RESEARCH ARTICLE SUMMARY◥

PLASMA ASTROPHYSICS

Electron-scale measurements ofmagnetic reconnection in spaceJ. L. Burch,* R. B. Torbert, T. D. Phan, L.-J. Chen, T. E. Moore, R. E. Ergun,J. P. Eastwood, D. J. Gershman, P. A. Cassak, M. R. Argall, S. Wang, M. Hesse,C. J. Pollock, B. L. Giles, R. Nakamura, B. H. Mauk, S. A. Fuselier, C. T. Russell,R. J. Strangeway, J. F. Drake, M. A. Shay, Yu. V. Khotyaintsev, P.-A. Lindqvist,G. Marklund, F. D. Wilder, D. T. Young, K. Torkar, J. Goldstein, J. C. Dorelli,L. A. Avanov, M. Oka, D. N. Baker, A. N. Jaynes, K. A. Goodrich, I. J. Cohen,D. L. Turner, J. F. Fennell, J. B. Blake, J. Clemmons, M. Goldman, D. Newman,S. M. Petrinec, K. J. Trattner, B. Lavraud, P. H. Reiff, W. Baumjohann, W. Magnes,M. Steller, W. Lewis, Y. Saito, V. Coffey, M. Chandler

INTRODUCTION: Magnetic reconnection isa physical process occurring in plasmas inwhich magnetic energy is explosively con-verted into heat and kinetic energy. The effectsof reconnection—such as solar flares, coronalmass ejections,magnetospheric substorms andauroras, and astrophysical plasma jets—havebeen studied theoretically, modeled withcomputer simulations, and observed in space.However, the electron-scale kinetic physics,which controls how magnetic field linesbreak and reconnect, has up to now eludedobservation.

RATIONALE: To advance understanding ofmagnetic reconnectionwith a definitive exper-

iment in space, NASAdeveloped and launchedtheMagnetosphericMultiscale (MMS)missionin March 2015. Flying in a tightly controlledtetrahedral formation, the MMS spacecraft cansample the magnetopause, where the inter-planetary and geomagnetic fields reconnect,and make detailed measurements of the plas-ma environment and the electric andmagneticfields in the reconnection region. Because thereconnection dissipation region at themagneto-pause is thin (a few kilometers) and movesrapidly back and forth across the spacecraft(10 to 100 km/s), high-resolutionmeasurementsare needed to capture the microphysics ofreconnection. The most critical measure-ments are of the three-dimensional electron

distributions, which must be made every30 ms, or 100 times the fastest rate previouslyavailable.

RESULTS: On 16 October 2015, the MMS tet-rahedron encountered a reconnection site onthe dayside magnetopause and observed (i)the conversion of magnetic energy to particlekinetic energy; (ii) the intense current andelectric field that causes the dissipation of mag-

netic energy; (iii) crescent-shaped electron velocitydistributions that carry thecurrent; and (iv) changesinmagnetic topology. Thecrescent-shaped featuresin the velocity distributions

(left side of the figure) are the result of demag-netization of solar wind electrons as they flowinto the reconnection site, and their accelera-tion and deflection by an outward-pointingelectric field that is set up at the magnetopauseboundary by plasma density gradients. As theyare deflected in these fields, the solar wind elec-tronsmix inwithmagnetospheric electrons andare accelerated along a meandering path thatstraddles the boundary, picking up the energyreleased in annihilating the magnetic field. Asevidence of the predicted interconnection ofterrestrial and solar wind magnetic fields, thecrescent-shaped velocity distributions are divertedalong the newly connectedmagnetic field linesin a narrow layer just at the boundary. This di-version along the field is shown in the rightside of the figure.

CONCLUSION:MMShas yielded insights intothe microphysics underlying the reconnection

between interplanetary and terres-trial magnetic fields. The persist-ence of the characteristic crescentshape in the electron distributionssuggests that the kinetic processescausing magnetic field line recon-nection are dominated by electrondynamics, which produces the elec-tric fields and currents that dissi-patemagnetic energy. The primaryevidence for this magnetic dissipa-tion is the appearance of an electricfield and a current that are parallelto one another and out of the planeof the figure. MMS has measuredthis electric field and current, andhas identified the important role ofelectrondynamics in triggeringmag-netic reconnection.▪

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The list of author affiliations is available inthe full article online.*Corresponding author. Email: jburch@swri.eduCite this article as J. L. Burch et al., Science352, aaf2939 (2016). DOI: 10.1126/science.aaf2939

Electron dynamics controls the reconnection between the terrestrial and solar magnetic fields.The pro-cess of magnetic reconnection has been a long-standing mystery. With fast particle measurements, NASA’sMagnetospheric Multiscale (MMS) mission has measured how electron dynamics controls magnetic recon-nection.Thedata in the circles showelectronswith velocities from0 to 104 km/s carrying current out of the page onthe left side of the X-line and then flowing upward and downward along the reconnectedmagnetic field on the rightside.Themost intense fluxes are red and the least intense are blue.The plot in the center showsmagnetic field linesand out-of-plane currents derived from a numerical plasma simulation using the parameters observed by MMS.

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RESEARCH ARTICLE◥

PLASMA ASTROPHYSICS

Electron-scale measurements ofmagnetic reconnection in spaceJ. L. Burch,1* R. B. Torbert,1,2 T. D. Phan,3 L.-J. Chen,4 T. E. Moore,5 R. E. Ergun,6

J. P. Eastwood,7 D. J. Gershman,5 P. A. Cassak,8 M. R. Argall,2 S. Wang,4 M. Hesse,5

C. J. Pollock,5 B. L. Giles,5 R. Nakamura,9 B. H. Mauk,10 S. A. Fuselier,1 C. T. Russell,11

R. J. Strangeway,11 J. F. Drake,4 M. A. Shay,12 Yu. V. Khotyaintsev,13 P.-A. Lindqvist,14

G. Marklund,14 F. D. Wilder,6 D. T. Young,1 K. Torkar,9 J. Goldstein,1 J. C. Dorelli,5

L. A. Avanov,5 M. Oka,3 D. N. Baker,6 A. N. Jaynes,6 K. A. Goodrich,6 I. J. Cohen,10

D. L. Turner,15 J. F. Fennell,15 J. B. Blake,15 J. Clemmons,15 M. Goldman,16

D. Newman,16 S. M. Petrinec,17 K. J. Trattner,6 B. Lavraud,18 P. H. Reiff,19

W. Baumjohann,9 W. Magnes,9 M. Steller,9 W. Lewis,1 Y. Saito,20

V. Coffey,21 M. Chandler21

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

Magnetic reconnection is an energy con-version process that operates in manyastrophysical environments, producingenergetic phenomena such as geomag-netic storms and aurora, solar flares and

coronal mass ejections, x-ray flares inmagnetars,andmagnetic interactions between neutron starsand their accretion disks. Reconnection is alsocrucially important in laboratory plasma phys-ics, where it has proved to be an impediment tothe achievement of magnetic-confinement fusion

through the sawtooth crashes that it triggers. Abetter understanding of reconnection is an im-portant goal for plasma physics on Earth and inspace, but a complete experiment is impossibleto conduct in most environments, which are toodistant, too hot, or too small for comprehensivein situ measurements (1).Earth’s magnetosphere has been explored by

many spacecraft missions, some of which havemade multipoint measurements in and aroundregions containing collisionless magnetic recon-nection (2–7). Results from these missions haveverified many of the predictions about magneticreconnection phenomena on the magnetohydro-dynamic (MHD) and ion scales. However, tomakemajor progress in the study of collisionless re-connection in space, it is necessary to extend themeasurements to the electron scale and makeaccurate three-dimensionalmeasurements of elec-tric and magnetic fields. Also required are accu-rate ion composition measurements, which canhelp to determine the role of ionospheric parti-cles in reconnection, as well as energetic particlemeasurements, which can help to determinewheremagnetic fields interconnect and how particlesare accelerated to high energies.NASA’sMagnetosphericMultiscale (MMS)mis-

sion (8) was designed to perform a definitive ex-periment in space on magnetic reconnection atthe electron scale, at which dissipation of mag-netic energy and the resulting interconnection of

magnetic fields occur. Electron-scale kinetic phys-ics in the region around the reconnection site(or the X-line) where field line breaking andreconnection occur has not previously been in-vestigated experimentally in space, owing to in-sufficiently detailedmeasurements. Our knowledgeof this region at the electron scale has comemainly from computer simulations (9–13) andlaboratory experiments (14, 15). The higher reso-lution of MMS measurements in both time andspace relative to previous missions offers an op-portunity to investigate the cause of reconnectionby resolving the structures and dynamics withinthe X-line region.The data set obtained by MMS incorporates

the following advances: (i) four spacecraft in aclosely controlled tetrahedron formationwith ad-justable separations down to 10 km; (ii) three-axis electric and magnetic field measurementswith accurate cross-calibrations allowing for themeasurement of spatial gradients and time varia-tions; and (iii) all-sky plasma electron and ionvelocity-space distributions with time resolutionof 30 ms for electrons and 150 ms for ions.The four MMS spacecraft were launched to-

gether on 13 March 2015 (UT) into a highly el-liptical (inclination 28°) orbit with perigee at 1.2Earth radii (RE) and apogee at 12 RE (both geo-centric). The mission is being conducted in twophases, the first phase targeting the dayside outerboundary of Earth’s magnetosphere (the magne-topause) and the second phase targeting the geo-magnetic tail, for which the apogee is raised to25RE. This article focuses onmagnetopausemea-surements during the first science phase of themission, which began on 1 September 2015. Forthis phase, a region of interest was identified asgeocentric radial distances of 9 to 12 RE, duringwhich all instruments are operated at their fast-est cadence, producing burst-mode data. Withinthe region of interest, the four spacecraft aremain-tained in a tetrahedral formation with separa-tions variable between 160 and 10 km. A qualityfactor for the tetrahedra, defined by the ratioof their surface area to their volume, is main-tained to within 80% of the ratio for a regulartetrahedron.By 14December 2015, the spacecraft had crossed

themagnetopausemore than 2000 times. On thebasis of detection of plasma jetting and heatingwithin the magnetopause current sheets, we in-fer that at least 50% of the crossings encounteredmagnetic reconnection. Most crossings occur-red in the reconnection exhaust downstream ofthe X-line, but a few of them passed very closeto the X-line. The data for one of these events(16 October 2015, 13:07 UT) are presented hereas an example of the electron-scalemeasurementsof the reconnection diffusion/dissipation regionaround an X-line.

MMS measurements

The set ofmeasurementsmade on each of the fourMMS spacecraft are listed in Table 1. The improve-ment in time resolution for three-dimensionalplasma distribution measurements was substan-tial: 30 ms for electrons and 150 ms for ions, as

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1Southwest Research Institute, San Antonio, TX, USA.2University of New Hampshire, Durham, NH, USA. 3Universityof California, Berkeley, CA, USA. 4University of Maryland,College Park, MD, USA. 5NASA, Goddard Space Flight Center,Greenbelt, MD, USA. 6University of Colorado LASP, Boulder,CO, USA. 7Blackett Laboratory, Imperial College London,London, UK. 8West Virginia University, Morgantown, WV,USA. 9Space Research Institute, Austrian Academy ofSciences, Graz, Austria. 10Johns Hopkins University AppliedPhysics Laboratory, Laurel, MD, USA. 11University ofCalifornia, Los Angeles, CA, USA. 12University of Delaware,Newark, DE, USA. 13Swedish Institute of Space Physics,Uppsala, Sweden. 14Royal Institute of Technology, Stockholm,Sweden. 15Aerospace Corporation, El Segundo, CA, USA.16University of Colorado, Boulder, CO, USA. 17LockheedMartin Advanced Technology Center, Palo Alto, CA, USA.18Institut de Recherche en Astrophysique et Planétologie,Toulouse, France. 19Department of Physics and Astronomy,Rice University, Houston, TX, USA. 20Institute for Space andAstronautical Sciences, Sagamihara, Japan. 21NASA, MarshallSpace Flight Center, Huntsville, AL, USA.*Corresponding author. Email: jburch@swri.edu

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compared to previous resolutions in the few-second range. This improvement required theuse of multiple analyzers rather than one spin-ning analyzer, resulting in stringent requirementson their absolute calibration and intercalibration.

Two benefits of this approach are the ability tomake accurate measurements of currents and ofelectron drift velocities. Another advance is theaccuratemeasurement of three-axis electric fields,which are crucially important for the investiga-

tion of reconnection. Data taken at the highestmeasurement resolution are referred to as burst-mode data, and all instruments operate in burstmode whenever the spacecraft are beyond a geo-centric distance of 9 RE on the dayside of Earth.

A reconnection dissipation regionencountered by MMS

Figure 1 shows summary plasma and field datafor MMS1 at a time resolution of ~4 s on 16 Oc-tober 2015. Because of data downlink limitations,only 2 to 4% of the burst-mode data can be trans-mitted to Earth. Data selection is made with twomechanisms: (i) an onboard system, which eval-uates 10-s intervals of burst-mode data and pri-oritizes them according to expected reconnectionsignatures; and (ii) a scientist-in-the-loop systemby which scientists view summary data (such asshown in Fig. 1) to select boundary crossings, flowjets, and other features that might have been

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Table 1. Measurements made on each MMS spacecraft.

Fields Three-dimensional electric and magnetic field measurements at time

resolution of <1 ms (direct current) and waves to 6 kHz (B) and 100 kHz (E)... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .

Fast plasma Full-sky viewing of plasma electrons and ions at 32 energies (10 eV to 30 keV):

electrons in 30 ms, ions in 150 ms... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .

Energetic particles Full-sky viewing of ion and electron energetic particles (20 to 500 keV)

with composition... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .

Plasma composition Composition-resolved 3D ion distributions (1 eV to 40 keV) for H+, He2+, He+,

and O+. Full sky at 10 s... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .

Potential control Maintenance of spacecraft potential to ≤4 V using ion emitter... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .

Fig. 1. Summary data in GSM (geocentric solar magnetospheric) coordinates fromMMS1 on 16 October 2015.The GSM coordinate system has X towardthe Sun, Z the projection of Earth’s magnetic dipole axis (positive = north) onto the plane perpendicular to X, and Y completing the right-hand system(approximately toward dusk). (A) Magnetic field vector. (B) Magnetic field magnitude. (C) Ion energy-time spectrogram in energy flux (eVcm−2 sr−1 s−1).(D) Electron energy-time spectrogram in energy flux (eVcm−2 sr−1 s−1). (E) Ion density. (F) Ion velocity vector. (G) Electric field vector.

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missed by the onboard system. Bothmechanismshave been effective in identifying possible recon-nection sites. Several magnetopause crossingswere observed on this day. The particular eventchosen for further analysis (near 13:07 UT) isnoted by the blue box in Fig. 1.Figure 2A shows the orbit as projected onto

the ecliptic plane; Fig. 2B shows the tetrahedronoccupied by the four spacecraft at 13:07:00UT on16 October 2015. During this time period, the sep-aration among the spacecraft was controlled at10 km. As shown in Fig. 2, MMS4 was located~10 km south (toward –Z) of MMS2 and MMS3.The detailed electron distribution functions fromthe four spacecraft show evidence that the recon-nection X-line was located to the north of MMS4and to the south ofMMS2 andMMS3.MMS1wasdisplaced toward negative values of X (towardEarth) so that as themagnetopausemoved inwardacross the four spacecraft, MMS1 detected thedissipation region slightly later than the otherthree spacecraft (Fig. 2).The 10-kmaverage separation of the four space-

craft amounted to ~6 electron skin depths (thedepth in a plasma towhich electromagnetic radia-tion can penetrate) based on a magnetosheath(shocked solarwind) density of ~12 cm−3. At suchsmall spacecraft separations, the plasma andfields measured by the four spacecraft are nearlyidentical throughout most of the regions, exceptin thin electron-scale layers near the reconnec-tion X-line.

Overview of two magnetopause crossings

Figure 3 shows MMS2 data during two en-counters with themagnetopause over a period ofalmost 2 min. The magnetopause crossings aredenoted by the two pairs of vertical blue dashed

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Fig. 2. Locations of the four MMS spacecraft during the magnetopause crossing investigated in this study. (A) Ecliptic-plane projection of MMS orbit ingeocentric-solar-ecliptic (GSE) coordinates on 16 October 2015.The beige area is the MMS region of interest where burst-mode data are taken. Hours of the dayare noted along the orbit. (B) MMS tetrahedral formation in GSE coordinates (X toward Sun, Z perpendicular to ecliptic plane,Y toward dusk).

Movie 1. An electromagnetic particle-in-cell simulationwith parameters corresponding to the event is per-formed with the P3D code (27). Particles are advancedusing a relativistic Boris stepper with electromagneticfields extrapolated to the particles’ positions (28).Electromagnetic fields are evolved using the trap-ezoidal leapfrog scheme on Maxwell’s equations withsecond-order spatial derivatives.The simulation is two-dimensional with periodic boundary conditions in bothdirections. Magnetic fields in the simulation are nor-malized to the magnitude of the L component of themagnetosheath magnetic field, 23 nT. Densities arenormalized to the magnetosheath density, 11.3 cm−3.Distances are normalized to 67.8 km (the magneto-sheath ion inertial scale di,sh = c/wpi,sh), and currentdensities to 0.270 mA/m2.The initial conditions for theupstream values of the L and M components of themagnetic field, the densities, and the electron and iontemperatures on both sides of the current sheet aretaken tomatch the event to the extent possible:BL,ms =39 nT, BL,sh = 23 nT, BM,ms = BM,sh = 2.278 nT, nms =0.7 cm−3, and nsh = 11.3 cm−3, where “ms” denotes themagnetospheric side and “sh” denotes the magneto-sheath side. For the temperatures, magnetosheath val-ues are Tp,sh = 320 eV and Te,sh = 28 eV to match theMMS data. For the magnetosphere, the low density

makes measuring temperatures difficult, so for the purposes of the simulation we defined the mag-netospheric temperature as that required to balance total pressure in the fluid sense with a protontemperature 6 times the electron temperature: Tp,ms = 1800 eV, Te,ms = 300 eV. No bulk flow of theupstream plasma is included in the initial conditions.The profiles for the initial conditions had double tanhprofiles for the magnetic field and temperature, and the density profile is chosen to enforce pressurebalance in the fluid sense.The domain size is 40.96 × 20.48 in normalized units and the grid scale is 0.01in both directions.The time step is 0.001 in units of the magnetosheath inverse ion cyclotron frequencyWci,sh

−1 and is run until t = 40.The time step on the electromagnetic fields is half that of the particles toresolve lightwaves.The simulation is initializedwith 500 particles per grid.The ion-to-electronmass ratio is100 and the ratio of the speed of light to the initial magnetosheath Alfvén speed is 15 (wpi,sh/Wci,sh = 15);these differ from the realistic values of 1836and 2000, respectively, but it is common to use smaller valuesfor numerical expediency and is not expected to adversely affect the simulations. http://bcove.me/o51zgjqt.

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lines. The diagram on the right side of Fig. 3shows the typical structure of a magnetopause inwhich asymmetric reconnection is occurring, takenfrom a numerical plasma simulation shown inMovie 1 for the observed magnetosheath andmagnetospheric conditions of the completeMMSmagnetopause crossing at 13:05:30 UT. The dia-gram shows the northward magnetic field onthe magnetosphere side of the boundary and thesouthward magnetic field on the magnetosheathside. The shear angle between themagnetosphereand magnetosheath magnetic fields is very large(~170°), implying a crossing with low guide field(the magnetic field component normal to theplane of the diagram). The converging plasmaflows carry the two nearly oppositely directedmagnetic field domains toward each other. An

X-line directed normal to the plane of the dia-gram denotes the small region in the reconnec-tion planewhere the field lines interconnect, andthis X-line is likely to extend by hundreds tothousands of kilometers in the east-west direc-tion (16), which is why a large number of exhaustregions are typically crossed by spacecraft nearthe magnetopause. Another reason why recon-nection events are routinely observed is the pres-ence of the exhaust jets (red arrows) flowingnorthward and southward from the X-line andthe nearby dissipation region (or diffusion re-gion). Although the results of reconnection arereadily observed with measurements at the fluidand ion scales, it is the electron-scale phenomenaacting within the dissipation region that deter-mine how reconnection occurs.

The color scale in the plasma simulation resultin Fig. 3 shows the plasma current normal to theplane of the picture (JM), which is nearly all due tofast-moving electrons generated by the reconnec-tion process. The strong –JM values (shown ingreen) are highly localized at the dissipation re-gion andX-line. The approximate path of theMMStetrahedron, based on the plasma and field mea-surements, is shownbyabluedashedcurve.Becausethe velocity of themagnetopause is approximately100 times the spacecraft velocities, theMMS pathshown is produced entirely by the motion of themagnetopause along L and N (directions definedin the Fig. 3 caption). For themagnetopause cross-ing centered at 13:07 UT, the spacecraft traversedboth exhaust jet regions and passed through thedissipation region between them.

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Fig. 3. Summary data for two magnetopause crossings of MMS2 on 16October 2015. The crossings are shown by the vertical blue dashed lines.Boundary-normal coordinates (L,M, N) are used with N normal to the bound-ary and away from Earth, L perpendicular toN and in the plane of reconnection(nearly along the magnetospheric magnetic field direction), and M normal tothe L, N plane (generally westward).These directions were determined from aminimum variance analysis of the magnetic field data between 13:05:40 and13:06:09 UT. The (x, y, z) GSE components of the L, M, and N axes are L =(0.3665, –0.1201, 0.9226) GSE,M = (0.5694, –0.7553, –0.3245) GSE, andN =(0.7358, 0.6443, –0.2084) GSE. Panel data include (A) magnetic field vectors,(B) energy-time spectrogram of ion energy flux, (C) energy-time spectrogram

of electron energy flux, (D) total plasma density, (E) ion flow velocity vectors,(F)magnitudes of electron and ion convection velocities, (G) current computedfrom velocity moments of ions and electrons, (H) current computed from∇ ×B, (I) parallel and perpendicular (toB) electron temperatures, and (J) electricfield vectors. In the very-low-density region to the left of the first vertical bluedashed line, spacecraft charging effects on plasma moment calculations mayaffect the data. The diagram to the right is the result of a numerical plasmasimulation (Movie 1) using parameters from the magnetopause crossingcentered on 13:05:52 UT. Spatial coordinates in the diagram are shown bothin kilometers and in ion diffusion lengths, L(di). Color scale indicates JMcurrent density.

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In comparison, the magnetopause crossingcentered at about 13:05:52 UT occurred com-pletely southward of the X-line in the exhaust,so that although it did include a traversal fromwell within the magnetosphere to well out intothe magnetosheath, it did not encounter the dis-sipation region. This difference is best seen in theion flow velocities in Fig. 3E, which when high inmagnitude denote the exhaust jets. For the firstmagnetopause crossing, only southward jets(ViL < 0) were observed, whereas for the secondevent (13:07 UT) both southward and northwardjets were observed, with no gap between them.During the flow reversal near 13:07 UT, the re-connecting magnetic field (BL) component wasclose to zero, suggesting that the spacecraft wasin close proximity to theX-line. The red highlightbar at the top of Fig. 3E shows this reversal.Another strong indicator of the dissipation re-

gion is the aforementioned –JM current, whichcan be seen in Fig. 3, G and H. The green trace is–JM (eastward current), which is predicted by thesimulation to peak at the dissipation region.WithMMS there are two ways to measure the cur-rents: (i) ∇ ×B calculated from themagnetic fielddata from the four spacecraft (as in Fig. 3A), or(ii) qn(Vi – Ve) using the computed moments ofthe ion and electron distribution functions, whereq is the electronic charge and n is the plasmadensity. The correspondence between the twomethods shown in Fig. 3 required a high level ofcalibration and cross-calibration of the variousplasma instruments. Differences between the twomethods at the smallest scales aremostly becauseeven with 10-km separation, the currents are notcompletely uniformacross theMMS tetrahedron,which is assumed in the ∇ × B calculation.The third strong indicator for a dissipation re-

gion is the enhancement of –EM (the reconnec-tion electric field), which is shown by the greentrace in Fig. 3J. The size of the EM bursts of morethan 10 mV/m is substantially larger than thecorrection due to X-line motion, which is on theorder of 1 mV/m or less. There are also strong ENcomponents bracketing 13:07 UT, which are elec-tric fields pointing outward and normal to themagnetopause. This normal component is pre-dicted by simulations (17), and in a simple senseis caused by the deeper penetration into the mag-netosphere of themagnetosheath ions because oftheir larger gyroradius relative to electrons withsimilar energies. The resulting charge separationproduces an ambipolar electric field, EN.There are important differences between the

reconnection exhaust at 13:05:52 UT and the re-gion surrounding the X-line near 13:07 UT. First,as shown in Fig. 3I, the degree of electron heat-ing (relative to the magnetosheath temperature)near theX-line (DTe|| ~ 120 eV) is substantially high-er than the heating in the exhaust (DTe|| ~20 eV).Second, the electron flow speed perpendicular tothe magnetic field, which largely tracks the ionperpendicular speed in the magnetosheath andthe exhaust, significantly exceeds the ion flowspeed near the X-line (Fig. 3F), resulting in a cur-rent that is much larger near the X-line than inthe exhaust. These differences further support

the identification of the X-line regions near13:07 UT.

Details of plasma and field observationsfrom MMS2

Figure 4 shows the 4 s marked with the red barin Fig. 3E of MMS2 data near the X-line. Figure4A shows that a deep magnetic field minimumoccurred just after 13:07:02.4 UT; Fig. 4B shows astrong plasma current (JM) starting at 13:07:02.1UT (on the magnetosphere side of the X-line)and extending through the minimum magneticfield. Figure 4C shows vector electric fields. In-side the –JM current layer, the EN component,which points outward from the magnetopauseas described above, is the strongest. It is alsonoteworthy that EM, the reconnection electricfield, is negative, as is the JM current. Figure 4Dshows a comparison betweenEM and –(Ve ×B)M.There is excellent agreement except near the dis-sipation region. Figure 4E shows the electric fieldcomponent parallel to B, which is strongest inthe region of the JM plasma current. Figure 4Fshows J · E′, where E′ = E + Ve × B, along withits parallel and perpendicular components. J ·E′has been referred to as the “dissipation quantity”in simulation results (18). The plot in Fig. 4Fshows clearly that the reconnection dissipationis caused by the strong –JM current and –EMelectric field, which are perpendicular toB in thedissipation region as B is dominated by BL inthat region. Because reconnection is known to bea dissipative process that converts magnetic en-ergy to heat and particle kinetic energy, the ob-servation that J · E ≈ JMEM > 0 provides a formof “smoking gun” for a reconnection dissipationregion.Shown in Fig. 4, G to I, are energy-time spec-

trograms of electrons moving parallel, perpen-dicular, and antiparallel to the localmagnetic fielddirection, respectively. In the region of dissipa-tion (13:07:02.15 to 13:07:02.29 UT), the parallelfluxes shift to lower energies, the perpendicularfluxes rise in intensity and shift to lower ener-gies, and the antiparallel fluxes remain at highenergies. All of the fluxes drop to lowermagneto-sheath levels after exiting the dissipation region.The electron velocity-space distribution functionsin Fig. 4, J to L, show three cuts through 3Ddistributions at 30-ms intervals through thedissipation region. Figure 4J shows cuts perpen-dicular toB, whereVperp1 = (b× v) ×b andVperp2 =–v × b (b and v are unit vectors of the magneticfield and the electron velocity moment, respec-tively). Shown in Fig. 4, K and L, are two or-thogonal cuts containing the magnetic fielddirection Vpara.Before MMS, the best 3D plasma measure-

ment resolution was 3 s; that is, only a single plas-ma distribution would have been measured insuch a brief interval. In comparison, MMS mea-sured 26 ion distributions and 133 electron dis-tributions in this interval. Movie 2 shows all ofthe MMS2 electron velocity-space distributionsin video form for a 3-s interval centered in Fig. 4.Themovie demonstrates that the 30-ms time res-olution of MMS is necessary for performing this

type of definitive investigation of the reconnec-tion dissipation region.The first column of distribution functions (on

the magnetosphere side of the X-line) shows acrescent-shaped distribution in the perpendicu-lar plane centered along the +Vperp1 axis in Fig.4J, parallel heating in Fig. 4, K and L, along witha vertical cut through the crescent along +Vperp1

in Fig. 4K. This type of crescent-shaped distribu-tion has been predicted from a simulation (9)that showed reduced distribution functions (inte-grated along Vpara). In that simulation, the elec-trons in the crescent population were describedas “meanderingparticles” consisting of acceleratedmagnetosheath electrons. Subsequent distribu-tion functions in Fig. 4J show that as theX-line isapproached along the path sketched in Fig. 3, thecrescent-shaped distribution wraps around theorigin and becomes a ring, whichmoves to lowerenergies. This energy shift is shown in the spec-trogram in Fig. 4H.Shown in Fig. 4, K and L, is a result that had

not been predicted: the formation of a crescent-shaped distribution along Vpara, indicating thetransition of the perpendicular crescent elec-trons to field-aligned flow. Such a transition isstrong evidence for the opening of magnetic fieldlines. At the end of the dissipation-region interval,the parallel crescent also begins to wrap aroundthe origin and move to lower energies, as shownby the spectrogram in Fig. 4G. The crescent elec-tronsaremovingalongB (+Vpara)while theelectronsalong –Vpara continue to show the electron-heatingfeature (the elongation along –Vpara), which ex-tends to higher energies than the crescent popu-lation. The direction of the field-aligned flowof the crescent electrons indicates that MMS2has moved above the X-line while still within thedissipation region (Fig. 3K). Because the electronspectrograms in Fig. 4, G to I, plot energy flux,they are much more sensitive to the high-energyparts of the distribution, and this fact explainswhy the antiparallel energy fluxes in Fig. 4I re-main level throughout the dissipation region.In summary, the data in Fig. 4 establish that

MMS2 passed through a reconnection dissipa-tion region around anX-line. The flow directionsshown in the electron velocity-space distributions(crescent shifting fromperpendicular to alongB)suggest that MMS2 moved northward from ap-proximately the same L location as the X-line.Parallel electric fields (Fig. 4E) also occur in thedissipation region. In addition, there is a strongnormal electric field, EN, whichmay be related tothe normal electric field predicted theoreticallyto result from magnetopause pressure gradientsalong the entire magnetopause (19). The crescent-shaped distributions are due to finite Larmorradius effects of magnetosheath electrons thathave been accelerated toward themagnetosphereby EN (Fig. 4C) in the weak magnetic field regionnear the X-line. This Larmor motion, togetherwith acceleration by the reconnection electricfield, results in a net out-of-plane electron bulkmotion or electron current –JM that is observed(Fig. 4B). As the high-density magnetosheathelectrons penetrate more deeply into the region

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Fig. 4. MMS2 plasma and field data on 16 October 2015. (A) Magnetic field vector. (B) Currents from plasma measurements. (C) Electric field vector.(D)ComparisonofMcomponent ofEand–Ve×B. (E)E||. (F)J ∙E. (G) Electron energy-timespectrogram(pitch angle=0° to 12°). (H) Electron energy-time spectrogram(pitch angle = 84° to 96°). (I) Electron energy-time spectrogram (pitch angle = 168° to 180°). (J) Electron velocity-space distribution (Vperp1, Vperp2). (K) Electronvelocity-space distribution (Vpara, Vperp1). (L) Electron velocity-space distribution (Vpara, Vperp2). Vperp1 is in the (b × v) × b direction, which is a proxy for E × B.

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with BL > 0, they gain more energy from EN, andthis effect can be clearly seen by comparing thefirst and fifth columns of Fig. 4J. Because theX-linemoved along theN direction across the spacecrafttetrahedron, the energy of accelerated magneto-sheath electrons increases from right to left inFig. 4. As can be seen in Movie 2, the electrondistributions at the magnetic field minimum(~13:07:02.45 UT) are isotropic at very low energy,indicating demagnetization, as would be expect-ed. As the electrons move inward (Earthward)toward the dissipation region, they gain succes-sively more energy as they cross the open fieldlines at the outer part of the dissipation region(parallel crescent) and then develop the highest-energy perpendicular crescent as the parallelcrescent disappears at the inner part (the mag-netosphere side) of the dissipation region.

Multi-spacecraft observations of thedissipation region

Figure 5 showsmulti-spacecraft plasma and fielddata for the same 4-s time period as in Fig. 4. Thevector magnetic field data in Fig. 5A show thatMMS 2, 3, and 4 all passed through the mag-netopause together, with MMS1 following themby ~0.2 s. The spatial scale (along the magneto-pause normal direction) of the various electronlayers can be estimated as follows: The consecu-tive detections by the four spacecraft of the BL

gradient at 13:07:02.2 to 13:07:02.4 reveal thatthis structuremoved at a speed of ~45 km/s alongthe normal direction. Thus, the width of the re-gionwhere seven crescent distributions (each sam-pled over 30 ms) were observed is ~9 km, or 6electron skin depths. The region of strong dissi-pation is even narrower, as shown by the electricfield measurements in Fig. 5.All four spacecraft measured somewhat sim-

ilar electric field and currents with important

differences in their amplitudes and duration, par-ticularly with the trailing MMS1. Shown in Fig. 5,B to D, are EM, EN, and E||, respectively. Near thedissipation region, MMS2, MMS3, and MMS4measured the perpendicular and parallel electricfields at various levels and strong JM current.The strongest J ·E′ peakwas detected byMMS2,indicating its deepest penetration into the dis-sipation region.The same types of electron velocity-space dis-

tributions presented in Fig. 4 for MMS2 were ob-served by MMS3 and MMS4. In Fig. 5, H to J,MMS4 distributions are shown in the first col-umn, and MMS3 distributions are displayed inthe other three columns. The first column of Fig.5H shows that as MMS4 was entering the dissi-pation region, it saw the perpendicular crescentnearly wrapped around the origin as a ring. Atthe same time, Fig. 5J shows the formation of aparallel crescent as in Fig. 3, but in this case, it iscentered on the –Vpara axis. This shift indicatesthatMMS4was located below (or southward of)the X-line, which, as noted before, is consistentwith its location shown in Fig. 2B. Also consist-ent with Fig. 2B are the parallel crescents thatform in the third and fourth columns of Fig. 5, Iand J, which are centered along the +Vpara axis,as was the case with MMS2. This direction indi-cates that MMS3, like MMS2, was located above(or northward of) the X-line. Exactly as forMMS2,the MMS3 perpendicular crescent in the secondcolumn of Fig. 5H appears first without a corre-sponding parallel crescent, but then evolves to-ward a ring in columns 3 and 4 as the parallelcrescents develop.

Energetic electron evidence for theopening of magnetic field lines

For the analysis of magnetopause reconnection,very-high-energy electron features can be a val-

uable adjunct to analyses of lower-energy par-ticles, because such electrons still have relativelysmall gyroradii (relative to the large-scale cur-rent sheet thickness) and are not expected to beperturbed by the strong electric fields in the vi-cinity of the electron diffusion region. The com-plex magnetic geometries of a reconnection siteare expected to redirect energetic electrons in afashion that reflects the magnetic geometry andtopology of the small region. Interesting energeticelectron signatures were indeed observed in thevicinity of the X-line by the Fly’s Eye EnergeticParticle Spectrometer (FEEPS), a part of the En-ergetic Particle Detector (EPD) investigation(20, 21). The bottom panel of Fig. 6 shows themagnetic field data and identifies the locationof the electron dissipation region (EDR). The toppanel shows a pitch-angle distribution of >50-keVelectrons, in which particles that travel paralleland antiparallel to the magnetic field are respec-tively located near the bottom and the top of theplot, and particles that gyrate nearly perpendic-ular to the magnetic field are near the verticalcenter of the plot. Our expectation has been thatnear an EDR, electrons might stream outwardfrom the energetic particle populations residingon the Earthward side toward themagnetosheathside along field lines that are reconnected closeto the EDR. Electrons are indeed streaming alongfield lines, and in the context of the spacecrafttrajectory relative to the EDRdeveloped elsewherein this paper (Fig. 3K), the electrons appear to betraveling away from Earth. Specifically, startingaround 13:06:55 UT, enhanced fluxes of >50-keVelectrons were streaming primarily in the paralleldirection with respect to the magnetic field andaway from the Earthward side, based on the in-ferred location of the EDR. After MMS passedthrough the EDR around 13:07:02 UT, these elec-trons exhibited streaming in the magneticallyopposite direction, predominantly antiparallel tothe magnetic field, but the inferred trajectory ofthe spacecraft through the EDR (Fig. 3K) indicatesthat the electrons are again traveling away fromEarth. These observations lend support to theidea that field lines connecting the magneto-sphere and magnetosheath sides are generatedthrough the reconnection process over small spatialscales dictated by electron processes.Note that the time resolution of FEEPS (2.5 s)

does not allow measurements within the EDR,but rather shows the reversal of the magneticfield–alignedmotionof the>50-keV electrons fromsouthward of the EDR to northward of it asMMS2made this traversal. In this sense, the FEEPS dataprovide independent confirmation of the openingofmagnetic field lines across the EDR, as deducedfrom the appearance of the parallel crescent inthe low-energy electron data.

Data interpretation

The existence of the crescent-shaped electron dis-tributions in the plane perpendicular to B, asshown in Figs. 4 and 5, can be explained concep-tually as follows. There is typically a large ionpressure gradient across the magnetopause. Dur-ingmagnetic reconnection, this pressure gradient

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Movie 2. Three-second segment of burst-mode electron distributions keyed to a plot of plasma and fielddata covering the same time period as Fig. 4.One hundred electron velocity-space distributions are shownover this period. Previous missions that used the spacecraft spin to cover the full sky could only acquireone or fewer distributions over a time period of this length. This factor of 100 increase in electron timeresolution is an important reason why MMS is able to investigate the electron-scale physics ofreconnection. http://bcove.me/9fkcpfn1.

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Fig. 5. Line plots ofMMS field data from all four spacecraft on 16 October 2015.At the bottom are electron velocity-space distributions for MMS4 andMMS3.Panels are as in Fig.4, except that the electron energy-time spectrograms are not shown.The parallel crescent in (J) forMMS4 is oriented in the opposite directionto that ofMMS3,which is consistentwithMMS4being southward of theX-line andMMS3being northward of it so that the electron flows are in opposite directions.

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produces a large normal electric EN in an LMNcoordinate system that points toward the Sun.This electric field balances the ion pressure gra-dient and keeps ions flowing from the magneto-sheath from penetrating the magnetosphere. Inthe vicinity of the X-line, EN modestly overlapsthe null field region (BL = BN = 0). The strongout-of-plane current JM during magnetopausereconnection actually peaks not at the X-line butdisplaced to themagnetosphere side of the X-linewhere EN peaks. The high JM is carried by high-velocity electrons with a crescent-shaped distri-bution in the VM-VN (perpendicular to B) planethat is symmetric across the VM axis. This cres-cent distribution results from cusp-like orbits ofelectrons associated with the motion in theM-Nplane controlled byEN(N ) andBL(N ) (22, 23). Themotion is similar to that of pickup ions in thesolar wind (24). Electrons around BL = 0 are ac-celerated toward the magnetosphere by EN. Asthey gain energy,BL causes them to turn in theMdirection. Eventually they turn around, reachinga peak velocity along M that is around twice theE ×B velocityVEB = cEN/BL. The electrons returnto BL = 0 with zero velocity (ignoring their ther-mal spread) and repeat their cusp-like motion.The electrondistribution function canbe calculatedanalytically. With increasing distance from thenull region to the turning point in N, it transi-tions fromahot thermal distribution to a horseshoe-like distribution (with more particles at higherVM and a depletion of particles around VM, VN =0) and then to a crescent centered at a velocityVM that increaseswith distance into the region ofhigh EN and narrows in the VM direction.

As the magnetopause moves inward in thisevent, the crescent-shaped electron populationenters a region of very weak magnetic field con-taining open field lines in the inner part of theelectron exhaust. In the exhaust region, newlyreconnected field lines move rapidly away fromthe X-line northward and southward—a phenom-enon that has been described as a double sling-shot (1) or simply a magnetic slingshot (2). It islikely that these exhaust region dynamics are re-sponsible for redirection of the perpendicularcrescents into the observed parallel crescents.Although the perpendicular crescents (averagedover Vparallel) were predicted in simulations (9),the parallel crescents have not been. Their directobservation by MMS therefore represents a newtarget for simulations.

Summary and implications

The MMS mission, which was designed to per-form a definitive experiment on magnetic recon-nection in space, has investigated electron-scalephysics in an encounter with the dissipation re-gion near a reconnection X-line at Earth’s mag-netopause. The high temporal resolution andaccuracy of the MMS plasma and field measure-ments were both necessary and sufficient for theinvestigation of the electron physics controllingreconnection.Using measurements of plasma currents and

reconnection electric fields, we have shown thatJ ·E′> 0 in the vicinity of the X-line, as predictedfor the dissipative nature of reconnection. Elec-tron distribution functions were found to containcharacteristic crescent-shaped features in velocity

space as evidence for the demagnetization andacceleration of electrons by an intense electricfield near the reconnection X-line. MMS has di-rectly determined the current density based onmeasured ion and electron velocities, which al-lowed the resolution of currents and associateddissipation on electron scales. These scales aresmaller than the spacecraft separation distancesand hence smaller than currents that can be de-termined by ∇ × B. The X-line region exhibitselectron demagnetization and acceleration (byboth EN and EM), which results in intense JMcurrent that is carried by the crescent-shapedelectron distributions. Kinetic simulations hadpredicted some elements of the crescent distribu-tions near theX-line, which raises the prospect ofactive interplay between theory and experiment,because the two techniques are now on a similarfooting. TheMMSmeasurements have led to dis-coveries about the evolution of electron accelera-tion in the dissipation region, aswell as the escapeof energized electrons away from the X-line intothe downstream exhaust region. The latter wasdetected by at least two MMS spacecraft locatedon opposite sides of the X-line. The observedstructures of the normal electric field and elec-trondynamicsnear theX-line by the four spacecraftare 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 re-gion is important not only for the initiation ofreconnection (breaking of the electron frozen-incondition), but also for electron acceleration andenergization, leading to much stronger electronheating and acceleration than seen in the down-stream exhaust. The details of the electron distri-bution functions, which show the rapid transition(within 30 ms) of the perpendicular crescent dis-tributions to parallel crescents, provide experi-mental evidence for the opening up of magneticfield lines while also demonstrating that it is theelectron dynamics that drives reconnection. Be-cause of the importance of reconnection inmanyastrophysical and laboratory environments andthe improvement achieved by its measurementresolution (25, 26), MMS has opened up a newwindow on the universal plasma process of mag-netic reconnection.

Materials and methods

The science phase ofMMS began on 1 September2015, when the orbit apogee precessed beyondthe duskmeridian toward the dayside, after whichit skimmed the magnetopause for 6 months. Thescientific strategy was to position the four space-craft in a tetrahedral formation at radial distancesfrom 9 to 12 RE, first at the ion scale (160 km) andprogressing to the smaller electron scale (10 km),so that magnetic reconnection could be investi-gated as themagnetopause crossed back and forthacross the tetrahedron in response to variationsin the solarwind dynamic pressure. This strategybore fruit as severalmagnetopause crossings wereobserved onmost days, with many of these cross-ings showing evidence for magnetic reconnec-tion based on the appearance of plasma jetting. A

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Fig. 6. Energetic electron data measured by FEEPS. Top: Pitch angle versus time spectrogram ofenergy fluxcarried by ~50-keVelectrons.Bottom:Magnetic vectors inGSMcoordinates andmagnetic fieldmagnitude.

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small subset of these reconnection events wassampleddirectlywhen theMMSspacecraft crossednear or through the electron dissipation regionwithin which magnetic energy is converted toparticle kinetic energy. Effective sampling at theelectron scale requires measurements at the high-est instrument data rate, termed a “burst mode.”Whenever the spacecraft are between 9 and 12RE

(the region of interest), all instruments are run attheir maximum data rates.Because of data downlink volume limitations

coupled with the unprecedentedly high internaldata rate of the MMS instruments, careful se-lection of data to be downlinked is necessary.Two methods are used for the downlink dataselection, both of which involve the use of a 96-GBonboard memory, which contains all the burst-mode data for two or more orbits of MMS. Thefirst method of data selection involves the re-porting of data evaluations by each instrumenton a 10-s time scale, resulting in figures of meritfor each interval, which are combined to gener-ate a spacecraft figure of merit. These figures ofmerit are transmitted to the ground along withsummary data for entire orbits. The summarydata are similar to those shown in Fig. 1. Aggre-gate figures of merit for the four spacecraft arecombined with ground software to generate amission-level figure of merit. These automati-cally generated figures of merit then determinethe priorities by which burst data are transmittedduring 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 thesummary data for each day, with the goal of op-timizing the data downlink selection by eitheradjusting the figures of merit or identifying newhigh-priority intervals that were not selected bythe onboard system. Both systems are effectiveand both are being used throughout the mission.The data from all the independent sensors oneach satellite, and between the four satellites, areintensively 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 cal-ibrated data are placed online at this site within30 days of their transmission to the MMS Sci-ence Operations Center. The data are archived intheNASACommonData Format (CDF) and so canbe plotted using a number of different data displaysoftware packages that can use CDF files. A verycomprehensive system called the Space Physics En-vironment Data Analysis System (SPEDAS) is avail-able by downloadinghttp://themis.ssl.berkeley.edu/socware/bleeding_edge/andselectingspdsw_latest.

zip. Training sessions on the use of SPEDAS areheld on a regular basis at space physics–relatedscientific meetings. All of the data plots in thispaper were generated with SPEDAS software ap-plied to the publicly available MMS database, sothey could readily be duplicated.

REFERENCES AND NOTES

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ACKNOWLEDGMENTS

The dedicated efforts of the entire MMS team are greatlyappreciated. We are especially grateful to the leadership ofC. Tooley, B. Robertson, and R. Black. Special thanks are dueto C. Pankratz and K. Larsen of the University of Colorado for theirleadership of the MMS Science Operations Center. Supported byNASA contract NNG04EB99C at Southwest Research Institute,which funded work at most of the co-author institutions in theUnited States. The IRAP contribution to MMS was supported byCNES. The Austrian contributions to the MMS mission aresupported by grants from the Austrian Research PromotionAgency FFG. The UK work was supported by the UK Science andTechnology Facilities Council through grants ST/K001051/1 andST/N000692/1. The work by NASA GSFC authors was supportedby the NASA Solar Terrestrial Probes program. The work of theGSFC-resident University of Maryland co-authors was supportedby NASA Goddard Planetary Heliophysics Institute contractNNG11PL02A. Work at U.C. Berkeley was supported by NASAMMS-IDS grant NNX08AO83G through the University of California.Work at the University of Colorado by M.G. and D.N. wassupported by NASA MMS-IDS Grant NNX08AO84G through theUniversity of Colorado. Work at the Swedish Institute for SpacePhysics and the Royal Institute of Technology was supported bythe Swedish National Space Board. Work at West Virginia Universitywas supported by NSF grants AGS-0953463 and AGS-1460037and by NASA grants NNX16AG76G and NNS16AF75G.

20 January 2016; accepted 3 May 2016Published online 12 May 201610.1126/science.aaf2939

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Electron-scale measurements of magnetic reconnection in space

Baumjohann, W. Magnes, M. Steller, W. Lewis, Y. Saito, V. Coffey and M. ChandlerFennell, J. B. Blake, J. Clemmons, M. Goldman, D. Newman, S. M. Petrinec, K. J. Trattner, B. Lavraud, P. H. Reiff, W.Goldstein, J. C. Dorelli, L. A. Avanov, M. Oka, D. N. Baker, A. N. Jaynes, K. A. Goodrich, I. J. Cohen, D. L. Turner, J. F. Strangeway, J. F. Drake, M. A. Shay, Yu. V. Khotyaintsev, P.-A. Lindqvist, G. Marklund, F. D. Wilder, D. T. Young, K. Torkar, J.R. Argall, S. Wang, M. Hesse, C. J. Pollock, B. L. Giles, R. Nakamura, B. H. Mauk, S. A. Fuselier, C. T. Russell, R. J. J. L. Burch, R. B. Torbert, T. D. Phan, L.-J. Chen, T. E. Moore, R. E. Ergun, J. P. Eastwood, D. J. Gershman, P. A. Cassak, M.

originally published online May 12, 2016DOI: 10.1126/science.aaf2939 (6290), aaf2939.352Science 

, this issue p. 10.1126/science.aaf2939; see also p. 1176Scienceincluding those in fusion reactors, the solar atmosphere, solar wind, and the magnetospheres of Earth and other planets.the process is driven by the electron-scale dynamics. The results will aid our understanding of magnetized plasmas,plasma properties within a reconnection event in Earth's magnetosphere (see the Perspective by Coates). They find that

used NASA's Magnetospheric Multiscale mission to probe theet al.details of how it occurs are poorly understood. Burch releasing energy and accelerating particles. Reconnection is important in a wide variety of physical systems, but the

Magnetic reconnection occurs when the magnetic field permeating a conductive plasma rapidly rearranges itself,Probing magnetic reconnection in space

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