Sense-checking the calibration of the Cluster FGM magnetometer spin-axis offsets using mirror mode waves in the magnetosheath

The calibrated data from the Cluster fluxgate magnetometer instruments (FGMs) aboard the four Cluster spacecraft are accessible through the European Space Agency (ESA) Cluster Science Archive (CSA). The FGM team at Imperial College – the PI institute that built and supports operation of the magnetometers – has regularly provided validated data to the CSA since its inception. The calibration and validation pipeline is well established and provides measurements at the highest instrument resolution within an uncertainty as low as 0.1 nT. New methods for magnetic field calibration have been proposed in the many years since Cluster’s commissioning. One of these uses mirror mode waves in the Earth’s magnetosheath to determine the spin-axis offsets of an in-flight magnetometer instrument. The FGM team applied this method to the Cluster instrument data during periods when the spacecraft spend a substantive proportion of their orbits in the magnetosheath, typically May-June and October-November. The offsets determined by this method were compared to those determined by the method already integrated into the pipeline. Good agreement was found between the two methods.

Due to the limitations in resource, the substantial effort that would be required to change calibration methods and re-deliver over 20 years of FGM data, and the potential impact on literature already published, the team would not recommend retroactive integration of the new method into the pipeline. However, the study provides a useful sense check of the pipeline and the data already delivered, as well as the remaining data to be delivered through to the end of the Cluster mission.