Sub-ion-gyro scale magnetic field compressions generated by the solar wind interaction with the moon

Short-period magnetic enhancements were detected by the MAP-LMAG magnetometer onboard Kaguya orbiting the moon in the solar wind at an altitude of 100 km. The duration was typically 10 s, which corresponds to 0.5 degrees in latitude along the Kaguya orbit and a scale size of 15 km. The magnitude of the magnetic field was enhanced up to 1.5–3.6 times as large as that of the preceding quiet periods. No such magnetic enhancements were found in the upstream solar wind magnetic field. The short-period magnetic enhancements were categorized into 2 groups. One is the sub-ion-gyro-scale limb compression detected at the terminator region of the moon in a nearly constant solar wind magnetic field. The magnetic field flared away from the moon consistently with the previously known limb compressions. The scale size deduced from the duration was 11 km, 85 times as small as that of previously reported limb compressions. It is significantly smaller than the typical proton gyroradius 50–100 km in the solar wind at 1AU. The other types of magnetic enhancements appeared at crossings of magnetic discontinuities of the solar wind. Some of them were found on the nightside of the moon. A possible explanation is that they were magnetic fields compressed by the solar wind ions reflected at the moon channeled back along the current sheet of an interplanetary tangential discontinuity, similar to the hot flow anomalies observed at the Earth’s bow shock. The reflected ions themselves were not detected on the nightside of the moon, while the magnetic field compressed by the expanding region can penetrate through the moon to be detected as magnetic field enhancements on the nightside of the moon. Graphical Abstract