D. L. Newman

Electron-Scale Measurements of Magnetic Reconnection in Space

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 NASAs Magnetospheric Multiscale mission to probe the plasma properties within a reconnection event in Earths 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 Magnetic reconnection is driven by the electron-scale dynamics occurring within magnetized plasmas. 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. 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.

Nonlinear two‐stream instabilities as an explanation for auroral bipolar wave structures

The evolution of two counter-streaming electron beams is shown by means of 2-D kinetic simulations to lead to electron distributions and coherent localized bipolar plasma wave structures with features similar to those measured by the FAST satellite in the auroral ionosphere. Electrostatic whistler waves are generated at later times when the bipolar structures begin to lose coherence and break up in the dimension transverse to the geomagnetic field.

Characteristics of parallel electric fields in the downward current region of the aurora

Direct measurements of parallel electric fields suggest that they are, in part, self-consistently supported as strong double layers in the auroral downward current region. The observed parallel electric fields have amplitudes reaching nearly 1 V/m and are confined to a thin layer of approximately 10 Debye lengths. The structures are moving at roughly the ion acoustic speed in the direction of the accelerated electrons, i.e., anti-earthward. On the high-potential side of the parallel electric field there is a clear signature of an accelerated electron beam which rapidly plateaus within a few hundred Debye lengths from the parallel electric field. Strong wave turbulence is observed in the vicinity of the plateaued electron distribution. Fast solitary waves, identified as a signature of electron phase-space holes, are seen farther away from the parallel electric field on the high-potential side. The observed ion distributions also indicate the presence of the parallel electric field. On the low-potential side of the double layer an ion beam is observed moving in the opposite direction of the electron beam and ion conics appear to be trapped between their mirror point and the moving double layer.Direct measurements of parallel electric fields suggest that they are, in part, self-consistently supported as strong double layers in the auroral downward current region. The observed parallel electric fields have amplitudes reaching nearly 1 V/m and are confined to a thin layer of approximately 10 Debye lengths. The structures are moving at roughly the ion acoustic speed in the direction of the accelerated electrons, i.e., anti-earthward. On the high-potential side of the parallel electric field there is a clear signature of an accelerated electron beam which rapidly plateaus within a few hundred Debye lengths from the parallel electric field. Strong wave turbulence is observed in the vicinity of the plateaued electron distribution. Fast solitary waves, identified as a signature of electron phase-space holes, are seen farther away from the parallel electric field on the high-potential side. The observed ion distributions also indicate the presence of the parallel electric field. On the low-potential sid...

Auroral particle acceleration by strong double layers: The upward current region

[1] Satellite observations have established that parallel electric fields of both upward and downward current regions of the aurora are supported, at least in part, by strong double layers. The purpose of this article is to examine the role of double layers in auroral electron acceleration using direct measurements of parallel electric fields and the accompanying particle distributions, electrostatic waves, and nonlinear structures; the concentration is on the upward current region. Direct observations of the ionospheric boundary of the auroral cavity suggest that a stationary, oblique double layer carries a substantial, albeit a minority fraction (∼10% to ∼50%) of the auroral potential. An order of magnitude density gradient results in an asymmetric electric field signature. Oblique double layers with amplitudes greater than 100 mV/m have been verified in ∼3% and may occur in up to 11% of auroral cavity crossings, so it is feasible that strong double layers are a principal acceleration mechanism. In this article we also present a second type of double layer that has a symmetric electric field signature and is seen inside of the auroral cavity. These structures are a possible signature of a midcavity or high-altitude acceleration mechanism. Numerical solutions of the Vlasov-Poisson equations support the possibility of midcavity double layers and indicate that trapped electrons can play an important role in the double-layer structure.

Evolution of electron phase-space holes in 3D

Electron phase-space holes are regions of de- pletedelectrondensitycommonlygeneratedduringthenon- linear stage of the two-stream instability. Recently, bipolar electric eld structures | a signature of electron holes | havebeenidentiedintheacceleration regionof theauroral ionosphere. This paper compares the evolution of electron holes in 2-D and 3-D using massively-parallel PIC simula- tions. In 2-D, the holes decay after hundreds of plasma periods while emitting electrostatic whistler waves. In the 3-Dsimulations,electronholesalsogounstableandgenerate whistlers but, due to physical processes not present in 2-D, energy flows out of the whistlers and into highly perpen- dicular lower hybrid modes. As a result of this dierence, 3-D holes do not decay as far as 2-D holes. The dierences between 2-D and 3-D evolution may have important impli- cationsforholelongevityandwavegenerationintheauroral ionosphere.

Two‐component model of strong Langmuir turbulence: Scalings, spectra, and statistics of Langmuir waves

A two‐component model of strong Langmuir turbulence is developed, in which intense coherent Langmuir wave packets nucleate from and collapse amid a sea of low‐level background waves. Power balance between these two components determines the overall scalings of energy density and power dissipation in the turbulence, and of the rate of formation, number density, volume fraction, and characteristic nucleation time of collapsing wave packets. Recent insights into the structure and evolution of collapsing wave packets are employed to estimate the spectra and field statistics of the turbulence. Extensive calculations using the Zakharov equations in two and three dimensions demonstrate that the predictions of the model are in excellent agreement with numerical results for scalings, spectra, and the distribution of fields in the turbulence in isotropic systems; strong support is thus found for the nucleation model. The scaling behavior proves to be insensitive to the form of the damping of the waves at large wave...

Parallel electric fields in the upward current region of the aurora: Numerical solutions

Direct observations of the parallel electric field by the Fast Auroral Snapshot satellite and the Polar satellite suggest that the ionospheric boundary of the auroral cavity is consistent with an oblique double layer that carries a substantial fraction (roughly 5% to 50%) of the auroral potential. A numerical solution to the Vlasov–Poisson equations of a planar, oblique double layer reproduces many of the properties of the observed electric fields, electron distributions, and ion distributions. The solutions indicate that the electron and ion distributions that emerge from the ionospheric side dominate the structure of the double layer. The ionospheric electron distribution includes scattered and reflected (mirrored) primaries, auroral secondaries, photoelectrons, and a cold population. A large fraction of the ionospheric electrons is reflected by the parallel electric field whereas the ionospheric ions are strongly accelerated. The steep density gradient between the ionosphere and the auroral cavity resu...

Quenching of the beam-plasma instability by large-scale density fluctuations in 3 dimensions

A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large-scale density fluctuations that are able to efficiently diffuse small-k beam-unstable Langmuir waves in phase space, and, second, on the presence of a significant isotropic non-thermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements. The calculations indicate that, for realistic parameters, the most unstable, small k modes are fully stabilized while some oblique mode with higher k and lower growth rate might remain unstable; thus a very broad range of levels of Langmuir waves is possible from levels of the order of enhanced spontaneous emission to the threshold level for nonlinear processes. On the other hand, from in situ measurements of the density fluctuations spectrum by ISEE-1 and 2 in the vicinity of the Earth, it is shown that measured 100 km scale fluctuations may be too effective in quenching the instability. If such strong density fluctuations are common in the solar wind, we show they must be highly anisotropic in order to allow the build-up of Langmuir waves to the observed mV m−1 range. Moreover, the anisotropy must be such that the strongest variations of density occur in a plane perpendicular to the magnetic field.

Scales of guide field reconnection at the hydrogen mass ratio

We analyze the signatures of component reconnection for a Harris current sheet with a guide field using the physical mass ratio of hydrogen. The study uses the fully kinetic particle in cell code IPIC3D to investigate the scaling with mass ratio of the following three main component reconnection features: electron density cavities along the separatrices, channels of fast electron flow within the cavities, and electron phase space holes due to the Buneman instability in the electron high speed channels. The width and strength of the electron holes and of the electron cavities are studied up the mass ratio proper of hydrogen, considering the effect of the simulation box size, and of the boundary conditions. The results compare favorably with the existing data from the Cluster and Themis missions and provide quantitative predictions for realistic conditions to be encountered by the planned magnetospheric multiscale mission.

Simulation of the collapse and dissipation of Langmuir wave packets

The collapse of isolated Langmuir wave packets is studied numerically in two dimensions using both particle‐in‐cell (PIC) simulations and by integrating the Zakharov partial differential equations (PDE’s). The initial state consists of a localized Langmuir wave packet in an ion background that either is uniform or has a profile representative of the density wells in which wave packets form during strong plasma turbulence. Collapse thresholds are determined numerically and compared to analytical estimates. A model in which Langmuir damping is significantly stronger than Landau damping is constructed which, when included in the PDE simulations, yields good agreement with the collapse dynamics observed in PIC simulations for wave packets with initial wave energy densities small compared to the thermal level. For more intense initial Langmuir fields, collapse is arrested in PIC simulations at lower field strengths than in PDE simulations. Neither nonlinear saturation of the density perturbation nor fluid elec...