2017
Ergun RE, Chen L-J, Wilder FD, Ahmadi N, Eriksson S, Usanova ME, Goodrich KA, Holmes JC, Sturner AP, Malaspina DM, et al. Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause. Geophysical Research Letters [Internet]. 2017;44 (7) :2978–2986.
Publisher's VersionAbstractObservations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large-amplitude parallel electric fields (E||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large-amplitude (\~100 mV/m) E|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.
Oka M, III WLB, Phan TD, Hull AJ, Amano T, Hoshino M, Argall MR, Contel LO, Agapitov O, Gershman DJ, et al. Electron Scattering by High-frequency Whistler Waves at Earth's Bow Shock. The Astrophysical Journal Letters [Internet]. 2017;842 (2) :L11.
Publisher's VersionAbstractElectrons are accelerated to non-thermal energies at shocks in space and astrophysical environments. While different mechanisms of electron acceleration have been proposed, it remains unclear how non-thermal electrons are produced out of the thermal plasma pool. Here, we report in situ evidence of pitch-angle scattering of non-thermal electrons by whistler waves at Earth's bow shock. On 2015 November 4, the Magnetospheric Multiscale ( MMS ) mission crossed the bow shock with an Alfvén Mach number ∼11 and a shock angle ∼84°. In the ramp and overshoot regions, MMS revealed bursty enhancements of non-thermal (0.5–2 keV) electron flux, correlated with high-frequency (0.2–0.4 \#\#IMG\#\# [
http://ej.iop.org/images/2041-8205/842/2/L11/apjlaa7759ieqn1.gif] \\$\\\backslashrm\\backslashOmega \\\\_\\backslashmathrm\ce\\\$\ , where \#\#IMG\#\# [
http://ej.iop.org/images/2041-8205/842/2/L11/apjlaa7759ieqn2.gif] \\$\\\backslashrm\\backslashOmega \\\\_\\backslashmathrm\ce\\\$\ is the cyclotron frequency) parallel-propagating whistler waves. The electron velocity distribution (measured at 30 ms cadence) showed an enhanced gradient of phase-space density at and around the region where the electron velocity component parallel to the magnetic field matched the resonant energy inferred from the wave frequency range. The flux of 0.5 keV electrons (measured at 1 ms cadence) showed fluctuations with the same frequency. These features indicate that non-thermal electrons were pitch-angle scattered by cyclotron resonance with the high-frequency whistler waves. However, the precise role of the pitch-angle scattering by the higher-frequency whistler waves and possible nonlinear effects in the electron acceleration process remains unclear.
Wilder FD, Ergun RE, Newman DL, Goodrich KA, Trattner KJ, Goldman MV, Eriksson S, Jaynes AN, Leonard T, Malaspina DM, et al. The nonlinear behavior of whistler waves at the reconnecting dayside magnetopause as observed by the Magnetospheric Multiscale mission: A case study. Journal of Geophysical Research: Space Physics [Internet]. 2017;122 (5) :5487–5501.
Publisher's VersionAbstractWe show observations of whistler mode waves in both the low-latitude boundary layer (LLBL) and on closed magnetospheric field lines during a crossing of the dayside reconnecting magnetopause by the Magnetospheric Multiscale (MMS) mission on 11 October 2015. The whistlers in the LLBL were on the electron edge of the magnetospheric separatrix and exhibited high propagation angles with respect to the background field, approaching 40°, with bursty and nonlinear parallel electric field signatures. The whistlers in the closed magnetosphere had Poynting flux that was more field aligned. Comparing the reduced electron distributions for each event, the magnetospheric whistlers appear to be consistent with anisotropy-driven waves, while the distribution in the LLBL case includes anisotropic backward resonant electrons and a forward resonant beam at near half the electron-Alfvén speed. Results are compared with the previously published observations by MMS on 19 September 2015 of LLBL whistler waves. The observations suggest that whistlers in the LLBL can be both beam and anisotropy driven, and the relative contribution of each might depend on the distance from the X line.
Shuster JR, Argall MR, Torbert RB, Chen L-J, Farrugia CJ, Alm L, Wang S, Daughton W, Gershman DJ, Giles BL, et al. Hodographic approach for determining spacecraft trajectories through magnetic reconnection diffusion regions. Geophysical Research Letters [Internet]. 2017;44 (4) :1625–1633.
Publisher's VersionAbstractWe develop an algorithm that finds a trajectory through simulations of magnetic reconnection along which input Magnetospheric Multiscale (MMS) spacecraft observations are matched. Using two-dimensional particle-in-cell simulations of asymmetric reconnection, the method is applied to a magnetopause electron diffusion region (EDR) encountered by the MMS spacecraft to facilitate interpretation of the event based on fully kinetic models. The recently discovered crescent-shaped electron velocity distributions measured by MMS in the EDR are consistent with simulation distributions at the corresponding time along the computed trajectory.
2016
Lavraud B, Zhang YC, Vernisse Y, Gershman DJ, Dorelli J, Cassak PA, Dargent J, Pollock C, Giles B, Aunai N, et al. Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause. Geophysical Research Letters [Internet]. 2016 :3042–3050.
Publisher's Version Matsui H, Erickson PJ, Foster JC, Torbert RB, Argall MR, Anderson BJ, Blake JB, Cohen IJ, Ergun RE, Farrugia CJ, et al. Dipolarization in the inner magnetosphere during a geomagnetic storm on 7 October 2015. Geophysical Research Letters [Internet]. 2016;43 (18) :9397–9405.
Publisher's VersionAbstractA dipolarization event was observed by the Magnetospheric Multiscale (MMS) spacecraft at L = 3.8 and 19.8 magnetic local time starting at ∼23:42:36 UT on 7 October 2015. The magnetic and electric fields showed initially coherent variations between the spacecraft. The sunward convection turned tailward after the dipolarization. The observation is interpreted in terms of the pressure balance or the momentum equation. This was followed by a region traversed where the fields were irregular. The scale length was of the order of the ion gyroradius, suggesting the kinetic nature of the fluctuations. Combination of the multi-instrument, multispacecraft data reveals a more detailed picture of the dipolarization event in the inner magnetosphere. Conjunction ionosphere-plasmasphere observations from DMSP, two-dimensional GPS total electron content, the Millstone Hill midlatitude incoherent scatter radar, and AMPERE measurements imply that MMS observations are located on the poleward edge of the ionospheric trough where Region 2 field-aligned currents flow.
Ergun RE, Holmes JC, Goodrich KA, Wilder FD, Stawarz JE, Eriksson S, Newman DL, Schwartz SJ, Goldman MV, Sturner AP, et al. Magnetospheric Multiscale observations of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the magnetopause. Geophysical Research Letters [Internet]. 2016;43 (11) :5626–5634.
Publisher's VersionAbstractWe report observations from the Magnetospheric Multiscale satellites of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the Earth's magnetopause. The observed waves have parallel electric fields (E||) with amplitudes on the order of 100 mV/m and display nonlinear characteristics that suggest a possible net E||. These waves are observed within the ion diffusion region and adjacent to (within several electron skin depths) the electron diffusion region. They are in or near the magnetosphere side current layer. Simulation results support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two stream instability, which is a consequence of mixing cold (\textless10 eV) plasma in the magnetosphere with warm (\~100 eV) plasma from the magnetosheath on a freshly reconnected magnetic field line. The frequent observation of these waves suggests that cold plasma is often present near the magnetopause.
Erickson PJ, Matsui H, Foster JC, Torbert RB, Ergun RE, Khotyaintsev YV, Lindqvist P-A, Argall MR, Farrugia CJ, Paulson KW, et al. Multipoint MMS observations of fine-scale SAPS structure in the inner magnetosphere. Geophysical Research Letters [Internet]. 2016;43 (14) :7294–7300.
Publisher's VersionAbstractWe present detailed observations of dynamic, fine-scale inner magnetosphere-ionosphere coupling at ∼3.9 RE in the Region 2 Birkeland field-aligned current (FAC). We find that observed electrodynamic spatial/temporal scales are primarily characteristic of magnetically mapped ionospheric structure. On 15 September 2015, conjugate Magnetospheric Multiscale (MMS) spacecraft and Millstone Hill radar observations show plasmasphere boundary region subauroral polarization stream (SAPS) electric fields at L = 4.0–4.2 near 21 MLT. MMS observations reveal high-altitude ∼1 mV/m fine-scale radial and azimuthal electric field perturbations over ≤0.15 L with high spatial coherence over ≥2–3 min and show outward motion within a broader FAC of ∼0.12 $μ$A/m2. Our analysis shows that MMS electric field fluctuations are most likely reflective of SAPS ionospheric structure at scales of ∼22 km and with ionospheric closure of small-scale filamentary FAC perturbations. The results highlight the ionosphere's importance in regulating fine-scale magnetosphere-ionosphere structure.
Le Contel O, Retinò A, Breuillard H, Mirioni L, Robert P, Chasapis A, Lavraud B, Chust T, Rezeau L, Wilder FD, et al. Whistler mode waves and Hall fields detected by MMS during a dayside magnetopause crossing. Geophysical Research Letters [Internet]. 2016;43 (12) :5943–5952.
Publisher's VersionAbstractWe present Magnetospheric Multiscale (MMS) mission measurements during a full magnetopause crossing associated with an enhanced southward ion flow. A quasi-steady magnetospheric whistler mode wave emission propagating toward the reconnection region with quasi-parallel and oblique wave angles is detected just before the opening of the magnetic field lines and the detection of escaping energetic electrons. Its source is likely the perpendicular temperature anisotropy of magnetospheric energetic electrons. In this region, perpendicular and parallel currents as well as the Hall electric field are calculated and found to be consistent with the decoupling of ions from the magnetic field and the crossing of a magnetospheric separatrix region. On the magnetosheath side, Hall electric fields are found smaller as the density is larger but still consistent with the decoupling of ions. Intense quasi-parallel whistler wave emissions are detected propagating both toward and away from the reconnection region in association with a perpendicular anisotropy of the high-energy part of the magnetosheath electron population and a strong perpendicular current, which suggests that in addition to the electron diffusion region, magnetosheath separatrices could be a source region for whistler waves.
Burch JL, Torbert RB, Phan TD, Chen L-J, Moore TE, Ergun RE, Eastwood JP, Gershman DJ, Cassak PA, Argall MR, et al. Electron-scale measurements of magnetic reconnection in space. Science [Internet]. 2016;352 (6290).
Publisher's VersionAbstractMagnetic 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\$\backslash$textquoteright\s Magnetospheric Multiscale mission to probe the plasma properties within a reconnection event in Earth\$\backslash$textquoteright\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. 1176INTRODUCTIONMagnetic reconnection is a physical process occurring in plasmas in which magnetic energy is explosively converted into heat and kinetic energy. The effects of reconnection\$\backslash$textemdash\such as solar flares, coronal mass ejections, magnetospheric substorms and auroras, and astrophysical plasma jets\$\backslash$textemdash\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.RATIONALETo 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.RESULTSOn 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.CONCLUSIONMMS 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\$\backslash$textquoteright\s Magnetospheric Multiscale (MMS) mission has measured how electron dynamics controls magnetic reconnection. The data in the circles show electrons with velocities from 0 to 104 km/s carrying current out of the page on the left side of the X-line and then flowing upward and downward along the reconnected magnetic field on the right side. The most intense fluxes are red and the least intense are blue. The plot in the center shows magnetic field lines and out-of-plane currents derived from a numerical plasma simulation using the parameters observed by MMS.Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA\$\backslash$textquoteright\s Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth\$\backslash$textquoteright\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.
Torbert RB, Burch JLL, Giles BLL, Gershman D, Pollock CJJ, Dorelli J, Avanov L, Argall MRR, Shuster J, Strangeway RJ, et al. Estimates of terms in Ohm's law during an encounter with an electron diffusion region. Geophysical Research Letters [Internet]. 2016;43 (12) :5918–5925.
Publisher's VersionAbstractWe present measurements from the Magnetospheric Multiscale (MMS) mission taken during a reconnection event on the dayside magnetopause which includes a passage through an electron diffusion region (EDR). The four MMS satellites were separated by about 10 km such that estimates of gradients and divergences allow a reasonable estimate of terms in the generalized Ohm's law, which is key to investigating the energy dissipation during reconnection. The strength and character of dissipation mechanisms determines how magnetic energy is released. We show that both electron pressure gradients and electron inertial effects are important, but not the only participants in reconnection near EDRs, since there are residuals of a few mV/m (\~30–50%) of E + Ue × B (from the sum of these two terms) during the encounters. These results are compared to a simulation, which exhibits many of the observed features, but where relatively little residual is present.
Farrugia CJ, Lavraud B, Torbert RB, Argall M, Kacem I, Yu W, Alm L, Burch J, Russell CT, Shuster J, et al. Magnetospheric Multiscale Mission observations and non-force free modeling of a flux transfer event immersed in a super-Alfvénic flow. Geophysical Research Letters [Internet]. 2016;43 (12) :6070–6077.
Publisher's VersionAbstractWe analyze plasma, magnetic field, and electric field data for a flux transfer event (FTE) to highlight improvements in our understanding of these transient reconnection signatures resulting from high-resolution data. The ∼20 s long, reverse FTE, which occurred south of the geomagnetic equator near dusk, was immersed in super-Alfvénic flow. The field line twist is illustrated by the behavior of flows parallel/perpendicular to the magnetic field. Four-spacecraft timing and energetic particle pitch angle anisotropies indicate a flux rope (FR) connected to the Northern Hemisphere and moving southeast. The flow forces evidently overcame the magnetic tension. The high-speed flows inside the FR were different from those outside. The external flows were perpendicular to the field as expected for draping of the external field around the FR. Modeling the FR analytically, we adopt a non-force free approach since the current perpendicular to the field is nonzero. It reproduces many features of the observations.
Wilder FD, Ergun RE, Goodrich KA, Goldman MV, Newman DL, Malaspina DM, Jaynes AN, Schwartz SJ, Trattner KJ, Burch JL, et al. Observations of whistler mode waves with nonlinear parallel electric fields near the dayside magnetic reconnection separatrix by the Magnetospheric Multiscale mission. Geophysical Research Letters [Internet]. 2016;43 (12) :5909–5917.
Publisher's VersionAbstractWe show observations from the Magnetospheric Multiscale (MMS) mission of whistler mode waves in the Earth's low-latitude boundary layer (LLBL) during a magnetic reconnection event. The waves propagated obliquely to the magnetic field toward the X line and were confined to the edge of a southward jet in the LLBL. Bipolar parallel electric fields interpreted as electrostatic solitary waves (ESW) are observed intermittently and appear to be in phase with the parallel component of the whistler oscillations. The polarity of the ESWs suggests that if they propagate with the waves, they are electron enhancements as opposed to electron holes. The reduced electron distribution shows a shoulder in the distribution for parallel velocities between 17,000 and 22,000 km/s, which persisted during the interval when ESWs were observed, and is near the phase velocity of the whistlers. This shoulder can drive Langmuir waves, which were observed in the high-frequency parallel electric field data.
Fisher MK, Argall MR, Joyce CJ, Smith CW, Isenberg PA, Vasquez BJ, Schwadron NA, Skoug RM, Sokół JM, Bzowski M, et al. A Survey of Magnetic Waves Excited by Newborn Interstellar He+ Observed by the ACE Spacecraft at 1 au. The Astrophysical Journal [Internet]. 2016;830 (1) :47.
Publisher's VersionAbstractWe report observations of low-frequency waves at 1 au by the magnetic field instrument on the Advanced Composition Explorer ( ACE /MAG) and show evidence that they arise due to newborn interstellar pickup He + . Twenty-five events are studied. They possess the generally predicted attributes: spacecraft-frame frequencies slightly greater than the He + cyclotron frequency, left-hand polarization in the spacecraft frame, and transverse fluctuations with minimum variance directions that are quasi-parallel to the mean magnetic field. Their occurrence spans the first 18 years of ACE operations, with no more than 3 such observations in any given year. Thus, the events are relatively rare. As with past observations by the Ulysses and Voyager spacecraft, we argue that the waves are seen only when the background turbulence is sufficiently weak as to allow for the slow accumulation of wave energy over many hours.
Nakamura R, Sergeev VA, Baumjohann W, Plaschke F, Magnes W, Fischer D, Varsani A, Schmid D, Nakamura TKM, Russell CT, et al. Transient, small-scale field-aligned currents in the plasma sheet boundary layer during storm time substorms. Geophysical Research Letters [Internet]. 2016;43 (10) :4841–4849.
Publisher's VersionAbstractWe report on field-aligned current observations by the four Magnetospheric Multiscale (MMS) spacecraft near the plasma sheet boundary layer (PSBL) during two major substorms on 23 June 2015. Small-scale field-aligned currents were found embedded in fluctuating PSBL flux tubes near the separatrix region. We resolve, for the first time, short-lived earthward (downward) intense field-aligned current sheets with thicknesses of a few tens of kilometers, which are well below the ion scale, on flux tubes moving equatorward/earthward during outward plasma sheet expansion. They coincide with upward field-aligned electron beams with energies of a few hundred eV. These electrons are most likely due to acceleration associated with a reconnection jet or high-energy ion beam-produced disturbances. The observations highlight coupling of multiscale processes in PSBL as a consequence of magnetotail reconnection.