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The Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission is a spin-off from NASA’s Medium-class Explorer (MIDEX) mission THEMIS, a five identical micro-satellite (hereafter termed “probe”) constellation in high altitude Earth-orbit since 17 February 2007. By repositioning two of the five THEMIS probes (P1 and P2) in coordinated, lunar equatorial orbits, at distances of ∼55–65 R E geocentric (∼1.1–12 R L selenocentric), ARTEMIS will perform the first systematic, two-point observations of the distant magnetotail, the solar wind, and the lunar space and planetary environment. The primary heliophysics science objectives of the mission are to study from such unprecedented vantage points and inter-probe separations how particles are accelerated at reconnection sites and shocks, and how turbulence develops and evolves in Earth’s magnetotail and in the solar wind. Additionally, the mission will determine the structure, formation, refilling, and downstream evolution of the lunar wake and explore particle acceleration processes within it. ARTEMIS’s orbits and instrumentation will also address key lunar planetary science objectives: the evolution of lunar exospheric and sputtered ions, the origin of electric fields contributing to dust charging and circulation, the structure of the lunar interior as inferred by electromagnetic sounding, and the lunar surface properties as revealed by studies of crustal magnetism. ARTEMIS is synergistic with concurrent NASA missions LRO and LADEE and the anticipated deployment of the International Lunar Network. It is expected to be a key element in the NASA Heliophysics Great Observatory and to play an important role in international plans for lunar exploration.

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is the fifth NASA Medium-class Explorer (MIDEX), launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. The mission employs five identical micro-satellites (hereafter termed “probes”) which line up along the Earth’s magnetotail to track the motion of particles, plasma and waves from one point to another and for the first time resolve space–time ambiguities in key regions of the magnetosphere on a global scale. The probes are equipped with comprehensive in-situ particles and fields instruments that measure the thermal and super-thermal ions and electrons, and electromagnetic fields from DC to beyond the electron cyclotron frequency in the regions of interest. The primary goal of THEMIS, which drove the mission design, is to elucidate which magnetotail process is responsible for substorm onset at the region where substorm auroras map (∼10 R E ): (i) a local disruption of the plasma sheet current (current disruption) or (ii) the interaction of the current sheet with the rapid influx of plasma emanating from reconnection at ∼25 R E . However, the probes also traverse the radiation belts and the dayside magnetosphere, allowing THEMIS to address additional baseline objectives, namely: how the radiation belts are energized on time scales of 2–4 hours during the recovery phase of storms, and how the pristine solar wind’s interaction with upstream beams, waves and the bow shock affects Sun–Earth coupling. THEMIS’s open data policy, platform-independent dataset, open-source analysis software, automated plotting and dissemination of data within hours of receipt, dedicated ground-based observatory network and strong links to ancillary space-based and ground-based programs. promote a grass-roots integration of relevant NASA, NSF and international assets in the context of an international Heliophysics Observatory over the next decade. The mission has demonstrated spacecraft and mission design strategies ideal for Constellation-class missions and its science is complementary to Cluster and MMS. THEMIS, the first NASA micro-satellite constellation, is a technological pathfinder for future Sun-Earth Connections missions and a stepping stone towards understanding Space Weather.

The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur. Surface waves on the boundary between a magnetosphere and the surrounding plasma might get trapped by the ionosphere forming an eigenmode. Here, Archer et al. show direct observations of this proposed mechanism at Earth’s magnetosphere by analyzing the response to an isolated fast plasma jet detected by the THEMIS satellites.

The global distribution of chorus wave amplitudes and their wave normal angles is investigated using high‐resolution wave spectra and waveform data from THEMIS for lower‐band and upper‐band chorus separately. Statistical results show that large amplitude chorus (>300 pT) occurs predominantly from premidnight to postdawn and is preferentially observed at lower L shells (<8) near the magnetic equator. However, strong or moderate chorus extends further into the afternoon sector and to higher L shells. For lower‐band chorus, strong waves (>50 pT) tend to have wave normal angles of <20° and their wave normal angles become even smaller with increasing wave amplitudes. For modest waves, the wave normal angles are distributed over a broad range with a major peak at <20° and a secondary peak at 60°–80°. Wave normal angles of lower‐band chorus are generally smaller on the dayside than on the nightside possibly due to the more uniform and more compressed magnetic field configuration on the dayside. Lower‐band chorus becomes more oblique with increasing latitude on the dayside, whereas on the nightside the probability of observing oblique chorus decreases at higher latitudes. Compared to lower‐band chorus, the properties of upper‐band chorus are somewhat different. Upper‐band chorus is considerably weaker in magnetic wave amplitudes, shows tighter confinement to the magnetic equator (<10°), and occurs at smaller L shells (<8). Furthermore, wave normal angles of upper‐band chorus are generally larger than those of lower‐band chorus, but the occurrence rate still peaks at wave normal angles of <20°, particularly for strong upper‐band chorus. Key Points Large amplitude chorus occurs from premidnight to dawn near the equator Strong lower‐band chorus typically has small wave normal angles (less than 20 deg) Upper‐band chorus is more oblique and much weaker than lower‐band chorus

The magnetopause boundary layer often exhibits flux enhancements in ≳30 $\\gtrsim 30$ keV electrons. Intriguingly, these enhancements frequently occur in the afternoon sector, which is typically magnetopause‐shadowed. They are usually attributed to local production by dayside reconnection, wave‐particle interactions, or radial diffusion by ultra‐low frequency waves. However, under standard magnetospheric conditions, these mechanisms fail to explain the rapid appearance of the electron fluxes and acceleration from magnetosheath energies (tens of eV) to tens of keV. Using data from the THEMIS mission, we report an energetic electron enhancement forming on hour timescales. A test‐particle simulation shows it can result from rapid, non‐diffusive radial transport driven by asymmetric drift‐orbit bifurcation. While this does not exclude alternative interpretations involving radial diffusion, the finding underscores the role of drift‐orbit bifurcation in controlling energetic electron dynamics near the magnetopause, which should be considered alongside conventional mechanisms.

Electromagnetic ion cyclotron (EMIC) waves can drive radiation belt depletion and Low‐Earth Orbit satellites can detect the resulting electron and proton precipitation. The ELFIN (Electron Losses and Fields InvestigatioN) CubeSats provide an excellent opportunity to study the properties of EMIC‐driven electron precipitation with much higher energy and pitch‐angle resolution than previously allowed. We collect EMIC‐driven electron precipitation events from ELFIN observations and use POES (Polar Orbiting Environmental Satellites) to search for 10s–100s keV proton precipitation nearby as a proxy of EMIC wave activity. Electron precipitation mainly occurs on localized radial scales (∼0.3 L), over 15–24 MLT and 5–8 L shells, stronger at ∼MeV energies and weaker down to ∼100–200 keV. Additionally, the observed loss cone pitch‐angle distribution agrees with quasilinear predictions at ≳250 keV (more filled loss cone with increasing energy), while additional mechanisms are needed to explain the observed low‐energy precipitation. Plain Language Summary Electromagnetic ion cyclotron (EMIC) emissions are a type of plasma wave that can be excited in the near‐Earth environment and interact with energetic electrons in the Earth's radiation belts. Through these wave‐particle interactions, electrons can be pushed into the loss cone and lost into the Earth's atmosphere (electron precipitation), where they deposit their energy by interacting with neutral atoms and cold charged particles. EMIC‐driven electron precipitation still needs to be fully characterized and understood. In this work, we use data from the Electron Losses and Fields InvestigatioN (ELFIN) CubeSats, which provide electron fluxes at high energy and pitch‐angle (look direction) resolution at ∼450 km of altitude. Our analysis reveals that precipitation is most efficient for ∼MeV electrons and is accompanied by weaker low‐energy precipitation down to ∼100–200 keV. Given the ELFIN CubeSats spin, we can also study the distribution of the precipitating electrons along different look directions (pitch‐angles). We find that the loss cone shape is well‐reproduced by quasilinear predictions of EMIC‐electron interactions at higher energies (≳250 keV), while quasilinear calculations underestimate the observed low‐energy precipitation. Key Points Energetic electron precipitation is observed by Electron Losses and Fields InvestigatioN nearby proton precipitation (a proxy for Electromagnetic ion cyclotron waves) primarily over 15–24 MLT Precipitation efficiency increases as a function of energy: weak ∼100s keV precipitation is concurrent with intense ∼MeV precipitation The observed pitch‐angle distribution shows a loss cone filling up with energy, similar to the pitch‐angle profiles from quasilinear theory

Using 2008–2011 data from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in Earth's subsolar magnetosheath, we study high-speed jets identified as intervals when the anti-sunward component of the dynamic pressure in the subsolar magnetosheath exceeds half of its upstream solar wind value. Based on our comprehensive data set of 2859 high-speed jets, we obtain the following statistical results on jet properties and favorable conditions: high-speed jets occur predominantly downstream of the quasi-parallel bow shock, i.e., when interplanetary magnetic field cone angles are low. Apart from that, jet occurrence is only very weakly dependent (if at all) on other upstream conditions or solar wind variability. Typical durations and recurrence times of high-speed jets are on the order of tens of seconds and a few minutes, respectively. Relative to the ambient magnetosheath, high-speed jets exhibit higher speed, density and magnetic field intensity, but lower and more isotropic temperatures. They are almost always super-Alfvénic, often even super-magnetosonic, and typically feature 6.5 times as much dynamic pressure and twice as much total pressure in anti-sunward direction as the surrounding plasma does. Consequently, they are likely to have significant effects on the magnetosphere and ionosphere if they impinge on the magnetopause.

We report on prolonged enhancements of electron fluxes at energies at or above 500 keV, observed in the magnetotail by the lunar‐orbiting Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) during the recovery phase of a magnetic storm with minimum Dst ${D}_{st}$ ≈ ${\\approx} $ −200 nT during periodic auroral electrojet (AE $AE$) activations. The enhanced energetic electron fluxes were omnidirectional and observed near the magnetic equator. No solar energetic particle background was detected. Although ARTEMIS detected earthward magnetic flux transport impulses exceeding 2 mV/m, along with associated broadband electrostatic fluctuations, no correlation was evident between these phenomena and the relativistic electron flux enhancements. Spectra obtained during the relativistic electron flux enhancements are fit by the Kappa function, κ $\\kappa $ = 3.75, similar to that of the quiet‐time plasma sheet electron population at lunar distance. Multiple reconnection events at large distances are, most likely, responsible for the electron heating.

Magnetospheric substorms are elemental processes of solar wind energy storage and explosive release in Earth's magnetosphere. They encompass fundamental plasma physics questions, are ubiquitous during all types of geomagnetic conditions, contribute significantly to magnetic storms, and are a key element of Space Weather applications. This paper reviews recent major advances enabled by modern multi‐point space‐based and ground‐based platforms. These datasets have also empowered a system‐wide perspective and advanced modeling. We particularly highlight progress in two areas: (1) substorm onset timing and evidence for current sheet preconditioning and destabilization and (2) fast flows and dipolarizations, including the role of entropy in magnetotail plasma propagation. Key Points Different processes interact to initiate the substorm onset and auroral breakup Multi‐scale disturbances result in, possibly, multiple paths to onset Flow bursts as the bubbles, important role of entropy in flow burst dynamics

High-angular-resolution measurements allow the direct observation of the scattering of energetic electrons by chorus waves in the magnetosphere, which causes quasiperiodic electron precipitation that gives rise to pulsating aurorae. Pulsating aurorae A pulsating aurora is a type of aurora that occurs in patches that blink on and off in an almost periodic fashion. They usually arise in the closing phase of an auroral display, at dawn, and cover up to several hundred kilometres of the sky, at an altitude of about 100 kilometres. Many such patches sometimes cover the entire sky. The pulsations arise from intermittent injections of energetic electrons into the upper atmosphere, but just how the injections happen has been unclear because of instrumental limitations on the observations. Satoshi Kasahara and colleagues report observations that show that the energetic electrons are quasiperiodically scattered by 'chorus waves'—intense electromagnetic plasma waves that arise at the magnetic equator and move towards the poles—at the same time as pulsating aurorae are seen from the ground. Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy 1 , 2 . These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn 1 , 3 . Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere 4 , 5 , 6 , 7 . A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves 8 , 9 , 10 , 11 . However, no direct observational evidence of this interaction has been obtained so far 12 . Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured 13 , 14 quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.