| Magnetic field is one of the basic physical quantities for studying energetic charged particle and plasma environment in space,and it is closely related to the internal state and evolution history of a planet.Accurate magnetic field measuring instruments can help us obtain high-precision spatial magnetic field information to study various physical phenomena related to magnetic fields,and to explore the environments of planets.The three-axis vector fluxgate magnetometer is one of the most widely used instruments for magnetic field measuring in space exploration missions.Since the fluxgate magnetometer has sensitivities errors,non-orthogonal errors of the angles between axes,and zero offsets,in order to calibrate the output value,we need to determine these parameters through ground calibration before the instrument launched.In addition,there exists stray fields generated from the satellite at the position of sensor installation and it cannot be identified and separated.So the static magnetic field and the instrument zero bias constitute a compound value,which is called the zero offset value.Since the zero offset trends to change slowly over time,it is necessary to calibrate the zero offset in-flight regularly to obtain true and reliable magnetic field data of environment.Based on the calibration project of Tianwen-1 Mars fluxgate magnetometer,this paper studies the ground and in-flight calibration of the spaceborne fluxgate magnetometer.The main research contents are organized as follows:1.Ground calibration of Mars Orbiter magnetometer of Tianwen-1Based on the research of various spaceborne magnetometer calibration schemes,we found that for the ground calibration when the sensor needs to be placed in a thermal chamber to determine the calibration parameters at different temperatures,the scalar calibration method that require full rotation of sensor is difficult to implement.The "thin shell" method or the vector calibration method using well-calibrated coil system is a better solution.The Mars magnetometer finally chose to use a fully calibrated coil system to obtain multiple sets of reference vector magnetic fields.The sensitivities and the angles between three axes,as well as their temperature coefficients were obtained through the vector calibration scheme.For the zero offset of the magnetometer,we are more concerned about the offset drifts with temperature changes rather than the absolute value in the ground test.We tested the zero offset drift under different electronic temperatures and sensor temperatures.2.Automatic procedure for the in-flight calibration of zero offsetRecently,the authors Wang and Pan(2021a)proposed a new method for zero offset in-orbit calibration based on the properties of Alfven waves,referred to as Wang-Pan method I.Wang and Pan(2021a)found that there exists an optimal offset line(OOL)determined by the properties of pure Alfven waves in an offset cube.The real zero offset point is exactly on this line,and the direction of OOL is consistent with the direction of the vector determined by the average magnetic field.Thus,the zero offset can be determined by the intersection of at least two non-parallel OOLs.Since no pure Alfven waves exist in the interplanetary magnetic field,we need to choose fluctuation events with highly Alfvénic properties as much as possible in practical applications.Therefore,this paper proposes an automatic procedure to select the fluctuation events with highly Alfvénic properties in the interplanetary magnetic field,and then calculate the zero offset based on the Wang-Pan method?.The automatic procedure consists of following three parts:(1)selection of highly Alfvénic fluctuation events,(2)determination of the corresponding OOL according to the selected fluctuation event,and a further evaluation for the applicability of selected event,(3)determination of the zero offset.We test our automatic procedure by applying it to the 90 days' magnetic field data measured by the fluxgate magnetometer onboard Venus Express.The test results show that our automatic procedure is able to achieve as good results as the Davis-Smith method.The probabilities of the difference between the zero offset determined by these two methods within-0.5 to 0.5 nT are about 64.2%,87.2%,and 73.9%for three axes respectively.One advantage of this procedure is that the criteria for selecting the highly Alfvenic fluctuation events are not complicated and the fluctuations can also be used for Davis-Smith method.3.Error analysis for the in-flight calibration of zero offset using linear magnetic hole structures in different spatial regionsA new method based on the assumption that the magnetic depression is only in the maximum variance direction was proposed by Wang and Pan(2021b)recently.Wang and Pan(2021b)found that each mirror mode structure has an OOL,which is expected to pass through the real zero offset point,and direction of OOL is consistent with the direction of the maximum variance of the mirror mode structure.Then,the intersection of multiple non-parallel OOLs can determine the zero offset value.We select the linear magnetic hole in the regions of the solar wind,terrestrial magnetosheath and magnetotail current sheet,and determine the corresponding OOLs according to the Wang-Pan method ?.We analyzed the error between the OOL and the real zero offset point.The results show that in the solar wind and magnetotail current sheet,the calculation error of the Wang-Pan method ? is as high as 70%within ±0.2 nT,and the calculation error is about an order of magnitude in the magnetosheath larger than that in the solar wind or magnetotail current sheet.In addition,we analyzed the influence of the typical characteristics of linear magnetic holes in different regions(the ratio of eigenvalues of the minimum variance analysis,the strength of the background magnetic field,the degree of the magnetic depression of the structure,and the duration of the magnetic hole structure)on the error of zero offset calibration.Our research results will provide a key reference for event selection in Wang-Pan method ?. |
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