Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session YI3: Eruptions and JetsInvited
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Chair: Auna Moser, General Atomics Room: Oglethorpe Auditorium |
Friday, November 20, 2015 9:30AM - 10:00AM |
YI3.00001: Laboratory identification of MHD eruption criteria in the solar corona Invited Speaker: Clayton E. Myers Ideal magnetohydrodynamic (MHD) instabilities such as the kink\footnote{Hood \& Priest, \textit{Geophys. Astrophys. Fluid Dynamics} \textbf{17}, 297 (1981)} and torus\footnote{Kliem \& T\"or\"ok, \textit{Phys. Rev. Lett.} \textbf{96}, 255002 (2006)} instabilities are believed to play an important role in driving ``storage-and-release" eruptions in the solar corona. These instabilities act on long-lived, arched magnetic flux ropes that are ``line-tied" to the solar surface. In spite of numerous observational and computational studies, the conditions under which these instabilities produce an eruption remain a subject of intense debate. In this paper, we use a line-tied, arched flux rope experiment to study storage-and-release eruptions in the laboratory\footnote{Myers, Ph.D. Thesis, Princeton University (2015).}. An \textit{in situ} array of miniature magnetic probes is used to assess the equilibrium and stability of the laboratory flux ropes. Two major results are reported here: First, a new stability regime is identified where torus-unstable flux ropes fail to erupt. In this ``failed torus" regime, the flux rope is torus-\textit{unstable} but kink-\textit{stable}. Under these conditions, a dynamic ``toroidal field tension force" surges in magnitude, causing the flux rope to contract. This tension force, which is missing from existing eruption models, is the $\mathbf{J}\!\times\!\mathbf{B}$ force between self-generated poloidal currents in the flux rope and the toroidal (guide) component of the vacuum field. Secondly, a clear torus instability threshold is observed in the kink-\textit{unstable} regime. This latter result, which is consistent with existing theoretical\footnote{Olmedo \& Zhang, \textit{Astrophys. J.} \textbf{718}, 433 (2010)} and numerical\footnote{T\"or\"ok \& Kliem, \textit{Astrophys. J.} \textbf{630}, L97 (2005)} results, verifies the key role of the torus instability in driving solar eruptions. [Preview Abstract] |
Friday, November 20, 2015 10:00AM - 10:30AM |
YI3.00002: A new class of three-dimensional ideal-MHD equilibria with current sheets Invited Speaker: Joaquim Loizu Ideal MHD predicts singular current densities in 3D equilibria with nested flux surfaces: a pressure-driven $1/x$ current density that arises around resonant rational surfaces, and a Dirac $\delta$-function current that develops at those surfaces. Only recently have these currents been computed numerically [1], and we provide details of the calculation. We show that locally-infinite shear (i.e. discontinuous rotational-transform) at the resonant surfaces ensures well-defined solutions. Singularities in the current density are allowed in ideal-MHD, but the current passing through any surface must be finite for any physically-acceptable equilibrium model. While the integral of the $\delta$-current density is finite, the $1/x$ current diverges over certain surfaces. This led to the conclusion that pressure gradients cannot exist in the vicinity of rational surfaces and thus that the possible pressure profiles are either fractal [2] or discontinuous [3]. In this talk, we present a new class of 3D, globally-ideal, MHD equilibria with (i) continuously nested surfaces, (ii) arbitrary, continuous and smooth pressure profiles, (iii) arbitrary, 3D boundaries, (iv) without unphysical currents, and which are (v) analytic functions of the boundary [4]. Examples of such equilibria, computed with the SPEC code, are shown and verified against generalized solutions to Newcomb equation, showing excellent convergence. The results imply that a resonant magnetic perturbation can penetrate all the way into the centre of a tokamak without being shielded at the resonant surface, even within ideal MHD.\\[4pt] [1] J. Loizu, S. Hudson, A. Bhattacharjee and P. Helander, Phys. Plasmas 22 022501 (2015) \\[0pt] [2] H. Grad, Phys. Fluids 10 (1):137 (1967)\\[0pt] [3] O. Bruno and P. Laurence, Commun. Pur. Appl. Math. 49 (7):717-764 (1996)\\[0pt] [4] J. Loizu, S. R. Hudson, A. Bhattacharjee, S. Lazerson and P. Helander, letter submitted to Phys. Plasmas [Preview Abstract] |
Friday, November 20, 2015 10:30AM - 11:00AM |
YI3.00003: Exploration of the Kinked Jet in the Crab Nebula with Scaled Laboratory Experiments Invited Speaker: Chikang Li X-ray images from the Chandra X-ray Observatory show that the South-East jet in the Crab nebula changes direction every few years. This remarkable phenomenon is also frequently observed for jets in other pulsar-wind nebulae and in other astrophysical objects. Numerical simulations suggest that it may be a consequence of current-driven, magnetohydrodynamic (MHD) instabilities taking place in the jet, yet that is just a hypothesis without verification in controlled experiments. To that end, we recently conducted scaled laboratory experiments that reproduced this phenomenon. In these experiments, a supersonic plasma jet was generated in the collision of two laser-produced plasma plumes, and this jet was radiographed from the side using 15-MeV and 3-MeV protons. It was observed that if self-generated toroidal magnetic fields around the jet were strong enough, they triggered plasma instabilities that caused substantial deflections throughout the jet propagation, mimicking the kinked jet structure seen in the Crab Nebula. We have modeled these laboratory experiments with comprehensive two- and three-dimensional numerical simulations, which in conjunction with the experiments provide compelling evidence that we have an accurate model of the most important physics of magnetic fields and MHD instabilities in the observed jet in the Crab Nebula. [Preview Abstract] |
Friday, November 20, 2015 11:00AM - 11:30AM |
YI3.00004: Magnetized Jets Driven By the Sun: The Structure of the Heliosphere Revisited Invited Speaker: Merav Opher The classic accepted view of the heliosphere is a quiescent, comet-like shape aligned in the direction of the Sun's travel through the interstellar medium (ISM) extending for thousands of astronomical units (AUs). Here, we show, based on magnetohydrodynamic (MHD) simulations, that the tension (hoop) force of the twisted magnetic field of the Sun confines the solar wind plasma beyond the termination shock and drives jets to the north and south very much like astrophysical jets. These jets are deflected into the tail region by the motion of the Sun through the ISM similar to bent galactic jets moving through the intergalactic medium. The interstellar wind blows the two jets into the tail but is not strong enough to force the lobes into a single comet-like tail, as happens to some astrophysical jets. Instead, the interstellar wind flows around the heliosphere and into the equatorial region between the two jets. As in some astrophysical jets that are kink unstable, we show here that the heliospheric jets are turbulent (due to large-scale MHD instabilities and reconnection) and strongly mix the solar wind with the ISM. The resulting turbulence has important implications for particle acceleration in the heliosphere. The two-lobe structure is consistent with the energetic neutral atom (ENA) images of the heliotail from IBEX where two lobes are visible in the north and south and the suggestion from the Cassini ENAs that the heliosphere is ``tailless.'' [Preview Abstract] |
Friday, November 20, 2015 11:30AM - 12:00PM |
YI3.00005: Laboratory Study of the Shaping and Evolution of Magnetized Episodic Plasma Jets Invited Speaker: Drew Higginson The expansion of hot, dense plasma (100 eV, 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}})$ into vacuum occupied by a strong magnetic field ($\beta \quad =$ P$_{\mathrm{kinetic}}$/P$_{\mathrm{mag}} \quad \approx $ 1) along the expansion axis is a seemingly elementary physics problem, yet it is one that has scarcely been investigated. As well as being a fundamental problem in plasma physics, understanding such a situation is important to provide an explanation of large-scale jets observed in the formation of young stellar objects (YSO). Additionally, the ability to manipulate such a situation (e.g. to optimize x-ray emission) may be essential to the feasibility of recently proposed inertial confinement fusion (ICF) schemes with an imposed magnetic field. To investigate these situations, a CF$_{\mathrm{2}}$ foil is irradiated with the ELFIE laser (10$^{\mathrm{13}}$ W/cm$^{\mathrm{2}}$, 0.6 ns) in an external axial magnetic field of 20 T. As the plasma expands radially it is restricted by magnetic pressure that creates a cavity with a shock at the expansion edge. This shock redirects flow back on axis and creates a strong, stationary, conical shock that collimates the flow into a jet traveling over 1000 km/s and extending many centimeters. The effect of episodic heating (e.g. from variable mass ejection in a YSO, or pulse shaping in ICF) was investigated by irradiating the target with a precursor laser (10$^{\mathrm{12}}$ W/cm$^{\mathrm{2}}$, 0.6 ns) at 9 to 19 ns prior to the main pulse. The addition of this relatively small addition of energy (\textless 20{\%} of the main pulse energy) changed the dynamics of the expansion dramatically by increasing the strength of the conical shock, reducing the forward expansion of the cavity and dramatically increasing emission. We also present MHD simulations that reproduce the experimental observables and help to understand dynamics of jet and cavity formation. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Friday, November 20, 2015 12:00PM - 12:30PM |
YI3.00006: Laboratory Measurements of Linear Electron Acceleration by Inertial Alfv\'en Waves Invited Speaker: J.W.R. Schroeder Alfv\'en waves occur in conjunction with a significant fraction of auroral electron acceleration [1]. Inertial mode Alfv\'en waves ($v_A > v_{te}$) in the auroral magnetosphere ($2-4 R_E$) with perpendicular scales on the order of the electron skin depth ($c/\omega_{pe}$) have a parallel electric field that, according to theory, is capable of nonlinearly accelerating suprathermal electrons to auroral energies [2]. Unfortunately, due to space-time ambiguities of rocket and satellite measurements, it has not yet been possible to fully verify how Alfv\'en waves contribute to the production of accelerated electrons. To overcome the limitations of \emph{in situ} spacecraft data, laboratory experiments have been carried out using the Large Plasma Device (LaPD), an NSF/DOE user facility at UCLA. An Electron Cyclotron Absorption (ECA) diagnostic has been developed to record the suprathermal parallel electron distribution function with 0.1\% precision [3]. The diagnostic records the electron distribution while inertial Alfv\'en waves simultaneously propagate through the plasma. Recent measurements have isolated oscillations of suprathermal electrons at the Alfv\'en wave frequency. Despite complications from boundary effects and the finite size of the experiment, a linear kinetic model has been produced that describes the experimental results. To our knowledge this is the first quantitative agreement between the measured and modeled linear response of suprathermal electrons to an inertial Alfv\'en wave. This verification of the linear physics is a necessary step before the nonlinear acceleration process can be isolated in future experiments. Presently, nonlinear effects cannot be detected because of limited Alfv\'en wave amplitudes. Ongoing work is focused on designing a higher-power antenna capable of efficiently launching larger-amplitude Alfv\'en waves with tunable perpendicular wavenumber and developing a theoretical understanding of the nonlinear acceleration process in LaPD plasma conditions.\\[4pt] [1] C. C. Chaston \emph{et. al.}, Geophys. Res. Lett. 34, L07101 (2007).\\[0pt] [2] C. A. Kletzing, J. Geophys. Res. 99, 11095--11104 (1994).\\[0pt] [3] D. J. Thuecks, F. Skiff, and C. A. Kletzing, Rev. Sci. Instrum. 83, 083503 (2012). [Preview Abstract] |
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