# The Value of the Moon for Heliophysics

Jasper Halekas, Menelaos Sarantos, Leonardo Regoli, Yuki Harada, Yihua Zheng, Masaki Nishino, Sarah Vines, Jamey Szalay, Peter Chi, Omar Leon, et al.

► To cite this version:

Jasper Halekas, Menelaos Sarantos, Leonardo Regoli, Yuki Harada, Yihua Zheng, et al. The Value of the Moon for Heliophysics. Bulletin of the American Astronomical Society, 2023, 55, 10.3847/25c2cfeb.ed6f0be4. insu-04237289

HAL Id: insu-04237289

https://insu.hal.science/insu-04237289

Submitted on 11 Oct 2023

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# The Value of the Moon for Heliophysics

## First Author:

Jasper S. Halekas, Department of Physics and Astronomy, University of Iowa

## Co-Authors:

M. Alves, M. Ridenti (Instituto Tecnológico de Aeronáutica)

R. Allen, D. Blewett, B. Denevi, G. Ho, M. Nord, P. Prem, L. Regoli, D. Waller, S. Vines
(Johns Hopkins University/Applied Physics Laboratory)

Y. Harada (Kyoto University)

F. Chu, A. Stanier (Los Alamos National Laboratory)

Vladimir Krasnoselskikh (LPC2E, CNRS)

L. Morrissey (Memorial University)

Y. Miyoshi (Nagoya University)

L.-J. Chen, J. Keller, M. Sarantos, P. Saxena, D. Sibeck, O. Tucker, Y. Zheng
(NASA/Goddard Space Flight Center)

H. Fuqua Haviland (NASA/Marshall Space Flight Center)

S. Yokota (Osaka University)

G. Kramer, X.-D. Zou (Planetary Science Institute)

J. Szalay (Princeton University)

C. Grava (Southwest Research Institute)

S. Fatemi (Umeå University)

L. Hood (University of Arizona)

S. Bale, R. Lillis, L. Liuzzo, A. Poppe, S. Xu (U.C. Berkeley)

P. Chi, V. Angelopoulos (UCLA)

X. Cao, J. Deca, M. Horányi, D. Malaspina, X. Wang (University of Colorado)

G. Hospodarsky, W. Kurth, R. Sawyer (University of Iowa)

C. Hartzell (University of Maryland)

O. Leon (University of Michigan)

A. Jordan (University of New Hampshire)

T. Dudok de Wit (University of Orléans)

M. Nishino (University of Tokyo)

## Synopsis:

The Moon represents an ideal heliophysics laboratory for the investigation of kinetic plasma physics, in situ measurements of the interplanetary and terrestrial environment, and remote measurements of geospace and beyond. Frequent future planned launches to the lunar vicinity, and extensive international interest, make the Moon an appealing and accessible target for valuable heliophysics investigations.
# I. Introduction

This white paper makes the case that the Moon represents an ideal heliophysics laboratory for the investigation of kinetic plasma physics, in situ measurements of the interplanetary and terrestrial environment, and remote measurements of geospace and beyond.

The Artemis-I mission, now on the verge of launch, represents the first step in a program of robotic and human exploration that aims to establish a human presence at the Moon. The anticipated increase in the frequency of launches to the Moon during the Artemis era should present numerous opportunities for quality heliophysics science. Heliophysics measurements of the radiation and plasma environment in lunar orbit and on the surface can in turn provide critical support for human exploration. Moreover, the Moon is not only a destination serendipitously enabled by human exploration, but a target worthy of sustained study in its own right.

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# II. Kinetic Plasma Physics at the Moon

*Wave-particle Interactions.* The Moon represents an ideal target for the study of plasma interactions with small scale structures and gradients. The Moon itself represents a meso-scale object, ~10-20 ion inertial lengths ($d_i$) in radius, but the lunar environment contains plasma structures down to the Debye length in scale. The solid surface of the Moon acts
primarily as an absorber, creating a trailing plasma wake structure. This wake, though tenuous, is permeated by beams of charged particles that refill the wake cavity both along (ions, electrons) and transverse (ions) to the magnetic field; these refilling beams drive a variety of plasma instabilities, primarily electrostatic in nature. In the upstream direction, the absorption of electrons by the surface also creates loss cone distributions that survive to significant distances from the Moon; these loss cones provide a source of anisotropy that generates electromagnetic instabilities. The Moon also represents a source of charged particles, produced variously by reflection/deflection of the solar wind ions and electrons from the lunar surface and magnetic and electric fields, photoemission and secondary electron emission from the surface, and ionization of the tenuous exosphere; these lunar-derived particles interact with the surrounding plasma to drive a wide range of instabilities across the electromagnetic spectrum. The presence of the Moon therefore results in numerous sources of free energy that produce an extended disturbed region that rings with a diverse mixture of electromagnetic and electrostatic plasma waves. However, not all of the observed waves and their sources have yet been identified, and gaps remain in existing measurements. The lunar region of influence thus represents a natural laboratory for future investigations of wave-particle interactions.

***Small-scale magnetic fields.*** At even smaller scales, the Moon has a patchwork of small-scale remanent crustal magnetic fields dotting its surface. These magnetized regions have lateral scales of 10s to 100s of km. Within even these small regions, the polarity of the field varies significantly, as demonstrated by Apollo LMAG measurements that revealed fields with completely different orientations at surface locations separated by less than a kilometer. As a result of their inhomogeneous character, lunar magnetic fields fall off rapidly in strength with altitude, and thus cannot withstand the solar wind dynamic pressure above altitudes of ~10-20 km or ~0.1-0.3 *d*₁. They therefore represent absolutely unique obstacles to the plasma flow, intrinsically kinetic in scale.

***Solar wind reflection.*** Despite their minute scale, the lunar magnetic fields can reflect/deflect a substantial fraction of the solar wind, with reflection fractions of up to 50% of the incoming ions observed above the strongest magnetic sources. The presence of “swirl” albedo markings in strongly magnetized regions of the lunar surface may further indicate that in some locations the reflection fraction approaches 100%. The upcoming Lunar Vertex investigation will attempt to test this hypothesis. Recent orbital observations and modern simulations suggest that the observed ion reflection may result at least in part from Hall electric fields caused by differential ion/electron motion in the lunar magnetic fields, with integrated potential changes of 100s of volts inferred from remote measurements; however, the structure of this potential remains unmeasured. In situ measurements also demonstrate that both ion and electron heating can occur, but we lack a complete accounting of how the energy of bulk plasma motion is converted to other forms in the magnetic field interaction regions. Furthermore, we do not yet have a full understanding of the temporal variability of the interaction and its response to changing solar wind conditions, nor of the effect of varying solar wind incidence angles (particularly near the terminator and poles, where the solar wind flows nearly tangential to the surface).
*Small-scale shocks and reconnection.* The lunar magnetic field interaction may involve multiple kinetic plasma processes, in vastly different physical contexts and parameter regimes than encountered in Earth's magnetosphere. At higher altitudes, observations dating back to the Apollo era reveal the formation of compressional structures that can extend to 100s of km in altitude and 1000s of km downstream. At present, it remains uncertain whether these structures represent collisionless shocks; if so, they would arguably represent the smallest such structures in the heliosphere. Alternatively, they may prove more analogous to foreshock structures such as “shocklets”. Meanwhile, in the magnetic field regions, one expects that ions should become demagnetized, while electrons should remain magnetized (tied to the magnetic field lines) until perhaps just above the surface. A similar scenario plays out in reconnection diffusion regions, and thus some have argued that the lunar magnetic field interaction contains physics similar in some ways to magnetic reconnection. We further speculate that the interaction between the interplanetary magnetic field and lunar magnetic fields may involve not just analogous physics, but in some cases could require actual magnetic reconnection, albeit perhaps of the electron-only type. At present, though, we lack the detailed low-altitude multi-point measurements needed to determine whether ion-scale and/or electron-scale magnetic reconnection takes place between solar wind and lunar magnetic fields. For the study of both small-scale shocks and reconnection, the lunar structures have the advantage of occurring at highly predictable locations, and they remain essentially fixed with respect to the Moon. Furthermore, they may not depend on macro-scale boundary conditions in the same way as foreshock structures or reconnection regions in larger-scale planetary magnetospheres, potentially allowing better isolation of the kinetic effects.

### III. In Situ Measurements of the Interplanetary and Terrestrial Environment

*Heliophysics measurements from the Moon.* The Moon represents not only a scientific target in its own right, but an excellent platform for the study of the interplanetary environment. By virtue of its orbit, the Moon passes through the pristine solar wind, the terrestrial foreshock, and the terrestrial magnetosheath and magnetotail. This location therefore enables the study of a wide range of topics, including the spatial and temporal structure of coronal mass ejections and stream interactions, solar wind turbulence, foreshock transients and their effect on the bow shock and magnetosphere, magnetic reconnection and plasmoid formation and motion in the distant magnetotail, etc. In many cases, the measurements from the lunar vicinity can be fruitfully combined with measurements from spacecraft in Earth orbit, the L1 Lagrange point, or elsewhere to flesh out our understanding of the dynamic structure of the heliosphere. Some of these topics have been and continue to be addressed by the two-probe THEMIS-ARTEMIS mission, but additional observations and vantage points would add to the scientific return from the Moon. Furthermore, THEMIS-ARTEMIS lacks several key measurements, including a complete suite of energetic particle observations, high frequency electromagnetic wave measurements (up to and above the plasma frequency), and ion mass composition, that would enable new scientific investigations. The Heliophysics Environmental and
Radiation Measurement Experiment Suite (HERMES) will fill in some of these gaps. The Lunar Surface Electromagnetics Experiment (LuSEE) investigation will likewise add some of these measurement capabilities (though only for one short lunar day!).

A partnership with exploration. The instrumentation and measurements that heliophysicists utilize for scientific purposes can also in many cases provide critical information for robotic and human explorers. For example, energetic particle sensors provide critical information on radiation environment that exploration will have to contend with, including not only the primary cosmic rays and solar energetic particles, but also secondary particles from the lunar surface. The Cosmic Ray Telescope for the Effects of Radiation (CRATER) on the Lunar Reconnaissance Orbiter (LRO) provides a recent example of the potential for collaboration between exploration and heliophysics. CRATER was initially selected and flown to measure a range of radiation of concern to human exploration and did so during LRO's prime mission; in LRO's continuing extended mission the instrument has made a wide range of important scientific measurements. Additional targeted energetic particle instrumentation could augment the energy range, add compositional information, and flesh out our understanding of the radiation environment on the lunar surface or even at the sub-surface. Similarly, plasma and electromagnetic field measurements from the lunar surface have important applications to exploration. The surface of the Moon, like any insulator in a plasma, charges to a floating potential with respect to the plasma, and is bounded by a non-neutral plasma sheath. The potentials and currents near the surface may affect sensitive instrumentation, particularly in the near-vacuum of the wake where there no photoemission exists to discharge any charge accumulated by plasma currents or tribo-charging. Furthermore, the near-surface plasma may in some situations become a dusty plasma (particularly when perturbed by landed systems), which is not only a possible concern for exploration, but also of interest from a fundamental physics perspective.

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#### IV. Remote Measurements of Geospace and Beyond

Just as we can use the Moon as a platform for in situ measurements, we can use it as a platform for remote measurements. The Moon provides a useful vantage point on geospace, as well as more distant locations, up to and including the edge of the heliosphere. For some measurements, such as Lyman alpha observations of the geocorona, the Moon provides a vantage point on phenomena not easily viewable from the Earth or even from a low-Earth or geosynchronous orbit. For other measurements that require a large effective aperture and a low electromagnetic noise background, the
lunar surface provides a convenient location for instrumentation of a size that could not be easily accommodated on an orbiting platform. For example, a radio array on the Moon could observe the Sun, other planets, and even astrophysical objects, including at wavelengths below ~10 MHz not visible from below the terrestrial ionosphere. Meanwhile, even for measurements that are possible from Earth orbit, such as ENA and EUV observations of the heliosheath, the Moon represents an appealing environment, given the very low noise background. Finally, for other types of measurements, such as observations of the Sun and its corona, the Moon provides an additional vantage point that can be usefully combined with observations from closer to the Earth.

## V. Conclusion

A variety of missions, dating back to the Apollo era, notably including significant recent international contributions, have made relevant heliophysics measurements from lunar orbit. These missions have made outstanding contributions to our scientific understanding, some of which we highlighted above. However, as also described above, significant questions remain. To answer these questions, we require additional measurements from some combination of different instrumentation, higher temporal/spatial resolution, different orbits, and/or multiple vantage points. The current exploration focus on the Moon, and the anticipated accompanying surge in new launch opportunities to cislunar space, therefore provides a new opportunity to make targeted measurements of and from the Moon. Furthermore, the Moon will almost certainly remain a primary target for international missions, enabling creative collaborative efforts. Focusing heliophysics effort on the Moon thus represents not just a “follow the money” or “follow the launches” strategy, but a path capable of paying substantial scientific dividends. Renewed lunar exploration presents an outstanding opportunity for major progress in heliophysics; it should be seized energetically.

## VI. Acknowledgments

This white paper owes a significant debt to the report “Heliophysics Science and the Moon” prepared for the NASA Advisory Council Heliophysics Subcommittee, which was the result of a sustained community effort in the 2006-2007 time period. Most of the themes in this white paper were anticipated in this previous report, and it provides greater detail on the value of the heliophysics investigations in many areas than this short white paper can possibly accomplish.

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