ARTEMIS Spacecraft Research Articles

Comparing Information Theory Analysis With Cross‐Correlation and Minimum Variance Analysis of the Solar Wind Structures

AbstractThe space weather effects at the Earth's magnetosphere are mostly driven by the solar wind that carries the interplanetary magnetic field (IMF). In this paper, we use 2 years of data in the solar wind from lunar orbiting ARTEMIS and MMS spacecraft upstream of the Earth's bow shock to study the structure of the IMF. We determine the lag times of IMF structures and their dependence on spacecraft positions by conducting an information theory analysis and comparing it with two traditional analysis methods: cross‐correlation (CC) analysis and minimum variance of magnetic field analysis (MVAB). For the events with long time intervals (i.e., >4 hr) and with small‐spatial separation between the MMS and ARTEMIS along the yGSM‐direction (i.e., <40Re, where Re is the Earth's radius), the lag times based on the CC and the mutual information (MI) analyses statistically agree with each other, with p‐values of 1.675 × 10−7 and 4.833 × 10−9, with the confidence of 95%. Both the results based on MI and CC have a large deviation from the results from MVAB. For some of the events, such a deviation could be improved by taking the fast mode speed into account; however, p‐tests showed that they were not statistically significant to the 95% confidence level.

Plasma Observations in the Distant Magnetotail During Intervals of Northward IMF

AbstractWe examine a 6‐day traversal of the magnetotail by the ARTEMIS satellites during an interval of prolonged northward IMF. The electrostatic analyzer (ESA) onboard the ARTEMIS spacecraft measures high ion and electron fluxes at approximately 60 RE downtail in regions of the magnetotail which would normally be the magnetotail lobe, containing open flux evacuated of plasma. We interpret these observations as trapped plasma on closed magnetic flux indicating that the magnetotail is closed or partially closed but extends at least as far as ∼60 RE downtail. We find that the occurrence of plasma in the magnetotail and the closure of the magnetosphere results in distinct changes to the magnetotail structure including a reduction in the magnetic field strength and pressure as well as a narrowing of the tail by approximately 20 RE.

Using machine learning to characterize solar wind driving of convection in the terrestrial magnetotail lobes

In order to quantitatively investigate the mechanism of how magnetospheric convection is driven in the region of magnetotail lobes on a global scale, we analyzed data from the ARTEMIS spacecraft in the deep tail and data from the Cluster spacecraft in the near and mid-tail regions. Our previous work revealed that, in the lobes near the Moon’s orbit, the convection can be estimated by using ARTEMIS measurements of lunar ions’ velocity. Based on that, in this paper, we applied machine learning models to these measurements to determine which upstream solar wind parameters significantly drive the lobe convection in magnetotail regions, to help us understand the mechanism that controls the dynamics of the tail lobes. The results demonstrate that the correlations between the predicted and measured convection velocities for the machine learning models (>0.75) are superior to those of the multiple linear regression model (∼0.23–0.43) in the testing dataset. The systematic analysis shows that the IMF and magnetospheric activity play an important role in influencing plasma convection in the global magnetotail lobes.

A Comprehensive Model for Pickup Ion Formation at the Moon

AbstractThe lunar exosphere is an ensemble of multiple overlapping, noninteracting neutral distributions that reflect the primary physical processes acting on the lunar surface. While previous observations have detected and constrained the behavior of some species, many others have only circumstantial evidence or theoretical modeling suggesting their presence. Many species are so tenuous as to be unobservable by direct neutral sampling, yet in comparison, measurements in their ionized form provide a particularly sensitive method of detection. To better aid the interpretation of past measurements and planning of future observations, we present a model for the production of lunar pickup ions from the Moon consisting of two components: An analytic model for the distributions of 18 neutral species produced by various mechanisms and an analytic model for the ionization and subsequent acceleration of 20 exospheric and surface‐sputtered pickup ion species. The dominant lunar pickup ions in the model are , He+, CO+, 40Ar+, Al+, Na+, K+, Si+, Ca+, and O+ with an asymmetric distribution favoring the positive interplanetary electric field hemisphere of the Moon. We compare the model predictions to statistically averaged pickup ion fluxes around the Moon as observed by the ARTEMIS spacecraft over the past decade. By filtering for interplanetary electric field‐aligned, high‐energy observations, we find that the pickup ion model lacks an additional source of heavy species. We suggest that a dense CO2 exosphere of 3 × 104 − 1 × 105 cm−3 could account for the missing pickup ion flux as part of the recycling of solar wind carbon ions incident to the Moon.

Multipoint Observation of the Solar Wind Interaction with Strong Lunar Magnetic Anomalies by ARTEMIS Spacecraft and Chang’E-4 Rover

A shock or a mini-magnetosphere was once thought to be formed by the solar wind interaction with strong lunar magnetic anomalies. However, the full structure of a mini-magnetosphere has never been verified and whether a mini-magnetosphere can be completely formed remains a controversy. In this work, we present a unique multipoint observation of such an interaction by the ARTEMIS spacecraft and the Chang’E-4 rover. Both solar wind deceleration and penetration are observed by the Chang’E-4 rover on the lunar surface near the magnetic anomaly. Meanwhile, a shock is observed by the ARTEMIS spacecraft downstream from the magnetic anomaly. It is suggested that the magnetic anomaly cannot stand off the solar wind, and there is no shock but just a boundary layer near the magnetic anomaly. Accordingly, a mini-magnetosphere is not completely formed and the downstream shock observed the ARTEMIS spacecraft just corresponds to a trailing shock.

Spatial Scales of the Velocity Shear Layer and Kelvin‐Helmholtz Waves on the Magnetopause: First Statistical Results

AbstractUsing data taken by the ARTEMIS spacecraft near Moon, we statistically study the properties of Kelvin‐Helmholtz (KH) waves and the thicknesses (ΔY) of initial velocity shear layers on the magnetopause at lunar distance. Based on a total of 48 KH wave events and 14 estimates of ΔY, the average KH wavelength (λX) and ΔY are found to be 16.05 RE and 0.53 RE at lunar distance, respectively, which are larger than those at near‐Earth magnetopause. The λX to ΔY ratio ranges from 18 to 41 at lunar distance, way beyond the range (6–13) predicted by the linear theory. This result indicates that the KH waves observed at lunar distance are not of the fastest growing mode according to linear theory. This study poses the first statistical study on λX and ΔY, providing insights into the generation and development of KH waves at lunar distance.

Rapid Refilling of the Lunar Wake under Transonic Plasma Flow: ARTEMIS Observations

Abstract The lunar wake is usually rapidly refilled when the background plasma is substantially subsonic. Here we present observational case studies, by the two ARTEMIS spacecraft, of the lunar wakes in the magnetosheath near the magnetopause under transonic plasma flows with relatively large plasma beta and different magnetic field orientations. The associated background plasma betas are near 1, suggesting that the plasmas are not strongly controlled by the magnetic fields. As a result, the part of the lunar wake under subsonic plasma was rapidly refilled. Pitch angle distributions reveal that a significant portion of the refilling ions are from the downstream wake. This is consistent with the fact that higher sound speed than bulk speed of the background plasma enables downstream particles to get access to upstream wake due to the pressure gradient. The other part under supersonic plasma remains a typical wake structure with high plasma density or flux depletions. Our results show that the Mach number and plasma beta primarily determine the rapid refilling of the lunar wake, which is allowed only in subsonic plasma with plasma beta greater than or close to 1 and hardly influenced at all by the magnetic field orientation.

Solar wind transient currents: statistical properties and impact on Earth’s magnetosphere

Solar wind discontinuities carry intense transient currents and significantly contribute to the turbulent spectrum and plasma heating. The most-investigated characteristics of these discontinuities are the magnetic field configuration and the current density, whereas plasma characteristics attract less attention. In this study, we utilize eight years of ARTEMIS spacecraft observations in the solar wind to investigate plasma density, velocity, and temperature variations across discontinuities. We also consider the role of discontinuities in the development of Earth’s magnetospheric perturbations. We show that observed discontinuities can be separated into two groups: (i) discontinuities with weak plasma density variations and a significant correlation between the solar wind and Alfven velocities, (ii) discontinuities with significant variations of plasma density and temperature. For most discontinuities, observed density variations anti-correlate with temperature variations, but larger density/temperature variations correspond to stronger current densities. Time intervals characterized by increased occurrence rate of solar wind discontinuities correspond to enhanced geomagnetic activity in the Earth’s magnetosphere as characterized by geomagnetic indices.

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.

Reply to: Comment on “The Dominant Role of Energetic Ions in Solar Wind Interaction With the Moon” by Poppe

AbstractIn all kinetic simulations of the solar wind interaction with the moon, dayside absorption of the core solar wind protons results in the formation of a numerically unstable vacuum in the tail requiring a fix. In Omidi et al. (2019, https://doi.org/10.1029/2018JA026243) it was shown that replacing the vacuum with a cold plasma results in unacceptable magnetic fields when compared to observations. It was also demonstrated that when the presence of energetic ions in the solar wind is accounted for, the tail is no longer a vacuum but contains a low‐density plasma. In addition, the resulting magnetic fields are in excellent agreement with the observations including the presence of the compressional wake. Poppe (2019) shows results from the Amitis, which employs a different vacuum model and produces a magnetic field profile in the central tail that agrees with observations. However, the results do not account for the presence of low‐density plasma in the tail and the formation of the compressional wake. To explain these shortcomings, Poppe (2019) questions the existence of plasma in the tail and attributes the compressional wake to lunar crustal fields. Here we address the criticism of plasma observations and provide additional data from ARTEMIS spacecraft that demonstrates the compressional wake is an inherent part of the lunar tail structure. As such, the Amitis vacuum simulations do not capture the full physics of the interaction. In contrast, when the energetic protons are included in the model, all aspects of the lunar tail structure are accounted for.

Statistical Study of Whistler Waves in the Solar Wind at 1 au

Abstract Whistler waves are intermittently present in the solar wind, while their origin and effects are not entirely understood. We present a statistical analysis of magnetic field fluctuations in the whistler frequency range (above 16 Hz) based on about 801,500 magnetic field spectra measured over 3 yr aboard the Artemis spacecraft in the pristine solar wind. About 13,700 spectra (30 hr in total) with intense magnetic field fluctuations satisfy the interpretation in terms of quasi-parallel whistler waves. We provide estimates of the whistler wave occurrence probability, amplitudes, frequencies, and bandwidths. The occurrence probability of whistler waves is shown to strongly depend on the electron temperature anisotropy. The whistler wave amplitudes are in the range from about 0.01 to 0.1 nT and typically below 0.02 of the background magnetic field. The frequencies of the whistler waves are shown to be below an upper bound that is dependent on β e . The correlations established between the whistler wave properties and local macroscopic plasma parameters suggest that the observed whistler waves can be generated in local plasmas by the whistler heat flux instability. The whistler wave amplitudes are typically small, which questions the hypothesis that quasi-parallel whistler waves are capable to regulate the electron heat flux in the solar wind. We show that the observed whistler waves have sufficiently wide bandwidths and small amplitudes, so that effects of the whistler waves on electrons can be addressed in the frame of the quasi-linear theory.

Can Enhanced Flux Loading by High‐Speed Jets Lead to a Substorm? Multipoint Detection of the Christmas Day Substorm Onset at 08:17 UT, 2015

AbstractThis paper describes highly interesting observations of the 25 December 2015 substorm onset at 08:17 UT during northward interplanetary magnetic field (IMF) while magnetosheath contained several intervals of negative Bz. A unique alignment of several spacecraft near the Earth‐Sun line together with magnetohydrodynamic simulations, ground‐based magnetometer, and auroral observations allow a comprehensive timing analysis of the events leading to substorm onset. Perplexingly, prior to substorm onset Geotail measured for 26 min positive IMF Bz just upstream of the bow shock, while simultaneously MMS spacecraft measured several intervals of strong negative Bz in the dayside magnetosheath. These strong pulses of negative Bz in the magnetosheath were associated with high dynamic pressure magnetosheath jets, likely created by foreshock transients during strongly radial IMF interval. Multipoint plasma and magnetic field measurements from ARTEMIS and THEMIS spacecraft were used to determine tail reconnection time at 8:14 and location at x=−33RE. Ground‐based observations of Pi2 pulsations and auroral brightening, with observations of a dipolarization front by THEMIS spacecraft, allowed determination of substorm onset to be at ≈08:17. All MMS spacecraft detected the same magnetosheath jet structure with Bz=−25 nT at ∼08:00 while IMF was northward. Based on DMSP observations and timing analysis we propose that these jets produced magnetopause reconnection leading to final, critical flux enhancement in the midtail region, which may have triggered reconnection 12–14 min later after jet observations.

Whistler Wave Generation by Halo Electrons in the Solar Wind

Abstract We present an analysis of simultaneous particle and field measurements from the ARTEMIS spacecraft, which demonstrates that quasi-parallel whistler waves in the solar wind can be generated locally by a bulk flow of halo electrons (whistler heat flux instability). ARTEMIS observes quasi-parallel whistler waves in the frequency range ∼0.05−0.2f ce simultaneously with electron velocity distribution functions that are a combination of counter-streaming core and halo populations. A linear stability analysis shows that the plasma is stable when there are no whistler waves, and it is unstable in the presence of whistler waves. In the latter case, the stability analysis shows that the whistler wave growth time is from a few to 10 seconds at frequencies and wavenumbers that match the observations. The observations clearly demonstrate that the temperature anisotropy of halo electrons crucially affects the heat flux instability onset: a slight anisotropy T ∥/T ⊥ > 1 may quench the instability, while a slight anisotropy T ∥/T ⊥ < 1 may significantly increase the growth rate. These results demonstrate that heat flux inhibition is strongly dependent on the microscopic plasma properties.

Hot Ion Flows in the Distant Magnetotail: ARTEMIS Observations From Lunar Orbit to ∼−200 RE

AbstractPlasma energization in Earth's magnetotail is supported by acceleration processes in (and around) magnetic reconnection regions. Hot plasma flows and strong electromagnetic waves, generated by magnetic energy release during reconnection, transport energy necessary for current system intensification and particle acceleration in the inner magnetosphere. Earth's magnetotail configuration includes two main reconnection regions (X lines): the near‐Earth X line, which has been well studied by several multispacecraft missions, and the distant X line, which has been much less investigated. In this paper, we utilize the unique data set gathered by two ARTEMIS spacecraft in 2010 at radial distances between lunar orbit and ∼200 RE (Earth radii). We identify an X line at around ∼80 RE and collect statistics on hot plasma flows observed around and beyond this distance. Ion spectra within these flows are well fitted by a power law with the exponential tail starting above an energy ϵ0∼ 2–5 keV. Assuming that these spectra are originated at the distant X line, we examine the characteristics of the acceleration at the distant tail reconnection region.

Growth rate measurement of ULF waves in the ion foreshock

AbstractWe report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock. We employ the two identical ARTEMIS spacecraft orbiting the Moon to characterize crescent‐shaped reflected ion beams and relatively monochromatic ULF waves. The event presented here features spacecraft separation of ∼2.5 Earth radii (0.9 ± 0.1 wavelengths) in the solar wind flow direction along a nearly radial interplanetary magnetic field. The ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are also within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. These results will inform future missions near bow and interplanetary shocks as well as future nonlinear studies related to turbulence and dissipation in the heliosphere.

Sequentially released tilted flux ropes in the Earth's magnetotail

On 14 July 2011, the two ARTEMIS spacecraft observed a chain of flux ropes generated by reconnection in the Earth's magnetotail. The flux ropes are found to extend more than 90 ion inertial lengths out of the reconnection plane. We analyze six of these flux ropes by employing a novel combination of two-spacecraft-timing for their global orientation and Grad–Shafranov reconstruction (GSR) for their local orientation. We find that their diameter was ∼10 ion inertial lengths, temporal separations ∼100 ion gyroperiods, and that their speed increased from ∼30% to ∼70% of the Alfvén velocity. Given their temporal and spatial separations, we can infer that they were produced sequentially via a secondary instability. All of the flux ropes were tilted in the current sheet plane, suggesting that the secondary instability grew along the X-line in the direction of the electron current (−YGSM). The first five had a tilt between −14° and −26°. Based on the GSR axes their shape became increasingly complex over a period of ∼300 gyroperiods indicating kinking. The sixth event was again rather linear, and had a larger tilt (−38°), cross-section and core magnetic field; these changes are most likely related to a change in the boundary conditions.

Azimuthal extent and properties of midtail plasmoids from two‐point ARTEMIS observations at the Earth‐Moon Lagrange points

AbstractAlthough distant‐tail plasmoids are perceived to extend across most of the magnetotail (~40 RE), recent studies in the near‐Earth region (X > −30 RE) have revealed that near‐Earth reconnection (where plasmoids originate) is likely localized and takes place preferentially on the duskside. This discrepancy in plasmoid azimuthal extent suggests that a plasmoid may grow as it moves from near‐Earth to the distant tail. Comprehensive multipoint, midtail plasmoid observations can be used to test this hypothesis. Between October 2010 and July 2011 the ARTEMIS spacecraft (P1 and P2) at the Earth‐Moon Lagrange points (midtail, X ~ −45 to −65 RE) provided simultaneous two‐point observations across the magnetotail for 4 days every lunar month, with a large range of spacecraft separations (0.1 to 25 RE). We find that plasmoids near lunar orbit, like other near‐Earth reconnection‐related phenomena, occur preferentially on the duskside of the magnetotail. Two‐point ARTEMIS observations reveal that the typical plasmoid azimuthal size in our data set is about 5 to 10 RE, much smaller than expected from previous distant‐tail observations. Plasmoids with an azimuthal size greater than 9 RE also exist but only at geomagnetic activity levels higher (AEpeak > 400 nT) than typically found in our data set (median AEpeak ~ 230 nT for our plasmoid data set and median AE ~ 100 nT during the entire period of ARTEMIS magnetotail observations). We conclude that plasmoids during small to moderate substorms (AEpeak < 400 nT) do not grow beyond ~10 RE until they have moved tailward of ~ −45 to −65 RE. Plasmoids during large substorms (AEpeak > 400 nT), however, either grow beyond ~10 RE before they reach lunar distance or initially extend across a large portion of the magnetotail.

Tail reconnection region versus auroral activity inferred from conjugate ARTEMIS plasma sheet flow and auroral observations

AbstractPlasma sheet flow bursts have been suggested to correspond to different types of auroral activity, such as poleward boundary intensifications (PBIs), ensuing auroral streamers, and substorms. The flow‐aurora association leads to the important question of identifying the magnetotail source region for the flow bursts and how this region depends on magnetic activity. The present study uses the ARTEMIS spacecraft coordinated with conjugate ground‐based auroral imager observations to identify flow bursts beyond 45 RE downtail and corresponding auroral forms. We find that quiet‐time flows are directed dominantly earthward with a one‐to‐one correspondence with PBIs. Flow bursts during the substorm recovery phase and during steady magnetospheric convection (SMC) periods are also directed earthward, and these flows are associated with a series of PBIs/streamers lasting for tens of minutes with similar durations to that of the series of earthward flows. Presubstorm onset flows are also earthward and associated with PBIs/streamers. The earthward flows during those magnetic conditions suggest that the flow bursts, which lead to PBIs and streamers, originate from further downtail of ARTEMIS, possibly from the distant‐tail neutral line (DNL) or tailward‐retreated near‐Earth neutral line (NENL) rather than from the nominal NENL location in the midtail. We find that tailward flows are limited primarily to the substorm expansion phase. They continue throughout the period of auroral poleward expansion, indicating that the expansion‐phase flows originate from the NENL and that NENL activity is closely related to the auroral expansion of the substorm expansion phase.

Identifying the magnetotail source region leading to preonset poleward boundary intensifications

AbstractSubstorm auroral onset is often preceded by plasma sheet flow bursts, poleward boundary intensifications (PBIs), and subsequent auroral streamers, suggesting that enhanced transient flows in the plasma sheet propagating toward the near‐Earth plasma sheet play an important role in leading to substorm onset. This study aims at determining the magnetotail source region of preonset PBIs and plasma sheet flows by using the ARTEMIS spacecraft coordinated with the THEMIS all‐sky imager array. ARTEMIS has an advantage of measuring flows between the nominal near‐Earth neutral line (NENL) and distant neutral line (DNL) locations, and thus, the flow direction allows identifying which source region is dominant. We found three preonset conjunction events, and all cases show earthward flow bursts ~10–30 min before substorm onset when PBIs and streamers are detected in the conjugate ionosphere. This time lag is sufficiently long to bring plasma from the ARTEMIS location toward the near‐Earth plasma sheet before onset. The earthward flow direction indicates that preonset enhanced flow channels leading to precursor PBIs originate from further downtail of ARTEMIS, possibly from DNL rather than from NENL. The flow direction changes to tailward after the substorm onset, suggesting that NENL is not related to preonset flow and auroral activity but becomes active during the expansion phase.

Evolution of the out-of-plane amplitude for quasi-periodic trajectories in the Earth–Moon system

The out-of-plane amplitude along quasi-periodic trajectories in the Earth–Moon system is highly sensitive to perturbations in position and/or velocity as underscored recently by the ARTEMIS spacecraft. Controlling the evolution of the out-of-plane amplitude is non-trivial, but can be critical to satisfying mission requirements. The sensitivity of the out-of-plane amplitude evolution to perturbations due to lunar eccentricity, solar gravity, and solar radiation pressure is explored and a strategy for designing low-cost deterministic maneuvers to control the amplitude history is also examined. The method is sufficiently general and is applied to the L1 quasi-periodic orbit that serves as a baseline for the ARTEMIS P2 trajectory.