2023 ARTEMIS SCIENCE NUGGETS

Statistical Analysis of Lunar 1 Hz Waves Using ARTEMIS Observations

by Yuequn Lou
Institute of Space Science and Applied Technology, Harbin Institute of Technology, Shenzhen, China

Introduction

One hertz whistler-mode waves are observed near 1 Hz in the spacecraft frame with narrowband spectra. They have been observed extensively in the upstream of many solar system bodies, including both planetary bodies with and without intrinsic magnetic ﬁelds, i.e., Earth, Venus, Mercury, Saturn, Mars, Jupiter, and Uranus, and at interplanetary bow shocks. These waves are also called “narrowband whistlers” or “upstream whistler waves”. The Moon, which has neither a global-scale magnetic ﬁeld nor a dense atmosphere, provides a representative example of the interaction between the solar wind and an unmagnetized, airless body with ﬂowing plasma. Near the Moon, 1 Hz waves have also been reported based on observations from various spacecraft, including Win, Lunar Prospector (hereafter referred to as “LP”), Geotail, Kaguya, etc. Lunar 1 Hz waves are observed at an occurrence rate of ∼6.6%, with the regions where the waves often occur outside the lunar wake in the solar wind. The 1 Hz waves exhibit typical propagating and polarizing features of whistler-mode waves. Moreover, the waves are closely related to external magnetic enhancements and crustal magnetic ﬁelds. In addition, 1 Hz narrowband whistlers tend to occur at a high (40°–90°) solar zenith angle (SZA). The distributions of 1 Hz waves exhibit considerable north–south and dawn–dusk asymmetries. Furthermore, waves with strong amplitude are typically observed near the lunar crustal magnetic anomalies. This study comprehensively investigates the spatial distributions and propagating properties of lunar 1 Hz waves using well-accumulated data from ARTEMIS

Results

As shown in Figure 1, the amplitude of 1 Hz waves is generally 0.05-1 nT. For the wave distribution in the Earth-centered system (Figures 1a and 1b), the wave occurrence and amplitude exhibit no obvious dawn-dusk and north-south asymmetry. It is noteworthy that 1 Hz waves disappear at XGSE < ~-45 RE. As for the wave distribution in the Moon-centered system (Figures 1c and 1d), on the dayside, 1 Hz waves mainly occur near the Moon (XSSE < ~4 RL), and the wave strength of 1 Hz waves is most intense in this region. On the nightside, 1 Hz waves occur extensively with moderately stronger wave amplitudes near the Moon (about or greater than 0.1 nT). As |XSSE| increases, the waves generally occur closer to both sides of YSSE = 0. It should be noted that 1 Hz waves cannot be observed in the typical lunar wake region right behind the Moon. Furthermore, at √(X_SSE^2+Y_SSE^2 ) = ~11 RL, 1 Hz waves can be observed extensively with an occurrence peak in the midnight sector. As is shown in Figure 1d, in the X-Z plane of SSE coordinates, at |XSSE| < ~4 RL, 1 Hz waves are generally observed near the equator. As |XSSE| increases, the waves extend to higher latitudes. At |XSSE| = 8-12 RL on the nightside, 1 Hz waves prefer to occur at high latitudes on both hemispheres.

Figure 1. Spatial distributions of 1 Hz wave amplitude in the (a, c) X-Y and (b, d) X-Z planes of geocentric solar ecliptic (GSE) and selenocentric solar ecliptic (SSE) coordinates.

As is shown in Figure 2b, 1 Hz waves are preferable to be observed on the dawn sector (longitude range of ~-150° to -60°) of the Moon. In addition, the peak of occurrence rates occurs at high latitudes (>20 °) on both hemispheres, with occurrence rates up to ~8%. However, the corresponding number of satellite samples in the high latitudes is not abundant enough, thus decreasing the reliability of this result. Figure 2c shows the root-mean-square (RMS) wave amplitude distributions of 1 Hz waves in the longitude-latitude space. It can be seen from the figure that the amplitudes of 1 Hz waves are predominantly distributed in the range of 0.05-1 nT. Specifically, 1 Hz waves are more intense on the dayside (longitude equals -90° to 90°), with averaged wave amplitudes about or greater than ~0.7 nT. Furthermore, the wave amplitudes exhibit moderate dawn-dusk and north-south asymmetries, with larger wave amplitude at dawn and in the south. This distribution pattern is generally consistent with that of strong magnetic anomalies. Figure 2d illustrates the peak wave frequency of 1 Hz waves as a function of longitude and latitude. 1 Hz waves tend to peak at higher wave frequency on the nightside than that on the dayside, while no obvious dawn-dusk and north-south asymmetries are observed.

Figure 2. Spatial distributions of (a) total number of satellite observation points in each Longitude-Latitude bin (0° longitude represents the local noon) regardless of the presence of 1 Hz waves, (b) occurrence rates, (c) averaged wave amplitude, (d) peak wave frequency of 1 Hz waves. Local time corresponding to the longitude is marked by the red axes.

As shown in Figure 3, most (over 95%) power-weighted wave normal angles of 1 Hz waves are located at 0° - 60°. As the wave amplitudes increase, wave normal angles of 1 Hz waves tend to decrease. Specifically, the percentage of waves with small wave normal angle gradually increases, and that of large wave normal angle decreases correspondingly. For weak 1 Hz waves (0.03-0.1 nT), wave normal angles are distributed relatively evenly over a range of 10° - 50° (10° - 60° for nightside waves) with a percentage of ~20% in each wave normal angle bin. For strong waves (≥0.3 nT), wave normal angles are distributed more concentratedly with the peak of occurrence rate (>30%) located at 20° - 30° and over ~80% of the waves having a wave normal angle of 10° - 40°. Compared with nightside, dayside waves generally have a smaller wave normal angle. The ellipticities of 1 Hz waves (Figures 3d-3f) mainly distribute in the range of [-0.8, -0.3]. As the wave strength intensifies, the waves tend to be more left-hand circularly polarized, with the peak of occurrence rate accounting for ~65% at smaller ellipticity ([-0.7, -0.6]). For weak and moderate 1 Hz waves, dayside waves are slightly more left-hand circularly polarized, while nightside waves tend to be more left-hand circularly polarized for strong waves.

Figure 3. Occurrence rates of power-weighted (a-c) wave normal angle and (d-f) ellipticity on the dayside (blue) and nightside (red) for three different wave amplitude levels. From left to right, the wave amplitudes are sorted into three intervals: weak (0.03 - 0.1 nT), moderate (0.1 - 0.3 nT), and strong (> 0.3 nT). The occurrence rates are defined as the ratio between the number of 1 Hz wave events in each wave normal angle (ellipticity) bin and the total number of 1 Hz waves in the designated amplitude level.

Conclusion

Using 5.5-year observational data from ARTEMIS, we performed a detailed statistical analysis of the spatial distributions of 1 Hz waves and investigated the wave normal angle and ellipticity features of these 1 Hz emissions. The amplitude of 1 Hz waves is generally 0.05 - 1 nT. The waves can be observed over a broad range of 1.1-12 RL. It is noteworthy that 1 Hz waves cannot be observed in the lunar wake region, due to a lack of interaction with the solar wind. 1 Hz waves are favorable of occurring at the dawn sector with peak occurrence rates of ~8% at high latitude regions. The amplitude of 1 Hz waves is most intense on the dayside, with modest dawn-dusk and north-south asymmetries, showing a similar distribution pattern with strong magnetic anomalies. 1 Hz waves predominantly propagate at wave normal angles < 60° with ellipticity at [-0.8, -0.3]. As wave amplitudes increase, wave normal angles decrease, and the waves become more left-hand circularly polarized. The increase in latitudes plays the opposite role.

Besides the Moon, numerous studies have also been performed to analyze the characteristics of 1 Hz waves in planetary bodies. Their results generally show that 1 Hz waves exhibit similar properties on these planets. Specifically, 1 Hz waves mainly propagate at wave normal angles < ~55° to the magnetic field with most commonly left-handed polarization. The wave amplitude generally decreases with increasing distance from the shock. Furthermore, the wave frequencies decrease as the planets locate farther away from the Sun. As is shown in this study, the 1 Hz waves at the Moon generally show similar wave features. Our results might imply that although these planets and the Moon have different sizes and the nature of the obstacle to the solar wind, 1 Hz waves in these plasma environments might share similar generating mechanisms.

References

Lou, Y. Q., Gu, X. D., Cao, X., Wu, M. Y., Xiao, S. D., Wang, G. Q., Ni, B. B., and Zhang, T. L. (2023). Statistical Analysis of Lunar 1 Hz Waves Using ARTEMIS Observations. The Astrophysical Journal, 943(1), https://doi.org/10.3847/1538-4357/aca767.

Biographical Notes

Yuequn Lou is a post-doctoral researcher in Harbin Institute of Technology, Shenzhen. Her research focuses on the plasma waves and wave-particle interactions in the terrestrial and planetary magnetosphere.

Xudong Gu is a research associate professor in the Department of Space Physics in Wuhan University. His research interests include magnetospheric and planetary physics, space weather modeling, and very low frequency detection system.

Mingyu Wu is an associate professor in Harbin Institute of Technology, Shenzhen. His research involves basic processes in plasma physics, and planetary space environment.

Binbin Ni is a professor in the Department of Space Physics, Wuhan University. His research interests include radiation belt physics, planetary magnetospheric physics and wave-particle interactions.

Tielong Zhang is a professor in Harbin Institute of Technology, Shenzhen. His research focuses on the deep space exploration, the development of magnetometers and planetary physics.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong @ igpp.ucla.edu