Observing the Lunar Magnetic Field through the Lunar Magnetometer and Electro-Magnetic Compatibility (EMC)
The strength of the magnetic field caused by the lunar magnetic anomaly is very weak compared with the geomagnetic field. For instance, at the height of 100 km the magnet field is only about 1 nT (nano Tesla. Tesla is a unit of magnetic flux density, nano = 10-9), only one ten-thousandth of the geomagnetic field. In order to observe such a weak magnetic field with high accuracy, not only the observation instruments are required to have high performance, but also the satellite 適aguya・must be designed to have little electromagnetic noise. The LMAG is equipped with a highly sensitive flux gate type magnetic field sensor, which can measure a magnetic field as weak as one hundred-thousandth of the geomagnetic field accurately. It measures the three components of the magnetic field as the vector field. To maximize the performance of the high-sensitivity sensor, the LMAG is attached to the tip of the mast, about 12 m away from the body of the "Kaguya". Since the 適aguya・itself will become the biggest noise source during the magnetic field observations in space, the magnetic field sensor has to be installed as far as possible from the body. Since the "Kaguya" is a mass of electronic equipment, various induced magnetic field components containing the DC and AC, will interfere with the observation. Reducing such magnetic field interference caused by the satellite body to the desired level for observation is called Electro-Magnetic Compatibility (EMC). By extending the long mast in "Kaguya" and devising the parts, circuit and wiring, the EMC satisfying the required accuracy is achieved.
Figure 1 shows the observed data by the LMAG during mast deployment on October 28, 2007. Before the deployment, the magnetic field from the body of the satellite was as strong as about 700 nT. When the deployment started, the intensity of the magnetic field reduced rapidly, and was only a few nT at last. In general, the interplanetary magnetic field around the moon is about a few nT, so we could make sure that magnetic field interference was down to a level with no influences. Supported by such thorough EMC countermeasures, we could observe the magnetic field of the moon with the LMAG.
Dynamo of the Moon and the Polar Wander Indicated by the Magnetic Anomaly
Then let's talk about the actual data analysis. The fact that there are magnetic anomalies on the moon means that some of the lunar rocks have the property 杜agnetization・ a nature as a magnet. There are many physical processes for a rock to be magnetized. Specifically we call it as thermoremanent magnetization (TRM), when the rocks are magnetized during the cooling process under the ambient magnet fields from hot lava. The TRM is aligned with the magnetic field imposed at the time of TRM acquisition. As a magnetic field recorded by the TRM, the only plausible candidate is the magnetic field generated by the dynamo, which is able to exist stably for a long term and in a large scale. Therefore, we extract the magnetization information from the magnetic anomaly, and then investigate whether it is a record of the magnetic field due to the dynamo at that time. The most important information is the magnetization orientation. If the magnetization orientation is retrieved, we can estimate the magnetic pole at that time. The magnetic pole is the intersection of the lunar surface and the extension of the positive pole (same as the S pole in the geomagnetic field) or the negative pole (same as the N pole) of the bar magnet located at the center of the moon. As mentioned above, since there is no bar magnet inside the moon, it should be treated as the virtual magnetic pole. Since February 2009, 適aguya・has conducted observations at a low altitude of 20~50 km high. In general, the quality of the data will be better when the observations are done closer to the lunar surface, which is the source of the magnetic anomalies, so we are using the data obtained from the low-altitude observations for such detailed analysis.