Rapid cadence estimation of Earth's magnetic field structure reflecting core processes from the Iridium constellation

Characterization of Earth's magnetic field is key to understanding dynamics of core circulation and the dynamo. Space-flight measurements of the Earth's magnetic field normally use highly precise magnetometers on a few spacecraft to acquire data over the entire globe under quiescent conditions over periods of months to years. The advent of commercial satellite constellations of 10s of satellites may offer additional and complementary observations, even with low-grade magnetic field observations, by providing extensive, rapid coverage. In this analysis, we assess whether the magnetic field data acquired from the Iridium Communications constellation of 66 low Earth orbiting satellites can be used to determine the geometry of Earth's main field. The Iridium satellites are in near polar, 86° inclination, 780 km altitude, circular orbits, distributed with 11 satellites in each of six orbit planes evenly spaced in longitude. We use data from the first generation Iridium satellites, launched in the late 1990s, and acquired for scientific analysis beginning in January 2010. Although the magnetometers were digitized with 30 nT resolution, we demonstrate that the uncertainties in the data are random errors so that the statistics, 300,000 samples/day over the Earth, allow determination of the average magnetic field in a given 9° latitude by 9° longitude bin to about 3 nT. The data reduction, inter-calibration, coverage, quiet interval selection, and uncertainty assessment are presented. Time series of spherical harmonic coefficients are derived to illustrate the specification of the field geometry that these data allow.