Session NM9: Mini-Conference: Plasma Energization - Interactions Between Fluid and Kinetic Scales IV

Fully implicit, energy-conserving electromagnetic particle-in-cell simulations in multiple dimensions

We discuss a new, implicit 2D-3V particle-in-cell (PIC) algorithm for non-radiative, electromagnetic kinetic plasma simulations, based on the Vlasov-Darwin model.\footnote{Nielson and Lewis, \textit{Methods Comput. Phys.} \textbf{16} p.367 (1976)} The Vlasov-Darwin model avoids radiative noise issues, but is elliptic and renders explicit time integration unconditionally unstable.\footnote{Nielson, Lewis (1976)} Absolutely stable, fully implicit, charge and energy conserving PIC algorithms for both electrostatic and electromagnetic regimes have been recently developed in 1D.\footnote{Chen, Chac\'on, and Barnes, \textit{JCP} \textbf{230} p.7018 (2011)}\textsuperscript{,}\footnote{Chen and Chac\'on, \textit{CPC} \textbf{185} p.2391 (2014)} In this study, we build on these recent successes to develop a multi-D, fully implicit PIC algorithm for the Vlasov-Darwin model.\footnote{Chen and Chacon, \textit{CPC}, submitted (2015)} The algorithm conserves global energy, local charge, and particle canonical-momentum exactly. The nonlinear iteration is effectively accelerated with a fluid preconditioner, allowing the efficient use of large timesteps compared to the explicit CFL. We demonstrate the potential of the approach with various numerical examples in 2D-3V.   

Multiscale Processes Energizing Plasmas during Reconnection: 3D Simulations in preparation for the MMS mission

Magnetic reconnection is a mechanism to convert magnetic energy to particle energy in the form of heat and directed flows. We study here where reconnection and particle energization are found in full 3D models of a reconnecting plasma sheet. Three regions emerge as the primary loci of energy conversion: the separatrices [1], the dipolarization fronts [2] and the electron diffusion region near x-points[3]. We consider two scenarios: one where the exhaust from multiple x-lines forms a plasmoid (a flux rope in 3D) and one where the exhaust encounters pristine unreconnected plasma and forms a pile-up front. A key process intrinsically 3D, not present in 2D, is the development of an instability in the outflow leading to the formation of secondary reconnection sites [2] that further enhance energy conversion. The MMS mission of NASA was launched on March 12 of this year with the stated goal of finding these regions. We will soon know if we are right in predicting these additional regions of dissipation in the reconnection outflow. [1] Lapenta, G, et al. J. Plasma Phys 81.01 (2015): 325810109. [2] Lapenta, G. et al., Nature Physics, 20 July, 2015. [3] Goldman, M. V., et al., Space Sci Rev (2015): 1-38.   

The importance of kinetic ions in macro to micro-scale coupling during flux-rope interactions

Simulation studies of thin kinetic-scale reconnecting current sheets have found the rate of reconnection to be independent of both system-size and the specific mechanism that violates the frozen-in condition for ions and electrons. However, these studies typically neglect the formation of the sheet, and the potential coupling of the kinetic physics to the MHD-scale driver. Here we show for the magnetic island coalescence problem, which naturally includes this formation and coupling, that ion kinetic effects are crucial to describe many key features of this reconnection test-problem: the peak and average rates, pile-up field, outflow velocity, and global evolution of the system. These features can not be accurately decribed by the usual two-fluid models, such as Hall-MHD. The results presented are conceivably important for many reconnecting systems in nature, where macro to micro-scale coupling is important.   

Magnetic pumping of the solar wind

The transport of matter and radiation in the solar wind and terrestrial magnetosphere is a complicated problem involving competing processes of charged particles interacting with electric and magnetic fields. Given the rapid expansion of the solar wind, it would be expected that superthermal electrons originating in the corona would cool rapidly as a function of distance to the Sun. However, this is not observed, and various models have been proposed as candidates for heating the solar wind. In the compressional pumping mechanism explored by Fisk and Gloeckler particles are accelerated by random compressions by the interplanetary wave turbulence. This theory explores diffusion due to spatial non-uniformities and provides a mechanism for redistributing particle. For investigation of a related but different heating mechanism, magnetic pumping, in our work we include diffusion of anisotropic features that develops in velocity space. The mechanism allows energy to be transferred to the particles directly from the turbulence. Guided by kinetic simulations a theory is derived for magnetic pumping. At the heart of this work is a generalization of the Parker Equation to capture the role of the pressure anisotropy during the pumping process.   

Electron energization during asymmetric magnetic reconnection

Bulk electron heating and energetic electron generation during asymmetric reconnection are studied with space observations, laboratory measurements, and numerical simulations. In space, the increase of the bulk electron temperature is about two percent of the incoming magnetic energy, which is consistent with the previous report by Phan et al. 2013 [1]. During storm time events, the energy increase in the electron tail population is larger than that of the bulk electrons, which indicates that a significant incoming magnetic energy is converted to the energetic electrons in these events. In laboratory, the electron temperature increase is about 5 percent of the incoming magnetic energy, which is more consistent with the recent PIC simulation results [2]. The electron temperature profile becomes asymmetric with a higher temperature on the low-density side. Where and how electrons are energized during asymmetric reconnection will be discussed by using data from 2D numerical simulations.\\[4pt] [1] Phan et al. Geophys. Res. Lett. 40, 4475 (2013).\\[0pt] [2] Shay et al. Phys. Plasmas 21, 122902 (2014).   

Study of Plasma Energization during Magnetic Reconnection in the FLARE (Facility for Laboratory Reconnection Experiments)

Various regimes or \lq\lq phases" are identified in a magnetic reconnection \lq\lq phase diagram" which classifies different coupling mechanisms from the global system scales to the local dissipation scales [H. Ji \& W. Daughton, Phys. Plasmas {\bf 18}, 111207 (2011)]. The FLARE device (http://flare.pppl.gov) is a new intermediate-scale plasma experiment under construction at Princeton to provide access to all of these phases directly relevant to space, solar, astrophysical, and fusion plasmas. Study of plasma energization during magnetic reconnection is one of major topics for the FLARE facility, which is planned to be a user facility. The motivating major physics questions regarding plasma energization and the planned collaborative research on these topics will be presented and discussed.   

Mini-Conference Poster Preview

Lead authors for each mini-conference poster will be given the opportunity to present a short 2 to 3 minute verbal summary of their poster presentation.