69 results found
Robertson SL, Eastwood JP, Stawarz JE, et al., 2021, Electron Trapping in Magnetic Mirror Structures at the edge of Magnetopause Flux Ropes, Journal of Geophysical Research: Space Physics, ISSN: 2169-9380
Quijia P, Fraternale F, Stawarz J, et al., 2021, Comparing turbulence in a Kelvin-Helmholtz instability region across the terrestrial magnetopause, Monthly Notices of the Royal Astronomical Society, ISSN: 0035-8711
Stawarz JE, Matteini L, Parashar TN, et al., 2021, Comparative analysis of the various generalized Ohm's law terms in magnetosheath turbulence as observed by magnetospheric multiscale, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-14, ISSN: 2169-9380
Decomposing the electric field (E) into the contributions from generalized Ohm's law provides key insight into both nonlinear and dissipative dynamics across the full range of scales within a plasma. Using high‐resolution, multi‐spacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent E spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to E over the accessible length scales, which extend to scales smaller than the electron inertial length at the greatest extent, with the Hall and electron pressure terms dominating at sub‐ion scales. The strength of the non‐ideal electron pressure contribution is stronger than expected from linear kinetic Alfvén waves and a partial anti‐alignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents in the turbulence. The relative contribution of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping E for the intervals examined in this study. Overall, the sum of the Ohm's law terms and measured E agree to within ∼ 20% across the observable scales. These results both confirm general expectations about the behavior of E in turbulent plasmas and highlight features that should be explored further theoretically.
Eastwood JP, Goldman M, Phan TD, et al., 2020, Energy flux densities near the electron dissipation region in asymmetric magnetopause reconnection, Physical Review Letters, Vol: 125, Pages: 1-6, ISSN: 0031-9007
Magnetic reconnection is of fundamental importance to plasmas because of its role in releasing and repartitioning stored magnetic energy. Previous results suggest that this energy is predominantly released as ion enthalpy flux along the reconnection outflow. Using Magnetospheric Multiscale data we find the existence of very significant electron energy flux densities in the vicinity of the magnetopause electron dissipation region, orthogonal to the ion energy outflow. These may significantly impact models of electron transport, wave generation, and particle acceleration.
Pouquet A, Rosenberg D, Stawarz JE, 2020, Interplay between turbulence and waves: large-scale helical transfer, and small-scale dissipation and mixing in fluid and Hall-MHD turbulence, ATTI Della Accademia Nazionale Dei Lincei Rendiconti Lincei Scienze Fisiche e Naturali, Vol: 31, Pages: 949-961, ISSN: 2037-4631
Novel features of turbulent flows have been analyzed recently, for example: (1) the possibility of an ideal invariant, such as the energy, to be transferred both to the small scales and to the large scales, in each case with a constant flux; (2) the existence of non-Gaussian wings in Probability Distribution Functions of kinetic, magnetic, and temperature fluctuations, together with their gradients, thus displaying large-scale as well as small-scale intermittency; and (3) the linear dependence on the control parameter of the effective dissipation in turbulence when non-linear eddies and waves interact. We shall briefly review these results with examples stemming from Solar Wind data, the atmosphere and the ocean with either magnetic fields, stratification, and/or rotation. In a second part, we shall examine numerically the inverse cascades of magnetic and of generalized helicity for Hall-MHD in the presence of forcing. These helical invariants in the ideal non-dissipative case involve various cross-correlations between the velocity and vorticity, the magnetic field, and the magnetic potential. For an ion inertial length larger than the forcing scale, the effect of the waves is significant. It leads to an exponential attenuation of the inverse cascade to large scales, since, through the velocity and vorticity, small scales play an increasing dynamical role for a strong Hall current.
Woodham L, Horbury T, Matteini L, et al., 2020, Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere, Astronomy and Astrophysics: a European journal, ISSN: 0004-6361
Context. Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the helio-sphere. Recent observations by Parker Solar Probe(PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by ‘quieter’ radial fields. Aims. We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona. Methods. We fit 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, Tp,‖, and perpendicular, Tp,⊥, temperature. Results. We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in Tp,‖, while Tp,⊥remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. Conclusions. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced Tp,‖that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.
Desai R, Zhang H, Davies E, et al., 2020, Three dimensional simulations of solar wind preconditioning and the 23 July 2012 Interplanetary Coronal Mass Ejection, Solar Physics: a journal for solar and solar-stellar research and the study of solar terrestrial physics, Vol: 295, Pages: 1-14, ISSN: 0038-0938
Predicting the large-scale eruptions from the solar corona and theirpropagation through interplanetary space remains an outstanding challenge in solar- and helio-physics research. In this article, we describe three dimensional magnetohydrodynamic simulations of the inner heliosphere leading up to and including the extreme interplanetary coronal mass ejection (ICME) of 23 July 2012, developed using the code PLUTO. The simulations are driven using the output of coronal models for Carrington rotations 2125 and 2126 and, given the uncertainties in the initial conditions, are able to reproduce an event of comparable magnitude to the 23 July ICME, with similar velocity and densityprofi les at 1 au. The launch-time of this event is then varied with regards to an initial 19 July ICME and the effects of solar wind preconditioning are found to be signi ficant for an event of this magnitude and to decrease over a time-window consistent with the ballistic re filling of the depleted heliospheric sector. These results indicate that the 23 July ICME was mostly unaffected by events prior, but would have travelled even faster had it erupted closer in time to the 19 July event where it would have experienced even lower drag forces. We discuss this systematic study of solar wind preconditioning in the context of space weatherforecasting.
Ergun RE, Ahmadi N, Kromyda L, et al., 2020, Observations of Particle Acceleration in Magnetic Reconnection-driven Turbulence, ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X
Ergun RE, Ahmadi N, Kromyda L, et al., 2020, Particle Acceleration in Strong Turbulence in the Earth's Magnetotail, ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X
Franci L, Stawarz JE, Papini E, et al., 2020, Modeling MMS observations at the Earth's magnetopause with hybrid simulations of Alfvénic turbulence, The Astrophysical Journal, Vol: 898, ISSN: 0004-637X
Magnetospheric Multiscale (MMS) observations of plasma turbulence generated by a Kelvin–Helmholtz (KH) event at the Earth's magnetopause are compared with a high-resolution two-dimensional (2D) hybrid direct numerical simulation of decaying plasma turbulence driven by large-scale balanced Alfvénic fluctuations. The simulation, set up with four observation-driven physical parameters (ion and electron betas, turbulence strength, and injection scale), exhibits a quantitative agreement on the spectral, intermittency, and cascade-rate properties with in situ observations, despite the different driving mechanisms. Such agreement demonstrates a certain universality of the turbulent cascade from magnetohydrodynamic to sub-ion scales, whose properties are mainly determined by the selected parameters, also indicating that the KH instability-driven turbulence has a quasi-2D nature. The fact that our results are compatible with the validity of the Taylor hypothesis, in the whole range of scales investigated numerically, suggests that the fluctuations at sub-ion scales might have predominantly low frequencies. This would be consistent with a kinetic Alfvén wave-like nature and/or with the presence of quasi-static structures. Finally, the third-order structure function analysis indicates that the cascade rate of the turbulence generated by a KH event at the magnetopause is an order of magnitude larger than in the ambient magnetosheath.
AkhavanTafti M, Palmroth M, Slavin JA, et al., 2020, Comparative analysis of the vlasiator simulations and MMS observations of multiple X‐line reconnection and flux transfer events, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-22, ISSN: 2169-9380
The Vlasiator hybrid‐Vlasov code was developed to investigate global magnetospheric dynamics at ion‐kinetic scales. Here, we focus on the role of magnetic reconnection in the formation and evolution of the magnetic islands at the low‐latitude magnetopause, under southward interplanetary magnetic field (IMF) conditions. The simulation results indicate that: 1) the magnetic reconnection ion kinetics, including the Earthward‐pointing Larmor electric field on the magnetospheric‐side of an X‐point and anisotropic ion distributions, are well‐captured by Vlasiator, thus enabling the study of reconnection‐driven magnetic island evolution processes, 2) magnetic islands evolve due to continuous reconnection at adjacent X‐points, ‘coalescence’ which refers to the merging of neighboring islands to create a larger island, ‘erosion’ during which an island loses magnetic flux due to reconnection, and ‘division’ which involves the splitting of an island into smaller islands, and 3) continuous reconnection at adjacent X‐points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross‐sectional growth rates up to +0.3 RE2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion‐scale flux transfer events (FTEs) sandwiched between two dominant X‐lines. The MMS measurements similarly reveal: 1) anisotropic ion populations, and 2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion‐scale FTEs may grow Earth‐sized within ~10 minutes, which is comparable to the average transport time for FTEs formed in the subsolar region to the high‐latitude magnetopause. Future simulations shall revisit reconnection‐driven island evolution processes with improved spatial resolutions.
The spectral properties associated with laminar, anti-parallel reconnection are examined using a 2.5D kinetic particle in cell simulation. Both the reconnection rate and the energy spectrum exhibit three distinct phases: an initiation phase where the reconnection rate grows, a quasi-steady phase, and a declining phase where both the reconnection rate and the energy spectrum decrease. During the steady phase, the energy spectrum exhibits approximately a double power law behavior, with a slope near −5/3 at wave numbers smaller than the inverse ion inertial length and a slope steeper than −8/3 for larger wave numbers up to the inverse electron inertial length. This behavior is consistent with a Kolmogorov energy cascade and implies that laminar reconnection may fundamentally be an energy cascade process. Consistent with this idea is the fact that the reconnection rate exhibits a rough correlation with the energy spectrum at wave numbers near the inverse ion inertial length. The 2D spectrum is strongly anisotropic with most energy associated with the wave vector direction normal to the current sheet. Reconnection acts to isotropize the energy spectrum, reducing the Shebalin angle from an initial value of 70° to about 48° (nearly isotropic) by the end of the simulation. The distribution of energy over length scales is further analyzed by dividing the domain into spatial subregions and employing structure functions.
Nakamura TKM, Stawarz JE, Hasegawa H, et al., 2020, Effects of Fluctuating Magnetic Field on the Growth of the Kelvin-Helmholtz Instability at the Earth's Magnetopause, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
Pouquet A, Stawarz JE, Rosenberg D, 2020, Coupling large eddies and waves in turbulence: Case study of magnetic helicity at the ion inertial scale, Publisher: arXiv
In turbulence, for neutral or conducting fluids, a large ratio of scales isexcited because of the possible occurrence of inverse cascades to large, globalscales together with direct cascades to small, dissipative scales, as observedin the atmosphere and oceans, or in the solar environment. In this context,using direct numerical simulations with forcing, we analyze scale dynamics inthe presence of magnetic fields with a generalized Ohm's law including a Hallcurrent. The ion inertial length epsilon_H serves as the control parameter atfixed Reynolds number. Both the magnetic and generalized helicity -- invariantsin the ideal case -- grow linearly with time, as expected from classicalarguments. The cross-correlation between the velocity and magnetic field growsas well, more so in relative terms for a stronger Hall current. We find thatthe helical growth rates vary exponentially with epsilon_H, provided the ioninertial scale resides within the inverse cascade range. These exponentialvariations are recovered phenomenologically using simple scaling arguments.They are directly linked to the wavenumber power-law dependence of generalizedand magnetic helicity, k^(-2), in their inverse ranges. This illustrates andconfirms the important role of the interplay between large and small scales inthe dynamics of turbulent flows.
Gingell I, Schwartz SJ, Eastwood JP, et al., 2020, Statistics of reconnecting current sheets in the transition region of earth's bow shock, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-14, ISSN: 2169-9380
We have conducted a comprehensive survey of burst mode observations of Earth's bow shock by the Magnetospheric Multiscale mission to identify and characterize current sheets associated with collisionless shocks, with a focus on those containing fast electron outflows, a likely signature of magnetic reconnection. The survey demonstrates that these thin current sheets are observed within the transition region of approximately 40% of shocks within the burst mode data set of Magnetospheric Multiscale. With only small apparent bias toward quasi‐parallel shock orientations and high Alfvén Mach numbers, the results suggest that reconnection at shocks is a universal process, occurring across all shock orientations and Mach numbers. On examining the distributions of current sheet properties, we find no correlation between distance from the shock, sheet width, or electron jet speed, though the relationship between electron and ion jet speed supports expectations of electron‐only reconnection in the region. Furthermore, we find that robust heating statistics are not separable from background fluctuations, and thus, the primary consequence of reconnection at shocks is in relaxing the topology of the disordered magnetic field in the transition region.
Ergun RE, Hoilijoki S, Ahmadi N, et al., 2019, Magnetic Reconnection in Three Dimensions: Observations of Electromagnetic Drift Waves in the Adjacent Current Sheet, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 10104-10118, ISSN: 2169-9380
Ergun RE, Hoilijoki S, Ahmadi N, et al., 2019, Magnetic Reconnection in Three Dimensions: Modeling and Analysis of Electromagnetic Drift Waves in the Adjacent Current Sheet, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 10085-10103, ISSN: 2169-9380
Stawarz J, Eastwood JP, Phan TD, et al., 2019, Properties of the turbulence associated with electron-only magnetic reconnection in Earth's magnetosheath, Letters of the Astrophysical Journal, Vol: 877, ISSN: 2041-8205
Turbulent plasmas generate intense current structures, which have long been suggested as magnetic reconnection sites. Recent Magnetospheric Multiscale observations in Earth's magnetosheath revealed a novel form of reconnection where the dynamics only couple to electrons, without ion involvement. It was suggested that such dynamics were driven by magnetosheath turbulence. In this study, the fluctuations are examined to determine the properties of the turbulence and if a signature of reconnection is present in the turbulence statistics. The study reveals statistical properties consistent with plasma turbulence with a correlation length of ~10 ion inertial lengths. When reconnection is more prevalent, a steepening of the magnetic spectrum occurs at the length scale of the reconnecting current sheets. The statistics of intense currents suggest the prevalence of electron-scale current sheets favorable for electron reconnection. The results support the hypothesis that electron reconnection is driven by turbulence and highlight diagnostics that may provide insight into reconnection in other turbulent plasmas.
Pouquet A, Rosenberg D, Stawarz JE, et al., 2019, Helicity Dynamics, Inverse, and Bidirectional Cascades in Fluid and Magnetohydrodynamic Turbulence: A Brief Review, Publisher: AMER GEOPHYSICAL UNION
Gingell I, Schwartz SJ, Eastwood JP, et al., 2019, Observations of magnetic reconnection in the ransition region of quasi-parallel hocks, Geophysical Research Letters, Vol: 46, Pages: 1177-1184, ISSN: 0094-8276
Using observations of Earth's bow shock by the Magnetospheric Multiscale mission, we show for the first time that active magnetic reconnection is occurring at current sheets embedded within the quasi‐parallel shock's transition layer. We observe an electron jet and heating but no ion response, suggesting we have observed an electron‐only mode. The lack of ion response is consistent with simulations showing reconnection onset on sub‐ion time scales. We also discuss the impact of electron heating in shocks via reconnection.
Nakamura R, Genestreti KJ, Nakamura T, et al., 2019, Structure of the current sheet in the 11 July 2017 Electron Diffusion Region Event, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 1173-1186, ISSN: 2169-9380
The structure of the current sheet along the Magnetospheric Multiscale (MMS) orbit is examined during the 11 July 2017 Electron Diffusion Region (EDR) event. The location of MMS relative to the X‐line is deduced and used to obtain the spatial changes in the electron parameters. The electron velocity gradient values are used to estimate the reconnection electric field sustained by nongyrotropic pressure. It is shown that the observations are consistent with theoretical expectations for an inner EDR in 2‐D reconnection. That is, the magnetic field gradient scale, where the electric field due to electron nongyrotropic pressure dominates, is comparable to the gyroscale of the thermal electrons at the edge of the inner EDR. Our approximation of the MMS observations using a steady state, quasi‐2‐D, tailward retreating X‐line was valid only for about 1.4 s. This suggests that the inner EDR is localized; that is, electron outflow jet braking takes place within an ion inertia scale from the X‐line. The existence of multiple events or current sheet processes outside the EDR may play an important role in the geometry of reconnection in the near‐Earth magnetotail.
Torbert RB, Burch JL, Phan TD, et al., 2018, Electron-scale dynamics of the diffusion region during symmetric magnetic reconnection in space, Science, Vol: 362, Pages: 1391-1395, ISSN: 0036-8075
Magnetic reconnection is an energy conversion process that occurs in many astrophysical contexts including Earth’s magnetosphere, where the process can be investigated in situ by spacecraft. On 11 July 2017, the four Magnetospheric Multiscale spacecraft encountered a reconnection site in Earth’s magnetotail, where reconnection involves symmetric inflow conditions. The electron-scale plasma measurements revealed (i) super-Alfvénic electron jets reaching 15,000 kilometers per second; (ii) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures in the velocity distributions; and (iii) the spatial dimensions of the electron diffusion region with an aspect ratio of 0.1 to 0.2, consistent with fast reconnection. The well-structured multiple layers of electron populations indicate that the dominant electron dynamics are mostly laminar, despite the presence of turbulence near the reconnection site.
Vasquez BJ, Forman MA, Coburn JT, et al., 2018, The Turbulent Cascade for High Cross-helicity States at 1 au. II. Minor Energy, ASTROPHYSICAL JOURNAL, Vol: 867, ISSN: 0004-637X
Smith CW, Coburn JT, Vasquez BJ, et al., 2018, Correlation Scales of the Turbulent Cascade at 1 AU, 17th Annual International Astrophysics Conference (AIAC) on Dissipative and Heating Processes in Collisionless Plasma - The Solar Corona, The Solar Wind, and The Interstellar Medium, Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Stawarz JE, Eastwood JP, Genestreti KJ, et al., 2018, Intense electric fields and electron‐scale substructure within magnetotail flux ropes as revealed by the Magnetospheric Multiscale mission, Geophysical Research Letters, Vol: 45, Pages: 8783-8792, ISSN: 0094-8276
Three flux ropes associated with near‐Earth magnetotail reconnection are analyzed using Magnetospheric Multiscale observations. The flux ropes are Earthward propagating with sizes from ∼3 to 11 ion inertial lengths. Significantly different axial orientations are observed, suggesting spatiotemporal variability in the reconnection and/or flux rope dynamics. An electron‐scale vortex, associated with one of the most intense electric fields (E) in the event, is observed within one of the flux ropes. This E is predominantly perpendicular to the magnetic field (B); the electron vortex is frozen‐in with E × B drifting electrons carrying perpendicular current and causing a small‐scale magnetic enhancement. The vortex is ∼16 electron gyroradii in size perpendicular to B and potentially elongated parallel to B. The need to decouple the frozen‐in vortical motion from the surrounding plasma implies a parallel E at the structure's ends. The formation of frozen‐in electron vortices within reconnection‐generated flux ropes may have implications for particle acceleration.
Eastwood J, Mistry R, Phan TD, et al., 2018, Guide field reconnection: exhaust structure and heating, Geophysical Research Letters, Vol: 45, Pages: 4569-4577, ISSN: 0094-8276
Magnetospheric Multiscale (MMS) observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X‐line in the Earth's magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near one edge of the exhaust and containing electron flow toward the X‐line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust but within the cavity, electron cooling and enhanced parallel ion heating was found. This is explained in terms of the parallel electric field, which inhibits electron mixing within the cavity on newly reconnected field lines, but accelerates ions. Consequently, guide field reconnection causes inhomogeneous changes in ion and electron temperature across the exhaust.
Smith CW, Vasquez BJ, Coburn JT, et al., 2018, Correlation Scales of the Turbulent Cascade at 1 au, ASTROPHYSICAL JOURNAL, Vol: 858, ISSN: 0004-637X
Ergun RE, Goodrich KA, Wilder FD, et al., 2018, Magnetic Reconnection, Turbulence, and Particle Acceleration: Observations in the Earth's Magnetotail, Geophysical Research Letters, Vol: 45, Pages: 3338-3347, ISSN: 0094-8276
We report observations of turbulent dissipation and particle acceleration from large-amplitude electric fields (E) associated with strong magnetic field (B) fluctuations in the Earth's plasma sheet. The turbulence occurs in a region of depleted density with anti-earthward flows followed by earthward flows suggesting ongoing magnetic reconnection. In the turbulent region, ions and electrons have a significant increase in energy, occasionally > 100 keV, and strong variation. There are numerous occurrences of |E| > 100 mV/m including occurrences of large potentials ( > 1 kV) parallel to B and occurrences with extraordinarily large J · E (J is current density). In this event, we find that the perpendicular contribution of J · E with frequencies near or below the ion cyclotron frequency (f ci ) provide the majority net positive J · E. Large-amplitude parallel E events with frequencies above f ci to several times the lower hybrid frequency provide significant dissipation and can result in energetic electron acceleration.
Gershman DJ, F-Vinas A, Dorelli JC, et al., 2018, Energy partitioning constraints at kinetic scales in low-beta turbulence, PHYSICS OF PLASMAS, Vol: 25, ISSN: 1070-664X
Nakamura R, Varsani A, Genestreti KJ, et al., 2018, Multiscale Currents Observed by MMS in the Flow Braking Region, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 1260-1278, ISSN: 2169-9380
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