In-flight calibration of the spaceborne fluxgate magnetometer in the Martian magnetosheath

In-flight calibration of the ze ro offset is crucial for ensuring high-precision measure ment of the spaceborne fluxgate magnetomete r.Tianwen-1 is China’s first Mars mission,and its orbiter will re main out of the solar wind for tens of days each year.Previous in-flight calibration methods might not be suitable for this orbiter during such a period.Recently,a new method was proposed by Wang GQ(2022 b),which we refer to as the Wang method Ⅱ for ease of description.Here,we test the performance of this method in the Martian magnetosheath by using magnetic field data measured by the Mars Atmosphere and Volatile EvolutioN(MAVEN) spacecraft.We find that the accuracy of the Wang method Ⅱ is affected by the number of magnetic field subinterval events,the eigenvalues of the minimum variance analysis for each event,and the position of the spacecraft in the magnetosheath.The estimated zero offset varies over a period of~27 days and has a 57.3% probability of accuracy within 2.0 nT.After being smoothed with a temporal window of 27 days,the ze ro offset has a 48.4%(99.3%) probability of accuracy within 1.0(2.0) nT.Our tests suggest that the Wang method Ⅱ provides an option for the Tianwen-1 orbiter to perform in-flight calibration when the orbiter remains out of the solar wind for an extended pe riod of time.

Characteristics of the coseismic geomagnetic disturbances recorded during the 2008 M_w 7.9 Wenchuan Earthquake and two unexplained problems

Twenty-seven FHDZ-M15 combined geomagnetic observation systems(each of which is equipped with a fluxgate magnetometer and a proton magnetometer)had been installed in the China geomagnetic network before the 2008 Wenchuan earthquake,during which coseismic disturbances were recorded by 26 fluxgate magnetometer observatories.The geomagnetic disturbances have similar spatial and temporal patterns to seismic waves,except for various delays.Six proton magnetometer observatories recorded coseismic disturbances with very small amplitudes.In addition,fluxgate magnetometers registered largeamplitude disturbances that are likely to have included responses to seismic waves.However,two problems remain unresolved.First,why do these geomagnetic disturbances always arrive later than P waves?Second,why do the geomagnetic disturbances have spatial and temporal directivity similar to the main rupture direction of the earthquake?Solving these two problems may be crucial to find the mechanism responsible for generating these geomagnetic anomalies.

Magnetic field data processing methods of the China Seismo-Electromagnetic Satellite

The High Precision Magnetometer(HPM) on board the China Seismo-Electromagnetic Satellite(CSES) allows highly accurate measurement of the geomagnetic field; it includes FGM(Fluxgate Magnetometer) and CDSM(Coupled Dark State Magnetometer)probes. This article introduces the main processing method, algorithm, and processing procedure of the HPM data. First, the FGM and CDSM probes are calibrated according to ground sensor data. Then the FGM linear parameters can be corrected in orbit, by applying the absolute vector magnetic field correction algorithm from CDSM data. At the same time, the magnetic interference of the satellite is eliminated according to ground-satellite magnetic test results. Finally, according to the characteristics of the magnetic field direction in the low latitude region, the transformation matrix between FGM probe and star sensor is calibrated in orbit to determine the correct direction of the magnetic field. Comparing the magnetic field data of CSES and SWARM satellites in five continuous geomagnetic quiet days, the difference in measurements of the vector magnetic field is about 10 nT, which is within the uncertainty interval of geomagnetic disturbance.

Automatic calculation of the magnetometer zero offset using the interplanetary magnetic field based on the Wang-Pan method

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.