Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, ...this process occurs in a minuscule electron-scale diffusion region
. On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed
. Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region
. In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales
. However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.
Plasma turbulence at scales of the order of the ion inertial length is mediated by several mechanisms, including linear wave damping, magnetic reconnection, the formation and dissipation of thin ...current sheets, and stochastic heating. It is now understood that the presence of localized coherent structures enhances the dissipation channels and the kinetic features of the plasma. However, no formal way of quantifying the relationship between scale-to-scale energy transfer and the presence of spatial structures has been presented so far. In the Letter we quantify such a relationship analyzing the results of a two-dimensional high-resolution Hall magnetohydrodynamic simulation. In particular, we employ the technique of space filtering to derive a spectral energy flux term which defines, in any point of the computational domain, the signed flux of spectral energy across a given wave number. The characterization of coherent structures is performed by means of a traditional two-dimensional wavelet transformation. By studying the correlation between the spectral energy flux and the wavelet amplitude, we demonstrate the strong relationship between scale-to-scale transfer and coherent structures. Furthermore, by conditioning one quantity with respect to the other, we are able for the first time to quantify the inhomogeneity of the turbulence cascade induced by topological structures in the magnetic field. Taking into account the low space-filling factor of coherent structures (i.e., they cover a small portion of space), it emerges that 80% of the spectral energy transfer (both in the direct and inverse cascade directions) is localized in about 50% of space, and 50% of the energy transfer is localized in only 25% of space.
Magnetic reconnection in the Earth's magnetosphere accelerates electrons. And yet energetic electrons are not created during reconnection in the solar wind. Observations from the Cluster spacecraft ...now suggest that electron acceleration is caused by repeated bursts of plasma flow, which only occur in situations where the magnetic reconnection is unsteady. PUBLICATION ABSTRACT
Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from XGSM = −30 RE to XGSM = −7 RE. How these DFs are formed is still poorly understood. Three possible mechanisms have ...been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near‐Earth region (XGSM > −10 RE) are primarily attributed to jet braking, while the DFs detected in the mid‐ or far‐tail region (XGSM < −15 RE) are primarily attributed to transient reconnection or spontaneous formation. In the jet‐braking mechanism, the high‐speed flow “pushes” the preexisting plasmas to produce the DF so that there is causality between high‐speed flow and DF. In the transient‐reconnection mechanism, there is no causality between high‐speed flow and DF, because the frozen‐in condition is violated.
Key Points
DFs are observed inside reconnection diffusion region
Three formation mechanisms of DF are compared
Causality between flow and DF is discussed
Magnetic reconnection is a universal process leading to energy conversion in plasmas. It occurs in the Solar System, in laboratory plasmas and is important in astrophysics . Reconnection has been ...observed so far only at large-scale boundaries between different plasma environments . It is not known whether reconnection occurs and is important in turbulent plasmas where many small-scale boundaries can form. Solar and laboratory measurements as well as numerical simulations indicate such possibility. Here we report, for the first time, in situ evidence of reconnection in a turbulent plasma. The turbulent environment is the solar wind downstream of the Earth's bow shock. We show that reconnection is fast and electromagnetic energy is converted into heating and acceleration of particles. This has significant implications for laboratory and astrophysical plasmas where both turbulence and reconnection should be common.
We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong ...core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration.
Key Points
Strong core fields, density depletion, intense currents inside magnetic islands
Energetic electron increase in the thin current sheet, and magnetic island
Energetic electrons are first accelerated in thin current sheet, then in island
We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at ∼1500 magnetic local time (MLT) by the Geotail spacecraft and at ∼1900 MLT by the Cluster ...spacecraft, which detected rolled‐up MHD‐scale vortices generated by the Kelvin‐Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD‐scale and ion‐scale structures of the vortices, and the formation mechanism of the low‐latitude boundary layer (LLBL). The observed KH wavelength (>4 × 104 km) is considerably longer than predicted by the linear theory from the thickness (∼1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high‐latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length ∼100 km and of magnetic shear ∼60° existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvénic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field‐aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk‐flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.
Complex magnetic structures are ubiquitous in turbulent astrophysical plasmas. Such structures can be host to many dynamic processes, such as magnetic reconnection and energy dissipation. Thus, ...revealing the 3D topologies of these structures is necessary. In this study, we propose a new method to reconstruct complex magnetic topologies in quasi-steady space plasmas, by utilizing eight-point measurements of magnetic fields and particles. Such a method, based on the Second-Order Taylor Expansion (SOTE) of a magnetic field, is nonlinear; it is constrained by and , where is from particle moments. A benchmark test of this method, using the simulation data, shows that the method can give accurate reconstruction results within an area about three times the size of a spacecraft tetrahedron. By comparing to the previous First-Order Taylor Expansion (FOTE) method, this method (SOTE) gives similar results for reconstructing quasilinear structures but exhibits better accuracy in reconstructing nonlinear structures. Such a method will be useful to the multi-scale missions, such as the future European Space Agency's "cross-scale" mission and China's "self-adaptive" mission. Also, it can be applied to four-point missions, such as Cluster and the Magnetospheric Multiscale Mission. We demonstrated how to apply this method to the four-point missions. In principle, this method will be useful to study shocks, magnetic holes, dipolarization fronts, and other nonlinear structures in space plasmas.
Whistler waves are believed to play an important role during magnetic reconnection. Here we report the near‐simultaneous occurrence of two types of the whistler‐mode waves in the magnetotail Hall ...reconnection region. The first type is observed in the magnetic pileup region of downstream and propagates away to downstream along the field lines and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type, propagating toward the X line, is found around the separatrix region and probably is generated by the electron beam‐driven whistler instability or Čerenkov emission from electron phase‐space holes. These observations of two different types of whistler waves are consistent with recent kinetic simulations and suggest that the observed whistler waves are a consequence of magnetic reconnection.
Key Points
Two types of whistler waves are observed in the reconnection diffusion region
First one is in pileup region, and second one is around separatrix
Whistlers are the consequences of magnetic reconnection
The solar wind plasma is a fully ionized and turbulent gas ejected by the outer layers of the solar corona at very high speed, mainly composed by protons and electrons, with a small percentage of ...helium nuclei and a significantly lower abundance of heavier ions. Since particle collisions are practically negligible, the solar wind is typically not in a state of thermodynamic equilibrium. Such a complex system must be described through self-consistent and fully nonlinear models, taking into account its multi-species composition and turbulence. We use a kinetic hybrid Vlasov-Maxwell numerical code to reproduce the turbulent energy cascade down to ion kinetic scales, in typical conditions of the uncontaminated solar wind plasma, with the aim of exploring the differential kinetic dynamics of the dominant ion species, namely protons and alpha particles. We show that the response of different species to the fluctuating electromagnetic fields is different. In particular, a significant differential heating of alphas with respect to protons is observed. Interestingly, the preferential heating process occurs in spatial regions nearby the peaks of ion vorticity and where strong deviations from thermodynamic equilibrium are recovered. Moreover, by feeding a simulator of a top-hat ion spectrometer with the output of the kinetic simulations, we show that measurements by such spectrometer planned on board the Turbulence Heating ObserveR (THOR mission), a candidate for the next M4 space mission of the European Space Agency, can provide detailed three-dimensional ion velocity distributions, highlighting important non-Maxwellian features. These results support the idea that future space missions will allow a deeper understanding of the physics of the interplanetary medium.
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