Session BI2: Laboratory Plasma Astrophysics I

Large-Scale Electron Acceleration by Parallel Electric Fields During Magnetic Reconnection

During reconnection in magnetized plasma stress in the magnetic field is reduced through changes in the field line topology. The process is often accompanied by an explosive release of magnetic energy and is implicated in a range of astrophysical phenomena. In the Earth's magnetotail, reconnection energizes electrons up to hundreds of keV and solar flare events can channel up to 50\% of the magnetic energy into the electrons resulting in superthermal populations in the MeV range. Electron energization is also fundamentally important to astrophysical applications yielding a window into the extreme environments. The conventional wisdom has been that magnetic-field-aligned electric field ($E_{||}$) during reconnection are confined to small regions around the diffusion region and along separatrices, and it has been argued that direct acceleration is too small to explain observations. In contrast, here we show that during reconnection powerful energization of electrons by $E_{||}$ can occur over spatial scales which hugely exceed previous theories and simulations. In our kinetic simulation $E_{||}$ is supported by non-thermal and strongly anisotropic features in the electron distributions not permitted in standard fluid formulations, but routinely observed by spacecraft in the Earth's magnetosphere. This allows for electron energization in spatial regions that exceed the regular $d_e$ scale electron diffusion region by at least three orders of magnitude [1]. \\[1ex] [1] J Egedal, et al., Nature Physics 8, 321-324, 2012.   

Ion energization during magnetic reconnection in the RFP laboratory plasma

Particle energization via the conversion of magnetic energy to kinetic energy is a widely occurring phenomena in space, astrophysical, and laboratory plasmas. In the MST Reversed-Field Pinch, a device that magnetically confines a high-temperature plasma, ions are strongly heated during impulsive magnetic reconnection events. Three new experimental observations may help distinguish among theoretical explanations. First, spatially localized spectroscopic measurements of impurity C$^{+6}$ ions reveal that the thermal heating is anisotropic, with the perpendicular temperature always increasing more than the parallel temperature. Second, measurements of neutral particle energy spectra and neutron flux show the generation of a high-energy tail on the distribution function of the majority ions during reconnection events. The high-energy ion density is typically a few percent of thermal ion density, and the high-energy ions have a power-law energy spectrum. Possibly related is the recent observation of acceleration of neutral-beam-injected fast ions to energies above their injection energy (25 keV) during reconnection events. The ion acceleration mechanism may be distinct from the thermal heating mechanism, although both phenomena exhibit characteristics that are clearly dependent on plasma density. Third, spectroscopic measurements of various impurity ions (C, N, O, and Al) made in the MST edge plasma suggest that the change in impurity ion temperature during a reconnection event may have a charge/mass dependence. There are several possible theoretical ion energization mechanisms; these new observations will be compared to predictions from these theories.   

The structure of the magnetic reconnection exhaust boundary

Switch-off slow shocks (SSS) are the key structure of driving the outflow in Petschek's reconnection model. Observations of reconnection in the solar wind in particular seem to suggest that reconnection X-lines and associated exhausts grow to very large scales and resemble the open outflow geometry predicted by Petschek. However, direct observations of SSSs in the Earth's magnetosphere and the solar wind are infrequent. Since it is the release of magnetic energy downstream from the X-line that ultimately drives the outflow rather than the dynamics close to the X-line, the absence of the SSS in observations and kinetic simulations calls into question the conjecture that fast collisionless reconnection actually can scale to very large systems. Thus, a key requirement for demonstrating the fast energy release of reconnection that takes place in large systems is to pin down the specific mechanism driving the Alfvenic outflow. We present a large 2-D reconnection simulation and its companion Riemann problem using a Particle-In-Cell code. The self-generated firehose-sense temperature anisotropy ({\it i.e.,} $T_\| > T_\perp$) by counterstreaming ions is found to be important in determining the structure of reconnection exhausts. This temperature anisotropy slows down the intermediate mode while speeds up the slow mode, and consequently prevents the formation of classical Switch-off Slow shocks in Petschek's model. Instead, the nonlinear coupling between the slow and intermediate waves constitutes the shock transition. A plateau of the firehose stability parameter $1-(\beta_\|-\beta_\perp)/2$ at value 0.25 is observed inside these hybrid waves, which should also be observable in Earth's magnetotail and the solar wind. This special value is significant because it is the degeneracy point of slow and intermediate waves in anisotropic plasmas. The anisotropic Rankine-Hugoniot jump conditions are derived and compared with our simulations, while the pseudo-potentials of shocks are analyzed to explain the dynamics of forming these transition structures. The Wal\'en relation is shown to fail at the core of reconnection exhausts, where the firehose instability is unstable. ({\it Ref: Yi-Hsin Liu, J. F. Drake and M. Swisdak, The structure of the magnetic reconnection exhaust boundary, Phys. of Plasmas, 19, 022110, 2012})   

Demonstration of Anisotropic Fluid Closure Capturing the Kinetic Structure of Magnetic Reconnection

Magnetic reconnection in collisionless plasmas plays an important role in space and laboratory plasmas. Allowing magnetic stress to be reduced by a rearrangement of magnetic line topology, this process is often accompanied by a large release of magnetic field energy, which can heat the plasma, drive large scale flows, or accelerate particles. Reconnection has been widely studied through fluid models and kinetic simulations. While two-fluid models often reproduce the fast reconnection that is observed in nature and seen in kinetic simulations, it is found that the structure surrounding the electron diffusion region and the electron current layer differ vastly between fluid models and kinetic simulations [1]. Recently, using an adiabatic solution of the Vlasov equation, a new fluid closure has been obtained for electrons that relate parallel and perpendicular pressures to the density and magnetic field [2]. Here we present the results of fluid simulation, developed using the HiFi framework [3], that implements new equations of state for guide-field reconnection. The new fluid closure accurately accounts for the anisotropic electron pressure that builds in the reconnection region due to electric and magnetic trapping of electrons. In contrast to previous fluid models, our fluid simulation reproduces the detailed reconnection region as observed in fully kinetic simulations [4]. We hereby demonstrate that the new fluid closure self-consistently captures all the physics relevant to the structure of the reconnection region, providing a gateway to a renewed and deeper theoretical understanding for reconnection in weakly collisional regimes.\\[4pt] [1] Daughton W et al., Phys. Plasmas 13, 072101 (2006).\\[0pt] [2] Le A et al., Phys. Rev. Lett. 102, 085001 (2009). \\[0pt] [3] Lukin VS, Linton MG, Nonlinear Proc. Geoph. 18, 871 (2011). \\[0pt] [4] Ohia O, et al., Phys. Rev. Lett. In Press (2012).   

Plasmoid Instability in High-Lundquist-Number Magnetic Reconnection

Our understanding of magnetic reconnection in resistive magnetohydrodynamics has gone through a fundamental change in recent years. The conventional wisdom is that magnetic reconnection mediated by resistivity is slow in high Lundquist ($S$) plasmas, due to the $S^{-1/2}$ scaling of reconnection rate predicted by the classical Sweet-Parker theory. However, recent studies showed that when $S$ exceeds a critical value $\sim10^{4}$, the Sweet-Parker current sheet is unstable to a super-Alfvenic plasmoid instability, with a growth rate that increases with $S$ [1]. Consequently, the reconnection layer changes to a chain of plasmoids connected by secondary current sheets that, in turn, may become unstable again. Eventually the reconnection layer will tend to a statistical steady state governed by complex dynamics of plasmoid formation and plasmoid loss due to advection and coalescence. The averaged reconnection rate in this regime is nearly independent of $S$ [2,3], and the distribution function $f(\psi)$ of magnetic fluxes $\psi$ in plasmoids follows a power-law $f\sim\psi^{-1}$. When Hall effects are included, the plasmoid instability may trigger onset of Hall reconnection even when the conventional criterion for onset is not satisfied. In addition to the usual single X-point topology of Hall reconnection, our large-scale resistive Hall MHD simulations reveal a novel intermediate regime, where formation of new plasmoids is observed after onset of Hall reconnection [4]. Qualitatively similar results have also been found when resistivity is replaced by hyper-resistivity. Our findings suggest that plasmoid formation may be a generic feature of magnetic reconnection in large systems, regardless of the mechanism of breaking the frozen-in condition. (In collaboration with A. Bhattacharjee and B. P. Sullivan).\\[4pt] [1] N. F. Loureiro, A. A. Schekochihin, and S. C. Cowley, Phys. Plasmas 14, 100703 (2007).\\[0pt] [2] A. Bhattacharjee, Y.-M. Huang, H. Yang, and B. Rogers, Phys. Plasmas 16, 112102 (2009).\\[0pt] [3] Y.-M. Huang and A. Bhattacharjee, Phys. Plasmas 17, 062104 (2010).\\[0pt] [4] Y.-M. Huang, A. Bhattacharjee, and B. P. Sullivan, Phys. Plasmas 18, 072109 (2011).   

Stirring An Unmagnetized Plasma for Magnetorotational Instability Studies

The efficient outward transport of angular momentum in accretion disks is thought to be the result of turbulence generated by the magnetorotational instability (MRI). The MRI arises when a differentially-flowing, conducting fluid is permeated by a weak magnetic field. The instability has been the subject of extensive analytical and numerical investigations for several decades, yet experimental verification of the MRI remains elusive. Recently, a new method for stirring a hot (Te=10 eV), unmagnetized plasma has been demonstrated experimentally [RSI 83, 063502 (2012)], making it possible to study the MRI in a laboratory plasma for the first time. In the experiment, plasma is confined by a cylindrical, axisymmetric, multicusp magnetic field. Azimuthal flows (up to 6 km/s) are driven by JxB torque using biased, heated filaments at a single toroidal position in the magnetized edge. Measurements show that momentum couples viscously from the magnetized edge to the unmagnetized core, and that flow is axisymmetric [PRL 108, 115001 (2012)]. Collisional ion viscosity must overcome the drag due to ion-neutral collisions for the plasma to rotate. Additional electrodes at the inner boundary will create the sheared flow necessary for exciting the MRI. This experiment has already achieved magnetic Reynolds numbers of Rm$\sim $50 and magnetic Prandtl numbers of Pm $\sim$ 0.3-6, which are approaching regimes shown to excite the MRI in local linear analysis and global Hall-MHD numerical simulations [POP 18, 062904 (2011)]. Experiments characterizing the MRI will compare the onset threshold to theoretical and numerical predictions, look for altered velocity profiles due to momentum transport during nonlinear saturation, and identify two fluid effects expected to arise from the Hall term and plasma-neutral interactions (important in protoplanetary accretion disks).