2021 ARTEMIS SCIENCE NUGGETS
Investigating the Moon's Interaction with the Terrestrial Magnetotail Lobe Plasma
by Lucas Liuzzo
Space Sciences Lab, UC Berkeley
Introduction
Earth's Moon is exposed to a wide range of particle and magnetic field environments during its orbit around Earth. When the Moon is located outside of Earth's magnetosphere, it is exposed to the rapidly flowing, dense plasma from the solar wind. However, for a few days each month, the Moon enters Earth's magnetosphere and is shielded from the solar wind, instead exposed to low-density magnetotail lobe plasma. In this study, we report observations from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft orbiting the Moon when exposed to these magnetotail lobe conditions. We find that dense, particles from the Moon's ionosphere strongly perturb the local electromagnetic environment, as occurs at multiple moons throughout the outer solar system. On the lunar nightside, we provide the first observation of an extended, cold plasma wake tilted out of the Moon's optical shadow. This geometry allowed ARTEMIS to measure an otherwise invisible, ambient magnetospheric ion population that is responsible for shaping the lunar plasma interaction. Hence, the Moon acts as a natural filter for plasma observations in the Earth's magnetotail, an effect that may therefore be used to understand loss processes of Earth's ionospheric plasma.
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| Figure 1. ARTEMIS trajectories, and observations on (a-g) October 30, 2012, and (h-n) June 12, 2014. Pink and red stars, and colored trajectory segments, highlight locations of observed low-energy "hidden" plasma and ionospheric pickup ion bursts, respectively. Gray shading and dashed lines denote the cylindrical lunar optical shadow. |
Observations
ARTEMIS consists of two probes orbiting the moon approximately once each day. Figure 1 shows the location of these probes on two separate days (October 30, 2012, and June 12, 2014), as well as observations from the P1 probe on these days. Notably on each of these days when the probes are located outside of the lunar shadow (gray shading in Figure 1), the Electrostatic Analyzers are unable to detect low-energy ions, since the Sun charges the spacecraft to positive potentials, thereby shielding low-energy ions from reaching the instruments. However, when in shadow, the spacecraft potential drops to near zero, and a previously "hidden," low-energy population of terrestrial magnetotail plasma becomes visible on both of these days. As the probe traveled closer to the Moon on October 30, 2012, this low-energy ion population disappeared, clearly making P1's entry into a plasma wake located behind the Moon. Luckily, on this day, properties of the Moon's interaction with the magnetotail lobe plasma caused this plasma wake to be tilted out of the solar shadow by ~25°, allowing P1 to initially detect the "hidden" low-energy ions before entering the downstream plasma wake.
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| Figure 2. ARTEMIS measurements and hybrid modeling of the October 30, 2012, P1 encounter. A clear cavity in the plasma density is visible in the data as well as model results, with an associated magnetic field enhancement to conserve the wakeside pressure. Additionally, the 2D structure and properties of particles from the lunar ionosphere are visible in the model results, as detected along the P1 trajectory. In the model, plasma flows from right to left. |
Modeling
To understand the three-dimensional nature of the Moon's plasma interaction on these days, and to further contextualize the ARTEMIS observations, we apply a hybrid model. This hybrid technique treats ions as individual particles (solving Newton's equations of motion to understand their dynamics), whereas electrons are treated as a massless, charge-neutralizing fluid. Figure 2 shows ARTEMIS data along with modeling results for the October 30, 2012, encounter of P1. Panel 2g illustrates that magnetospheric plasma is absorbed at the Moon's ramside (at x=+1; plasma flows from right to left), generating a density wake ~2 lunar radii wide downstream. Concurrently, the model shows that the magnetic field within this wake is enhanced, in order to maintain pressure balance. Dotted lines in the panels illustrate this wake, which is visibly tilted out of the Moon's solar shadow (denoted in panel 2g by dashed lines). Hence, using the Moon to block out sunlight, ARTEMIS was able to briefly detect the "hidden" population of low-energy ions comprising the plasma of the terrestrial magnetotail lobes before it was affected by the Moon's plasma interaction (which is responsible for forming this density wake).
Figure 3 shows model results for the June 12, 2014, encounter. This figure shows that the presence of the Moon's ionosphere causes the local magnetic environment to be perturbed. These perturbations are asymmetric around the Moon, centered rather around the x=+1 line, corresponding to the location of the densest ionospheric plasma. In panel 3d, two regions of reduced magnetotail lobe plasma density extend from the lunar surface. These "plasma absorption wings" are generated by magnetospheric particles with large field-aligned velocities that impact the Moon in one hemisphere, causing a wake-like depletion region on the opposite side of the surface. Similar features have also been observed at Saturn's moons by the Cassini spacecraft.
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| Figure 3. Model results of the June 12, 2014, ARTEMIS encounter. Note that the P1 trajectory was tilted out of the plane shown here, with regions where P1 was below/above this plane denoted by dashed/solid lines, respectively. |
This study has reported the first detection of a cold ion wake located behind the Moon when exposed to the plasma from Earth's magnetotail lobes. Properties of the lunar interaction with this magnetotail lobe plasma, as well as the location of the Moon's optical shadow during these encounters, allowed for the unique opportunity to measure a "hidden" population of low-energy magnetotail plasma that may originate from Earth's ionosphere being transported far downstream into the magnetotail. When located in the tenuous magnetotail lobe plasma, the Moon's local environment resembles those of many outer planet moons, including Saturn's moons Tethys, Rhea, and Dione. Correspondingly, many properties of the lunar plasma interaction are strikingly similar to those of the Saturnian moons, including an extended plasma wake downstream, as well as "plasma absorption wings" extending from their surfaces. Importantly, the ten-year-long ARTEMIS data set is therefore an invaluable resource: with continually-expanding data from Moon encounters, we can investigate analogous plasma regimes experienced by multiple outer planet moons—for which only a handful of encounters, e.g., from Cassini, exist—with an active spacecraft that is in our own backyard.
Conclusion
This study has reported the first detection of a cold ion wake located behind the Moon when exposed to the plasma from Earth's magnetotail lobes. Properties of the lunar interaction with this magnetotail lobe plasma, as well as the location of the Moon's optical shadow during these encounters, allowed for the unique opportunity to measure a "hidden" population of low-energy magnetotail plasma that may originate from Earth's ionosphere being transported far downstream into the magnetotail. When located in the tenuous magnetotail lobe plasma, the Moon's local environment resembles those of many outer planet moons, including Saturn's moons Tethys, Rhea, and Dione. Correspondingly, many properties of the lunar plasma interaction are strikingly similar to those of the Saturnian moons, including an extended plasma wake downstream, as well as "plasma absorption wings" extending from their surfaces. Importantly, the ten-year-long ARTEMIS data set is therefore an invaluable resource: with continually-expanding data from Moon encounters, we can investigate analogous plasma regimes experienced by multiple outer planet moons- for which only a handful of encounters, e.g., from Cassini, exist- with an active spacecraft that is in our own backyard.
References
Liuzzo, L, A. R. Poppe, J. Halekas, S. Simon, and X. Cao (2021). Investigating the Moon's interaction with the terrestrial magnetotail lobe plasma. Geophysical Research Letters, 48, e2021GL093566, doi:10.1029/2021GL093566.Biographical Note
Lucas Liuzzo is a Postdoctoral Researcher at the Space Sciences Laboratory located at the University of California, Berkeley. His research focuses on the interactions of moons throughout the solar system with the plasmas impinging onto them, applying a combination of data analysis and modeling techniques.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu