2022 ARTEMIS SCIENCE NUGGETS
Spectral Properties of Whistler-Mode Waves at the Moon
by Wataru Sawaguchi and Yuki Harada
Department of Geophysics, Graduate School of Science, Kyoto University
Introduction
Unlike the Earth, the Moon does not possess a global magnetic field or a thick atmosphere, leading to direct interaction of the lunar surface with the ambient plasma. At first glance, the plasma interaction with the Moon appears vastly different from the solar wind interaction with the Earth’s magnetic field forming the terrestrial magnetosphere. However, we observe a variety of plasma disturbances driven by the Moon, which have many similarities to plasma processes operating in the Earth’s magnetosphere. Whistler-mode waves are one of the plasma waves observed around both the Moon and Earth. At the Moon, the whistler-mode waves are excited by the anisotropy of upward-traveling electrons along a magnetic field line (Figure 1).
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| Figure 1. Schematic illustration of whistler-mode wave excitation at the Moon. The surface absorption and magnetic reflection of incoming electrons lead to the anisotropy (more perpendicular electrons than parallel electrons with respect to the magnetic field) of electrons traveling outward from the Moon. These anisotropic electrons drive whistler-mode waves propagating toward the Moon via cyclotron resonance. |
Whistler-mode chorus waves in the Earth’s magnetosphere have been extensively studied. Characteristic spectral properties of the chorus waves include (a) fine frequency-time structures such as rising tone elements and (b) a gap at half the electron cyclotron frequency. Meanwhile, the lunar whistler mode waves have not been investigated in terms of their spectral properties. To find out whether the lunar whistler-mode waves show the characteristics (a) and (b), we analyze magnetic field and electron data obtained by the ARTEMIS probes, which are equipped with comprehensive instruments well-suited for such plasma wave investigation.
Results
(a) On the rising tone elements:
By utilizing high-time-resolution waveform data obtained by ARTEMIS, we identify discrete rising tone elements of whistler-mode waves around the Moon as shown in Figure 2 (Sawaguchi et al., 2021). Two-point measurements by the dual ARTEMIS probes indicate that these waves are Moon-related; the waves were detected only by the probe magnetically connected to the Moon, not by the other unconnected probe. The observed rising tone elements agree very well with the frequency sweep profiles predicted by a non-linear theory developed for the terrestrial chorus waves (Omura et al., 2008). The characteristic chirps can be heard when the signals are converted into sound in the audio range
(https://youtu.be/Y1Fmb-D5MVc)
.
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| Figure 2. Discrete rising tone elements of lunar whistler-mode waves observed by ARTEMIS (adapted from Sawaguchi et al. (2021)). The black curves in the top panel show theoretically predicted frequency sweep profiles based on Omura et al. (2008). . |
(b) On the gap at half the electron cyclotron frequency:
We statistically analyze long-term ARTEMIS data to search for the gap at half the electron cyclotron frequency. The whistler-mode wave events are categorized according to their spectral shapes. Figure 3 shows one of the results (Sawaguchi et al., 2022). An important result is that almost no “banded” events (with the gap) are detected while we find many “no-gap” events (without the gap) for the lunar whistler-mode waves. This makes a stark contrast to the terrestrial chorus, for which “banded” waves are more common than “no-gap” waves.
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| Figure 3. Spatial distributions of whistler-mode waves for different categories of spectral shapes observed by ARTEMIS in the terrestrial magnetotail (Sawaguchi et al., 2022). The occurrence of waves is shown in the magnetic field line coordinates; “B Distance” < 1 R_L indicates that the magnetic field line is connected to the Moon. |
Conclusion
The ARTEMIS observations reveal that the lunar whistler-mode waves exhibit (a) rising tone elements similar to the terrestrial chorus but (b) no gap at half the electron cyclotron frequency in contrast to the terrestrial chorus. The former implies the universality of the non-linear generation processes for frequency sweeps and the latter may provide new constraints on the generation mechanisms of the gap at half the electron cyclotron frequency.
References
Omura, Y., Katoh, Y., and Summers, D. (2008). Theory and simulation of the generation of whistler-mode chorus. Journal of Geophysical Research, 113, A04223. https://doi.org/10.1029/2007JA012622Sawaguchi, W., Harada, Y., and Kurita, S. (2021). Discrete rising tone elements of whistler-mode waves in the vicinity of the Moon: ARTEMIS observations. Geophysical Research Letters, 48, e2020GL091100. https://doi. org/10.1029/2020GL091100
Sawaguchi, W., Harada, Y., Kurita, S., and Nakamura, S. (2022). Spectral properties of whistler-mode waves in the vicinity of the Moon: A statistical study with ARTEMIS. Journal of Geophysical Research: Space Physics, 127, e2022JA030582. https://doi. org/10.1029/2022JA030582
Biographical Note
Wataru Sawaguchi was a graduate student in the Department of Geophysics, Graduate School of Science, Kyoto University. Yuki Harada is an assistant professor in the Department of Geophysics, Graduate School of Science, Kyoto University. Their research interests include plasma dynamics at unmagnetized and weakly magnetized bodies such as the Moon, Mars, and Mercury.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu