An optimal method for in-flight calibration of the fluxgate magnetometer when the total magnetic field of Alfven waves has a drift trend

High-precision magnetic field measurement is of great significance for deep space exploration. However, the zero offset of the fluxgate magnetometer on the satellite will change slowly with time, which requires in-flight calibration. Recently, a new in-flight calibration method has been proposed, namely the Wang-Pan method. It is found that there is an optimal offset line with a high linearity in the offset cube for an Alfven wave, and the zero offset of magnetometer can be calculated by using the optimal offset lines of several Alfven waves. However, this method does not take into account the drifting tendency of the total magnetic field of the selected events. Based on numerical analysis, we found that the optimal offset line will be far away from the real zero offset and even no longer has a high linearity if the total magnetic field of the Alfven wave has a drift trend, which results in calculation errors. Nevertheless, the calculation error of the Wang-Pan method can be significantly reduced after removing such a drift trend. We verify this optimal method by using the magnetic field data obtained from the Venus Express. The results of this paper will be helpful for the application of the Wang-Pan method to the terrestrial magnetospheric satellites.