Smallest Scale Magnetic Field Compressions Generated by the Solar Wind Interaction with the Moon

Abstract Short-period magnetic enhancements were detected by the MAP-LMAG magnetometer onboard Kaguya orbiting the moon in solar wind at an altitude of 100 km. The duration was typically 10 seconds, 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 to 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 smallest 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. It was detected in a low dynamic pressure of the solar wind, which is a favorable condition for the detection of limb compressions, with no recurrence. No intense local magnetic field was identified at the foot of the magnetic field line of the limb compression. The scale size deduced from the duration was as small as 11 km, 85 times as small as that of previously reported limb compressions. The other types of magnetic enhancements appeared at the magnetic discontinuities of the solar wind magnetic field, similar to the hot flow anomalies observed at the Earth’s bow shock. A similar high-pressure structure that compresses the ambient magnetic field can be generated by the solar wind ions reflected at the moon channeled back along the current sheet of a tangential discontinuity when the motional electric field points toward the current sheet. The hot 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.