ARTEMIS Observations Research Articles

Giant Kelvin‐Helmholtz (KH) Waves at the Boundary Layer of the Coronal Mass Ejections (CMEs) Responsible for the Largest Geomagnetic Storm in 20 Years

AbstractStarting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm in 20 years. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event. The IMF ranged from −60 to +40 nT both with hour to minutes periodicity with plasma jets propagating in ‐direction within multi‐scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi‐scale density and magnetic field fluctuations including reconnection jets. The wavelengths varied from 60 to 270 , suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading.

Dependence of Lunar Pickup Ion Flux on Source Location: ARTEMIS Observations

The Moon is enveloped in an exosphere, which is comprised of a variety of neutral atoms and molecules. Once exospheric neutrals are ionized by photons, protons, or electrons from the Sun, the resulting ions are accelerated by the electromagnetic fields of their surroundings and can thereby travel away from their source locations. These ions are the so-called pickup ions and are frequently observed by the two Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft. In this study, we identify 115 events from an 11 yr period of ARTEMIS observations, which contain a total of 11,987 samples for our statistics. By using analytical ion trajectory calculations, we trace the source location of each pickup ion observation. Most pickup ion trajectories originate near the subsolar point, consistent with the efficiency of sputtering. We find that the flux of pickup ions strongly anticorrelates with the source altitude, providing indirect evidence of decreasing exospheric ion flux with increasing altitude. We also find that the flux of pickup ions does not show a significant relationship with the crustal magnetic field intensity. This implies that a depression of sputtering efficiency or the trapping of near-surface freshly born ions by a crustal magnetic anomaly may not reduce the subsequent pickup ion flux as effectively as expected. In summary, the present paper provides a statistical view of lunar pickup ion fluxes in association with the altitude, local time, and local crustal magnetic field of their source locations.

Thin Current Sheets in the Magnetotail at Lunar Distances: Statistics of ARTEMIS Observations

AbstractThe magnetotail current sheet's (CSs) spatial configuration and stability control the onset of magnetic reconnection ‐ the driving process for magnetospheric substorms. The near‐Earth CS has been thoroughly investigated by numerous missions, whereas the midtail CS has not been adequately explored. This is especially the case for the long‐term variation of its configuration in response to the solar wind. We present a statistical analysis of 1261 magnetotail CS crossings by the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission orbiting the moon (X ∼ −60 RE), collected during the entirety of Solar Cycle 24. We demonstrate that the magnetotail CS typically remains extremely thin, with a characteristic thickness comparable to the thermal ion gyroradius, even at large distances from Earth's dipole. We also find that a substantial fraction (∼one quarter) of the observed CSs have a partially force‐free magnetic field configuration, with a significant contribution of the magnetic field shear component to the pressure balance. Further, we quantify the impact of the changing solar wind driving conditions on the properties of the midtail around the lunar orbit. During active solar wind driving conditions, we observe an increase in the occurrence rate of thin CSs, whereas quiet solar wind driving conditions seem to favor the formation of partially force‐free CSs.

Limits on the Density of the Lunar Ionosphere: ARTEMIS Observations

The Moon of our Earth has a tenuous atmosphere, known as an exosphere. The ionization of this exosphere is speculated to possibly form a weak ionosphere. Some radio occultation (RO) experiments have suggested the presence of a dense ionosphere with an electron density on the order of hundreds of cm−3 near the surface. Using in situ measurements from the ARTEMIS mission during 2012–2021, we conduct statistical analyses and case studies to investigate the plasma density at near-surface altitudes. ARTEMIS measurements reveal no plasma densities at altitudes between 10 and 50 km that exceed 35 cm−3, and therefore they provide no evidence for a steady-state or global lunar ionosphere at the level suggested by some RO observations. Density profiles with local time and altitude show higher density in the sunlit sector than in the shadowed sector. These observations suggest that the natural variation of solar wind plasma flux with solar zenith angle plays a critical role in controlling the plasma population near the surface. This research provides a reference for a comparison with RO observations and a statistical view of the low-altitude plasma environment near the lunar surface.

Characteristics of Lunar Surface Electrons Inferred From ARTEMIS Observations: 1. Backscattered Electrons

AbstractLunar surface charging is a scientifically and practically important topic at the Moon that is largely determined by the electron currents near the surface. Among those electron populations, lunar photoelectrons (PHE) and backscattered electrons (BSE) produced by incident electrons that make up the high‐energy tail of lunar emitted electrons are not well characterized yet. Recently, Xu et al. (2021, https://doi.org/10.1029/2020je006790) reported oxygen Auger electron observations at the Moon by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun spacecraft, which provides a unique feature to identify lunar photoelectrons. We utilize this feature to isolate cases of emitted electrons dominated by BSE over PHE. With selected BSE cases, we characterize how the backscattering coefficient η varies with primary electron energy, which decreases with increasing energy. Our results also reveal η to be dependent on the magnetic dip angle, as a fraction of BSE re‐impact the surface in a magnetized environment. The characterization of the backscattering coefficient not only gives insights into the lunar surface properties and lunar surface charging but could also be potentially applied to other airless bodies.

Interaction of Interplanetary Shocks With the Moon: Hybrid Simulations and ARTEMIS Observations

AbstractWhen in the solar wind, the Moon is exposed to its embedded discontinuities such as interplanetary (IP) shocks. In this study we utilize 3‐D electromagnetic hybrid (kinetic ions, fluid electrons) simulations and observations of two events, by THEMIS/ARTEMIS spacecraft to understand the interaction of IP shocks with the Moon. Simulation parameters are based on one of the ARTEMIS events and include the presence of supra‐thermal ions in the solar wind. The results show that the absorption of the cold solar wind protons on the Moon's dayside leads to a density hole in the shock front behind the Moon. This density hole refills and recovers as the IP shock moves down the lunar tail. Penetration of the interplanetary magnetic field through Moon's body leads to the survival of a steepened magnetic structure associated with the shock surface. However, due to the lower pressures in the lunar tail the structure broadens in space and is expected to steepen when the IP shock is beyond the tail. The IP shock is also found to interact with the energetic ions in the lunar tail resulting in their acceleration to higher energies. Comparing the densities measured by ARTEMIS in the solar wind and lunar tail shows the absence of a shock front in density in the tail consistent with the formation of a density hole at the shock front. Similarly, comparing the magnetic field profiles in the solar wind and lunar tail shows the expected broadening of the magnetic field shock front.

Survey of Electron Heating and Implications for Wave‐Particle Interactions Near the Lunar Surface: ARTEMIS Observations

AbstractInteractions between the incident solar wind plasma and lunar crustal magnetic fields can lead to modifications in electron and ion dynamics that can result in the generation of electrostatic and electromagnetic waves. The resulting waves can then interact with the ambient electrons, leading to perpendicular and/or parallel electron heating. We analyze 10 years of data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun mission, when the spacecraft was within 200 km of the dayside lunar surface and within the solar wind, in order to characterize the near‐Moon plasma environment. We find that as the reflected ion density increases, electrostatic waves associated with plasma conditions favorable for the electron cyclotron drift instability play an increasingly important role in perpendicular electron heating, while electromagnetic interactions display the opposite trend. Additionally, we find that electrostatic waves associated with parallel heating exhibit plasma characteristics that are consistent with both the electron two‐stream instability and the modified two‐stream instability.

Kelvin–Helmholtz Waves on the Magnetopause at the Lunar Distances under Southward IMF: ARTEMIS Observations

The Kelvin–Helmholtz (KH) instability, a common phenomenon widely observed at the magnetopause, plays an important role in plasma transport while reconnection at low latitude is less efficient during the northward interplanetary magnetic field (IMF). In this study, we analyze the magnetic field and plasma observations obtained by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft located near the lunar orbit and find KH waves under the southward IMF at the lunar-orbit magnetopause. We also calculate the dominant period, phase velocity, and wavelength of the KH waves and further compare this event with the KH waves seen at the flank magnetopause under the southward IMF, which indicates that the wavelength increases as the distance from the subsolar point increases. The observations also show that the KH waves at lunar distance under the southward IMF are characterized by irregularity and intermittence.

ARTEMIS Observations of Lunar Nightside Surface Potentials in the Magnetotail Lobes: Evidence for Micrometeoroid Impact Charging

AbstractThe Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun mission observes anomalously low lunar nightside surface potentials while in the terrestrial magnetotail lobes. Observed potential magnitudes are between 15% and 40% that expected from ambient tail‐plasma charging, are highly concentrated on the lunar dawn hemisphere, and are correlated with low ambient plasma densities. These characteristics suggest an additional, highly asymmetric source of cold current to the lunar surface. Given these characteristics, we identify micrometeoroid impact‐generated plasmas as the likely source of this additional current. Using laboratory measurements of impact charge yields and models of the micrometeoroid flux to the Moon constrained by in situ measurements, we show that currents due to micrometeoroid impact plasmas have the necessary magnitude and spatial distribution to explain the observed surface potential measurements. Micrometeoroid impact‐generated currents may contribute to surface charging at airless bodies with intense micrometeoroid bombardment and/or low ambient plasma densities.

Bombardment of Lunar Polar Crater Interiors by Out-of-ecliptic Ions: ARTEMIS Observations

Abstract Permanently shadowed regions (PSRs) inside lunar polar craters likely harbor volatiles which are of great interest for both science and resource utilization. The origin and evolution of this resource is in part driven by external space weathering, including the bombardment by space plasma ions. Previous work has investigated the flux of horizontal solar wind ions that are deflected into lunar polar craters by plasma wake electrostatic fields. Here, we constrain the flux of ions that travel northward or southward in the natural environment encountered by the Moon along its orbit. Using 9 years of in situ ion measurements gathered by the ARTEMIS mission, we find that all locations inside lunar craters are altered by a flux of out-of-ecliptic ions of 10−3 times the solar wind flux. In particular, the central floor of Shackleton’s crater is weathered by a non-null ion flux. The origin of northward and southward ions impacting the Moon is identified as (i) shocked thermalized solar wind ions in the terrestrial magnetosheath and (ii) the isotropic component of ion distributions in the terrestrial magnetotail. The energy spectrum of out-of-ecliptic ions is found to be flatter than the solar wind spectrum. Thus, sputtering of pure water ice irradiated by out-of-ecliptic ions would be created for more than 30% by >3 keV protons, unlike equatorial locations that are mostly sputtered by 1 keV protons. Future work may investigate the influence of the low, but nonzero flux of out-of-ecliptic ions reported here on the budget of volatiles in lunar PSRs.

ARTEMIS Observations of Plasma Waves in Laminar and Perturbed Interplanetary Shocks

Abstract The “Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun” mission provides a unique opportunity to study the structure of interplanetary shocks and the associated generation of plasma waves with frequencies between ∼50 and 8000 Hz due to its long duration electric and magnetic field burst waveform captures. We compare wave properties and occurrence rates at 11 quasi-perpendicular interplanetary shocks with burst data within 10 minutes (∼3200 proton gyroradii upstream, ∼1900 downstream) of the shock ramp. A perturbed shock is defined as possessing a large amplitude whistler precursor in the quasi-static magnetic field with an amplitude greater than 1/3 the difference between the upstream and downstream average magnetic field magnitudes; laminar shocks lack these large precursors and have a smooth, step function-like transition. In addition to wave modes previously observed, including ion acoustic, whistler, and electrostatic solitary waves, waves in the ion acoustic frequency range that show rapid temporal frequency change are common. Three shocks had burst captures in the ramp; of these, the two laminar shocks contained a wide range of large amplitude wave modes in the ramp whereas the one perturbed shock contained no such waves. Thus, energy dissipation through wave–particle interactions is more prominent in these two laminar shocks than in the perturbed shock. Based on observations from all 11 shocks, the wave occurrence rates for laminar shocks are higher in the transition region, especially the ramp, than downstream. In contrast, perturbed shocks have approximately 2–3 times the wave occurrence rate downstream than laminar shocks.

Plasma Convection in the Terrestrial Magnetotail Lobes Measured Near the Moon's Orbit

AbstractIn order to study the plasma convection in the deep magnetotail lobes near lunar orbit, we investigated ions originating from the tenuous exosphere and surface of the Moon, which are measured by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft. Directly measuring the plasma convection in the tail lobes is difficult, due to the typically large positive spacecraft potential. In this work we show that in the terrestrial magnetotail near the Moon, the convection velocity can be estimated by measuring the velocity of lunar ions. Determining what factors control the lobe convection is important in understanding the linkage between the upstream conditions and the dynamics of the tail lobes. Based on systematic analysis of multiple ARTEMIS observations and OMNI data, we find that the interplanetary magnetic field (IMF) and magnetospheric activity plays an important role in controlling plasma convection in the near‐Moon lobes.

Energetic Electron Acceleration by Ion-scale Magnetic Islands in Turbulent Magnetic Reconnection: Particle-in-cell Simulations and ARTEMIS Observations

Abstract Particle-in-cell simulations show that numerous ion-scale magnetic islands can be formed in a turbulent magnetic reconnection region. These islands can confine and accelerate electrons to high energies effectively, causing the energetic electron energy flux to peak at the center of each island. ARTEMIS spacecraft observations of turbulent reconnection outflows in Earth’s magnetotail reveal that peaks in the energetic electron energy flux are well correlated with bipolar signatures of reconnected magnetic field, which are indicative of ion-scale magnetic islands, providing observational evidence for this acceleration process.

The Acceleration of Lunar Ions by Magnetic Forces in the Terrestrial Magnetotail Lobes

AbstractIn order to study the acceleration of ions originating from the tenuous exosphere and surface of the Moon, we analyzed data from the ElectroStatic Analyzer (ESA) and Flux Gate Magnetometer (FGM) carried by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft. Previous investigations have modeled the acceleration of lunar ions by the motional electric field of the surrounding plasma. However, in the terrestrial magnetotail, where the lunar ion density can equal or even exceed the ambient plasma density, other forces may play an important role in the tenuous plasma environment. Determining what forces govern lunar ion motion is important in understanding their interaction with the ambient plasma in the unique environment of the magnetotail. Based on a detailed analysis of two individual ARTEMIS observations, we find that magnetic pressure and magnetic tension forces may play an important role in accelerating the lunar ions.

ARTEMIS Observations of Well-structured Lunar Wake in Subsonic Plasma Flow

Abstract Typical lunar wakes are usually formed in the supersonic background plasma in the solar wind or distant magnetosheath because the plasmas do not have enough time to refill into the wake with thermal velocities. Here we present ARTEMIS observations of a well-structured lunar wake under subsonic plasma near the Earth’s magnetopause, where the background plasma flows are nearly field-aligned and have relatively low plasma beta. This lunar wake is quite different from the typical lunar wakes in the supersonic solar wind. The boundaries of this lunar wake are clearly outside the boundaries of the optical shadow, indicating that there is no gradual refilling of ambient plasma into the wake. Meanwhile, no enhancement of magnetic strength in the lunar wake is evident for the absence of diamagnetic currents at the boundaries of the lunar wake. The significantly strong gradient of plasma pressure at the wake boundary cannot be maintained under the subsonic condition because the rarefaction wake can propagate upstream and thus the Mach cone outside the wake will disappear so that the upstream plasma pressure supplement will decrease. Our results also show that the very low plasma beta plays a crucial role in the formation of the lunar wake in subsonic plasma.

Comment on “The Dominant Role of Energetic Ions in Solar Wind Interaction With the Moon” by Omidi et al.

AbstractOmidi et al. (2019) recently published a case study analysis of the lunar wake crossing by the ARTEMIS P2 spacecraft that occurred on 24 June 2015 by comparing P2 observations of plasma and magnetic fields in the lunar wake with a hybrid plasma model using upstream plasma conditions measured by ARTEMIS P1. As presented in Omidi et al., the ARTEMIS P2 ion moments during the lunar wake crossing show anomalously high temperatures and low velocities. This is due to both the lack of subtraction of background counts from penetrating radiation and the application of moment analysis during times when no ion flux was observed by ARTEMIS P2. Additionally, the hybrid results show large discrepancies with the ARTEMIS data, from which Omidi et al. concluded that an additional smaller population of suprathermal ions was required to accurately reproduce the ARTEMIS observations. This conclusion is at odds with previous investigations and, thus, deserves further study. Here we have used the Amitis hybrid plasma model to simulate the lunar wake with upstream plasma conditions determined from the ARTEMIS P1 reference observations, identical to those used in the Omidi et al. “Run‐0” hybrid model. We found that the Amitis hybrid model can successfully reproduce the structure of the lunar wake and the ARTEMIS P2 observations with good fidelity, without the inclusion of any additional high energy ion populations. We therefore conclude that energetic ions do not dominate the structure and behavior of the lunar wake, in contrast to the conclusions of Omidi et al.

Magnetotail boundary crossings at lunar distances: ARTEMIS observations

We compare results from a preliminary analysis of two years of ARTEMIS magnetopause boundary crossings at lunar distances with available empirical models. We remove the effects of variable solar wind flow directions and aberration angles to study the magnetotail cross-section as a function of solar wind conditions. The average magnetopause distance from the central axis is 26 RE, but this distance ranges from 10 RE for high solar wind dynamic pressures and strong northward IMF orientations to 39 RE for low solar wind dynamic pressures and weak southward IMF orientations compared to the nominal solar wind conditions. The time-independent Howe and Binsack (1972) model describes the average location of the crossings very well. For high solar wind dynamic pressures, the Lu et al., (2011) model performs best, while for low pressures the Petrinec and Russell (1996) model gives the closest prediction. As predicted by theory and seen in past studies, the magnetotail cross-section is suggestive of prolate during intervals of strong IMF By, but oblate during intervals of strong IMF Bz. Any asymmetric variation of the tail boundary with respect to the sign of IMF By was not observed. The decreasing size of boundary with the increasing dynamic pressure was found when dynamic pressures are smaller than 2 nPa. Although the scatter is larger, the tail size for pressures larger than 2 nPa suggests a constant radius. The tail boundary size decreases as the strength of IMF Bz increases regardless of its polarity. However, it was also observed that an even stronger southward IMF Bz can cause larger magnetopause size in the presence of large dynamic pressures.

The Energetic Particle Environment of the Lunar Nearside: Influence of the Energetic Ions from Earth’s Bow Shock

Abstract Energetic particles from Earth’s bow shock can frequently access the lunar orbit. These energetic particles are mainly ions. Since they are mostly field-aligned, they have much greater impacts on the lunar near side than on its far side. We present a statistical study of these upstream energetic ions at the lunar orbit using ARTEMIS observations. During the five-year time interval from 2012 to 2016, 496 energetic ion events with time durations ≥30 minutes were identified. The average duration of an event is about 1.93 hr. Most events occurred at the dawn-looking quadrants of Earth, showing an asymmetric dawn–dusk distribution in space. In 490 of the 496 events, the magnetic field lines directly extend to the bow shock. This is very important in order for energetic ions to arrive at the lunar orbit along field lines. The highest energies of these upstream energetic ions range from 101.5 to 658.5 keV based on the energy channels of ARTEMIS. The spatial distribution of the events depends on the highest energy. Events with higher energies tend to occur near the subsolar region and are related to greater AE indices, indicating stronger disturbances of the geomagnetosphere. By taking into account upstream energetic ions, in addition to galactic cosmic rays and solar energetic particles, our results provide a more comprehensive understanding of the energetic particle environment of the lunar near side.

Observations of Kelvin‐Helmholtz Waves in the Earth's Magnetotail Near the Lunar Orbit

AbstractKelvin‐Helmholtz waves (KHWs), which have been widely observed at the magnetopause in the region near the Earth, play an essential role in the transport of solar wind plasma and energy into the magnetosphere under dominantly northward interplanetary magnetic field (IMF) conditions. In this study, we present simultaneous observations of KHWs under the northward IMF observed by both the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft in the Earth's magnetotail around the lunar orbit (at X ~ −50RE, Y ~ 30RE, dusk side) and the Geotail in the near‐Earth space (at X ~ −5RE, Y ~ −10RE, dawn side). The KHWs are quantitatively characterized by their dominant period, phase velocity, and wavelength, utilizing wavelet analysis and an approximation of their center‐of‐mass velocity. Our results suggest that the phase velocity and spatial scale of KHWs may increase as they propagate along the boundary layer toward the tail. Alternatively, the differences between the ARTEMIS and Geotail observations may indicate the possibility of dawn‐dusk asymmetry in the excited KHWs in this study. Our results strongly evidence the existence of the development of KHWs in terms of their wave frequency and scale size in the magnetotail and provide insight to the time evolution of KHWs along the magnetopause.

The Energetic Particle Environment of the Lunar Nearside: SEP Influence

Abstract The energetic particle environment of the lunar nearside is quite different from that of the lunar farside. Due to the shielding of the Earth’s magnetosphere and the Moon, the lunar nearside may receive much fewer energetic particles from the Sun. It is currently impossible to directly measure the received energetic particle flux of the whole lunar surface. By using the ARTEMIS and Wind observations, we qualitatively studied the shielding effect of the Earth’s magnetosphere and the lunar body from solar energetic particles (SEPs). We found that the Earth’s magnetosphere can effectively shield SEPs with energies up to 4 Mev during the SEP event. However, in the solar wind, the Moon can provide partial shielding from SEPs with energies ≤100 KeV. SEPs with energies above 150 KeV in the lunar shadow show no difference in flux from in the solar wind, which suggests that the nearside and farside are the same in receiving SEPs with energies above 150 KeV during the SEP event.