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- Title
Diffuse Whistler‐Mode Waves Detected by Kaguya in the Lunar Polar Region.
- Authors
Nakagawa, T.; Takahashi, F.; Shimizu, H.; Saito, Y.
- Abstract
The solar wind particles reflected by the lunar magnetic field are the major energy source of electromagnetic wave activities, such as the 100 s magnetohydrodynamic waves and the 1 Hz whistler‐mode waves generated by protons and the non‐monochromatic whistler‐mode waves generated by mirror‐reflected electrons. Kaguya found a new type of whistler‐mode waves at 100 km altitude above the polar regions of the Moon with a broad frequency range of 1–16 Hz. The waves appear diffuse in both the time and frequency domains, and their occurrence is less sensitive to the magnetic connection to the lunar surface. The polarization is right‐handed with respect to the background magnetic field, and the wave number vector is nearly parallel to the magnetic field perpendicular to the solar wind flow. The diffuse waves are thought to be generated by the solar wind ions reflected by the lunar magnetic field through cyclotron resonance. The resonant ions are expected to have a velocity component parallel to the magnetic field larger than the solar wind bulk speed; however, such ions were not always simultaneously detected by Kaguya. The waves may have been generated above the dayside of the Moon and then propagated along the magnetic field being convected by the solar wind to reach the polar regions to be detected by Kaguya. Plain Language Summary: Unlike Earth, the Moon is not shielded by a global magnetic field; hence, solar wind particles can access the lunar surface. Although most of these particles are absorbed by the surface, a small fraction is scattered by the surface or reflected by intense lunar magnetic fields (magnetic anomalies) back into the solar wind and can become an energy source of wave activities. The protons reflected by these magnetic anomalies generate 0.01 Hz ultra‐low frequency waves and 1 Hz electron cyclotron waves. The electrons reflected by the lunar magnetic field form a field‐aligned beam that generates broadband electromagnetic waves, whose detection depends on the magnetic connection of spacecraft to the lunar surface. Kaguya found a new type of electromagnetic waves with a broad frequency range like the waves generated by the electrons, but less sensitive to the magnetic connection like the waves generated by the reflected protons. These electromagnetic waves are preferentially observed above the polar region of the Moon, not above intense magnetic anomalies. They are thought to be generated by the solar wind ions reflected by the lunar magnetic fields and propagate along the solar wind magnetic field to the polar region being convected down the solar wind flow. Key Points: Diffuse emission of whistler‐mode waves from 1 to 16 Hz is found over the polar regions of the Moon in the solar windRight‐hand polarized waves propagate parallel to the background magnetic field without a significant Doppler shiftThe waves are thought to be generated by the ions reflected by the Moon and propagate along field lines convected by the solar wind
- Subjects
KAGUYA (Artificial satellite); SOLAR wind; DOPPLER effect; MAGNETIC fields; MOON
- Publication
Radio Science, 2022, Vol 57, Issue 1, p1
- ISSN
0048-6604
- Publication type
Article
- DOI
10.1029/2021RS007369