Long‐Term Trends in the Calibration of the Cluster Fluxgate Magnetometer Instruments

Abstract The calibration parameters of the Cluster mission's fluxgate magnetometer instruments have been determined and updated on a per‐orbit basis throughout the 24 years of the mission. Since there is no absolute field reference for vector measurements in‐flight, various spacecraft‐to‐spacecraft and instrument‐to‐instrument comparisons have been used to assess the accuracy of the measurements. Relative accuracy—consistency of the FGM measurements between spacecraft—is typically 1% for fields up to 4,000 nT, which covers most of the key scientific regions visited by the Cluster spacecraft. Absolute accuracy is harder to quantify but may be as good as 2% over the same range. An analysis of long‐term trends in the calibration parameters shows variations on annual and mission‐length timescales, particularly for sensor offsets, while sensor gains and angles are remarkably stable. Offset variations are correlated with sensor temperature—for the Cluster 1 instrument in particular—and to a lesser extent with electronics temperature; coefficients are of the order 0.1 and 0.01 nT/C respectively. After correction for temperature effects, there is no clearly measurable offset drift over the mission lifetime. Statistics for all four spacecraft are presented. These data could be used to inform the design and implementation of future missions employing fluxgate magnetometers. The spacecraft are found to be magnetically “clean,” which allows in‐flight calibration techniques to be applied with confidence and precision, leading to consistently high‐quality magnetic‐field data sets available through the Cluster Science Archive.