Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this ...dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub‐ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O‐lines) and the separatrices. Inside each current filament, kinetic‐scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X‐lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O‐lines but not X‐lines.
Key Points
Strong current, turbulence, and energy dissipation at O‐lines
No current, turbulence, and energy dissipation at X‐lines
The current‐driven turbulence at O‐lines leads to dissipation
Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how ...electrons are accelerated to high energies and what processes determine the power-law index
δ
. Here, we review previous observations of the power-law index
δ
in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the ‘above-the-looptop’ solar hard X-ray source and the plasma sheet in Earth’s magnetotail), the spectra are typically soft (
δ
≳
4
). This is in contrast to the typically hard spectra (
δ
≲
4
) that are observed in coincidence with shocks. The difference implies that shocks are more efficient in producing a larger non-thermal fraction of electron energies when compared to magnetic reconnection. A caveat is that during active times in Earth’s magnetotail,
δ
values seem spatially uniform in the plasma sheet, while power-law distributions still exist even in quiet times. The role of magnetotail reconnection in the electron power-law formation could therefore be confounded with these background conditions. Because different regions have been studied with different instrumentations and methodologies, we point out a need for more systematic and coordinated studies of power-law distributions for a better understanding of possible scaling laws in particle acceleration as well as their universality.
ABSTRACT Intermittent structures, such as thin current sheets, are abundant in turbulent plasmas. Numerical simulations indicate that such current sheets are important sites of energy dissipation and ...particle heating occurring at kinetic scales. However, direct evidence of dissipation and associated heating within current sheets is scarce. Here, we show a new statistical study of local electron heating within proton-scale current sheets by using high-resolution spacecraft data. Current sheets are detected using the Partial Variance of Increments (PVI) method which identifies regions of strong intermittency. We find that strong electron heating occurs in high PVI (>3) current sheets while no significant heating occurs in low PVI cases (<3), indicating that the former are dominant for energy dissipation. Current sheets corresponding to very high PVI (>5) show the strongest heating and most of the time are consistent with ongoing magnetic reconnection. This suggests that reconnection is important for electron heating and dissipation at kinetic scales in turbulent plasmas.
Data from the Cassini spacecraft identify strong electron acceleration as the solar wind approaches the magnetosphere of Saturn. This so-called bow shock unexpectedly occurs even when the magnetic ...field is roughly parallel to the shock-surface normal. Knowledge of the magnetic dependence of electron acceleration will aid understanding of supernova remnants.
Chen and Wolf (1999) used a thin‐filament theory to construct a 2D model of a bursty bulk flow (BBF) motion inside the plasma sheet. The modeling revealed that the low‐entropy filament overshoots its ...equilibrium position and executes a heavily damped oscillation about that position. In this letter we demonstrate, for the first time, the multiple overshoot and rebound of a BBF observed by the five THEMIS probes on 17 March 2008 just after 10:22 UT. We found that the BBF oscillatory braking was accompanied by interlaced enhancements and depletions of radial pressure gradients. The earthward and tailward flow bursts caused formation of vortices with opposite sense of rotation.
It has long been suggested that whistler waves play an active role during magnetic reconnection. However, all previous observations were based on case studies and could not give a detailed picture as ...to where the whistler waves occur in the reconnection region. In this paper, a statistical study by using the Cluster data is performed to investigate the spatial distribution and the occurrence rate of whistler waves in the magnetotail reconnection region. It is found that the occurrence rate of the whistler waves is large in the separatrix region (|Bx/B0| > 0.4) and in the pileup region (|Bx/B0| < 0.2, |θ| > 45°, where θ = arctan(Bz/Bx)), but is very small in the vicinity of the X‐line. These statistical results are well consistent with recent kinetic simulations and with previous observational case studies.
Key Points
Occurrence rate of the whistler waves is revealed in reconnection region
Occurrence rate is large in the separatrix and pileup region, but low in the X‐line region
Statistical results are well consistent with the previous case study and kinetic simulations
In this study, we apply a new method—the first‐order Taylor expansion (FOTE)—to find magnetic nulls and reconstruct magnetic field topology, in order to use it with the data from the forthcoming MMS ...mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the FOTE method can find a null both inside and outside the tetrahedron and also deduce its drift velocity. In addition, the FOTE method can (1) avoid limitations of the PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3‐D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2‐D (X and O); (3) reconstruct the magnetic field topology. We quantitatively test the accuracy of FOTE in positioning magnetic nulls and reconstructing field topology by using the data from 3‐D kinetic simulations. The influences of SC separation (0.05~1 di) and null‐SC distance (0~1 di) on the accuracy are both considered. We find that (1) for an isolated null, the method is accurate when the SC separation is smaller than 1 di, and the null‐SC distance is smaller than 0.25~0.5 di; (2) for a null pair, the accuracy is same as in the isolated‐null situation, except at the separator line, where the field is nonlinear. We define a parameter ξ ≡ |( λ1 + λ2 + λ3 )|/|λ|max in terms of the eigenvalues (λi) of the null to quantify the quality of our method—the smaller this parameter the better the results. Comparing to the previously used parameter (η≡|∇ ⋅ B|/|∇ × B|), ξ is more relevant for null identification. Using the new method, we reconstruct the magnetic field topology around a radial‐type null and a spiral‐type null, and find that the topologies are well consistent with those predicted in theory. We therefore suggest using this method to find magnetic nulls and reconstruct field topology with four‐point measurements, particularly from Cluster and the forthcoming MMS mission. For the MMS mission, this null‐finding algorithm can be used to trigger its burst‐mode measurements.
Key Points
We develop a new method to find magnetic nulls and reconstruct field topology
We quantitatively test this method, and compare it with the Poincare index
This method is useful for MMS mission, particularly for triggering burst mode
ABSTRACT
The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the ...interaction between a supercritical, perpendicular shock and pre-existing, fully developed plasma turbulence, employing a novel combination of magnetohydrodynamic and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed. We find that the behaviour of shock ripples, i.e. shock surface fluctuations with short (a few ion skin depths, di) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock–turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multispacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterized by the presence of shock waves interacting with plasma turbulence.
Plasmas in Earth's outer magnetosphere, magnetosheath, and solar wind are essentially collisionless. This means particle distributions are not typically in thermodynamic equilibrium and deviate ...significantly from Maxwellian distributions. The deviations of these distributions can be further enhanced by plasma processes, such as shocks, turbulence, and magnetic reconnection. Such distributions can be unstable to a wide variety of kinetic plasma instabilities, which in turn modify the electron distributions. In this paper, the deviation of the observed electron distributions from a bi‐Maxwellian distribution function is calculated and quantified using data from the Magnetospheric Multiscale spacecraft. A statistical study from tens of millions of electron distributions shows that the primary source of the observed non‐Maxwellianity is electron distributions consisting of distinct hot and cold components in Earth's low‐density magnetosphere. This results in large non‐Maxwellianities at low densities. However, after performing a statistical study we find regions where large non‐Maxwellianities are observed for a given density. Highly non‐Maxwellian distributions are routinely found at Earth's bowshock, in Earth's outer magnetosphere and in the electron diffusion regions of magnetic reconnection. Enhanced non‐Maxwellianities are observed in the turbulent magnetosheath, but are intermittent and are typically not correlated with local processes. The causes of enhanced non‐Maxwellianities are investigated.
Key Points
Electron non‐Maxwellianity is computed for 6 months of data (85 million electron distributions)
Electron non‐Maxwellianity is typically large in the magnetosphere due to hot and cold electron populations
Enhanced non‐Maxwellianity is found in reconnection regions, the bowshock, and magnetosheath turbulence
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