Magnetometer in-flight offset accuracy for the BepiColombo spacecraft

Recently ESA and JAXXA launched the two-spacecraft mission BepiColombo to explore the plasma and magnetic field environment of Mercury. Both spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO, also referred to as Mio), are equipped with fluxgate magnetometers, to provide in-situ data for the characterization of the internal magnetic field origin as well as its dynamic interaction with the solar wind. To achieve this goal, accurate magnetic field measurements are thus of crucial importance, which require proper in-flight calibration. In particular the magnetometer offset, which relates relative fluxgate readings into an absolute value, needs to be determined with high accuracy. Usually, the magnetometer offsets are evaluated from Alfvénic fluctuations observed in the pristine solar wind. However, while Mio's orbit will indeed partially reside in the solar wind, MPO will remain within the magnetosphere at most times during the main mission phase. Therefore, we examine an alternative offset determination method, based on the observation of highly compressional fluctuations, the so-called Mirror Mode Method. To evaluate the method performance in the Hermean environment, we analyze four years of MESSENGER magnetometer data, which are calibrated by the Alfvénic fluctuation method, and compare it with the accuracy and error of the offsets determined by the Mirror Mode Method in different plasma environments around Mercury. We show that the Mirror Mode Method yields the same offset estimates and thereby confirms its applicability. Furthermore, we also evaluate the spacecraft observation time within different regions necessary to obtain reliable offset estimates. Although the lowest percentage of strong compressional fluctuations are observed in the solar wind, this region is most suitable for an accurate offset determination with the Mirror Mode Method. 132 hours of solar wind data are sufficient to determine the offset to within 0.5nT, while thousands of hours are necessary to reach this accuracy in the magnetosheath or within the magnetosphere. We conclude that in the solar wind the Mirror Mode Method might be a good complementary approach to the Alfvénic fluctuation method to determine the (spin-axis) offset of the Mio magnetometer. However, although the Mirror Mode Method requires considerably more data within the magnetosphere, it might also be for the MPO magnetometer one of the most valuable tools to determine the offsets accurately.