2025 ARTEMIS SCIENCE NUGGETS
ARTEMIS Observations of Electron Cyclotron Harmonics in the Lunar Plasma Environment
by A. R. Poppe
Space Sciences Laboratory, University of California, Berkeley
Introduction
Since the Moon has no atmosphere, its surface is directly exposed to charged particles and electromagnetic fields present in interplanetary space. When charged particles interact with the lunar surface, they can be absorbed or reflected. Such particles can also liberate electrons from the lunar surface itself, a process known as `secondary electron emission’. The absorption of ambient particles and the production of new secondary electrons often leads to plasma instabilities, which in turn generate a variety of waves in the electrostatic and/or electromagnetic fields. Here, we report ARTEMIS observations of a particular type of wave known as an electron cyclotron harmonic (ECH) wave. We show that these waves are often present in correlation with times when the ARTEMIS spacecraft are magnetically connected to the lunar surface, which also tends to correlate with secondary electron and ambient loss cone distributions. Finally, through a statistical analysis, we show that a vast majority (~97%) of the ECH waves occur while the Moon is passing through the Earth’s magnetotail.
ECH Observations Near the Moon
Figure 1 shows an example ARTEMIS observation of ECH waves in the lunar plasma environment. As shown in panels 1a and 1b, the Moon was located downstream of the Earth in the magnetotail and the P1 spacecraft traversed through the lunar shadow. The P1 spacecraft was magnetically connected to the lunar surface during this time (panels 1c, 1i; see also black bar between panels 1i and 1j). In panel 1j, ECH waves are observed as the multiple horizontal bands occurring at times between ~15:00 and 15:25 between the electron cyclotron frequency (pink line) and the electron plasma frequency (green line). The narrowband emission of the ECH waves occurs at approximately the (n+1/2) values of the electron cyclotron frequency. In addition to the electrostatic waves, the ARTEMIS probes also observed significant modification to the electron distributions, as seen in panels 1f, g, and h. Panels 1f and 1g show the down-going (towards the lunar surface) and upgoing (away from the lunar surface) electron distributions, respectively. The down-going distribution is a typical and relatively undisturbed population characteristic of the magnetotail, with a ~100 eV temperature. In contrast, the upcoming electron distribution has been modified via interaction with the lunar surface. Here, high energy electrons (>~200 eV) have been absorbed by the lunar surface and a narrow (i.e., cold) additional beam of electrons is present (see also the ratio of upgoing over down-going electron fluxes shown in panel 1h). The narrow beam of upcoming electrons are presumably secondary electrons produced at the lunar surface by incident primary electrons and subsequently accelerated away from the lunar surface. Ultimately, the combination of the absorption of field-aligned electrons (known as a `loss-cone distribution’) and/or the presence of the additional upcoming secondary electron beam from the lunar surface is responsible for generating the observed ECH wave emissions.
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| Figure 1. ARTEMIS P1 observations of ECH waves and associated electron distributions on September 30, 2012. Panels (a) and (b) show the Moon’s position with respect to the Earth and the ARTEMIS P1 position with respect to the Moon, respectively. Panel (c) shows the magnetic field vector. Panel (d) shows the total plasma density. Panel (e) shows the differential ion energy flux. Panels (f), (g), and (h) show the downgoing electron spectra, upgoing electron spectra, and upgoing over downgoing ratio, respectively. Panel (i) shows the distance along the magnetic field line from ARTEMIS P1 to the lunar surface while magnetically connected. Finally, panel (j) shows the electrostatic wave spectrum with the ECH waves present as narrowband features in between the electron cyclotron frequency (pink line) and electron plasma frequency (green line). |
Statistical Distribution of ECH Waves Near the Moon
We have also explored the spatial distribution of ECH waves that occur in the lunar plasma environment in order to better understand their nature and underlying generation mechanisms. We identified all ECH wave emissions observed by the ARTEMIS spacecraft between August 2011 and December 2024. Overall, >2,000 ECH events were identified and catalogued in this time range. For each event, we also recorded the position of the ARTEMIS spacecraft with respect to the Moon and the position of the Moon (and ARTEMIS probes) with respect to the Earth.
Figure 2a shows the fraction of time that ECH observations were detected by the ARTEMIS probes with respect the Moon, located in this panel at [0,0]. Here, a majority of the ECH events occur either upstream or downstream of the Moon within ~1.5 lunar radii. A smaller number of events occur farther away. Figure 2b shows the fraction of times that ECH waves were observed near the Moon with respect to the Earth and its magnetosphere (solid and dashed lines represent the average terrestrial bow shock and magnetopause, respectively). In this frame, ECH waves occur almost exclusively (>97% of the time) with the terrestrial magnetosphere, i.e., within the dotted lines denoted the magnetopause boundary.
Taken together, these two distributions strongly suggest that ECH waves are most favorably generated during passage through the unique plasma environment of the Earth’s magnetotail along magnetic field lines that are connected to the lunar surface. Such field lines typically occur along the Moon-Earth direction (i.e., +/- X direction in panel 2a) when the Moon is within the terrestrial magnetotail lobes. The plasma environment in this region is often relatively low density (<1 cm-3) and hot (>~50 eV) which may be conducive to ECH formation. Additional detailed plasma instability and wave mode modeling is required to fully validate this hypothesis.
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| Figure 2. Spatial distributions of ECH wave occurrence in the lunar plasma environment. Panel (a) shows the frequency of ECH occurrence in the lunar-centered frame (where the +X direction points from the Moon to the Sun). Panel (b) shows the frequency of ECH occurrence in the Earth-centered frame with the terrestrial bow shock and magnetopause denoted as the solid and dashed lines, respectively. |
Conclusion
This study has presented ARTEMIS observations of electron cyclotron harmonic (ECH) waves in the lunar plasma environment. Individual case study observations of ECH waves show multiple harmonics in between the electron cyclotron (fce) and electron plasma (fp) frequencies occurring at odd-half harmonics (i.e., ~(n+1/2)*fce). Concurrent measurements of the electron distributions also show both loss-cone distributions, caused by absorption of ambient electrons by the lunar surface, and upgoing electron beams, likely secondary electrons emitted from the lunar surface and accelerated upwards away from the Moon. A statistical analysis of over 13 years of ARTEMIS observations shows that ECH waves occur primarily along the Moon-Earth line during times within the terrestrial magnetotail. This suggests that magnetic connection to the lunar surface while exposed to the low density and hot magnetotail lobe plasmas is the most conducive environment for ECH formation at the lunar surface. Overall, the collected ARTEMIS ECH wave observations, comprised of over 2,000 individual observations, is a robust dataset that can be used in future theoretical comparisons to further understand the fundamental nature of space plasmas and how the Moon alters its nearby environment.
References
Poppe, A. R., J. S. Halekas, D. M. Malaspina, X. Zhang, and V. Angelopoulos, ARTEMIS Observations of Electron Cyclotron Harmonic Waves in the Lunar Plasma Environment, Geophys. Res. Lett., 52, 2025Biographical Note
Andrew R. Poppe is an Associate Research Scientist at the Space Sciences Laboratory (SSL) located at the University of California, Berkeley. His research focuses on plasma interactions with planetary bodies throughout the solar system, including the Earth’s Moon. He also serves as the Deputy PI of the ARTEMIS mission.
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