Probing magnetic reconnection in space Magnetic reconnection occurs when the magnetic field permeating a conductive plasma rapidly rearranges itself, releasing energy and accelerating particles. Reconnection is important in a wide variety of physical systems, but the details of how it occurs are poorly understood. Burch et al. used NASA's Magnetospheric Multiscale mission to probe the plasma properties within a reconnection event in Earth's magnetosphere (see the Perspective by Coates). They find that the process is driven by the electron-scale dynamics. The results will aid our understanding of magnetized plasmas, including those in fusion reactors, the solar atmosphere, solar wind, and the magnetospheres of Earth and other planets. Science, this issue p. 10.1126/science.aaf2939; see also p. 1176

Structured Abstract INTRODUCTION Magnetic reconnection is a physical process occurring in plasmas in which magnetic energy is explosively converted into heat and kinetic energy. The effects of reconnection—such as solar flares, coronal mass ejections, magnetospheric substorms and auroras, and astrophysical plasma jets—have been studied theoretically, modeled with computer simulations, and observed in space. However, the electron-scale kinetic physics, which controls how magnetic field lines break and reconnect, has up to now eluded observation. RATIONALE To advance understanding of magnetic reconnection with a definitive experiment in space, NASA developed and launched the Magnetospheric Multiscale (MMS) mission in March 2015. Flying in a tightly controlled tetrahedral formation, the MMS spacecraft can sample the magnetopause, where the interplanetary and geomagnetic fields reconnect, and make detailed measurements of the plasma environment and the electric and magnetic fields in the reconnection region. Because the reconnection dissipation region at the magnetopause is thin (a few kilometers) and moves rapidly back and forth across the spacecraft (10 to 100 km/s), high-resolution measurements are needed to capture the microphysics of reconnection. The most critical measurements are of the three-dimensional electron distributions, which must be made every 30 ms, or 100 times the fastest rate previously available. RESULTS On 16 October 2015, the MMS tetrahedron encountered a reconnection site on the dayside magnetopause and observed (i) the conversion of magnetic energy to particle kinetic energy; (ii) the intense current and electric field that causes the dissipation of magnetic energy; (iii) crescent-shaped electron velocity distributions that carry the current; and (iv) changes in magnetic topology. The crescent-shaped features in the velocity distributions (left side of the figure) are the result of demagnetization of solar wind electrons as they flow into the reconnection site, and their acceleration and deflection by an outward-pointing electric field that is set up at the magnetopause boundary by plasma density gradients. As they are deflected in these fields, the solar wind electrons mix in with magnetospheric electrons and are accelerated along a meandering path that straddles the boundary, picking up the energy released in annihilating the magnetic field. As evidence of the predicted interconnection of terrestrial and solar wind magnetic fields, the crescent-shaped velocity distributions are diverted along the newly connected magnetic field lines in a narrow layer just at the boundary. This diversion along the field is shown in the right side of the figure. CONCLUSION MMS has yielded insights into the microphysics underlying the reconnection between interplanetary and terrestrial magnetic fields. The persistence of the characteristic crescent shape in the electron distributions suggests that the kinetic processes causing magnetic field line reconnection are dominated by electron dynamics, which produces the electric fields and currents that dissipate magnetic energy. The primary evidence for this magnetic dissipation is the appearance of an electric field and a current that are parallel to one another and out of the plane of the figure. MMS has measured this electric field and current, and has identified the important role of electron dynamics in triggering magnetic reconnection. Electron dynamics controls the reconnection between the terrestrial and solar magnetic fields. The process of magnetic reconnection has been a long-standing mystery. With fast particle measurements, NASA’s Magnetospheric Multiscale (MMS) mission has measured how electron dynamics controls magnetic reconnection. The data in the circles show electrons with velocities from 0 to 104 km/s carrying current out of the page on the left side of the X-line and then flowing upward and downward along the reconnected magnetic field on the right side. The most intense fluxes are red and the least intense are blue. The plot in the center shows magnetic field lines and out-of-plane currents derived from a numerical plasma simulation using the parameters observed by MMS.

Abstract Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA’s Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth’s magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.

Magnetic reconnection is an energy conversion process that operates in many astrophysical environments, producing energetic phenomena such as geomagnetic storms and aurora, solar flares and coronal mass ejections, x-ray flares in magnetars, and magnetic interactions between neutron stars and their accretion disks. Reconnection is also crucially important in laboratory plasma physics, where it has proved to be an impediment to the achievement of magnetic-confinement fusion through the sawtooth crashes that it triggers. A better understanding of reconnection is an important goal for plasma physics on Earth and in space, but a complete experiment is impossible to conduct in most environments, which are too distant, too hot, or too small for comprehensive in situ measurements (1).

Earth’s magnetosphere has been explored by many spacecraft missions, some of which have made multipoint measurements in and around regions containing collisionless magnetic reconnection (2–7). Results from these missions have verified many of the predictions about magnetic reconnection phenomena on the magnetohydrodynamic (MHD) and ion scales. However, to make major progress in the study of collisionless reconnection in space, it is necessary to extend the measurements to the electron scale and make accurate three-dimensional measurements of electric and magnetic fields. Also required are accurate ion composition measurements, which can help to determine the role of ionospheric particles in reconnection, as well as energetic particle measurements, which can help to determine where magnetic fields interconnect and how particles are accelerated to high energies.

NASA’s Magnetospheric Multiscale (MMS) mission (8) was designed to perform a definitive experiment in space on magnetic reconnection at the electron scale, at which dissipation of magnetic energy and the resulting interconnection of magnetic fields occur. Electron-scale kinetic physics in the region around the reconnection site (or the X-line) where field line breaking and reconnection occur has not previously been investigated experimentally in space, owing to insufficiently detailed measurements. Our knowledge of this region at the electron scale has come mainly from computer simulations (9–13) and laboratory experiments (14, 15). The higher resolution of MMS measurements in both time and space relative to previous missions offers an opportunity to investigate the cause of reconnection by resolving the structures and dynamics within the X-line region.

The data set obtained by MMS incorporates the following advances: (i) four spacecraft in a closely controlled tetrahedron formation with adjustable separations down to 10 km; (ii) three-axis electric and magnetic field measurements with accurate cross-calibrations allowing for the measurement of spatial gradients and time variations; and (iii) all-sky plasma electron and ion velocity-space distributions with time resolution of 30 ms for electrons and 150 ms for ions.

The four MMS spacecraft were launched together on 13 March 2015 (UT) into a highly elliptical (inclination 28°) orbit with perigee at 1.2 Earth radii (R E ) and apogee at 12 R E (both geocentric). The mission is being conducted in two phases, the first phase targeting the dayside outer boundary of Earth’s magnetosphere (the magnetopause) and the second phase targeting the geomagnetic tail, for which the apogee is raised to 25 R E . This article focuses on magnetopause measurements during the first science phase of the mission, which began on 1 September 2015. For this phase, a region of interest was identified as geocentric radial distances of 9 to 12 R E , during which all instruments are operated at their fastest cadence, producing burst-mode data. Within the region of interest, the four spacecraft are maintained in a tetrahedral formation with separations variable between 160 and 10 km. A quality factor for the tetrahedra, defined by the ratio of their surface area to their volume, is maintained to within 80% of the ratio for a regular tetrahedron.

By 14 December 2015, the spacecraft had crossed the magnetopause more than 2000 times. On the basis of detection of plasma jetting and heating within the magnetopause current sheets, we infer that at least 50% of the crossings encountered magnetic reconnection. Most crossings occurred in the reconnection exhaust downstream of the X-line, but a few of them passed very close to the X-line. The data for one of these events (16 October 2015, 13:07 UT) are presented here as an example of the electron-scale measurements of the reconnection diffusion/dissipation region around an X-line.

MMS measurements The set of measurements made on each of the four MMS spacecraft are listed in Table 1. The improvement in time resolution for three-dimensional plasma distribution measurements was substantial: 30 ms for electrons and 150 ms for ions, as compared to previous resolutions in the few-second range. This improvement required the use of multiple analyzers rather than one spinning analyzer, resulting in stringent requirements on their absolute calibration and intercalibration. Two benefits of this approach are the ability to make accurate measurements of currents and of electron drift velocities. Another advance is the accurate measurement of three-axis electric fields, which are crucially important for the investigation of reconnection. Data taken at the highest measurement resolution are referred to as burst-mode data, and all instruments operate in burst mode whenever the spacecraft are beyond a geocentric distance of 9 R E on the dayside of Earth. Table 1 Measurements made on each MMS spacecraft. View this table:

Data interpretation The existence of the crescent-shaped electron distributions in the plane perpendicular to B, as shown in Figs. 4 and 5, can be explained conceptually as follows. There is typically a large ion pressure gradient across the magnetopause. During magnetic reconnection, this pressure gradient produces a large normal electric E N in an LMN coordinate system that points toward the Sun. This electric field balances the ion pressure gradient and keeps ions flowing from the magnetosheath from penetrating the magnetosphere. In the vicinity of the X-line, E N modestly overlaps the null field region (B L = B N = 0). The strong out-of-plane current J M during magnetopause reconnection actually peaks not at the X-line but displaced to the magnetosphere side of the X-line where E N peaks. The high J M is carried by high-velocity electrons with a crescent-shaped distribution in the V M -V N (perpendicular to B) plane that is symmetric across the V M axis. This crescent distribution results from cusp-like orbits of electrons associated with the motion in the M-N plane controlled by E N (N) and B L (N) (22, 23). The motion is similar to that of pickup ions in the solar wind (24). Electrons around B L = 0 are accelerated toward the magnetosphere by E N . As they gain energy, B L causes them to turn in the M direction. Eventually they turn around, reaching a peak velocity along M that is around twice the E × B velocity V EB = cE N /B L . The electrons return to B L = 0 with zero velocity (ignoring their thermal spread) and repeat their cusp-like motion. The electron distribution function can be calculated analytically. With increasing distance from the null region to the turning point in N, it transitions from a hot thermal distribution to a horseshoe-like distribution (with more particles at higher V M and a depletion of particles around V M , V N = 0) and then to a crescent centered at a velocity V M that increases with distance into the region of high E N and narrows in the V M direction. As the magnetopause moves inward in this event, the crescent-shaped electron population enters a region of very weak magnetic field containing open field lines in the inner part of the electron exhaust. In the exhaust region, newly reconnected field lines move rapidly away from the X-line northward and southward—a phenomenon that has been described as a double slingshot (1) or simply a magnetic slingshot (2). It is likely that these exhaust region dynamics are responsible for redirection of the perpendicular crescents into the observed parallel crescents. Although the perpendicular crescents (averaged over V parallel ) were predicted in simulations (9), the parallel crescents have not been. Their direct observation by MMS therefore represents a new target for simulations.

Summary and implications The MMS mission, which was designed to perform a definitive experiment on magnetic reconnection in space, has investigated electron-scale physics in an encounter with the dissipation region near a reconnection X-line at Earth’s magnetopause. The high temporal resolution and accuracy of the MMS plasma and field measurements were both necessary and sufficient for the investigation of the electron physics controlling reconnection. Using measurements of plasma currents and reconnection electric fields, we have shown that J · E′ > 0 in the vicinity of the X-line, as predicted for the dissipative nature of reconnection. Electron distribution functions were found to contain characteristic crescent-shaped features in velocity space as evidence for the demagnetization and acceleration of electrons by an intense electric field near the reconnection X-line. MMS has directly determined the current density based on measured ion and electron velocities, which allowed the resolution of currents and associated dissipation on electron scales. These scales are smaller than the spacecraft separation distances and hence smaller than currents that can be determined by ∇ × B. The X-line region exhibits electron demagnetization and acceleration (by both E N and E M ), which results in intense J M current that is carried by the crescent-shaped electron distributions. Kinetic simulations had predicted some elements of the crescent distributions near the X-line, which raises the prospect of active interplay between theory and experiment, because the two techniques are now on a similar footing. The MMS measurements have led to discoveries about the evolution of electron acceleration in the dissipation region, as well as the escape of energized electrons away from the X-line into the downstream exhaust region. The latter was detected by at least two MMS spacecraft located on opposite sides of the X-line. The observed structures of the normal electric field and electron dynamics near the X-line by the four spacecraft are highly variable spatially and/or temporally, even on electron scales. Among the implications of this initial MMS experiment is the discovery that the X-line region is important not only for the initiation of reconnection (breaking of the electron frozen-in condition), but also for electron acceleration and energization, leading to much stronger electron heating and acceleration than seen in the downstream exhaust. The details of the electron distribution functions, which show the rapid transition (within 30 ms) of the perpendicular crescent distributions to parallel crescents, provide experimental evidence for the opening up of magnetic field lines while also demonstrating that it is the electron dynamics that drives reconnection. Because of the importance of reconnection in many astrophysical and laboratory environments and the improvement achieved by its measurement resolution (25, 26), MMS has opened up a new window on the universal plasma process of magnetic reconnection.

Materials and methods The science phase of MMS began on 1 September 2015, when the orbit apogee precessed beyond the dusk meridian toward the dayside, after which it skimmed the magnetopause for 6 months. The scientific strategy was to position the four spacecraft in a tetrahedral formation at radial distances from 9 to 12 R E , first at the ion scale (160 km) and progressing to the smaller electron scale (10 km), so that magnetic reconnection could be investigated as the magnetopause crossed back and forth across the tetrahedron in response to variations in the solar wind dynamic pressure. This strategy bore fruit as several magnetopause crossings were observed on most days, with many of these crossings showing evidence for magnetic reconnection based on the appearance of plasma jetting. A small subset of these reconnection events was sampled directly when the MMS spacecraft crossed near or through the electron dissipation region within which magnetic energy is converted to particle kinetic energy. Effective sampling at the electron scale requires measurements at the highest instrument data rate, termed a “burst mode.” Whenever the spacecraft are between 9 and 12 R E (the region of interest), all instruments are run at their maximum data rates. Because of data downlink volume limitations coupled with the unprecedentedly high internal data rate of the MMS instruments, careful selection of data to be downlinked is necessary. Two methods are used for the downlink data selection, both of which involve the use of a 96-GB onboard memory, which contains all the burst-mode data for two or more orbits of MMS. The first method of data selection involves the reporting of data evaluations by each instrument on a 10-s time scale, resulting in figures of merit for each interval, which are combined to generate a spacecraft figure of merit. These figures of merit are transmitted to the ground along with summary data for entire orbits. The summary data are similar to those shown in Fig. 1. Aggregate figures of merit for the four spacecraft are combined with ground software to generate a mission-level figure of merit. These automatically generated figures of merit then determine the priorities by which burst data are transmitted during the next ground contact. The second data downlink selection method builds on the first one by using a scientist-in-the-loop to examine the figures of merit and the summary data for each day, with the goal of optimizing the data downlink selection by either adjusting the figures of merit or identifying new high-priority intervals that were not selected by the onboard system. Both systems are effective and both are being used throughout the mission. The data from all the independent sensors on each satellite, and between the four satellites, are intensively intercalibrated (25, 26). Beginning on 1 March 2016, the entire MMS data set has been available online at https://lasp.colorado.edu/mms/sdc/public/links/. Fully calibrated data are placed online at this site within 30 days of their transmission to the MMS Science Operations Center. The data are archived in the NASA Common Data Format (CDF) and so can be plotted using a number of different data display software packages that can use CDF files. A very comprehensive system called the Space Physics Environment Data Analysis System (SPEDAS) is available by downloading http://themis.ssl.berkeley.edu/socware/bleeding_edge/andselectingspdsw_latest.zip. Training sessions on the use of SPEDAS are held on a regular basis at space physics–related scientific meetings. All of the data plots in this paper were generated with SPEDAS software applied to the publicly available MMS database, so they could readily be duplicated. Movie 1 An electromagnetic particle-in-cell simulation with parameters corresponding to the event is performed with the P3D code (27). Particles are advanced using a relativistic Boris stepper with electromagnetic fields extrapolated to the particles’ positions (28). Electromagnetic fields are evolved using the trapezoidal leapfrog scheme on Maxwell’s equations with second-order spatial derivatives. The simulation is two-dimensional with periodic boundary conditions in both directions. Magnetic fields in the simulation are normalized to the magnitude of the L component of the magnetosheath magnetic field, 23 nT. Densities are normalized to the magnetosheath density, 11.3 cm−3. Distances are normalized to 67.8 km (the magnetosheath ion inertial scale d i,sh = c/ω pi,sh ), and current densities to 0.270 μA/m2. The initial conditions for the upstream values of the L and M components of the magnetic field, the densities, and the electron and ion temperatures on both sides of the current sheet are taken to match the event to the extent possible: B L ,ms = 39 nT, B L ,sh = 23 nT, B M ,ms = B M ,sh = 2.278 nT, n ms = 0.7 cm−3, and n sh = 11.3 cm−3, where “ms” denotes the magnetospheric side and “sh” denotes the magnetosheath side. For the temperatures, magnetosheath values are T p,sh = 320 eV and T e,sh = 28 eV to match the MMS data. For the magnetosphere, the low density makes measuring temperatures difficult, so for the purposes of the simulation we defined the magnetospheric temperature as that required to balance total pressure in the fluid sense with a proton temperature 6 times the electron temperature: T p,ms = 1800 eV, T e,ms = 300 eV. No bulk flow of the upstream plasma is included in the initial conditions. The profiles for the initial conditions had double tanh profiles for the magnetic field and temperature, and the density profile is chosen to enforce pressure balance in the fluid sense. The domain size is 40.96 × 20.48 in normalized units and the grid scale is 0.01 in both directions. The time step is 0.001 in units of the magnetosheath inverse ion cyclotron frequency Ω ci,sh −1 and is run until t = 40. The time step on the electromagnetic fields is half that of the particles to resolve light waves. The simulation is initialized with 500 particles per grid. The ion-to-electron mass ratio is 100 and the ratio of the speed of light to the initial magnetosheath Alfvén speed is 15 (ω pi,sh /Ω ci,sh = 15); these differ from the realistic values of 1836 and 2000, respectively, but it is common to use smaller values for numerical expediency and is not expected to adversely affect the simulations. http://bcove.me/o51zgjqt Movie 2 Three-second segment of burst-mode electron distributions keyed to a plot of plasma and field data covering the same time period as Fig. 4. One hundred electron velocity-space distributions are shown over this period. Previous missions that used the spacecraft spin to cover the full sky could only acquire one or fewer distributions over a time period of this length. This factor of 100 increase in electron time resolution is an important reason why MMS is able to investigate the electron-scale physics of reconnection. http://bcove.me/9fkcpfn1.

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Acknowledgments: The dedicated efforts of the entire MMS team are greatly appreciated. We are especially grateful to the leadership of C. Tooley, B. Robertson, and R. Black. Special thanks are due to C. Pankratz and K. Larsen of the University of Colorado for their leadership of the MMS Science Operations Center. Supported by NASA contract NNG04EB99C at Southwest Research Institute, which funded work at most of the co-author institutions in the United States. The IRAP contribution to MMS was supported by CNES. The Austrian contributions to the MMS mission are supported by grants from the Austrian Research Promotion Agency FFG. The UK work was supported by the UK Science and Technology Facilities Council through grants ST/K001051/1 and ST/N000692/1. The work by NASA GSFC authors was supported by the NASA Solar Terrestrial Probes program. The work of the GSFC-resident University of Maryland co-authors was supported by NASA Goddard Planetary Heliophysics Institute contract NNG11PL02A. Work at U.C. Berkeley was supported by NASA MMS-IDS grant NNX08AO83G through the University of California. Work at the University of Colorado by M.G. and D.N. was supported by NASA MMS-IDS Grant NNX08AO84G through the University of Colorado. Work at the Swedish Institute for Space Physics and the Royal Institute of Technology was supported by the Swedish National Space Board. Work at West Virginia University was supported by NSF grants AGS-0953463 and AGS-1460037 and by NASA grants NNX16AG76G and NNS16AF75G.