Magnetic field is one of the essential physical parameters to study the space physics and evolution of the solar system. There are several methods to measure the magnetic field in the space by spacecraft and rockets. Fluxgate magnetometer has been most generally used out of them because it measures the vector field accurately and does not need much weight and power budgets. When we try more difficult missions such as multi-satellite observation, landing on the celestial body and exploration in the area of severe environment, we have to modify the magnetometer or develop new techniques to make the instrument adequate for those projects. For example, we developed a 20-bit delta-sigma analogue-to-digital converter for MGF-I on the BepiColombo MMO satellite, to achieve the wide-range (±2000 nT) measurement with good resolution in the high radiation environment. For further future missions, we have examined the digitalizing of the circuit, which has much potential to drastically reduce the instrument weight, power consumption and performance dependence on the temperature. Key words: Magnetometer, magnetic field, space science. Magnet Fields in the SpaceThe magnetic fields in the space are classified into three types according to the generation sources. One is the field having the source inside or on the surface of celestial body, e.g., the Sun, planets and satellites in the solar system. In the cases of the Earth, Mercury and outer planets than Mars, magnetic dynamo is working in the liquid core, and the magnetic moment is placed at the center of the planets. The area where the magnetic field is effective spreads in the range of the planetary scale and is called "magnetosphere". Meanwhile the Moon and Mars do not have significant magnetic moment. There are weakly "magnetized" parts on the surface.The second category is the magnetic field caused by the electric current carried by plasma. For example, in the Earth's magnetosphere, magnetic field lines coming from the Antarctic region or going to the Arctic region are extended in the night direction. This configuration is represented by the summation of the magnetic fields generated by the Earth's intrinsic moment and the field along the SunEarth direction. The latter field is maintained by the electric current flowing from dawn to dusk in the plasmasheet.The third is the inductive field. One of the Maxwell's equation, rotE = −(∂B)/(∂t), represents that the rotation of the electric field is accompanied by the temporal change of the magnetic field.The time scale of the magnetic fields having the source inside or on the surface of the planets is substantially long, although they are not perfectly constant. The polarity of the earth magnetic dipole is known to have changed on the time scale of million years. The magnetic fields generated by the Copyright c TERRAPUB, 2013. electrical currents vary on the relatively short time scale, although the frequency and amplitude depend on the source phenomena. The time and spatial scales of the inductive fields, those are often ob...

One of the design challenges for future magnetospheric satellite missions is optimizing the mass, size, and power consumption of the instruments to meet the mission requirements. We have developed a digital-type fluxgate (DFG) magnetometer that is anticipated to have significantly less mass and volume than the conventional analog-type. Hitherto, the lack of a space-grade digital-to-analog converter (DAC) with good accuracy has prevented the development of a high-performance DFG. To solve this problem, we developed a high-resolution DAC using parts whose performance was equivalent to existing space-grade parts. The developed DAC consists of a 1-bit second-order sigma-delta modulator and a fourth-order analog low-pass filter. We tested the performance of the DAC experimentally and found that it had better than 17-bits resolution in 80% of the measurement range, and the linearity error was 2−13.3 of the measurement range. We built a DFG flight model (in which this DAC was embedded) for a sounding rocket experiment as an interim step in the development of a future satellite mission. The noise of this DFG was 0.79 nTrms at 0.1–10 Hz, which corresponds to a roughly 17-bit resolution. The results show that the sigma-delta DAC and the DFG had a performance that is consistent with our optimized design, and the noise was as expected from the noise simulation. Finally, we have confirmed that the DFG worked successfully during the flight of the sounding rocket.

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