The Magnetic Field of the Earth: Performance Considerations for Space-Based Observing Systems

The core and crustal magnetic fields of the Earth show featureswith spatial-frequency characteristics and, in the case of the corefield, temporal variation which have been studied from satellites. Wereview some of the considerations and problems inherent in makingmagnetic observations from space. We find that useful observations ofthe core and crustal fields are possible at the peak of the solar cycleeven though the greatest useful data volume is obtained at the minimum.During the last three cycles, the proportion of data with Kp < 2at solar maximum compared to minimum varied between 0.4 and 0.8.Orbit determination at the current state of the art effectively eliminatesorbit error as a problem in field measurement. However, the magneticsignature of the carrier and the plasma environment in which the mea-surements are made must be considered on a case by case basis. Wequantify the spatial resolution obtained for crustal-field anomalies atdifferent altitudes and show that observing altitude can be used to se-lect the range of spatial structures to be resolved. For bodies separatedby 350 km, the increase in resolution in going from Magsat altitude( ~ 350 km) to 150-km altitude is about an order of magnitude. Orbitalinclination, while not necessarily of critical importance for crustal-fieldmeasurements, can be a severe limitation on the determination of thespatial moments (Gauss coefficients) of the core field. Measurements atinclinations below 450, by themselves, do not allow a good determina-tion of the moments.