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Past Seminars

Numerical Simulations of the Initiation of Coronal Mass Ejections

Yuhong Fan: HAO/NCAR

Friday February 18 2011, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Eruptive flares and coronal mass ejections (CMEs) are believed to correspond to a sudden, explosive release of the free magnetic energy stored in the previously quasi-equilibrium, twisted/sheared coronal magnetic fields. However, the detailed magnetic field structure for the eruption precursors and the physical causes for their sudden disruption remain fundamental unanswered questions under investigation. I present three-dimensional MHD simulations of the evolution of the magnetic field in the corona where the emergence of a twisted magnetic flux rope is driven at the lower boundary into a pre-existing coronal potential arcade field. Through a sequence of simulations in which a varying amount of twisted flux is transported into the corona before the emergence is stopped, I investigate the conditions that lead to a dynamic eruption of the resulting coronal flux rope. It is found that the eruption is triggered when the flux rope in the corona rises to a critical height where the corresponding potential field declines with height at a sufficiently steep rate, a mechanism consistent with the onset of the torus instability. The simulations suggest that S (or inverse S) shaped current sheets develop under the flux rope during the quasi-static phase before the eruption, and reconnections in the current sheet effectively reduce the anchoring of the flux rope, allowing it to rise quasi-statically to the critical height, and the dynamic eruption ensues. Through tracing reconnected field lines during the eruption, the evolution and morphology of the X-ray post-flare loops and their foot-points corresponding to the flare ribbons are deduced, which reproduce some of the commonly observed features associated with eruptive flares in regions with pre-existing X-ray sigmoids. I also present preliminary results from MHD simulations that model qualitatively the magnetic field evolution of the eruptive flare occurred on December 13, 2006 in the emerging delta-sunspot region NOAA 10930 observed by the Hinode satellite.

THEMIS-C Observations of Ion and Electron Velocity Signatures within Solar Wind Reconnection Exhausts

Stefan Eriksson: Laboratory for Atmospheric & Space Physics (LASP)

Friday February 11 2011, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

We present the first joint electron and ion velocity observations from solar wind reconnection exhausts as recorded by TH-C in 2009-2010. The MVAB method is used to find the reference frame of the exhaust where L and N correspond to eigenvectors of maximum and minimum field variance, and M=NxL. Many cases (60) now confirm for the first time that the electrons match the measured ion velocity signature along the jet direction (VL) in the solar wind rest frame of the exhaust. This is expected within exhaust channels where ions and electrons are predicted to move together at the ExB drift. We also present examples where electrons move faster than ions (VLe>VLi) at both exhaust edges. Initial results suggest that a majority of events do not display electrons moving toward the X-line at the exhaust edges. This feature is either too thin to be resolved by the 3-s (1200 km at 400 km/s solar wind) instrument resolution or else is only present closer to the X-line consistent with a general lack of Hall magnetic field signatures at solar wind exhausts. Ions and electrons are generally consistent with the predicted V=V0+/-dV velocity at the two correlated (+dV) and anti-correlated (-dV) exhaust edges. V0 is an external reference velocity and dV is derived from the Walen relation based on electron density and proton mass [Paschmann et al., 1986]. However, we show a case where ions and electrons seemed to follow separate +/- branches of the predicted Walen relation on one side of an exhaust. The electrons (ions) displayed a negative (positive) correlation between VL and BL when a positive correlation was expected for both. This event and cases of faster electron than ion speeds at both exhaust edges suggest that the two species may display separate turn-over points of the VL-BL phase transition within exhausts.

Control of gradient-driven instabilities through nonlinear interaction with Alfven waves

Troy Carter: UCLA

Thursday January 27 2011, 11:00 - 12:00pm in Gamow Tower, Room F931 Show Abstract

Recent progress and remaining challenges for understanding the solar dynamo

Mausumi Dikpati: High Altitude Observatory

Friday January 21 2011, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Abstract: Astrophysical bodies -- planets, stars and galaxies -- havetheir own dynamos that generate, maintain and evolve their magnetic fields. Best known to us are the dynamos of the earth's interior and the Sun. These have certain common features, such as magnetic polarity reversals, but they operate in very different physical regimes. The geodynamo operates in a fully nonlinear magnetohydrodynamic regime in which primarily the magnetic fields govern the dynamics, while the Sun works primarily in a hydrodynamical regime. That is, the Sun's global differential rotation, meridional circulation and some smaller scale turbulent motions provide dynamo action, and the back-reaction from induced magnetic fields affects them relatively little. This difference means that a kinematic 'mean-field' approach to the solar dynamo can be very successful, but won't work for the geodynamo.

Over the past half-century, solar dynamo theory has proceeded along two parallel tracks: axisymmetric mean-field models, particularly the so-called 'flux-transport' models, and full 3D MHD models. Breakthroughs using mean-field models include explaining the solar cycle period, how the fields reverse, and what features can be predicted. Full 3D MHD models now produce cyclic evolution of the fields. I will review recent developments for both model types.

The biggest remaining challenge is how to include the effects of unresolved small scale processes, such as the rising of magnetic flux-tubes and MHD turbulence. Current full 3D MHD models are themselves mean-field models -- very sophisticated but very expensive -- that are very hard to use to advance our understanding of the solar dynamo. But axisymmetric kinematic flux-transport models also have limitations. There is a third class of model, intermediate in complexity and expense, that offers the opportunity to extend our understanding of the solar cycle by explaining global departures from axisymmetry, such as 'active longitudes', 'sector boundaries', and 'tilted dipole' structures. I will describe how this model can be built as a generalization of axisymmetric flux transport dynamo models.

Challenging problems in kinetic simulation of turbulence and transport in tokamaks

Yang Chen: CIPS - University of Colorado at Boulder

Friday December 10 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Transport in tokamaks is anomalous, caused by small scale plasma turbulence. Particle-in-Cell simulations have been the primary tool for studying micro-turbulence and predicting the anomalous transport level in future devices such as ITER. Many people believe that now is the time for pushing for truly first principle simulations of tokamak plasmas on the transport time scale, thanks to the development of the gyrokinetic model and the delta-f method, and supercomputers with tens of thousands of processors. In this talk I will describe the status of a gyrokinetic delta-f PIC code GEM, explain the unique algorithm used in GEM that solves numerical difficulties arising from the fast electron motion along the magnetic field, and present future plans for GEM development. I will discuss what I perceive to be the most important challenges to a transport time scale simulation, namely (1) the difficulty with determining long wavelength radial electric field in gyrokinetics and (2) lack of scale separation in a global simulation. I will speculate on ways to solve these challenging problems.

Role of waves and instabilities in magnetic reconnection

Johanan Lapenta: CIPS - University of Colorado at Boulder

Friday December 03 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Reconnection is a key process in laboratory and astrophysics, it converts vast amounts of magnetic energy into kinetic energy, efficiently and fast. The spectacular events of reconnection, in solar flares, in geomagnetic storms, in the jets from supermassive black holes are just examples of sites where tremendous energies are converted into relativistic particles and heat. The community has been faced for decades to explain why this is possible. According to standard macroscopic theory, reconnection should be slow. To name an example a solar flare should last decades not minutes. A vast gulf of several orders of magnitude separate our understanding from reality. Recently, great advances have been made by going beyond macroscopic theory to include kinetic scale events. The fascinating discovery is that kinetic scales do not simply act as turbulent noise akin to collisions, as it had always been imagined. Rather, kinetic effects change profoundly the system at large scales. Here we focus on one type of processes, the instabilities dues to drifts in the region of reconnection: lower hybrid drift instability, Buneman instability and anisotropy-driven instabilities.

[1] G. Lapenta, J.U. Brackbill, Nonlinear Evolution of the Lower Hybrid Drift Instability: Current Sheet Thinning and Kinking, Physics of Plasmas, 9, 1544-1554, 2002.

[2] P. Ricci, J.U. Brackbill, W.S. Daughton, G. Lapenta, Influence of the Lower-Hybrid Drift Instability on the onset of Magnetic Reconnection, Physics of Plasmas, 11, 4489-4500, 2004.

[3] W. Daughton, G. Lapenta, P. Ricci, Nonlinear Evolution of the Lower-hybrid Drift Instability in a Current Sheet, Physical Review Letters, 93, 105004, 2004.

[4] G. Lapenta, J. King, Study of Current Intensification by Compression in the Earth Magnetotail, Journal of Geophysical Research, 112, A12204, doi:10.1029/2007JA012527, 2007.

[5] G. Lapenta, Large scale momentum exchange by microinstabilities: a process happening in laboratory and space plasmas, Physica Scripta, 80, 035507, 2009.

Breaking field lines during magnetic reconnection: it's turbulent electron viscosity and not anomalous resistivity

Haihong Che: CIPS - University of Colorado at Boulder

Friday November 05 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

The dissipation mechanism that breaks magnetic field lines during reconnection has remained a mystery since the first models of reconnection were proposed in the 1950s. Classical resistivity is too small to explain reconnection observations in tokamak sawteeth, the solar corona and heliosphere. 3-D particle-in-cell simulations of magnetic reconnection reveal that strong currents and associated high electron-ion streaming velocities that develop near the x-line can drive instabilities. The electron scattering caused by this turbulence produces an enhanced drag, "anomalous resistivity", that has been widely invoked as the dissipation mechanism. We have demonstrated with simulations and analytic modeling that during low-$\beta$ reconnection with a guide field that electron current layers become strongly turbulent. The surprise, however, is that the turbulence driven by an electron sheared-flow instability completely dominates traditional streaming instabilities and the associated turbulent driven strong transverse momentum transport, dubbed "anomalous viscosity", balances the reconnection electric field and therefore breaks field lines. The turbulence modestly enhances the rate of reconnection. This instability was not seen in earlier simulations because of the limited scale size of earlier computational domains. The instability is electromagnetic, is part of the whistler branch and therefore falls below the electron cyclotron frequency. The ions play no significant role. A second surprise is that a guide field is required for the instability to exist so that reconnection with a guide field exhibits stronger turbulence than anti-parallel reconnection. Signatures of this turbulence that could be explored in laboratory reconnection experiments and satellite observations are discussed.

Natural Fueling of the Core and Edge in a Tokamak Fusion Reactor

Weigang Wan: CIPS - University of Colorado at Boulder

Friday October 29 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

A natural fueling mechanism that helps to maintain the main core deuterium and tritium (DT) density profiles in a tokamak fusion reactor is presented. In H-mode plasmas dominated by ion-temperature gradient (ITG) driven turbulence, cold DT ions near the edge will naturally pinch radially inward towards the core. The mechanism is investigated using the gyrokinetic turbulence code GEM and is analyzed using quasilinear theory. At the edge, this pinch effect of cold ions could help to explain the pedestal density buildup. Recent DEGAS 2 calculations indicate the neutrals in the pedestal are colder than the background ions. We have shown that near to the pedestal top the pinch flow velocity of recycling ion source is significantly higher than that of the outgoing main ions, and is dependent on its cold temperature.

Structure of an Exploding Laser-Produced Plasma

Andrew Collette: CCLDAS/LASP/CU

Friday October 15 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

The behavior of expanding dense plasmas has long been a topic of interest in space plasma research, particularly in the case of expansion within a magnetized background plasma. Expansion perpendicular to B causes a wide range of effects, including a 'diamagnetic bubble' or localized reduction of the background field, as well as visible periodic structures on the expanding plasma surface. A recent series of experiments at the UCLA Large Plasma Device (LaPD) studied these phenomena via a laser-produced plasma immersed in a large magnetized background plasma. The structure of the expanding plasma is diagnosed in three dimensions via a high-resolution in-plasma probe drive. Currents within the expanding plasma are found to have complex structure in three dimensions; in particular, an unexpected current system along the background field was discovered at the cavity surface. In addition to measurement of the plasma structure, the time behavior of large-scale periodic structures on the plasma surface was investigated via two-probe correlation analysis, revealing that the structures are static and translate with the bubble across the background field.

The Equations of State in Collisionless Reconnection and their Implication for Electron Energization

Jan Egedal: MIT

Wednesday October 13 2010, 11:00 - 12:00pm in Gamow Tower, Room F931 Show Abstract

Wind and Cluster spacecraft observations of reconnecting current sheets in the Earth`s magnetotail show strong electron temperature anisotropy. This anisotropy is accounted for in a solution of the Vlasov equation that was recently derived for general reconnection geometries with magnetized electrons in the limit of fast transit time [1]. A necessary ingredient is a parallel electric field structure, which maintains quasi-neutrality by regulating the electron density, traps a large fraction of thermal electrons, and heats electrons in the parallel direction. Based on the expression for the electron phase space density, equations of state provide a fluid closure that relates the parallel and perpendicular pressures to the density and magnetic field strength [2]. This new fluid model agrees well with fully kinetic simulations of guide-field reconnection, where the parallel electron temperature becomes many times greater than the perpendicular temperature. In addition, the equations of state relate features of the electron diffusion region that develop during anti-parallel reconnection to the upstream electron beta. They impose strong constraints on the electron Hall currents and magnetic fields [3]. For plasmas with low electron beta gradients in the anisotropic pressure can support large parallel electric fields over extended regions. This is important for energization of super-thermal electrons in the Earth magnetotail [4] and perhaps also for fast electrons observed during reconnection events at the sun.

[1] J. Egedal, N. Katz, et al., J. Geophys. Res. 113, A12207 (2008).

[2] A. Le, J. Egedal, et al., Phys. Rev. Lett., 102, 085001 (2009).

[3] A. Le, J. Egedal, et al., Geophys. Res. Lett. 37, L03106 (2010).

[4] J. Egedal, A. Le, et al., Geophys. Res. Lett. 37, L10102 (2010).

High-energy gamma-ray emission in compact binaries

: CIPS - University of Colorado at Boulder

Friday October 01 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Four variable gamma-ray sources (GeV-TeV) have been associated with binary systems in our Galaxy: the "microquasar" Cygnus X-3 and the "gamma-ray binaries" LS I +61 303, LS 5039 and PSR B1259-63. These objects are all composed of a massive companion star and a compact object of unknown nature, possibly a young pulsar or an accreting black hole. After a brief introduction on gamma-ray astronomy, I will present a comprehensive theoretical model for the high-energy gamma-ray emission and variability in these systems. In this model, the high-energy radiation is produced by inverse Compton scattering of stellar photons on ultra-relativistic electron-positron pairs injected by a young pulsar in gamma-ray binaries and in a relativistic jet in microquasars. I will show that this model explains well the TeV gamma-ray emission observed in LS 5039, but cannot account for the gamma-ray emission in LS I +61 303 and PSR B1259-63. Other processes may dominate in these more complex systems. In Cygnus X-3, the gamma-ray radiation is convincingly reproduced by relativistic Doppler-boosted Compton emission of pairs in a jet. Gamma-ray binaries and microquasars provide a novel environment for the study of pulsar winds and relativistic jets at very small spatial scales.

Kinetic Theory and Simulation of Component Magnetic Reconnection

Marty Goldman, David Newman and Giovanni Lapenta: CIPS - University of Colorado at Boulder

Friday September 17 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

A necessary condition for magnetic reconnection to occur is the breaking of the "frozen-in" condition for particles flowing in with field lines towards the current sheet separating oppositely-directed magnetic fields. Early simulations of magnetic reconnection relied on resistive MHD to unfreeze ions from field lines by permitting diffusion. Later, the so-called Hall term - usually omitted from MHD - was shown to give rise to separate electron and ion "diffusion" regions. Next, full kinetic (PIC) simulations suggested that kinetic wave turbulence and pressure agyrotropy may affect the reconnection. Most of these simulations were in 2D and assumed antiparallel rather than component reconnection, in which there is an initial out-of-plane magnetic guide field, Bg. We examine the consequences of introducing a realistic initial Bg into 2D and 3D implicit PIC simulations with physical ion-to-electron mass-ratio (1836). Among the new discoveries to be described are the deflection of elongated midplane electron-velocity-jets and the disruption of acompanying highly elongated external "diffusion" regions by very small guide fields commonly found in the magnetosphere. Other electron-scale features of reconnection to be discussed include electron velocity distributions, kinetic instabilities and the spatial structure of electric fields. A number of these features may be detectable by the NASA MMS spacecraft to be launched in 2014.

Kinetic MHD simulation of tearing modes

Jianhua Cheng: CIPS - University of Colorado at Boulder

Friday September 10 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

We have developed a Lorentz force ion, fluid electron kinetic MHD simulation. Different from traditional kinetic or MHD codes, this hybrid model includes full kinetic ions and could be applied to study MHD scale physics. To eliminate the constraint on the timestep due to the fast electron compressional wave, a second-order accuracy implicit method is employed.In this talk, I will present the main equations we are solving, several numerical issues and physics results. For benchmarking, we first studied Alfv'en waves, ion sound waves and whistler waves. Linear results agree well with the analytical theory. Also investigated are the nonlinear evolution of the resistive tearing mode starting from the Harris sheet equilibrium configuration. The linear growth rate and mode structure agree favorably with the resistive MHD theory. In the nonlinear regime, several stages are identified including the secondary island formation, its coalescence with the main island and the nonlinear saturation. In particular, we measured the Rutherford growth rate and the saturation island width for various parameters.

A Visit to La Palma and the Dutch Open Telescope

Alan Kiplinger: CIPS - University of Colorado at Boulder

Friday September 03 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Dr. Alan Kiplinger is a solar physicist at the Center for Integrated Plasma Studies, University of Colorado. He will talk about his recent invited trip to the solar Dutch Open Telescope which is on the island of La Palma off the coast of Northwest Africa. La Palma is a fascinating tropical island with an ancient past, and it has a bright future in astronomy with a myriad of telescopes. These include the surprisingly powerful solar telescope known as the Dutch Open Telescope (DOT) which Dr. Kiplinger was asked to work with in May 2010. The telescope's observations are now dedicated, for 2010, to solar prominences under the coordination efforts of Sara Martin (HelioResearch, La Crescenta, CA) representing the international 'Prominence Research: Observations and Models' team (PROM), and the U. of Utrecht with Dr. Rob Hammerslag. The telescope is itself an amazing mechanical marvel and it is now collecting multiwavelength data of prominence dynamics at unprecedented rates. Dr. Kiplinger will describe experiences on the island, special views of prominences and the DOT.

The Fusion Simulation Program

John Cary: Tech-X and M. Greenwald: PSFC/MIT

Friday August 27 2010, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

The Fusion Simulation Program (FSP) Definition Project has been underway for about one year, with the purpose of defining in more detail the Fusion Simulation Program. The purpose of this seminar is to present the status and direction of the planning and to gather community input on the planning and any other related items.

Some aspects of the magnetic reconnection theory

Grigory Vekstein: University of Manchester

Friday July 09 2010, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

This talk will cover three topics.

Collisionless reconnection: inertia of electrons versus their non-scalar thermal pressure tensor as the reconnection mechanism.

Energetics of forced magnetic reconnection: forced reconnection as a trigger of magnetic relaxation in an MHD-stable magnetic configuration.

On a possible role of secondary tearing instability and plasmoids formation in forced magnetic reconnection.

Momentum transport and the radial electric field in tokamaks

Peter Catto: MIT

Wednesday May 05 2010, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

It is well known that the size of turbulent eddies depends strongly on the velocity shear, making the transport of momentum a key ingredient in any turbulent simulation. However, a self-consistent formulation that includes both the turbulence and the transport of momentum has not been available until recently. One problem is that the turbulent fluctuations have very short wavelengths, on the order of the characteristic size of the microscopic motion of magnetized ions. Thus, a macroscopic fluid description is not valid, and more sophisticated kinetic models are required. The current methodology requires accuracies of 1e-10 to self-consistently evolve the average velocity profiles in a tokamak, whereas the extended formulation to be presented only requires an accuracy of 1e-2.

Magnetic reconnection in high-energy-density plasmas with strong radiative cooling.

Dmitri Uzdensky: CIPS - University of Colorado at Boulder

Friday April 30 2010, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Magnetic reconnection is a fundamental plasma-physics process involving dramatic rearrangement of magnetic topology and often leading to a violent release of magnetic energy. It is responsible for many disruptive phenomena in laboratory-, space-, and solar physics (e.g., solar flares and magnetospheric substorms). Traditional reconnection research, geared towards these relatively tenuous environments, has so far been limited to electron-ion plasmas with no photons. In contrast, in various astrophysical situations where reconnection is believed to be important, the dissipated energy density is so high that photons can not be ignored. This, together with some recent laser-plasma laboratory experiments, motivates a new direction of research --- magnetic reconnection in High-Energy-Density (HED) plasmas, where radiation effects (radiative cooling, Compton drag, radiation pressure, and, in more extreme astrophysical cases, pair creation) become important. In this talk, I will first outline the basic general physical ideas behind this new area of research and will then explain in more detail the effect on reconnection of one particular radiative process --- optically-thin radiative cooling.

Fluctuations and Flows in the Colorado FRC Experiment

Adam Light: CIPS - University of Colorado at Boulder

Friday April 23 2010, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

This talk outlines the goals of the Colorado Field-Reversed Configuration (FRC) Experiment and presents the latest measurements. We use merged spheromaks to study flows and fluctuations in self-organized plasmas. Preliminary measurements using a multi-point (16 positions x 3 axes) magnetic diagnostic indicate that a variety of waves are generated during the merging process. Dispersion relations for coherent waves are extracted from the magnetic data by using a cross-spectral-density correlation technique. Observations during merging include signatures of magnetosonic, L-mode, and ion-cyclotron waves. In order to highlight flow-driven processes, we have designed and constructed a two-point biasing probe for driving bulk E x B flows at subsonic to supersonic speeds. We present first results from the use of the biasing probe.

Modeling the solar wind interaction with Jupiter's magnetosphere

Mariel Desroche: Department of Physics - University of Colorado at Boulder

Friday April 16 2010, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Magnetic reconnection at Earth's dayside magnetopause creates an "open" magnetosphere, resulting in magnetic flux transfer to the magnetotail. The dynamics of this interaction are dominated by the properties of the solar wind and the interplanetary magnetic field. At Jupiter, the large size of the magnetosphere, rapid rotation, and large internal plasma source ensure that internal processes have a non-negligible effect on the solar wind interaction. The extent of the effect of magnetospheric properties on the solar wind interaction is not well understood; this, along with limited plasma and magnetic field measurements in the outer magnetosphere and magnetosheath has made it difficult to characterize the dynamics at Jupiter's magnetopause. The Cooling study described the motion of reconnected flux tubes on Earth's magnetopause; a proposal to adapt this study to explore the roles of reconnection and viscous interactions at Jupiter's magnetopause will be discussed.