Session NI2: Reconnection: Experiments and Observations

Anomalous heating and plasmoid formation in pulsed power driven magnetic reconnection experiments

Magnetic reconnection is an important process occurring in various plasma environments, including high energy density plasmas. In this talk we will present results from a recently developed magnetic reconnection platform driven by the MAGPIE pulsed power generator (1 MA, 250 ns) at Imperial College London [1,2]. In these experiments, supersonic, sub-Alfv\'enic plasma flows collide, bringing anti-parallel magnetic fields into contact and producing a well-defined, elongated reconnection layer. This layer is long-lasting ($>$200 ns, $>$ 10 hydrodynamic flow times) and is diagnosed using a suite of high resolution, spatially and temporally resolved diagnostics which include laser interferometry, Thomson scattering and Faraday rotation imaging. We observe significant heating of the electrons and ions inside the reconnection layer, and calculate that the heating must occur on time-scales far faster than can be explained by classical mechanisms. Possible anomalous mechanisms include in-plane electric fields caused by two-fluid effects, and enhanced resistivity and viscosity caused by kinetic turbulence. We also observe the repeated formation of plasmoids in the reconnection layer, which are ejected outwards along the layer at super-Alfv\'enic velocities. The O-point magnetic field structure of these plasmoids is determined using in situ magnetic probes, and these plasmoids could also play a role in the anomalous heating of the electrons and ions. In addition, we present further modifications to this experimental platform which enable us to study asymmetric reconnection or measure the out-of-plane magnetic field inside the plasmoids. \newline \newline [1] Suttle, L. G., Hare, J. D., Lebedev, S. V. (2016). Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows. PRL, 116, 225001 \newline \newline [2] Hare, J. D., Suttle, L., Lebedev, S. V. (2017). Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment. PRL, 118, 85001 \newline \newline This work was performed in collaboration with: S. V. Lebedev, L. G. Suttle, N. F. Loureiro, A. Ciardi, G. C. Burdiak, J. P. Chittenden, T. Clayson, S. J. Eardley, C. Garcia, J. W. D. Halliday, N. Niasse, T. Robinson, R. A. Smith, N. Stuart, F. Suzuki-Vidal, G. F. Swadling, J. Ma, and J. Wu   

Experimental demonstration of collisionless plasmoids at the electron scale during high Lundquist number magnetic reconnection.

The dynamics of magnetic reconnection can vary greatly depending on the collisionality of the plasma. While resistivity alone provides force balance during collisional reconnection, it cannot account for the reconnection rate during collisionless reconnection. As the collisionality decreases, kinetic processes, such as electron pressure anisotropy\footnote{Egedal J., Nature Phys., \textbf{8}, 321 (2012).} can develop unimpeded and provide pressure balance across the current sheet\footnote{Le A., Phys. Plasmas \textbf{21}, 012103 (2014).}. Recent PIC simulations have shown that more unique structures, driven by pressure anisotropy, can develop only if the electrons do not collide as they traverse the reconnection region\footnote{Le A., J. Plasma Phys. \textbf{81}, 305810108 (2015).}. More precisely, this collisionless regime exists when the characteristic Lundquist number is above $S>10 \epsilon (m_{i}/m_{e}) L/d_{i}$ (for anti-parallel reconnection), where $\epsilon<1$ is an experimental scale factor and $L$ is the system size. The Terrestrial Reconnection EXperiment (TREX) has been specifically designed to operate in this regime, where the Lundquist number is set by the applied reconnection drive. Early experiments in low collisional plasmas with $S\sim10^3$ showed evidence of magnetic island formation (plasmoids) occurring below characteristic ion length scales\footnote{Olson J., Phys. Rev. Lett. \textbf{116}, 255001 (2016).}. The experiments demonstrate that the plasmoid instability is still active for relatively small system size compared to predictions from either extended MHD or fully kinetic PIC simulations. Furthermore, in recent experiments with $S > 10^4$, we document a transition to a regime where the current sheet shrinks to the electron scale, $\delta_{J} \sim 2-4c/\omega_{pe}$, consistent with results from kinetic simulations showing that such electron layers are related to strong pressure anisotropy.   

Electron heating and acceleration during magnetic reconnection

Magnetic reconnection is thought to be an important driver of energetic particles in a variety of astrophysical phenomena such as solar flares and magnetospheric storms. However, the observed fraction of energy imparted to a nonthermal component can vary widely in different regimes. We use kinetic particle-in-cell (PIC) simulations to demonstrate the important role of the non-reversing (guide) field in controlling the efficiency of electron acceleration in collisionless reconnection. In reconnection where the guide field is smaller than the reconnecting component, the dominant electron accelerator is a Fermi-type mechanism that preferentially energizes the most energetic particles. In strong guide field reconnection, the field-line contraction that drives the Fermi mechanism becomes weak. Instead, parallel electric fields are primarily responsible for driving electron heating but are ineffective in driving the energetic component of the spectrum. Three-dimensional simulations reveal that the stochastic magnetic field that develops during 3D guide field reconnection plays a vital role in particle acceleration and transport. The reconnection outflows that drive Fermi acceleration also expel accelerating particles from energization regions. In 2D reconnection, electrons are trapped in island cores and acceleration ceases, whereas in 3D the stochastic magnetic field enables energetic electrons to leak out of islands and freely sample regions of energy release. A finite guide field is required to break initial 2D symmetry and facilitate escape from island structures. We show that reconnection with a guide field comparable to the reconnecting field generates the greatest number of energetic electrons, a regime where both (a) the Fermi mechanism is an efficient driver and (b) energetic electrons may freely access acceleration sites. These results have important implications for electron acceleration in solar flares and reconnection-driven dissipation in turbulence.   

Anisotropic Electron Tail Generation during Tearing Mode Magnetic Reconnection

Magnetic reconnection (MR) plays an important role in particle transport, energization, and acceleration in space, astrophysical, and laboratory plasmas. In the MST RFP, discrete MR events release large amounts of energy from the equilibrium magnetic field, a large fraction of which is transferred to the ions in a non-collisional process. Key features are anisotropic heating, mass and charge dependence, and energetic ion tail formation. Unlike the ions, the thermal electron temperature decreases at MR events, which is consistent with enhanced electron heat transport due to increased magnetic stochasticity. However, new high-speed x-ray spectrum measurements reveal transient formation of a non-Maxwellian energetic electron tail during MR. The energetic tail is characterized by a power-law, E$^{\mathrm{-\gamma }}$, with the spectral index ($\gamma )$ decreasing from 4.2 to 2.2 at MR, and then increasing rapidly to 6.8 due to increased stochastic transport. The x-ray emission peaks in a radial view and is symmetric in the toroidal direction, indicating an anisotropic electron tail is generated. The toroidal symmetry of the electron tail implies runaway acceleration is not a dominant process, consistent with the net emf, $\eta $J$_{\mathrm{ll}}$, being smaller than the Dreicer field. Modeling of bremsstrahlung emission shows that a power-law electron tail distribution that is localized near the magnetic axis will yield strong perpendicular anisotropy, consistent with x-ray measurements in the radial and toroidal views. A strong correlation between high energy x-ray flux and tearing mode dynamics suggests a turbulent mechanism is active. This implies that the electron tail formation most likely results from a turbulent wave-particle interaction.   

Experimental verification of the role of electron pressure in fast magnetic reconnection with a guide field

Magnetic reconnection enables explosive conversion of magnetic field energy to plasma kinetic energy in space and laboratory plasmas. In many reconnecting plasmas in space, solar, and laboratory plasmas, reconnection proceeds in the presence of a finite guide field (GF) such that the magnetic field lines meet at an angle less than 180°, and in magnetic fusion devices the guide field can be the largest component of the field. We report detailed laboratory observations of the structure of reconnection current sheets in a two-fluid plasma regime with a guide magnetic field. We observe and quantitatively analyze the quadrupolar electron pressure variation in the ion-diffusion region, as originally predicted by extended magnetohydrodynamics simulations. The projection of the electron pressure gradient parallel to the magnetic field contributes significantly to balancing the parallel electric field, and the results demonstrate how parallel and perpendicular force balance are coupled in guide field reconnection and confirm basic theoretical models of the importance of electron pressure gradients for obtaining fast magnetic reconnection. I discuss connections to observations of reconnection with finite guide field by spacecraft missions, and implications for two-fluid reconnection in magnetic fusion devices.   

Diffusion region in magnetopause reconnection observed by the MMS mission

The diffusion region is the primary location where the plasmas are energized to dissipate the magnetic energy in reconnection. The NASA Magnetospheric Multiscale (MMS) mission, capable of resolving sub-gyroscales of both electrons and ions, has created new frontiers in the state-of-the-art understanding of the diffusion region. The MMS detection of reconnection at Earth's magnetopause will be discussed to highlight the roles of demagnetized particle orbits and wave fluctuations in the reconnection dynamics. When the guide field is significantly weaker than the reconnecting magnetic field, the reconnection current layer is gyro-resistive and the electron distribution functions exhibit strong finite-gyroradius effects with crescent and counterstreaming characteristics. When the guide field is comparable to the reconnecting component, the electron jets are mainly the E cross B drift due to the polarization electric field and the guide magnetic field, and the energy conversion at the jet reversal is dominated by the wave electric field near the lower hybrid frequency. Insensitive to the guide-field, the dense magnetosheath electrons in the reconnection exhaust are transported, by wave turbulence, across the magnetospheric separatrix to modify the plasma properties and field structures in the magnetosphere. The MMS results will be compared with available laboratory measurements from the Magnetic Reconnection Experiment in Princeton, and challenges in diffusion region physics will be discussed.