F is the total field associated witht he hydromagnetic solar wind flow past the Moon. The fields of equation 1 can also be classed as external (B_E), permanent (B_S), induced (Bμ, B_P, and B_T) and solar wind interaction (B_D, and B_F).

The Apollo 16 lunar surface magnetometer (LSM), deployed at the Apollo lunar surface experiments package (ALSEP) site was operating during all LPM measurements. Because the LSM continuously measures three field vectors per second, it monitors all the secondary fields; that is, once B_S has been determined by vector subtraction. Therefore, equation 1 can be used to calculate the steady remnant field B_S at each LPM deployment site

For the experimental technique, the self contained LPM shown in figure 1 is used to measure the steady magnetic field at different points along the lunar traverse of the astronauts. The LPM field measurements are a vector sum of the steady remnant field from the lunar crust and of the time-varying components of the field; these components are later subtracted from the LPM measurements to give the desired resultant steady field values caused by the magnetized crustal material. The LPM consists of a set of three orthogonal fluxgate sensors mounted on top of a tripod; the sensor-tripod assembly is connected by means of a 15-m ribbon cable to the electronics box, which is mounted on the lunar roving vehicle (Rover). The 15-m cable length was determined from magnetic properties tests of the Rover. The LPM has been calibrated by using by using magnetic reference instruments directly traceable to the National Bureau of Standards. The pertinent LPM characteristics are listed in table 1.

The fluxgate sensor, shown schematically in figure 2, is used to measure the vector components of the magnetic field in the magnetometer experiment. Three fluxgate sensors (references 12-7 and 12-8) are orthogonally mounted in the sensor block as sown in figure 12-1. Each sensor weighs 18g and uses 15 mW of power during operation. The sensor consists of a flattened toroidal core of Permalloy that is driven to saturation by a square wave at a frequency f₀ = 7250 Hz. This constant-voltage square wave drives the core to saturation during alternate half cycles and modulates the permeability at twice the drive frequency. The voltage induced in the sense windings is equal to the time rate of change of the net flux contained in the area enclosed by the sense winding. This net flux is the superposition of the flux from the drive winding and the ambient magnetic field. The signal generated in the sense winding at the second harmonic of the drive signal will be amplitude modulated at a magnitude proportional to the ambient magnetic field. The sensor electronics amplifies and filters the 2f₀ sense-winding signal and synchronously demodulates it to derive a voltage proportional to the ambient magnetic field. After demodulation, the resulting signal is amplified and used to drive the feedback winding to null out the ambient field within the sensor. Operating at null increases thermal stability by making the circuit independent of core permeability variations with temperature.

The sensor block, mounted on the top of a tripod, is positioned 75 cm above the lunar surface. The tripod assembly consists of a latching device to hold the sensor block, a bubble level with 1 degree annular rings, and a shadowgraph with 3 degree markings used to align the device along the Moon-to-Sun line.

The magnetometer electronics is self-contained with a set of mercury cells for power and three digital displays for visual read-out of the magnetic field components. A block diagram of the instrument is shown in figure 3. The sensors are driven into saturation by a 7.25 kHz square wave, and a 14.5 kHz pulse is used to demodulate the second harmonic signal from the sense windings. The amplifier output is synchronously demodulated, producing a direct-current output voltage proportional to the amplitude of the ambient magnetic field. This demodulated output is used to drive the feedback winding of the sensor so that the sensor can be operated at null conditions. The demodulated output from each channel is passed through a low pass filter with a time constant of 20 sec. Upon actuation of the READ switch, this filtered analog signal is converted to a digital 9-bit binary number. The output of the analog-to-digital converter then goes to a storage register for display by the light-emitting diode numeric indicator. Three numeric indicators are used for each axis and read out in octal from 000 to 777 for magnetic field values from -256 to +256 γ

The astronaut operation is crucial to the execution of the experiment. The following measurement sequence is conducted (figure 4). Leaving the electronics box on the Rover, the astronaut turns the power switch on, reads the digital displays in sequence, and verbally relays the data back to Earth. At the first site only, two sets of additional readings are taken with the sensor block first rotated 180 degrees about a horizontal axis and then rotated 180 degrees about a vertical axis. These additional readings allow determination of a zero offset for each axis.