EPP

As one of the seven scientific payloads on board the Tianwen-1 orbiter, the Mars Orbiter Magnetometer (MOMAG) will measure the magnetic fields of and surrounding Mars to study its space environment and the interaction with the solar wind. The instrument consists of two identical triaxial fluxgate magnetometer sensors, mounted on a 3.19 meter-long boom with a seperation of about 90 cm. The dual-magnetometers configuration will help eliminate the magnetic field interference generated by the spacecraft platform and payloads. The sensors are controlled by an electric box mounted inside the orbiter. Each magnetometer measures the ambient vector magnetic field over a wide dynamic range (to 10,000 nT per axis) with a resolution of 1.19 pT. Both magnetometers sample the ambient magnetic field at an intrinsic frequency of 128 Hz, but will operate in a model with alternating frequency between 1 and 32 Hz to meet telemetry allocations.

The space-borne fluxgate magnetometer (FGM) requires regular in-flight calibration to obtain its zero offset. Recently, Wang GQ and Pan ZH (2021a) developed a new method for the zero offset calibration based on the properties of Alfvén waves. They found that an optimal offset line (OOL) exists in the offset cube for a pure Alfvén wave and that the zero offset can be determined by the intersection of at least two nonparallel OOLs. Because no pure Alfvén waves exist in the interplanetary magnetic field, calculation of the zero offset relies on the selection of highly Alfvénic fluctuation events. Here, we propose an automatic procedure to find highly Alfvénic fluctuations in the solar wind and calculate the zero offset. This procedure includes three parts: (1) selecting potential Alfvénic fluctuation events, (2) obtaining the OOL, and (3) determining the zero offset. We tested our automatic procedure by applying it to the magnetic field data measured by the FGM onboard the Venus Express. The tests revealed that our automatic procedure was able to achieve results as good as those determined by the Davis–Smith method. One advantage of our procedure is that the selection criteria and the process for selecting the highly Alfvénic fluctuation events are simpler. Our automatic procedure could also be applied to find fluctuation events for the Davis–Smith method.