Preface to Special Topic: Plasma Physics from the Magnetospheric Multiscale Mission

Launched on 13 March 2015, the Magnetospheric Multiscale (MMS) mission is a formation of four identical Earth-orbiting spacecraft with the primary science goal to “understand the microphysics of magnetic reconnection by determining the kinetic processes occurring in the electron diffusion region (EDR) that are responsible for collisionless magnetic reconnection, especially how reconnection is initiated.”¹ While the primary science target of MMS is magnetic reconnection, its unique capabilities—notably, (1) interspacecraft separations approaching the characteristic length scales of the electrons in the plasma, (2) unprecedented high-resolution 3D particle distribution function measurements for both positively charged ions and electrons, and (3) 3D electric field measurements—have solidified MMS as an invaluable tool for the study of a wide range of fundamental plasma physics phenomena that are broadly applicable across many space, astrophysical, and laboratory plasmas. Some of these phenomena include magnetic reconnection, plasma turbulence, collisionless shocks, waves, instabilities, kinetic-scale plasma structures, and collisionless energy conversion and dissipation, just to name a few.

The four MMS spacecraft carry a full complement of in situ plasma instrumentation capable of characterizing the electromagnetic fields and particles in the plasma environments surrounding Earth. The FIELDS instrument suite² consists of Flux Gate (FGM)³ and Search Coil (SCM)⁴ Magnetometers measuring the low and high frequency magnetic fields (⁠ $B$⁠), respectively, axial⁵ and spin-plane⁶ Electric Field Double Probes (EDP) providing 3D electric field (⁠ $E$⁠) measurements, and an Electron Drift Instrument (EDI)⁷ measuring $E$ and $B$ based on the timing and drift of electron beams that are emitted and return to the spacecraft. The Fast Plasma Investigation (FPI)⁸ is an electrostatic analyzer with multiple sensor heads distributed around each of the four spinning spacecraft, capable of measuring 3D ion and electron particle distribution functions and derived moments at unprecedented temporal resolutions. The Hot Plasma Composition Analyzer (HPCA)⁹ is a time-of-flight electrostatic analyzer capable of separating the particle distribution functions and supplying derived moments for different ion species. The solid-state Energetic Particle Detectors (EDP) consist of two types of sensors, the Energetic Ion Spectrometer (EIS)¹⁰ and Fly's Eye Energetic Particle Spectrometer (FEEPS),¹¹ which measure particle distributions for ions and electrons with energies above the range of FPI and HPCA. Additionally, the MMS spacecraft have Active Spacecraft Potential Control (ASPOC)¹² systems, which control the potential of the spacecraft using beams of indium ions in order to improve the measurements from the other instruments. Bandwidth limitations on transmitting data back to Earth coupled with the high-resolution measurements mean that it is not possible to downlink all of the highest resolution “burst” measurements from the spacecraft. MMS employs a system in which a “scientist-in-the-loop” manually selects the most relevant intervals of data to send down based on an examination of lower resolution “survey” data that are regularly downlinked from the spacecraft.¹³ 

The MMS spacecraft follow highly elliptical orbits around Earth, as illustrated in Fig. 1(a) for several example orbits taken from throughout the mission so far, allowing the spacecraft to dwell in the plasma environment near apogee for an extended period of time during each orbit. As Earth orbits the Sun, the location of the orbit's apogee precesses around Earth relative to the Sun–Earth line, as illustrated by the series of blue orbits in Fig. 1(a), such that at different times of year the spacecraft sample the plasma environment on the dayside (sunward) of the Earth and the nightside (anti-sunward) of the Earth. The apogee of the orbits has been varied over the course of the mission in order to target key regions of Earth's magnetosphere. In the first phase of the mission [blue orbits in Fig. 1(a)], the apogee was located at $∼ 12$ Earth radii (R_E), allowing the apogee to skim along the nominal location of the dayside magnetopause (the exact location of which depends on upstream solar wind conditions), where magnetic reconnection is expected to occur between the solar wind and Earth's magnetic field, as the orbit precesses through the dayside. In March/April 2017, the apogee was raised to $∼ 25 R E$ [green orbit in Fig. 1(a)] allowing MMS to be in the nominal location where near-Earth magnetic reconnection is expected to occur in the magnetotail as the spacecraft precesses through the nightside. In February 2019, the apogee was further adjusted to $∼ 28 R E$ [red orbit in Fig. 1(a)], where it has remained to present. These apogee distances have not only allowed MMS to target the various locations where magnetic reconnection is expected to occur in Earth's magnetosphere, but, due to the precession of the orbit around the Earth, have allowed MMS to sample the multitude of different plasma environments in near-Earth space, including the solar wind, Earth's bow shock, the dayside and flanks of Earth's magnetosheath and magnetopause, and the plasma sheet and lobes of Earth's magnetotail. Further details about these different regions of near-Earth space are briefly discussed in Sec. II.

The orbits of each individual spacecraft are constructed such that the four spacecraft typically achieve a closely spaced tetrahedral formation near apogee, as illustrated for an example day in Fig. 1(b). The separation between the spacecraft has been varied over the course of the mission, with interspacecraft separations reaching as small as $∼ 5$ km and as large as a few hundred km. These interspacecraft separations are selected such that they are smaller than the characteristic scales of the ions and approach the characteristic scales of the electrons for the environment—and, in some extreme cases in the magnetotail, the separations have even been comparable to the Debye scale.¹⁶ The orbits have also been adjusted at various points in the mission to achieve other useful formation shapes, with a notable example being a “string-of-pearls” configuration with pseudo-logarithmic interspacecraft separations, illustrated in Fig. 1(c), in which the spacecraft are separated in one-dimension along a nearly identical orbital track.

Multipoint spacecraft formations enable the application of analysis techniques that allow one to directly extract information about the spatial configuration of the system. Tetrahedral configurations, which feature interspacecraft separations in all three spatial dimensions, allow the computation of 3D spatial gradients in the plasma, essentially employing first-order finite difference methods.^17,18 Such multipoint measurements have been widely used, both with MMS and previous multi-spacecraft missions such as Cluster to estimate the current density (⁠ $j = ∇ × B / μ 0$⁠, where μ₀ is the vacuum permeability) using the so-called curlometer technique. However, particularly in the era of MMS, many other gradient quantities and even full dynamical equations that are key to disentangling the complex plasma dynamics have been successfully computed using these techniques.^18–23 Other advanced multi-spacecraft techniques, such as polynomial reconstructions of the magnetic field line topology^24–26 and k-filtering/wave telescope techniques, which can estimate 3D wave vector information,^27–29 have also been employed using the tetrahedral configuration. While the linear string-of-pearls configuration sacrifices the ability to extract 3D information about the spatial gradients, they supplement such analyses by allowing the comparison of structures across multiple length scales simultaneously, through the estimation of correlation functions or structure functions, that can be used to characterize multiscale processes, such as turbulence, in the plasma.^30,31

Beyond the multipoint measurements, another key strength of MMS, enabled by the high-quality and high-time-resolution measurements of the particle velocity distributions from FPI in combination with the 3D measurements of both the electromagnetic fields, has been the ability to cross-validate measurements using different techniques, strengthening the conclusions drawn from MMS data on otherwise subtle aspects of the dynamics. Examples of this cross-validation include the comparison of estimates of $j$ from the multi-spacecraft curlometer technique and those computed directly from the plasma moments or assessing the magnetization of the plasma (⁠ $E = − u s × B$⁠, where u_s is the flow velocity of species s) for different particle species (see Phan et al.³² for an example of such inter-comparisons from early in the mission, although numerous other examples are also present in the literature). The incorporation of a dedicated MMS Theory & Modeling Team³³ into the overall MMS Science Working Team has also provided invaluable context through numerical simulations that is necessary to place the extremely local measurements provided by MMS into the larger context of the plasma and the magnetosphere as a whole.

MMS encounters a variety of different plasma environments with different ambient plasma conditions and undergoing different types of dynamics that are formed through the interaction of Earth's magnetic field and the solar wind. These environments are illustrated schematically in Fig. 2(a). The intrinsic, nearly dipolar, magnetic field formed by Earth's dynamo acts as an obstacle to the fast flow of plasma emanating from the Sun known as the solar wind. The interaction of Earth's magnetic field with the solar wind compresses Earth's magnetic field Sunward of the Earth and stretches out the field downstream of Earth in a region called the magnetotail. The solar wind flow is both supersonic and super-Alfvénic causing the formation of a bow shock upstream of the Earth. The region of shocked solar wind plasma downstream of the bow shock is known as the magnetosheath and the boundary between this solar-wind-origin plasma and Earth's magnetic field is known as the magnetopause. The magnetotail can be divided into broad regions—the region of dense plasma called the plasma sheet surrounding the reversal in the stretched magnetic field lines from Earthward to anti-Earthward and the low density, magnetically dominated lobes northward and southward of the plasma sheet.

Some examples of these different regions of near-Earth space, as observed by MMS, are shown in Figs. 2(b) and 2(c). In the observations, particularly near apogee, the motion of the spacecraft in their orbits is typically much slower than those associated with the local plasma dynamics (advection of structures, motion of plasma boundaries, flapping of the magnetotail, etc.) and, therefore, much of the observed local variation is associated with the motion of the plasma as opposed to the motion of the spacecraft. However, over long enough time scales, comparable to the orbital time of the spacecraft, the orbital motion carries the spacecraft through different regions of the magnetosphere.

In Fig. 2(b), MMS traverses from the magnetosphere, through the magnetopause into the magnetosheath, and finally through the bow shock into the high-speed solar wind plasma. Earth's dayside magnetosphere typically has relatively hot, low-density plasma with a very slow bulk flow velocity and a large $| B |$ dominated by a northward (positive) $B z , GSE$ associated with Earth's intrinsic magnetic field. The magnetosheath, by comparison, has higher densities and lower temperatures than the magnetosphere. Flow velocities in the magnetosheath can be significant (typically around a couple of hundred km/s) but are slower than the supersonic and super-Alfvénic flows in the solar wind due to the action of the bow shock. The fast speed and comparatively low temperature of the solar wind ions mean the ions appear as a narrow beam with all of the particles at nearly the same energy, as can be readily seen in Fig. 2(b-i). Processes associated with the bow shock also tend to excite complex fluctuations with properties that may depend on the upstream geometry of the shock (i.e., quasi-parallel or quasi-perpendicular), which act as a driver of additional turbulent dynamics in the magnetosheath. The orientation of $B$ in the magnetosheath depends on the orientation of $B$ in the upstream solar wind, as well as the local dynamics within the magnetosheath. When $B$ is southward (⁠ $B z , GSE < 0$⁠) in the magnetosheath, as in the example in Fig. 2(b-v), resulting in a large magnetic shear with the northward magnetospheric magnetic field across the magnetopause, there is an enhanced likelihood that conditions will be favorable for magnetic reconnection at the magnetopause.

Figure 2(c) shows an MMS encounter with the plasma sheet and lobes in Earth's magnetotail. The spacecraft is primarily within the hotter and higher-density plasma sheet at the center of the magnetotail; however, several excursions into the lobe, apparent from the significant decreases in particle number densities and large $| B |$ dominated by the stretched out $B x , GSE$ component of the magnetic field, are present. Several fast tailward ion flows (negative $u i x , GSE$⁠) followed by Earthward ion flows (positive $u i x , GSE$⁠) are present, suggestive of ongoing magnetotail reconnection in which the spacecraft encountered both of the outflow jets. During the periods within the plasma sheet, several excursions toward the plasma sheet boundary without entering the lobe are also apparent from increases in the $B x , GSE$ component of the magnetic field without the associated large decreases in density.

A principal goal of the MMS mission is to understand the electron dynamics that enable magnetic reconnection for a broad range of plasma parameters.³⁷ Reconnection at the magnetopause is highly asymmetric, in that the inflowing plasmas from the magnetosheath and magnetosphere typically have significantly different densities, temperatures, and magnetic field strengths. Reconnection in the magnetotail plasma sheet is often nearly symmetric. Whereas magnetotail reconnection occurs between nearly anti-parallel magnetic fields, magnetopause reconnection is often observed when the shear angle between the magnetosheath and magnetospheric fields is low. Finally, the plasma beta (ratio of plasma thermal to magnetic pressures) is typically lower in the magnetotail than in the magnetopause, meaning that more free magnetic energy is available for particle acceleration during magnetotail reconnection. The initial orbit of MMS and subsequent maneuvers, discussed in Sec. I, were chosen to maximize the number of reconnection events MMS could observe in these dramatically different environments.

Many space plasmas are nearly collisionless, meaning that the mean free path for Coulomb collisions is comparable to or larger than the system size of interest. In the solar wind at 1 AU, the collisional mean-free-path is $∼ 3$ AU, which is larger than the distance between the Sun and Earth and much larger than many of the dynamical fluctuations of interest, placing the solar wind in a nearly collisionless regime.³⁸ For magnetospheric plasmas, collisional mean-free-paths can be orders of magnitude larger than the system size with values of $∼ 10 5 R E$ or more in Earth's magnetosheath and $∼ 10 10 R E$ or more in Earth's plasma sheet depending on the type of collision. In the absence of collisional resistivity and/or viscosity to diffuse gradients in the plasma, the length-scale of gradients can become comparable to the characteristic length scales associated with the motion of individual particles (e.g., ion and electron gyroradii or inertial lengths) or the timescales of the plasma dynamics can become comparable to the characteristic timescales associated with the motion of individual particles, violating the fluid approximations. As such, understanding the detailed changes in the dynamics that occur at progressively smaller structures within the plasma and the impact they have on the overall system are among the central challenges in space plasma physics. MMS is specifically designed to probe these small-scale collisionless processes within the plasma.

Below we provide a brief primer on the theoretical description of collisionless plasmas, which underpins how we conceptualize the plasma dynamics observed by MMS, followed by a brief discussion of how such collisionless dynamics manifest themselves for the example of magnetic reconnection.

From dimensional analysis, the “Ideal MHD” term is expected to be the dominant contribution to $E$ for length scales much larger than the proton inertial length and gives rise to the well-known single-fluid MHD behavior of the plasma in which the magnetic field can be interpreted as being frozen-in to $u$ in the absence of resistivity. The “Hall” and “Electron Pressure” terms are both expected to become relevant for scales roughly comparable to or smaller than the proton characteristic length scales, with the Hall term describing the effect of protons decoupling from the magnetic field while the magnetic field remains frozen-in to the electron flow and the electron pressure term describing effects such as diamagnetic drifts and ambipolar electric fields. The “Electron Inertia” term is expected to become important at scales comparable to or smaller than the electron inertial length and describes the electric field that arises from the difference between the ion and electron inertia and is associated with the finite mobility of electrons in the sense that the effect disappears if the electrons are massless. The combination of the ideal MHD and Hall terms can be expressed as $E = − u e × B$⁠, corresponding to the magnetic field being frozen-in to the electron flow. Therefore, the electron pressure and electron inertia terms are notable compared to the ideal MHD and Hall terms in that, in the absence of collisions, they are the only terms capable of decoupling the magnetic field from the motion of all of the particle species and are the only terms capable of generating parallel electric fields in the plasma. As such, the electron pressure and electron inertia terms are central to understanding collisionless magnetic reconnection and energy conversion/dissipation. In the presence of collisions, the final “Resistive” term gives rise to non-ideal electric fields that dissipate energy and diffuse gradients in the magnetic field. If the resistivity (⁠ $1 / σ$⁠) is large enough, the resistive electric field will become the dominant contribution to Ohm's law at scales larger than the scales where the Hall, electron pressure, and electron inertial terms are expected to become significant, thus making it difficult for the plasma dynamics to excite gradients for which those terms would be important in the plasma. However, the near absence of collisions in the space plasmas encountered by MMS, mean that the resistive term is expected to be small.

Collisionless magnetic reconnection occurs throughout the magnetosphere in thin plasma current sheets. Reconnection is a fundamental process of plasmas that drives the rapid transfer of energy from electromagnetic fields to particles via a topological change in the current sheet magnetic field. Reconnection is inherently a multiscale process, which is caused by—and, in turn, impacts—the dynamics of the magnetospheric plasma over the broadest possible range of scale sizes.

At the heart of magnetic reconnection is the so-called “diffusion region” (nested, shaded boxes in Fig. 3), wherein inflowing plasmas mix and their magnetic fields merge. Collimated outflow jets carry Poynting flux and accelerated plasma particles away from the diffusion region. Non-ideal-MHD forces in the diffusion region [ $E + u × B ≠ 0$ with $∇ × ( E + u × B ) ≠ 0$⁠, see Eqs. (8) and (4)] are critically important for magnetic reconnection as they allow magnetic fields to slip through the particles and merge. Furthermore, non-ideal electric fields are required to balance the electromagnetic energy continuity equation [Eq. (9)], with energy conversion driven by non-ideal fields [ $j · ( E + u × B ) > 0$] balancing the net negative divergence of Poynting flux (⁠ $∇ · S < 0$⁠). In collisionless plasmas, diffusion regions have a nested structure, with the ion diffusion region (IDR), wherein the electric field is predominately balanced by the Hall term, surrounding the electron diffusion region (EDR), which is dominated by electron-kinetic forces, such as the electron pressure and electron inertial terms in Eq. (8). Importantly, the Hall term alone is (1) insufficient to describe the decoupling of electron and magnetic field motions and (2) incapable of driving net energy transfer from the fields to the particles, given that $j × B · j = 0$⁠. The MMS mission was motivated, in part, by the need to resolve the electron-kinetic forces in the EDR that enable magnetic reconnection. As such, the physics of the EDR, including the force balance relationship and energy conversion, have received a great deal of attention.

Validating decades of theoretical and modeling predictions,^41–43 MMS has repeatedly observed that non-gyrotropic electron distribution functions in the EDR are critical for enabling reconnection at current sheets with nearly anti-parallel magnetic fields. During anti-parallel reconnection, which is pictured in Fig. 3, electrons take a meandering path through the EDR, bouncing around the magnetic field reversal as they transit from the inflow to outflow. The distribution of electrons in the EDR often have a crescent-shaped structure in velocity space.^44,45 The meandering electrons provide the necessary current density to support the sharp magnetic field reversal, and the divergence of the non-gyrotropic pressure provides the necessary force to violate the frozen-in condition.⁴⁶ Electrons gain energy from the electric field as they bounce,^45,47 providing the necessary source of energy conversion to balance the divergence of the Poynting flux.⁴⁸ 

In contrast to the anti-parallel case described above, signatures of electron non-gyrotropy are often barely discernible^49–51 or absent entirely during reconnection between weakly sheared magnetic fields. So-called guide magnetic field components (directed out of the plane of Fig. 3) do not vanish at the center of the EDR, and serve to magnetize the electrons and inhibit meandering. Isotropic electron pressure forces, inertial forces, and/or 3D effects may be required to break the frozen-in condition in this case.^51,52 Electrons are accelerated parallel to the guide magnetic field, and this streaming population provides the current density to support the field reversal.⁵⁰ 

While energy conversion in the EDR is important for enabling the magnetic reconnection process, the total amount of energy converted in the EDR itself is negligibly small due to the EDR's small volume. The majority of particle acceleration by reconnection happens outside the EDR, in the separatrices (boundaries between inflow and outflow regions), exhaust jets, jet fronts, and secondary plasma structures like flux ropes that are generated by multiple nearby reconnection lines.

The contents of this collection cover a broad range of topics, highlighting many ways that the high-resolution, multi-spacecraft measurements from MMS are shedding new light on the fundamental physical processes operating in collisionless plasmas. The contributions both include new, cutting-edge examinations of the MMS data, as well as numerical works that help to place the findings of MMS into a broader context. While many of these studies focus on the physics of magnetic reconnection, the papers also touch on subjects such as collisionless shocks, plasma turbulence, the Kelvin–Helmholtz instability, waves, and the nuances of collisionless dynamics—in many cases, shedding new light on the interplay between these processes.

The work in the study by Burch et al.⁵³ addresses some of the main objectives of the MMS mission by identifying an EDR that MMS encountered in Earth's magnetotail on 6 July 2017. The reconnection rate is estimated using two independent methods—one using the measured inflow electron velocities and the other using the measured electric fields, which give similar dimensionless reconnection rates, albeit using different normalizations involving the electron and ion Alfvén speeds, respectively. In conjunction with previous examinations of the reconnection rate at the magnetopause and in the magnetosheath,⁵⁴ as well as in the magnetotail,^45,55 the work provides insight into the variability of the reconnection rate across different types of reconnection events and highlights further questions about the relationship between different estimates of the reconnection rate. Genestreti et al.⁵⁶ examines EDR dynamics going beyond the steady state picture of magnetic reconnection by considering the origin of “patchy” energy conversion in the EDR as quantified by $j · E$⁠. Genestreti et al.⁵⁶ examines 22 EDR's encountered by MMS and demonstrates that the nonuniformity of the energy conversion is linked to variability in the inflowing magnetic field, as opposed to the quasi-steady characteristics of the reconnection events, such as guide field strength or asymmetries. Norgren et al.³⁶ examine electron phase space holes measured by MMS during an encounter with the plasma sheet boundary layer on the same day as the reconnection event observed by Burch et al.⁵³ Norgren et al.³⁶ associate these electron phase space holes with fluctuations excited at the separatrix of a magnetic reconnection event. Using a unique set of measurements, incorporating both the high-resolution electron distribution functions measured by FPI and data from EDI, direct evidence for the thermalization of electrons due to wave–particle interactions with the electron phase space holes is obtained, providing key insight into energy dissipation in the collisionless regime.

Several other papers explore the waves excited by the magnetic reconnection process. Wang et al.⁵⁷ use 2D and 3D particle-in-cell simulations to model conditions consistent with magnetotail reconnection and examine the interaction of ions and electrons with lower hybrid drift waves (or current sheet corrugations) driven unstable by the density gradients and resulting diamagnetic drifts at the reconnection separatrix. Such instabilities generate a train of vortices in the electron flow, which have been observed in spacecraft observations of magnetic reconnection.^58–60, Marshall et al.⁶¹ examines observations of similar lower hybrid drift waves at a dayside magnetopause reconnection event, identifying parallel electric fields associated with these fluctuations that are observed to be imparting energy to the electrons and accelerating electron beams. Choi et al.⁶² examine the whistler waves, which are among the many types of waves that have been observed in association with magnetic reconnection by MMS, using an example 2D particle-in-cell simulation of asymmetric guide field reconnection emulating conditions that may be encountered at the Earth's magnetopause. Choi et al.⁶² focus on using the simulation to characterize how the complex electron velocity distribution functions that are generated by the collisionless magnetic reconnection process are related to the whistler wave activity. The work highlights how fast super-Alfvénic electron beams, gyrotropic temperature anisotropy, and agyrotropic temperature distributions linked with the well-studied crescent distributions observed by MMS contribute to the whistler power in different regions both along the separatrix on the magnetospheric side of the reconnection event and within the EDR closer to the x-line.

The extended outflows of magnetic reconnection can also be an important region where particle energization and heating occurs and several studies focus on unique observations of the reconnection outflows provided by MMS. Øieroset et al.⁶³ examine a set of observations where the MMS spacecraft crossed through a reconnection outflow at the transition between the EDR and IDR, with each of the four spacecraft observing a cut across the current sheet at a different stage in the acceleration of the ion jets. Payne et al.⁶⁴ use a 2D particle-in-cell simulation in conjunction with a magnetic reconnection event observed by MMS in Earth's magnetotail on 17 June 2017 to examine the role of the so-called “generator” regions in the magnetic reconnection outflows, where the deceleration and remagnetization of the fast super-Alfvénic electron outflows as the electrons leave the EDR imparts energy from the particle flows back into the electromagnetic fields. Payne et al.⁶⁴ discuss how this process eventually results in the heating of particles in the reconnection outflows. Oka et al.⁶⁵ uses a series of magnetotail reconnection observations to explore how the energy dissipated by magnetic reconnection is partitioned between thermal heating and non-thermal particle acceleration, demonstrating that the fraction of non-thermal energization can vary from 20% to 60% of the particle energization and examining how this variation depends on various properties of the reconnection events.

Two studies focus, in particular, on so-called dipolarization fronts in the magnetotail, which are often interpreted as the boundary of reconnection outflows as they plow through the surrounding plasma sheet. Hosner et al.⁶⁶ perform a statistical study of 61 dipolarization fronts observed by MMS, examining how the thin gradients built up at the fronts can drive the lower-hybrid drift instability, while Alqeeq et al.⁶⁷ examines six dipolarization fronts in detail, using the MMS measurements to determine how the electric fields are generated by generalized Ohm's law and how those electric fields contribute to the energy exchange at the dipolarization fronts.

Beyond the study of the large-scale reconnection events present at Earth's magnetopause and in Earth's magnetotail, several authors explore the physics of collisionless shocks using MMS observations of Earth's bow shock. Wang et al.⁶⁸ perform a survey of ultra-low-frequency waves in the foreshock upstream of Earth's bow shock, using MMS to examine how these waves are excited by and interact with the different populations of ions associated with the solar wind and reflected ions from the bow shock. Building on previous work,^69, Kamaletdinov et al.⁷⁰ use the 3D electric field measurements from MMS to examine bipolar electrostatic structures at the bow shock. While the majority of such electrostatic structures observed at the shock have been identified as ion phase space holes, Kamaletdinov et al.⁷⁰ focus on a minority population of unexplained bipolar electrostatic structures that they identify as slow electron phase space holes. Such structures have been proposed as candidates for electron acceleration at high-Mach number collisionless shocks and Kamaletdinov et al.⁷⁰ demonstrate that the electron phase space holes are likely unstable at the bow shock, which limits their expected ability to contribute to electron acceleration in astrophysical shocks. Pollock et al.⁷¹ use MMS observations in the string-of-pearls configurations [see Fig. 1(c)] to examine Earth's bow shock during a period where the magnetic field was nearly anti-aligned with the solar wind flow. Such conditions can lead to particularly dynamic and deformed shocks and Pollock et al.⁷¹ use the MMS observations to consider different models for the bow shock structure, such as the shock undergoing quasi-periodic expansion and contraction, the shock having surface ripples, or the shock having a more complex porous surface. Motivated by observations,^72, Ng et al.⁷³ perform a 3D hybrid particle-in-cell simulation to examine how transient jets of plasma generated at the bow shock may lead to perturbations of the magnetopause, triggering magnetic reconnection.

Recent research on the fluctuations downstream of the shock with MMS^74–80 has also been advancing our understanding of plasma physics at the intersection of shock, plasma turbulence, and magnetic reconnection physics by demonstrating the existence of magnetic reconnection at the multitude of thin current sheets embedded within the turbulence in Earth's magnetosheath and several new additions to this exciting area of research are included in this collection. Roberts et al.⁸¹ examines the nature of the turbulent fluctuations in the magnetosheath downstream of Earth's bow shock using a variety of analysis techniques enabled by the high-time-resolution particle measurements and multipoint formation of MMS. The results demonstrate that, at least within the interval examined, the sub-proton scale turbulent fluctuations appear consistent with interacting kinetic Alfvén waves that are often invoked in theories of collisionless plasma turbulence and that Landau damping is likely playing a role in the dissipation of the turbulence. Gingell et al.³⁵ focuses on the thin current sheets embedded within the turbulent magnetosheath, which are thought to play a role in the dissipation of turbulence⁸² and are likely important to understanding the overall energy budget and particle heating associated with the collisionless shock process,⁸³ revealing how the density of current sheets is distributed across the magnetosheath. Stawarz et al.,³⁴ Bessho et al.,⁸⁴ and Bandyopadhyay et al.⁸⁵ all examine magnetic reconnection at these turbulent current sheets in the magnetosheath and their possible role within the shock and turbulence dynamics. Building on works demonstrating the existence of reconnection at turbulent current sheets in the magnetosheath, Stawarz et al.³⁴ performs a survey of turbulence-driven reconnection in the magnetosheath, identifying 256 reconnection events, and demonstrating that the magnetic correlation length of turbulence influences the extent to which ions are able to couple into the reconnection dynamics to form jets, leading to so-called “electron-only” magnetic reconnection in agreement with suggestions from previous numerical simualtions.⁸⁶ Stawarz et al.³⁴ further discusses the potential contribution these reconnection events may make to the dissipation of the turbulence. Bessho et al.⁸⁴ examines the magnetic reconnection events self-consistently generated downstream of a shock in a 2D particle-in-cell simulation, demonstrating that electron-only reconnection is generated, which has exceptionally strong reconnection electric fields in agreement with observations. Bandyopadhyay et al.⁸⁵ uses MMS's unique capabilities to directly measure the energy exchange terms from Eqs. (9)–(11) for turbulence-driven reconnection in the magnetosheath, as well as for an example magnetopause reconnection event.

On the flanks of Earth's magnetopause, where a strong velocity shear exists between the magnetosheath plasma and the magnetospheric plasma, MMS is enabling the examination of the Kelvin–Helmholtz instability and its development into a complex turbulent boundary layer through the action of secondary instabilities. Two papers in this collection explore this problem, focusing on the particularly complex case of the Kelvin–Helmholtz instability during a period where the solar wind magnetic field is directed southward (opposite to Earth's dipole), which is conducive to both the Kelvin–Helmholtz instability and magnetic reconnection at Earth's magnetopause. Blasl et al.⁸⁷ presents observations of such a Kelvin–Helmholtz event under southward magnetic field conditions as observed by MMS, while Nakamura et al.⁸⁸ presents a particle-in-cell simulation of the event, with both the observations and simulations demonstrating the potentially important role of secondary Rayleigh–Taylor and lower hybrid drift instabilities within the vorticies, particularly in the southward configuration.

Within Earth's dayside inner magnetosphere, Abid et al.⁸⁹ demonstrates how MMS observations allow us to examine wave–particle interactions, in this case for electromagnetic ion cyclotron waves which lead to the energization of cold ions.

Finally, several papers use MMS observations in conjunction with theory to get at the heart of some of the fundamental complexities associated with collisionless dynamics. Argall et al.⁹⁰ applies formulations of the kinetic entropy density for the particle distribution function in a plasma, as discussed by Liang et al.,⁹¹ to MMS observations and simulations of magnetic reconnection. The use of the kinetic entropy density as a tool for examining energy conversion and dissipation in collisionless plasmas and the impact that the finite and non-uniformly spaced velocity grid available from spacecraft observations have on its computation is examined. Shuster et al.⁹² develops a model for a kinetic equilibrium state that can be achieved in a collisionless plasma, which includes the presence of a temperature gradient. The model is compared directly to MMS observations of electron-scale current sheets, both in terms of the observed particle distribution functions and the gradients inferred from the multi-spacecraft measurements, with good agreement. Building on recent work done in Goldman et al.,^93, Goldman et al.⁹⁴ examines how to deal with and interpret the highly complex distribution functions that can develop in the absence of collisions. The authors consider so-called multi-beam distributions, made up of an arbitrary assortment of different populations or “beams” in velocity space. Three methods are developed for systematically identifying different beams, which can then be used to compute effective multi-beam moments that are taken as the sum of the moments computed for each beam. The methods are applied to both MMS observations and particle-in-cell simulations and the derived multi-beam moments are compared with the standard moments computed over the entire distribution function, demonstrating how the technique can be used to provide additional information about the energetics in collisionless plasmas.

A key challenge in the study of the natural plasmas within our heliosphere is that many of these environments, such as the planetary magnetospheres and solar wind, are in a collisionless or nearly collisionless regime. While the large-scale dynamics of such systems are often reasonably described in the context of fluid descriptions, such as magnetohydrodynamics, collisionless processes are ultimately responsible for the dissipation of fluid-scale fluctuations into plasma heating, non-thermal particle acceleration, and the partitioning of that energy between different particle species, as well as enabling the mixing of plasma between magnetically isolated regions through the breaking of the frozen-in condition (as, for example, at the interface between the Earth's magnetosphere and the solar wind). The behavior of fundamental plasma processes, such as magnetic reconnection, plasma turbulence, and collisionless shocks—all of which, at a basic level, couple the large-scale fluid dynamics to the small-scale kinetic dynamics—are, thus, intrinsically linked to the collisionless dynamics in these systems. MMS has ushered in a new era of observationally driven advances to our understanding of kinetic-scale plasma dynamics through a combination of high-time-resolution and closely spaced multipoint measurements at scales as small as $∼ 5$ km capable of directly probing the 3D gradients at scales typically approaching the electron scales in the plasma (and in some cases even smaller than the electron scales under extreme conditions). Alongside these high-quality measurements, a key strength of MMS has been the ability to employ these measurements across a broad range of plasma environments, particularly those associated with Earth's magnetosphere. Such measurements allow one to contrast the dynamics at play across a breadth of different plasma conditions, which as well as being germane to understanding the magnetospheric system, are representative of the general classes of plasma structure (shock processed plasma, interfaces between distinct plasma regions both with and without strong velocity shears, jets of plasma, etc.) that may be found throughout the Universe. The broad array of studies assembled in this collection, along with those that have been published more widely since the launch of the mission in 2015, are a testament to the many ways that MMS is bringing new insight into a range of plasma physics topics such as (1) how collisionless dynamics enable and support the magnetic reconnection process; (2) the dynamics and dissipation of plasma turbulence in the collisionless regime; (3) the interplay between plasma phenomena such as collisionless shocks, magnetic reconnection, turbulence, the Kelvin–Helmholtz instability, and more; and (4) how collisionless plasmas facilitate the exchange of energy between the electromagnetic fields, bulk flows, and internal energy and how that energy is partitioned between different particle species and populations in phase space.

While measurements such as those made by MMS have provided unprecedented insight into the kinetic-scale dynamics operating in localized regions of the plasma, one of the next frontiers in space plasma physics will come from unambiguously understanding the interplay between those local dynamics and the larger-scale dynamics of the system. One approach to this challenge, which has been extensively employed by the MMS mission and is highlighted by a number of studies in this collection, is the use of high-quality local measurements in conjunction with numerical simulations using various levels of refinement from global single-fluid MHD to fully kinetic. Previous^30,31 and upcoming campaigns with MMS are also using alternative formation strategies, such as the string-of-peals formation illustrated in Fig. 1(c), and data selection strategies to attempt to probe and link together the dynamics across multiple scales simultaneously. However, building off of the success of missions such as MMS and Cluster, which were designed to probe the 3D structure of the plasma at specific length scales with four spacecraft, the next leap forward, that will help to answer this crucial problem of how the local plasma physics couple into the larger meso-scale and system-scale dynamics, will come from larger constellations (or swarms) of spacecraft capable of simultaneously measuring multiple length scales at once. Such missions are already on the horizon, with the HelioSwarm mission, consisting of nine spacecraft in a loose formation with interspacecraft separations ranging from fluid to proton scales in the solar wind and magnetosheath, recently being selected for development by NASA and the Plasma Observatory mission, recently being proposed and selected for further study by ESA. Furthermore, while MMS is currently providing a unique dataset of solar wind observations, the distinct character of the solar wind fluctuations (notably, lower fluctuation amplitude and narrow angular width of the ion distribution function in the supersonic solar wind) relative to plasmas in the magnetosheath and magnetosphere, present challenges for missions, such as MMS, that are optimized to measure magnetospheric plasmas. Therefore, missions targeting electron-scale dynamics with instrumentation optimized for the solar wind plasma are also an important target for the next generation of space plasma physics missions.⁹⁵ Even with such new missions on the horizon, MMS has been and continues to be an immense achievement for the space and plasma physics communities and the dataset it has produced will no doubt continue to be an important resource for the study of fundamental plasma physics.

J.E.S. is supported by the Royal Society University Research Fellowship No. URF\R1\201286. K.J.G. is supported by NASA's MMS FIELDS Grant No. NNG04EB99C. Both authors would like to express their gratitude to the MMS mission team and everyone who has made MMS a success.

The authors have no conflicts to disclose.

Julia E. Stawarz: Writing – original draft (equal); Writing – review & editing (equal). Kevin J. Genestreti: Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are openly available from the MMS Science Data Center at https://lasp.colorado.edu/mms/sdc/public/, Refs. 1, 3, and 8.