Magnetometer data errors and lunar induction studies

In this paper we discuss errors in lunar magnetic field data obtained by the Apollo 12, 15 and 16 surface and the orbiting Explorer 35 Ames and Goddard magnetometers and the implications of these errors on investigations of electromagnetic induction in the moon. The Apollo surface instruments measure fields up to 300γ with fluctuations of about 15γ and the Explorer magnetometers measure fields of about 10γ with 5γ fluctuations. Typical errors of these field measurements are found to be on the order of 1γ or 1% of the largest fields measured. Comparison of data from the Apollo 12 and Explorer 35 Ames and Goddard magnetometers has allowed measurement of the relative gains and offsets of these three instruments between day 323 of 1969 and day 179 of 1970. During the first half of this period, (i.e., the first four lunations of Apollo 12 magnetometer operation), all three instruments generally are in good agreement with the gains and offsets differing by 1 to 2% and 0.5γ, respectively. Commencing with the fifth Apollo 12 lunation the two Explorer magnetometer gains and offsets are significantly different. The degraded operation of one or both magnetometers results in gain differences increasing up to 60% and offset differences over 1γ by the eight Apollo 12 lunation. Neither Explorer 35 magnetometer agrees well with the Apollo 12 instrument during this later period. The Apollo x Alsep axis gain coincides better with that of the Ames instrument but the y Alsep axis gain corresponds to the Goddard magnetometer. It is probable that both Explorer magnetometers begin to malfunction by the fifth lunation. Coincident with the increase in again and offset errors an additional data anomaly appears which is associated with exit of the spacecraft from the lunar sun shadow. This error in the spin plane attains a maximum value of 7γ, which decays monotonically within approximately 3 hours of shadow exit. This malfunction probably results from orientation error in the spin plane sensors. Three other errors in the Apollo surface magnetometer data are also discussed. The effects of magnetic data errors on electrical conductivity are estimated by determining the envelope of conductivity profiles resulting from the errors delineated by this study. Adjustment of previously published bulk lunar magnetic permeability results to account for magnetometer gain uncertainties, yield a lunar permeability of μ=1.012+/-0.011. We conclude that careful selection of magnetic data used in any study of the moon or its environment is the best procedure for minimizing the effects of these errors.