explanation      blue bibcodes open ADS page with paths to full text
Author name code: zweibel
ADS astronomy entries on 2021-05-31
author:"Zweibel, Ellen G." 

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Title: Extragalactic Magnetism with SOFIA (Legacy Program) -- I:
    The magnetic field in the multi-phase interstellar medium of M51
Authors: Borlaff, Alejandro S.; Lopez-Rodriguez, Enrique; Beck,
   Rainer; Stepanov, Rodion; Ntormousi, Eva; Hughes, Annie; Tassis,
   Konstantinos; Marcum, Pamela M.; Grosset, Lucas; Beckman, John E.;
   Proudfit, Leslie; Clark, Susan E.; Díaz-Santos, Tanio; Mao, Sui Ann;
   Reach, William T.; Roman-Duval, Julia; Subramanian, Kandaswamy; Tram,
   Le Ngoc; Zweibel, Ellen G.; SOFIA Legacy Team
2021arXiv210509315B    Altcode:
  The recent availability of high-resolution far-infrared (FIR)
  polarization observations of galaxies using HAWC+/SOFIA has facilitated
  studies of extragalactic magnetic fields in the cold and dense molecular
  disks.We investigate if any significant structural differences are
  detectable in the kpc-scale magnetic field of the grand design face-on
  spiral galaxy M51 when traced within the diffuse (radio) and the
  dense and cold (FIR) interstellar medium (ISM). Our analysis reveals a
  complex scenario where radio and FIR polarization observations do not
  necessarily trace the same magnetic field structure. We find that the
  magnetic field in the arms is wrapped tighter at 154um than at 3 and 6
  cm; statistically significant lower values for the magnetic pitch angle
  are measured at FIR in the outskirts (R > 7 kpc) of the galaxy. This
  difference is not detected in the interarm region. We find strong
  correlations of the polarization fraction and total intensity at FIR and
  radio with the gas column density and 12CO(1-0) velocity dispersion. We
  conclude that the arms show a relative increase of small-scale turbulent
  B-fields at regions with increasing column density and dispersion
  velocities of the molecular gas. No correlations are found with HI
  neutral gas. The star formation rate shows a clear correlation with
  the radio polarized intensity, which is not found in FIR, pointing
  to a small-scale dynamo-driven B-field amplification scenario. This
  work shows that multi-wavelength polarization observations are key
  to disentangling the interlocked relation between star formation,
  magnetic fields, and gas kinematics in the multi-phase ISM.

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Title: Regimes of cosmic-ray diffusion in Galactic turbulence
Authors: Reichherzer, P.; Merten, L.; Dörner, J.; Becker Tjus, J.;
   Pueschel, M. J.; Zweibel, E. G.
2021arXiv210413093R    Altcode:
  Cosmic-ray transport in astrophysical environments is often dominated
  by the diffusion of particles in a magnetic field composed of both a
  turbulent and a mean component. This process needs to be understood in
  order to properly model cosmic-ray signatures. One of the most important
  aspects in the modeling of cosmic-ray diffusion is that fully resonant
  scattering, the most effective such process, is only possible if the
  wave spectrum covers the entire range of propagation angles. By taking
  the wave spectrum boundaries into account, we quantify cosmic-ray
  diffusion parallel and perpendicular to the guide field direction at
  turbulence levels above 5% of the total magnetic field. We apply our
  results of the parallel and perpendicular diffusion coefficient to
  the Milky Way. We show that simple purely diffusive transport is in
  conflict with observations of the inner Galaxy, but that just by taking
  a Galactic wind into account, data can be matched in the central 5 kpc
  zone. Further comparison shows that the outer Galaxy at $>5$ kpc, on
  the other hand, should be dominated by perpendicular diffusion, likely
  changing to parallel diffusion at the outermost radii of the Milky Way.

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Title: The impact of magnetic fields on momentum transport and
    saturation of shear-flow instability by stable modes
Authors: Fraser, A. E.; Terry, P. W.; Zweibel, E. G.; Pueschel, M. J.;
   Schroeder, J. M.
2021PhPl...28b2309F    Altcode: 2020arXiv201011198F
  The Kelvin-Helmholtz (KH) instability of a shear layer with an
  initially-uniform magnetic field in the direction of flow is studied
  in the framework of 2D incompressible magnetohydrodynamics with finite
  resistivity and viscosity using direct numerical simulations. The shear
  layer evolves freely, with no external forcing, and thus broadens
  in time as turbulent stresses transport momentum across it. As with
  KH-unstable flows in hydrodynamics, the instability here features
  a conjugate stable mode for every unstable mode in the absence of
  dissipation. Stable modes are shown to transport momentum up its
  gradient, shrinking the layer width whenever they exceed unstable
  modes in amplitude. In simulations with weak magnetic fields, the
  linear instability is minimally affected by the magnetic field, but
  enhanced small-scale fluctuations relative to the hydrodynamic case
  are observed. These enhanced fluctuations coincide with increased
  energy dissipation and faster layer broadening, with these features
  more pronounced in simulations with stronger fields. These trends
  result from the magnetic field reducing the effects of stable modes
  relative to the transfer of energy to small scales. As field strength
  increases, stable modes become less excited and thus transport less
  momentum against its gradient. Furthermore, the energy that would
  otherwise transfer back to the driving shear due to stable modes
  is instead allowed to cascade to small scales, where it is lost
  to dissipation. Approximations of the turbulent state in terms of
  a reduced set of modes are explored. While the Reynolds stress is
  well-described using just two modes per wavenumber at large scales,
  the Maxwell stress is not.

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Title: The Magellanic Corona as the key to the formation of the
    Magellanic Stream
Authors: Lucchini, S.; D'Onghia, E.; Fox, A.; Bustard, C.;
   Bland-Hawthorn, J.; Zweibel, E.
2021AAS...23743406L    Altcode:
  The Magellanic Stream is one of the most complex gaseous structures in
  the Milky Way's immediate environment. The Large and Small Magellanic
  Clouds (LMC/SMC), through their mutual interactions over the past
  several billion years, have lost over a billion solar masses through
  tidal and ram pressure forces, and the Milky Way has stretched this gas
  into the Magellanic Stream we see today. It is a massive, multi-phase,
  filamentary, turbulent structure that we are only now beginning to
  fully understand. Recent work using absorption line spectroscopy along
  quasar sightlines has revealed a huge amount of ionized gas that cocoons
  the directly observable neutral hydrogen first mapped in 1974. This
  ionized component of the Stream contributes ~90% of the total mass,
  and until now there hasn't been an explanation for the source of this
  majority of the Stream in tidal models. Here we present novel N-body
  hydrodynamical simulations of the tidal and ram pressure interactions
  between the LMC, SMC, and Milky Way that lead to the formation of the
  Magellanic Stream and Leading Arm. We include, for the first time,
  a Magellanic Corona of warm, ionized gas surrounding the Magellanic
  Clouds throughout their interactions that can account for the currently
  observed mass and multi-phase nature of the Stream. This Magellanic
  Corona is well motivated by the discovery of dwarf galaxies associated
  with the Magellanic Group, the high mass of the LMC (~2×10<SUP>11</SUP>
  solar masses), and the warm circumgalactic gas found around LMC-like
  galaxies in cosmological simulations. We predict that this Magellanic
  Corona will be unambiguously observable via high-ionization absorption
  lines in the ultraviolet spectra of background quasars lying near
  the LMC. This prediction is directly testable with the Cosmic Origins
  Spectrograph on the Hubble Space Telescope.

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Title: Cosmic Ray Transport, Energy Loss, and Influence in the
    Multiphase Interstellar Medium
Authors: Bustard, Chad; Zweibel, Ellen G.
2020arXiv201206585B    Altcode:
  The bulk propagation speed of GeV-energy cosmic rays is limited by
  frequent scattering off hydromagnetic waves. Most galaxy evolution
  simulations that account for this confinement assume the gas is fully
  ionized and cosmic rays are well-coupled to Alfvén waves; however,
  multiphase density inhomogeneities, frequently under-resolved in
  galaxy evolution simulations, induce cosmic ray collisions and
  ionization-dependent transport driven by cosmic ray decoupling and
  elevated streaming speeds in partially neutral gas. How do cosmic
  rays navigate and influence such a medium, and can we constrain
  this transport with observations? In this paper, we simulate cosmic
  ray fronts impinging upon idealized, partially neutral clouds and
  lognormally-distributed clumps, with and without ionization-dependent
  transport. With these high-resolution simulations, we identify cloud
  interfaces as crucial regions where cosmic ray fronts can develop a
  stair-step pressure gradient sufficient to collisionlessly generate
  waves, overcome ion-neutral damping, and exert a force on the cloud. We
  find that the acceleration of cold clouds is hindered by only a
  factor of a few when ionization-dependent transport is included,
  with additional dependencies on magnetic field strength and cloud
  dimensionality. We also probe how cosmic rays sample the background
  gas and quantify collisional losses. Hadronic gamma-ray emission maps
  are qualitatively different when ionization-dependent transport is
  included, but the overall luminosity varies by only a small factor,
  as the short cosmic ray residence times in cold clouds are offset by
  the higher densities that cosmic rays sample.

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Title: Turbulence-level dependence of cosmic ray parallel diffusion
Authors: Reichherzer, P.; Becker Tjus, J.; Zweibel, E. G.; Merten,
   L.; Pueschel, M. J.
2020MNRAS.498.5051R    Altcode: 2019arXiv191007528R; 2020MNRAS.tmp.2453R
  Understanding the transport of energetic cosmic rays belongs to the
  most challenging topics in astrophysics. Diffusion due to scattering
  by electromagnetic fluctuations is a key process in cosmic ray
  transport. The transition from a ballistic to a diffusive-propagation
  regime is presented in direct numerical calculations of diffusion
  coefficients for homogeneous magnetic field lines subject to turbulent
  perturbations. Simulation results are compared with theoretical
  derivations of the parallel diffusion coefficient's dependences
  on the energy and the fluctuation amplitudes in the limit of weak
  turbulence. The present study shows that the widely used extrapolation
  of the energy scaling for the parallel diffusion coefficient to
  high turbulence levels predicted by quasi-linear theory does not
  provide a universally accurate description in the resonant-scattering
  regime. It is highlighted here that the numerically calculated diffusion
  coefficients can be polluted for low energies due to missing resonant
  interaction possibilities of the particles with the turbulence. Five
  reduced-rigidity regimes are established, which are separated by
  analytical boundaries derived in this work. Consequently, a proper
  description of cosmic ray propagation can only be achieved by using
  a turbulence-level-dependent diffusion coefficient and can contribute
  to solving the Galactic cosmic ray gradient problem.

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Title: HAWC+ Far-infrared Observations of the Magnetic Field Geometry
    in M51 and NGC 891
Authors: Jones, Terry Jay; Kim, Jin-Ah; Dowell, C. Darren; Morris,
   Mark R.; Pineda, Jorge L.; Benford, Dominic J.; Berthoud, Marc;
   Chuss, David T.; Dale, Daniel A.; Fissel, L. M.; Goldsmith, Paul F.;
   Hamilton, Ryan T.; Hanany, Shaul; Harper, Doyal A.; Henning, Thomas
   K.; Lazarian, Alex; Looney, Leslie W.; Michail, Joseph M.; Novak,
   Giles; Santos, Fabio P.; Sheth, Kartik; Siah, Javad; Stacey, Gordon
   J.; Staguhn, Johannes; Stephens, Ian W.; Tassis, Konstantinos; Trinh,
   Christopher Q.; Vaillancourt, John E.; Ward-Thompson, Derek; Werner,
   Michael; Wollack, Edward J.; Zweibel, Ellen G.; HAWC+ Science Team
2020AJ....160..167J    Altcode: 2020arXiv200807897J
  Stratospheric Observatory for Infrared Astronomy High-resolution
  Airborne Wideband Camera Plus polarimetry at 154 μm is reported for
  the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the
  polarization vectors generally follow the spiral pattern defined by the
  molecular gas distribution, the far-infrared (FIR) intensity contours,
  and other tracers of star formation. The fractional polarization is
  much lower in the FIR-bright central regions than in the outer regions,
  and we rule out loss of grain alignment and variations in magnetic
  field strength as causes. When compared with existing synchrotron
  observations, which sample different regions with different weighting,
  we find the net position angles are strongly correlated, the fractional
  polarizations are moderately correlated, but the polarized intensities
  are uncorrelated. We argue that the low fractional polarization in the
  central regions must be due to significant numbers of highly turbulent
  segments across the beam and along lines of sight in the beam in the
  central 3 kpc of M51. For NGC 891, the FIR polarization vectors within
  an intensity contour of 1500 $\mathrm{MJy}\,{\mathrm{sr}}^{-1}$ are
  oriented very close to the plane of the galaxy. The FIR polarimetry
  is probably sampling the magnetic field geometry in NGC 891 much
  deeper into the disk than is possible with NIR polarimetry and radio
  synchrotron measurements. In some locations in NGC 891, the FIR
  polarization is very low, suggesting we are preferentially viewing
  the magnetic field mostly along the line of sight, down the length
  of embedded spiral arms. There is tentative evidence for a vertical
  field in the polarized emission off the plane of the disk.

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Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena in Solar and
    Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
   Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
   A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
   Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
   L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
   Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
   G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
   K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
   J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
   A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
   Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
   J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
   Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
   Zenitani, S.; Zweibel, E.
2020arXiv200908779J    Altcode:
  Magnetic reconnection underlies many explosive phenomena in the
  heliosphere and in laboratory plasmas. The new research capabilities in
  theory/simulations, observations, and laboratory experiments provide the
  opportunity to solve the grand scientific challenges summarized in this
  whitepaper. Success will require enhanced and sustained investments
  from relevant funding agencies, increased interagency/international
  partnerships, and close collaborations of the solar, heliospheric,
  and laboratory plasma communities. These investments will deliver
  transformative progress in understanding magnetic reconnection and
  related explosive phenomena including space weather events.

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Title: Cosmic Ray Transport and Calorimetry in the Simulated LMC
Authors: Gardipee, J.; Bustard, C.; Zweibel, E.
2020AAS...23613602G    Altcode:
  Supernova-driven outflows are an important component of stellar
  feedback in dwarf galaxies, and cosmic-rays produced by supernovae
  are a promising driver of outflows. The Large Magellanic Cloud (LMC),
  a well-studied satellite galaxy of the Milky Way, may harbor such a
  cosmic-ray driven outflow, making it a great test subject to learn
  more about cosmic-rays and their influence on galaxies. In this
  project, we compare the known, diffuse gamma-ray luminosity of the
  LMC to predictions from FLASH magnetohydrodynamic (MHD) simulations,
  in order to test different cosmic ray transport models. Specifically,
  gamma-ray studies inform us that cosmic-rays in the LMC only lose ~1%
  of their energy to hadronic collisions, which is far below expectations
  from state-of-the-art simulations. This discrepancy may be resolved,
  however, by including physically-motivated, faster cosmic ray transport
  through multiphase gas, thereby decreasing hadronic collisions in
  the interstellar medium. I'll show synthetic gamma-ray emission maps
  and total gamma-ray luminosity for a set of LMC-specific outflow
  simulations. These simulations, building on the work of Bustard
  et al. 2020, include cosmic-ray production at supernovae and star
  formation motivated by the inferred star formation history of the
  LMC. New simulations take into account super-Alfvenic cosmic-ray
  streaming in partially neutral gas. We vary the factor by which the
  streaming velocity is boosted compared to the Alfven velocity and
  explore changes in gamma-ray production in each case.

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Title: ALMA-SPONGE: The Role of Neutral Hydrogen in Diffuse
    Interstellar Chemistry
Authors: Rybarczyk, D.; Stanimirovic, S.; Murray, C.; Babler, B.;
   Gerin, M.; Ostriker, E.; Gong, M.; Heiles, C.; Zweibel, E.
2020AAS...23623702R    Altcode:
  The diffuse interstellar medium (ISM) plays a pivotal role in the
  star formation process, but its temperature distribution, turbulent
  properties, and chemistry are still poorly understood. Recently,
  the 21-SPONGE survey measured Galactic neutral hydrogen (HI)
  absorption with exceptional sensitivity, quantifying the cold,
  warm, and thermally unstable gas (CNM, WNM, and UNM) fractions, the
  temperature distribution, and the turbulent properties of HI in the
  direction of 57 bright background sources. For a subset of sources, we
  have obtained molecular absorption spectra (CCH, HNC, HCN, and HCO+)
  with ALMA to characterize the chemistry of the molecular ISM along
  the same lines of sight as 21-SPONGE, where we have key constraints
  on the kinetic temperature, cold gas fraction, and interstellar
  turbulence. We have detected molecular gas in the direction of some
  sources, while some sources show no evidence for molecular gas. Both
  detections and non-detections are used to place constraints on the
  role of non-equilibrium chemistry in molecule formation, and to test
  predictions for molecular abundances based on recent MHD models. We
  also detect a variety of molecular species in the direction of 3C123,
  whose line of sight hosts AU-scale overdense and overpressured HI
  structures. Molecular probes of this AU-scale structure are vital to
  understand its formation and role in the ISM.

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Title: Cosmic-Ray-driven Outflows from the Large Magellanic Cloud:
    Contributions to the LMC Filament
Authors: Bustard, Chad; Zweibel, Ellen G.; D'Onghia, Elena; Gallagher,
   J. S., III; Farber, Ryan
2020ApJ...893...29B    Altcode: 2019arXiv191102021B
  In this paper, we build from previous work and present simulations of
  recent (within the past Gyr), magnetized, cosmic-ray driven outflows
  from the Large Magellanic Cloud (LMC), including our first attempts
  to explicitly use the derived star formation history of the LMC to
  seed outflow generation. We run a parameter set of simulations for
  different LMC gas masses and cosmic-ray transport treatments, and we
  make preliminary comparisons to published outflow flux estimates,
  neutral and ionized hydrogen observations, and Faraday rotation
  measure maps. We additionally report on the gas mass that becomes
  unbound from the LMC disk and swept by ram pressure into the Trailing
  Magellanic Stream. We find that, even for our largest outburst, the
  mass contribution to the Stream is still quite small, as much of the
  outflow-turned-halo gas is shielded on the LMCs far-side due to the
  LMCs primarily face-on infall through the Milky Way halo over the past
  Gyr. On the LMC's near-side, past outflows have fought an uphill battle
  against ram pressure, with the near-side halo mass being at least a
  factor of a few smaller than that of the far-side. Absorption-line
  studies probing only the LMC foreground, then, may be severely
  underestimating the total mass of the LMC halo formed by outflows.

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Title: Small-scale Structure Traced by Neutral Hydrogen Absorption
    in the Direction of Multiple-component Radio Continuum Sources
Authors: Rybarczyk, Daniel R.; Stanimirović, Snezana; Zweibel, Ellen
   G.; Murray, Claire E.; Dickey, John M.; Babler, Brian; Heiles, Carl
2020ApJ...893..152R    Altcode: 2020arXiv200210471R
  We have studied the small-scale distribution of atomic hydrogen
  (H I) using 21 cm absorption spectra against multiple-component
  background radio continuum sources from the 21-SPONGE survey and the
  Millennium Arecibo Absorption-Line Survey. We have found &gt;5σ
  optical depth variations at a level of ∼0.03-0.5 between 13 out
  of 14 adjacent sightlines separated by a few arcseconds to a few
  arcminutes, suggesting the presence of neutral structures on spatial
  scales from a few to thousands of au (which we refer to as tiny-scale
  atomic structure, TSAS). The optical depth variations are strongest
  in directions where the H I column density and the fraction of H I
  in the cold neutral medium (CNM) are highest, which tend to be at low
  Galactic latitudes. By measuring changes in the properties of Gaussian
  components fitted to the absorption spectra, we find that changes
  in both the peak optical depth and the linewidth of TSAS absorption
  features contribute to the observed optical depth variations, while
  changes in the central velocity do not appear to strongly impact the
  observed variations. Both thermal and turbulent motions contribute
  appreciably to the linewidths, but the turbulence does not appear
  strong enough to confine overpressured TSAS. In a majority of cases,
  the TSAS column densities are sufficiently high that these structures
  can radiatively cool fast enough to maintain thermal equilibrium with
  their surroundings, even if they are overpressured. We also find that a
  majority of TSAS is associated with the CNM. For TSAS in the direction
  of the Taurus molecular cloud and the local Leo cold cloud, we estimate
  densities over an order of magnitude higher than typical CNM densities.

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Title: Transport of High-energy Charged Particles through Spatially
    Intermittent Turbulent Magnetic Fields
Authors: Chen, L. E.; Bott, A. F. A.; Tzeferacos, P.; Rigby, A.;
   Bell, A.; Bingham, R.; Graziani, C.; Katz, J.; Koenig, M.; Li,
   C. K.; Petrasso, R.; Park, H. -S.; Ross, J. S.; Ryu, D.; White,
   T. G.; Reville, B.; Matthews, J.; Meinecke, J.; Miniati, F.; Zweibel,
   E. G.; Sarkar, S.; Schekochihin, A. A.; Lamb, D. Q.; Froula, D. H.;
   Gregori, G.
2020ApJ...892..114C    Altcode: 2018arXiv180804430C
  Identifying the sources of the highest energy cosmic rays requires
  understanding how they are deflected by the stochastic, spatially
  intermittent intergalactic magnetic field. Here we report measurements
  of energetic charged-particle propagation through a laser-produced
  magnetized plasma with these properties. We characterize the diffusive
  transport of the particles experimentally. The results show that the
  transport is diffusive and that, for the regime of interest for the
  highest energy cosmic rays, the diffusion coefficient is unaffected
  by the spatial intermittency of the magnetic field.

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Title: The Role of the Parker Instability in Structuring the
    Interstellar Medium
Authors: Heintz, Evan; Bustard, Chad; Zweibel, Ellen G.
2020ApJ...891..157H    Altcode: 2019arXiv191003588H
  The Parker instability, a Rayleigh-Taylor-like instability of thermal
  gas supported against gravity by magnetic fields and cosmic rays,
  is thought to be dynamically important for galaxy evolution, possibly
  promoting molecular cloud formation and the galactic dynamo. In previous
  work, we examined the effect of three different cosmic-ray transport
  models on the Parker instability: decoupled (γ<SUB>c</SUB> = 0),
  locked to the thermal gas (γ<SUB>c</SUB> = 4/3), and coupled to the
  gas with streaming by self-confinement. We expand on that work here by
  considering radiative cooling, a smooth gravitational potential, and
  simulations into the nonlinear regime. We determine that cosmic-ray
  transport away from compression points, whether by diffusion or
  streaming, is the largest driver of the instability. Heating due to
  cosmic-ray streaming is also destabilizing and especially affects
  the nonlinear regime. While cooling depressurizes the dense gas,
  streaming cosmic rays heat and inflate the diffuse extraplanar
  gas, greatly modifying the phase structure of the medium. In 3D,
  we find that the fastest growth favors short-wavelength modes in the
  horizontal direction perpendicular to the background magnetic field;
  this is imprinted on Faraday rotation measure maps that may be used
  to detect the Parker instability. The modifications to the Parker
  instability that we observe in this work have large implications for
  the structure and evolution of galaxies, and they highlight the major
  role that cosmic rays play in shaping their environments.

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Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
   F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
   Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
   Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
   Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
   Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
   Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2020arXiv200400079J    Altcode:
  This white paper summarizes major scientific challenges and
  opportunities in understanding magnetic reconnection and related
  explosive phenomena as a fundamental plasma process.

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Title: The Role of Pressure Anisotropy in Cosmic-Ray Hydrodynamics
Authors: Zweibel, Ellen G.
2020ApJ...890...67Z    Altcode: 2019arXiv191003052Z
  The mean free path of cosmic rays in diffuse interstellar and
  intracluster gas is determined primarily by pitch angle scattering
  from hydromagnetic waves with wavelength of order the cosmic-ray
  gyroradius. In the theory of cosmic-ray self confinement, the waves are
  generated by instabilities driven by the cosmic rays themselves. The
  dominant instability is due to bulk motion, or streaming, of the cosmic
  rays, parallel to the background magnetic field ${\boldsymbol{B}}$
  , and transfers cosmic-ray momentum and energy to the thermal gas
  as well as confining the cosmic rays. Classical arguments and recent
  numerical simulations show that self confinement due to the streaming
  instability breaks down unless the cosmic-ray pressure and thermal
  gas density gradients parallel to ${\boldsymbol{B}}$ are aligned,
  a condition that is unlikely to always be satisfied We investigate
  an alternative mechanism for cosmic-ray self confinement and heating
  of thermal gas based on pressure anisotropy instability. Although
  pressure anisotropy is demonstrably less effective than streaming
  instability as a self-confinement and heating mechanism on global
  scales, it may be important on mesoscales, particularly near sites of
  cosmic-ray injection.

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Title: Cosmic Ray Transport and the Galaxy Gas Cycle
Authors: Bustard, C.; Zweibel, E.; D'Onghia, E.; Gallagher, J.
2020AAS...23512304B    Altcode:
  Despite representing only a billionth of the gas population, cosmic
  rays have enough energy to significantly shape the structure of the
  ISM, the supernova-driven outflows that emanate from it, and the
  surrounding circumgalactic medium (CGM). They exert this influence,
  however, through micro-scale interactions with plasma fluctuations on
  scales of order 1 AU. To fully realize the macroscopic, observable
  effects of cosmic rays, my Ph.D. research leverages novel numerical
  techniques and plasma physics-based cosmic ray treatments to simulate
  cosmic rays in galaxies, specifically their roles in supernova-driven
  galactic winds. An intriguing case-study is the outflow-harboring Large
  Magellanic (LMC). Using FLASH magnetohydrodynamic simulations with an
  additional cosmic ray module, we simulate ram pressure stripping, cosmic
  ray driven outflows, and trailing filament formation from the LMC,
  explicitly using the resolved star formation history of the LMC to seed
  superbubble blowout. We find that thermally driven outflows primarily
  fall back to the disk as "fountains", whereas cosmic rays drive extended
  winds, developing a cosmic ray dominated halo above the LMC disk. Ram
  pressure stripping, although very inefficient without outflows, can
  transform even small fountain flows into expelled gas. This process is
  amplified when cosmic rays are included, expelling a significant amount
  of gas and cosmic rays from the LMC into the Magellanic Stream. Using
  mock observations, we constrain our simulations with recent data from
  the Wisconsin H-Alpha Mapper, absorption line studies, and Faraday
  rotation measure studies. Interestingly, our results may provide
  indirect evidence for a more gas-rich LMC, which tempers outflow
  strengths to reasonable levels. This work was supported by the NSF
  Graduate Research Fellowship Program under Grant No. DGE-1256259

---------------------------------------------------------
Title: Cosmic Ray Transport and the Galaxy Gas Cycle
Authors: Bustard, C.; Zweibel, E.; D'Onghia, E.; Gallagher, J.
2020AAS...23526002B    Altcode:
  Despite representing only a billionth of the gas population, cosmic
  rays have enough energy to significantly shape the structure of the
  ISM, the supernova-driven outflows that emanate from it, and the
  surrounding circumgalactic medium (CGM). They exert this influence,
  however, through micro-scale interactions with plasma fluctuations on
  scales of order 1 AU. To fully realize the macroscopic, observable
  effects of cosmic rays, my Ph.D. research leverages novel numerical
  techniques and plasma physics-based cosmic ray treatments to simulate
  cosmic rays in galaxies, specifically their roles in supernova-driven
  galactic winds. An intriguing case-study is the outflow-harboring Large
  Magellanic (LMC). Using FLASH magnetohydrodynamic simulations with an
  additional cosmic ray module, we simulate ram pressure stripping, cosmic
  ray driven outflows, and trailing filament formation from the LMC,
  explicitly using the resolved star formation history of the LMC to seed
  superbubble blowout. We find that thermally driven outflows primarily
  fall back to the disk as "fountains", whereas cosmic rays drive extended
  winds, developing a cosmic ray dominated halo above the LMC disk. Ram
  pressure stripping, although very inefficient without outflows, can
  transform even small fountain flows into expelled gas. This process is
  amplified when cosmic rays are included, expelling a significant amount
  of gas and cosmic rays from the LMC into the Magellanic Stream. Using
  mock observations, we constrain our simulations with recent data from
  the Wisconsin H-Alpha Mapper, absorption line studies, and Faraday
  rotation measure studies. Interestingly, our results may provide
  indirect evidence for a more gas-rich LMC, which tempers outflow
  strengths to reasonable levels. This work was supported by the NSF
  Graduate Research Fellowship Program under Grant No. DGE-1256259

---------------------------------------------------------
Title: SOFIA/HAWC+ Traces the Magnetic Fields in NGC 1068
Authors: Lopez-Rodriguez, Enrique; Dowell, C. Darren; Jones, Terry
   J.; Harper, Doyal A.; Berthoud, Marc; Chuss, David; Dale, Daniel A.;
   Guerra, Jordan A.; Hamilton, Ryan T.; Looney, Leslie W.; Michail,
   Joseph M.; Nikutta, Robert; Novak, Giles; Santos, Fabio P.; Sheth,
   Kartik; Siah, Javad; Staguhn, Johannes; Stephens, Ian W.; Tassis,
   Konstantinos; Trinh, Christopher Q.; Ward-Thompson, Derek; Werner,
   Michael; Wollack, Edward J.; Zweibel, Ellen G.; HAWC+Science Team
2020ApJ...888...66L    Altcode: 2019arXiv190706648L
  We report the first detection of galactic spiral structure by means
  of thermal emission from magnetically aligned dust grains. Our 89 μm
  polarimetric imaging of NGC 1068 with the High-resolution Airborne
  Wideband Camera/Polarimeter (HAWC+) on NASAs Stratospheric Observatory
  for Infrared Astronomy (SOFIA) also sheds light on magnetic field
  structure in the vicinity of the galaxy's inner-bar and active galactic
  nucleus (AGN). We find correlations between the 89 μm magnetic field
  vectors and other tracers of spiral arms, and a symmetric polarization
  pattern as a function of the azimuthal angle arising from the projection
  and inclination of the disk field component in the plane of the sky. The
  observations can be fit with a logarithmic spiral model with pitch angle
  of {16.9}<SUB>-2.8</SUB><SUP>+2.7\circ</SUP> and a disk inclination
  of 48° ± 2°. We infer that the bulk of the interstellar medium from
  which the polarized dust emission originates is threaded by a magnetic
  field that closely follows the spiral arms. Inside the central starburst
  disk (&lt;1.6 kpc), the degree of polarization is found to be lower
  than for far-infrared sources in the Milky Way, and has minima at the
  locations of most intense star formation near the outer ends of the
  inner-bar. Inside the starburst ring, the field direction deviates
  from the model, becoming more radial along the leading edges of the
  inner-bar. The polarized flux and dust temperature peak ∼3″-6″
  NE of the AGN at the location of a bow shock between the AGN outflow
  and the surrounding interstellar medium, but the AGN itself is weakly
  polarized (&lt;1%) at both 53 and 89 μm.

---------------------------------------------------------
Title: The Magellanic Corona as the key to the formation of the
    Magellanic Stream.
Authors: Lucchini, S.; D'Onghia, E.; Fox, A. J.; Bustard, C.;
   Bland-Hawthorn, J.; Zweibel, E.
2020Natur.585..203L    Altcode: 2020arXiv200904368L
  The dominant gaseous structure in the Galactic halo is the Magellanic
  Stream, an extended network of neutral and ionized filaments surrounding
  the Large and Small Magellanic Clouds (LMC/SMC), the two most massive
  satellite galaxies of the Milky Way. Recent observations indicate
  that the Clouds are on their first passage around our Galaxy, the
  Stream is made up of gas stripped from both the LMC and the SMC,
  and the majority of this gas is ionized. While it has long been
  suspected that tidal forces and ram-pressure stripping contributed
  to the Stream's formation, a full understanding of its origins has
  defied modelers for decades. Several recent developments, including
  the discovery of dwarf galaxies associated with the Magellanic Group,
  the high mass of the LMC, the detection of highly ionized gas toward
  stars in the LMC and the predictions of cosmological simulations all
  support the existence of a halo of warm ionized gas around the LMC at
  a temperature of $\sim5\times10^{5}\;\mathrm{K}$. Here we show that
  by including this "Magellanic Corona" in hydrodynamic simulations of
  the Magellanic Clouds falling onto the Galaxy, we can simultaneously
  reproduce the Stream and its Leading Arm. Our simulations explain
  the Stream's filamentary structure, spatial extent, radial velocity
  gradient, and total ionized gas mass. We predict that the Magellanic
  Corona will be unambiguously observable via high-ionization absorption
  lines in the ultraviolet spectra of background quasars lying near
  the LMC. This prediction is directly testable with the Cosmic Origins
  Spectrograph on the Hubble Space Telescope.

---------------------------------------------------------
Title: A Dynamical Study of Extraplanar Diffuse Ionized Gas in
    NGC 5775
Authors: Boettcher, Erin; Gallagher, J. S., III; Zweibel, Ellen G.
2019ApJ...885..160B    Altcode: 2019arXiv190911679B
  The structure and kinematics of gaseous, disk-halo interfaces
  are imprinted with the processes that transfer mass, metals, and
  energy between galactic disks and their environments. We study the
  extraplanar diffuse ionized gas (eDIG) layer in the interacting,
  star-forming galaxy NGC 5775 to better understand the consequences
  of star formation feedback on the dynamical state of the thick-disk
  interstellar medium. Combining emission-line spectroscopy from the
  Robert Stobie Spectrograph on the Southern African Large Telescope
  with radio continuum observations from Continuum Halos in Nearby
  Galaxies—an EVLA Survey, we ask whether thermal, turbulent, magnetic
  field, and cosmic-ray pressure gradients can stably support the eDIG
  layer in dynamical equilibrium. This model fails to reproduce the
  observed exponential electron scale heights of the eDIG thick disk and
  halo on the northeast ({h}<SUB>z,e</SUB>=0.6,7.5 kpc) and southwest
  ({h}<SUB>z,e</SUB>=0.8,3.6 kpc) sides of the galaxy at R &lt; 11
  kpc. We report the first definitive detection of an increasing eDIG
  velocity dispersion as a function of height above the disk. Blueshifted
  gas along the minor axis at large distances from the midplane hints
  at a disk-halo circulation and/or ram pressure effects caused by the
  ongoing interaction with NGC 5774. This work motivates further integral
  field unit and/or Fabry-Perot spectroscopy of galaxies with a range
  of star formation rates to develop a spatially resolved understanding
  of the role of star formation feedback in shaping the kinematics of
  the disk-halo interface. <P />Based on observations made with the
  Southern African Large Telescope (SALT) under programs 2015-1-SCI-023
  and 2016-2-SCI-029 (PI: E. Boettcher).

---------------------------------------------------------
Title: Cosmic ray acceleration of cool clouds in the circumgalactic
    medium
Authors: Wiener, Joshua; Zweibel, Ellen G.; Ruszkowski, Mateusz
2019MNRAS.489..205W    Altcode: 2019MNRAS.tmp.1948W; 2019arXiv190301471W
  We investigate a mechanism for accelerating cool (10<SUP>4</SUP> K)
  clouds in the circumgalactic medium (CGM) with cosmic rays (CRs),
  possibly explaining some characteristics of observed high-velocity
  clouds (HVCs). Enforcing CRs to stream down their pressure gradient into
  a region of slow streaming speed results in significant build-up of CR
  pressure which can accelerate the CGM. We present the results of the
  first two-dimensional magnetohydrodynamic (MHD) simulations of such `CR
  bottlenecks,' expanding on simpler simulations in 1D. Although much more
  investigation is required, we find two main results. First, radiative
  cooling in the interfaces of these clouds is sufficient to keep the
  cloud intact to CR wave heating. Secondly, cloud acceleration depends
  almost linearly with the injected CR flux at low values (comparable to
  that expected from a Milky Way-like star formation rate), but scales
  sublinearly at higher CR fluxes in 1D simulations. 2D simulations show
  hints of sublinear dependence at high CR fluxes but are consistent with
  pure linear dependence up to the CR fluxes tested. It may therefore be
  plausible to accelerate cool clouds in the CGM to speeds of hundreds
  of km s<SUP>-1</SUP>.

---------------------------------------------------------
Title: Some Recent Results on Cosmic-Ray Feedback
Authors: Zweibel, Ellen
2019cmms.confE..25Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Constraints on cosmic-ray transport in galaxy clusters from
    radio and γ-ray observations
Authors: Wiener, Joshua; Zweibel, Ellen G.
2019MNRAS.488..280W    Altcode: 2019MNRAS.tmp.1613W; 2018arXiv181202179W; 2019MNRAS.tmp.1664W
  The nature of cosmic rays (CRs) and their transport in galaxy clusters
  is probed by several observations. Radio observations reveal synchrotron
  radiation of cosmic-ray electrons (CRe) spiralling around cluster
  magnetic fields. γ-ray observations reveal hadronic reactions of
  cosmic-ray protons (CRp) with gas nuclei that produce pions. No such
  cluster-wide γ-ray signal has been measured, putting an upper limit
  on the density of CRp in clusters. But the presence of CRe implies
  some source of CRp, and consequently there must be some CRp-loss
  mechanism. We quantify the observational constraints on this mechanism
  assuming that losses are dominated by CR transport, ultimately deriving
  lower limits on this transport. Using the Coma cluster as an example, we
  find that bulk outward speeds of 10-100 km s<SUP>-1</SUP> are sufficient
  to reduce γ-radiation below current upper limits. These speeds are
  sub-Alfvénic and are consistent with a self-confinement model for CR
  transport if the magnetic field is coherent on large scales. If the
  transport is diffusive, we require minimum diffusion coefficients of
  10<SUP>31</SUP>-10<SUP>32</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP>. This
  is consistent with CRs free streaming at the speed of light along a
  field tangled on length-scales of a few kpc. We find that a model
  of the Coma cluster with a tangled field and the self-confinement
  picture together can be consistent with observations if the relative
  acceleration efficiency of CR protons is less than 15 times more than
  that of electrons of the same energy. This value is 3-6 times lower
  than the same quantity for Galactic cosmic rays.

---------------------------------------------------------
Title: The Tayler Instability in the Anelastic Approximation
Authors: Goldstein, J.; Townsend, R. H. D.; Zweibel, E. G.
2019ApJ...881...66G    Altcode: 2018arXiv180808958G
  The Tayler instability (TI) is a non-axisymmetric linear instability
  of an axisymmetric toroidal magnetic field in magnetohydrostatic
  equilibrium (MHSE). In a differentially rotating radiative region of
  a star, the TI could drive the Tayler-Spruit dynamo, which generates
  magnetic fields that can significantly impact stellar structure and
  evolution. Heuristic prescriptions disagree on the efficacy of the
  dynamo, and numerical simulations have yet to definitively agree
  upon its existence. The criteria for the TI to develop were derived
  using fully compressible magnetohydrodynamics, while numerical
  simulations of dynamical processes in stars frequently use an
  anelastic approximation. This motivates us to derive new anelastic
  Tayler instability criteria. We find that some MHSE configurations
  are unstable in the fully compressible case but become stable in
  the anelastic case. We find and characterize the unstable modes of
  a simple family of cylindrical MHSE configurations using numerical
  calculations, and we discuss the implications for fully nonlinear
  anelastic simulations.

---------------------------------------------------------
Title: Cosmic-Ray Transport between the Knee and the Ankle with
    CRPropa
Authors: Merten, L.; Tjus, J.; Bustard, C.; Zweibel, E.
2019ICRC...36..350M    Altcode: 2019PoS...358..350M
  No abstract at ADS

---------------------------------------------------------
Title: Hybrid Simulations of the Resonant and Non-Resonant Cosmic
    Ray Streaming Instability
Authors: Haggerty, C.; Caprioli, D.; Zweibel, E.
2019ICRC...36..279H    Altcode: 2019PoS...358..279H; 2019arXiv190906346H
  Using hybrid simulations (kinetic ions--fluid electrons), we test
  the linear theory predictions of the cosmic ray (CR) streaming
  instability. We consider two types of CR distribution functions:
  a "hot" distribution where CRs are represented by a drifting
  power law in momentum and an anisotropic "beam" of monochromatic
  particles. Additionally, for each CR distribution we scan over
  different CR densities to transition from triggering the resonant to
  the non-resonant (Bell) streaming instability. We determine the growth
  rates of these instabilities in simulations by fitting an exponential
  curve during the linear stage, and we show that they agree well with
  the theoretical predictions as a function of wave number agree. We also
  examine the magnetic helicity as a function of time and wave number,
  finding a general good agreement with the predictions, as well as some
  unexpected non-linear features to the instability development.

---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
   Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
   J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
   A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
   W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2019BAAS...51c...5J    Altcode: 2019astro2020T...5J
  This is a group white paper of 100 authors (each with explicit
  permission via email) from 51 institutions on the topic of magnetic
  reconnection which is relevant to 6 thematic areas. Grand challenges
  and research opportunities are described in observations, numerical
  modeling and laboratory experiments in the upcoming decade.

---------------------------------------------------------
Title: Extreme Plasma Astrophysics
Authors: Uzdensky, Dmitri; Begelman, Mitchell; Beloborodov, Andrei;
   Blandford, Roger; Boldyrev, Stanislav; Fiuza, Frederico; Giannios,
   Dimitrios; Kunz, Matthew; Loureiro, Nuno; Lyutikov, Maxim; Medvedev,
   Mikhail; Philippov, Alexander; Quataert, Eliot; Sironi, Lorenzo;
   Spitkovsky, Anatoly; Werner, Gregory; Zhdankin, Vladimir; Zweibel,
   Ellen
2019BAAS...51c.362U    Altcode: 2019astro2020T.362U; 2019arXiv190305328U
  This white paper describes the present status and emerging opportunities
  in Extreme Plasma Astrophysics — a study of astrophysically-relevant
  plasma processes taking place under extreme conditions that necessitate
  taking into account relativistic, radiation, and QED effects.

---------------------------------------------------------
Title: Supermassive Black Hole Feedback
Authors: Ruszkowski, Mateusz; Nagai, Daisuke; Zhuravleva, Irina;
   Brummel-Smith, Corey; Li, Yuan; Hodges-Kluck, Edmund; Yang, Hsiang-Yi
   Karen; Basu, Kaustuv; Chluba, Jens; Churazov, Eugene; Donahue, Megan;
   Fabian, Andrew; Faucher-Giguère, Claude-André; Gaspari, Massimo;
   Hlavacek-Larrondo, Julie; McDonald, Michael; McNamara, Brian; Nulsen,
   Paul; Mroczkowski, Tony; Mushotzky, Richard; Reynolds, Christopher;
   Vikhlinin, Alexey; Voit, Mark; Werner, Norbert; ZuHone, John;
   Zweibel, Ellen
2019BAAS...51c.326R    Altcode: 2019astro2020T.326R; 2019arXiv190309686R
  Galaxy evolution is critically shaped by the energy injection from
  supermassive black holes (SMBHs). A major challenge is unraveling how
  the energy released near the SMBHs is distributed throughout galaxies
  and their environments. This white paper discusses the prospect
  of tackling this problem using high-resolution X-ray/microwave
  observations.

---------------------------------------------------------
Title: [Plasma 2020 Decadal] The Material Properties of Weakly
    Collisional, High-Beta Plasmas
Authors: Kunz, M. W.; Squire, J.; Balbus, S. A.; Bale, S. D.; Chen,
   C. H. K.; Churazov, E.; Cowley, S. C.; Forest, C. B.; Gammie, C. F.;
   Quataert, E.; Reynolds, C. S.; Schekochihin, A. A.; Sironi, L.;
   Spitkovsky, A.; Stone, J. M.; Zhuravleva, I.; Zweibel, E. G.
2019arXiv190304080K    Altcode:
  This white paper, submitted for the Plasma 2020 Decadal Survey, concerns
  the physics of weakly collisional, high-beta plasmas -- plasmas in which
  the thermal pressure dominates over the magnetic pressure and in which
  the inter-particle collision time is comparable to the characteristic
  timescales of bulk motions. This state of matter, although widespread in
  the Universe, remains poorly understood: we lack a predictive theory
  for how it responds to perturbations, how it transports momentum
  and energy, and how it generates and amplifies magnetic fields. Such
  topics are foundational to the scientific study of plasmas, and are of
  intrinsic interest to those who regard plasma physics as a fundamental
  physics discipline. But these topics are also of extrinsic interest:
  addressing them directly informs upon our understanding of a wide
  variety of space and astrophysical systems, including accretion flows
  around supermassive black holes, the intracluster medium (ICM) between
  galaxies in clusters, and regions of the near-Earth solar wind. Specific
  recommendations to advance this field of study are discussed.

---------------------------------------------------------
Title: Plasma 2020 - Intracluster Medium Plasmas
Authors: Caprioli, Damiano; Brunetti, Gianfranco; Jones, Thomas W.;
   Kang, Hyesung; Kunz, Matthew; Oh, S. Peng; Ryu, Dongsu; Zhuravleva,
   Irina; Zweibel, Ellen
2019arXiv190308751C    Altcode:
  Galaxy clusters are the largest and most massive bound objects
  resulting from cosmic hierarchical structure formation. Baryons account
  for somewhat more than 10% of that mass, with roughly 90% of the
  baryonic matter distributed throughout the clusters as hot ($T&gt;1$
  keV), high-$\beta$, very weakly collisional plasma; the so-called
  "intracluster medium" (ICM). Cluster mergers, close gravitational
  encounters and accretion, along with violent feedback from galaxies
  and relativistic jets from active galactic nuclei, drive winds,
  gravity waves, turbulence and shocks within the ICM. Those dynamics,
  in turn, generate cluster-scale magnetic fields and accelerate and
  mediate the transport of high-energy charged particles. Kinetic-scale,
  collective plasma processes define the basic character and fundamental
  signatures of these ICM phenomena, which are observed primarily by
  X-ray and radio astronomers.

---------------------------------------------------------
Title: Probing the magnetoionized intragroup medium of NGC 2563
Authors: Williams, Anna; Wilcots, Eric; Zweibel, Ellen
2019AAS...23323906W    Altcode:
  We present the results of a study to observe magnetic fields within the
  intragroup medium of galaxy group NGC 2563. We use full polarization
  observations at S band (2-4 GHz) collected at the Karl G. Jansky Very
  Large Array to measure the polarization of extragalactic systems
  within 0.5 degrees of NGC 2563. Our survey detected 183 sources,
  22 of which are at least 1% polarized. None of these polarized
  sources are associated with known group members, and determined to
  be background sources. We use the Faraday rotation of the polarized
  sources to probe the intragroup medium. When we compare the Faraday
  rotation of polarized sources with sightlines that pass within 420 kpc
  of the group center to those that pass through the outer 420 kpc, we
  see a slight increase in Faraday dispersion towards the center. This
  increase in Faraday dispersion follows the observed increase in
  electron density towards the center of the group, and is likely due
  to a combination of this property as well as an increase the magnetic
  field strength. Furthermore, we find that polarized sightlines that pass
  within 120 kpc of a known group member also show an increase in Faraday
  rotation dispersion, which may be due to magnetic field amplifciation
  via tidal interactions, ram pressure stripping, or turbulent wakes as
  the galaxies move through the intragroup medium. While this is the
  first study of its kind, we expect future all-sky surveys like the
  VLA Sky Survey will rapidly contribute to the detection of polarized
  sources towards galaxy groups and clusters. This will greatly improve
  our statistical understanding of the magnetized medium with large-scale
  galaxy structures, and help us to unravel the origin of large-scale
  magnetic fields in and around galaxies.

---------------------------------------------------------
Title: SOFIA Far-infrared Imaging Polarimetry of M82 and NGC 253:
    Exploring the Supergalactic Wind
Authors: Jones, Terry Jay; Dowell, C. Darren; Lopez Rodriguez, Enrique;
   Zweibel, Ellen G.; Berthoud, Marc; Chuss, David T.; Goldsmith, Paul F.;
   Hamilton, Ryan T.; Hanany, Shaul; Harper, Doyal A.; Lazarian, Alex;
   Looney, Leslie W.; Michail, Joseph M.; Morris, Mark R.; Novak, Giles;
   Santos, Fabio P.; Sheth, Kartik; Stacey, Gordon J.; Staguhn, Johannes;
   Stephens, Ian W.; Tassis, Konstantinos; Trinh, Christopher Q.; Volpert,
   C. G.; Werner, Michael; Wollack, Edward J.; HAWC+ Science Team
2019ApJ...870L...9J    Altcode: 2018arXiv181206816J
  We present far-infrared polarimetry observations of M82 at 53 and
  154 μm and NGC 253 at 89 μm, which were taken with High-resolution
  Airborne Wideband Camera-plus (HAWC+) in polarimetry mode on the
  Stratospheric Observatory for Infrared Astronomy. The polarization of
  M82 at 53 μm clearly shows a magnetic field geometry perpendicular
  to the disk in the hot dust emission. For M82 the polarization at 154
  μm shows a combination of field geometry perpendicular to the disk
  in the nuclear region, but closer to parallel to the disk away from
  the nucleus. The fractional polarization at 53 μm (154 μm) ranges
  from 7% (3%) off nucleus to 0.5% (0.3%) near the nucleus. A simple
  interpretation of the observations of M82 invokes a massive polar
  outflow, dragging the field along, from a region ∼700 pc in diameter
  that has entrained some of the gas and dust, creating a vertical field
  geometry seen mostly in the hotter (53 μm) dust emission. This outflow
  sits within a larger disk with a more typical planar geometry that
  more strongly contributes to the cooler (154 μm) dust emission. For
  NGC 253, the polarization at 89 μm is dominated by a planar geometry
  in the tilted disk, with weak indication of a vertical geometry above
  and below the plane from the nucleus. The polarization observations
  of NGC 253 at 53 μm were of a insufficient signal-to-noise ratio for
  a detailed analysis.

---------------------------------------------------------
Title: Role of stable modes in driven shear-flow turbulence
Authors: Fraser, Adrian E.; Pueschel, M. J.; Terry, P. W.; Zweibel,
   E. G.
2018PhPl...25l2303F    Altcode: 2018arXiv180709280F
  A linearly unstable, sinusoidal E×B shear flow is examined in the
  gyrokinetic framework in both the linear and nonlinear regimes. In
  the linear regime, it is shown that the eigenmode spectrum is nearly
  identical to hydrodynamic shear flows, with a conjugate stable mode
  found at every unstable wavenumber. In the nonlinear regime, turbulent
  saturation of the instability is examined with and without the inclusion
  of a driving term that prevents nonlinear flattening of the mean flow
  and a scale-independent radiative damping term that suppresses the
  excitation of conjugate stable modes. From a variety of analyses, the
  nonlinear state is found to have a significant component associated
  with stable modes. The role of these modes is investigated through
  a simple fluid model that tracks how momentum transport and partial
  flattening of the mean flow scale with the driving term. From this
  model, it is shown that, except at high radiative damping, stable modes
  play an important role in the turbulent state and yield significantly
  improved quantitative predictions when compared with corresponding
  models neglecting stable modes.

---------------------------------------------------------
Title: Atomic and Ionized Microstructures in the Diffuse Interstellar
    Medium
Authors: Stanimirović, Snežana; Zweibel, Ellen G.
2018ARA&A..56..489S    Altcode: 2018arXiv181000933S
  It has been known for half a century that the interstellar medium
  (ISM) of our Galaxy is structured on scales as small as a few hundred
  kilometers, more than 10 orders of magnitude smaller than typical ISM
  structures and energy input scales. In this review we focus on neutral
  and ionized structures on spatial scales of a few to ∼10<SUP>4</SUP>
  AU, which appear to be highly overpressured, as these have the most
  important role in the dynamics and energy balance of interstellar gas:
  the tiny scale atomic structures (TSASs) and extreme scattering events
  (ESEs) as the most overpressured example of the tiny scale ionized
  structures (TSISs). We review observational results and highlight
  key physical processes at AU scales. We present evidence for and
  against microstructures as part of a universal turbulent cascade and
  as discrete structures, and we review their association with supernova
  remnants, the Local Bubble, and bright stars. We suggest a number of
  observational and theoretical programs that could clarify the nature of
  AU structures. TSAS and TSIS probe spatial scales in the range of what
  is expected for turbulent dissipation scales and are therefore of key
  importance for constraining exotic and not-well-understood physical
  processes that have implications for many areas of astrophysics. The
  emerging picture is one in which a magnetized, turbulent cascade,
  driven hard by a local energy source and acting jointly with phenomena
  such as thermal instability, is the source of these microstructures.

---------------------------------------------------------
Title: The Fate of Supernova-heated Gas in Star-forming Regions of
the LMC: Lessons for Galaxy Formation?
Authors: Bustard, Chad; Pardy, Stephen A.; D'Onghia, Elena; Zweibel,
   Ellen G.; Gallagher, J. S., III
2018ApJ...863...49B    Altcode: 2018arXiv180207263B
  Galactic winds and fountains driven by supernova-heated gas play an
  integral role in redistributing gas in galaxies, depositing metals in
  the circumgalactic medium, and quenching star formation. The interplay
  between these outflows and ram-pressure stripping (RPS) due to the
  galaxy’s motion through an ambient medium may enhance these effects by
  converting fountain flows into expelled gas. In this paper, we present
  controlled, 3D simulations of RPS combined with thermally driven,
  local outflows from clustered supernovae in an isolated disk galaxy
  modeled on the Large Magellanic Cloud (LMC), a dwarf satellite of the
  Milky Way on its first infall. Observational evidence of local outflows
  emanating from supergiant shells in the LMC and a trailing filament
  of H I gas originating from these regions—with no obvious Leading
  Arm counterpart—may represent a perfect example of this process. Our
  simulations present a proof of concept that ram pressure can convert
  fountain flows into expelled gas. We find that fountains launched near
  the peak star formation time of the LMC can comprise part of the LMC
  filament in the Trailing Stream but with lower column densities than
  observed. Larger, more numerous outflows from the LMC may be possible
  and may contribute more mass, but higher-inertia gas will lengthen the
  timescale for this gas to be swept away by ram pressure. Given the
  high-resolution observations, increased knowledge of star formation
  histories, and growing evidence of multiphase ionized outflows, the
  LMC is an ideal test bed for future wind models.

---------------------------------------------------------
Title: The Parker Instability with Cosmic-Ray Streaming
Authors: Heintz, Evan; Zweibel, Ellen G.
2018ApJ...860...97H    Altcode: 2018arXiv180300584H
  Recent studies have found that cosmic-ray transport plays an important
  role in feedback processes such as star formation and the launching
  of galactic winds. Although cosmic-ray buoyancy is widely held to be
  a destabilizing force in galactic disks, the effect of cosmic-ray
  transport on the stability of stratified systems has yet to be
  analyzed. We perform a stability analysis of a stratified layer for
  three different cosmic-ray transport models: decoupled (Classic Parker),
  coupled with γ <SUB> c </SUB> = 4/3 but not streaming (Modified
  Parker), and finally coupled with streaming at the Alfvén speed. When
  the compressibility of the cosmic rays is decreased the system becomes
  much more stable, but the addition of cosmic-ray streaming to the Parker
  instability severely destabilizes it. Through comparison of these
  three cases and analysis of the work contributions for the perturbed
  quantities of each system, we demonstrate that cosmic-ray heating
  of the gas is responsible for the destabilization of the system. We
  find that a 3D system is unstable over a larger range of wavelengths
  than the 2D system. Therefore, the Parker instability with cosmic-ray
  streaming may play an important role in cosmic-ray feedback.

---------------------------------------------------------
Title: The Propagation of Cosmic Rays from the Galactic Wind
Termination Shock: Back to the Galaxy?
Authors: Merten, Lukas; Bustard, Chad; Zweibel, Ellen G.; Becker
   Tjus, Julia
2018ApJ...859...63M    Altcode: 2018arXiv180308376M
  Although several theories exist for the origin of cosmic rays (CRs)
  in the region between the spectral “knee” and “ankle,” this
  problem is still unsolved. A variety of observations suggest that
  the transition from Galactic to extragalactic sources occurs in this
  energy range. In this work, we examine whether a Galactic wind that
  eventually forms a termination shock far outside the Galactic plane can
  contribute as a possible source to the observed flux in the region of
  interest. Previous work by Bustard et al. estimated that particles can
  be accelerated to energies above the “knee” up to R <SUB>max</SUB>
  = 10<SUP>16</SUP> eV for parameters drawn from a model of a Milky
  Way wind. A remaining question is whether the accelerated CRs can
  propagate back into the Galaxy. To answer this crucial question, we
  simulate the propagation of the CRs using the low-energy extension of
  the CRPropa framework, based on the solution of the transport equation
  via stochastic differential equations. The setup includes all relevant
  processes, including three-dimensional anisotropic spatial diffusion,
  advection, and corresponding adiabatic cooling. We find that, assuming
  realistic parameters for the shock evolution, a possible Galactic
  termination shock can contribute significantly to the energy budget in
  the “knee” region and above. We estimate the resulting produced
  neutrino fluxes and find them to be below measurements from IceCube
  and limits by KM3NeT.

---------------------------------------------------------
Title: Acoustic Disturbances in Galaxy Clusters
Authors: Zweibel, Ellen G.; Mirnov, Vladimir V.; Ruszkowski, Mateusz;
   Reynolds, Christopher S.; Yang, H. -Y. Karen; Fabian, Andrew C.
2018ApJ...858....5Z    Altcode: 2018arXiv180204808Z
  Galaxy cluster cores are pervaded by hot gas which radiates at far
  too high a rate to maintain any semblance of a steady state; this is
  referred to as the cooling flow problem. Of the many heating mechanisms
  that have been proposed to balance radiative cooling, one of the most
  attractive is the dissipation of acoustic waves generated by active
  galactic nuclei. Fabian et al. showed that if the waves are nearly
  adiabatic, wave damping due to heat conduction and viscosity must
  be well below standard Coulomb rates in order to allow the waves to
  propagate throughout the core. Because of the importance of this result,
  we have revisited wave dissipation under galaxy cluster conditions
  in a way that accounts for the self-limiting nature of dissipation by
  electron thermal conduction, allows the electron and ion temperature
  perturbations in the waves to evolve separately, and estimates kinetic
  effects by comparing to a semicollisionless theory. While these
  effects considerably enlarge the toolkit for analyzing observations
  of wavelike structures and developing a quantitative theory for wave
  heating, the drastic reduction of transport coefficients proposed in
  Fabian et al. remains the most viable path to acoustic wave heating
  of galaxy cluster cores.

---------------------------------------------------------
Title: Impact of Cosmic-Ray Transport on Galactic Winds
Authors: Farber, R.; Ruszkowski, M.; Yang, H. -Y. K.; Zweibel, E. G.
2018ApJ...856..112F    Altcode: 2017arXiv170704579F
  The role of cosmic rays generated by supernovae and young stars has very
  recently begun to receive significant attention in studies of galaxy
  formation and evolution due to the realization that cosmic rays can
  efficiently accelerate galactic winds. Microscopic cosmic-ray transport
  processes are fundamental for determining the efficiency of cosmic-ray
  wind driving. Previous studies modeled cosmic-ray transport either
  via a constant diffusion coefficient or via streaming proportional to
  the Alfvén speed. However, in predominantly cold, neutral gas, cosmic
  rays can propagate faster than in the ionized medium, and the effective
  transport can be substantially larger; i.e., cosmic rays can decouple
  from the gas. We perform three-dimensional magnetohydrodynamical
  simulations of patches of galactic disks including the effects of
  cosmic rays. Our simulations include the decoupling of cosmic rays
  in the cold, neutral interstellar medium. We find that, compared to
  the ordinary diffusive cosmic-ray transport case, accounting for the
  decoupling leads to significantly different wind properties, such
  as the gas density and temperature, significantly broader spatial
  distribution of cosmic rays, and higher wind speed. These results
  have implications for X-ray, γ-ray, and radio emission, and for the
  magnetization and pollution of the circumgalactic medium by cosmic rays.

---------------------------------------------------------
Title: Angular momentum transport by heat-driven g-modes in slowly
    pulsating B stars
Authors: Townsend, R. H. D.; Goldstein, J.; Zweibel, E. G.
2018MNRAS.475..879T    Altcode: 2017arXiv171202420T
  Motivated by recent interest in the phenomenon of waves transport in
  massive stars, we examine whether the heat-driven gravity (g) modes
  excited in slowly pulsating B (SPB) stars can significantly modify the
  stars' internal rotation. We develop a formalism for the differential
  torque exerted by g modes, and implement this formalism using the GYRE
  oscillation code and the MESASTAR stellar evolution code. Focusing
  first on a 4.21M<SUB>⊙</SUB> model, we simulate 1 000 yr of stellar
  evolution under the combined effects of the torque due to a single
  unstable prograde g mode (with an amplitude chosen on the basis of
  observational constraints), and diffusive angular momentum transport
  due to convection, overshooting, and rotational instabilities. We find
  that the g mode rapidly extracts angular momentum from the surface
  layers, depositing it deeper in the stellar interior. The angular
  momentum transport is so efficient that by the end of the simulation,
  the initially non-rotating surface layers are spun in the retrograde
  direction to ≈ 30 per cent of the critical rate. However, the
  additional inclusion of magnetic stresses in our simulations almost
  completely inhibits this spin-up. Expanding our simulations to cover
  the whole instability strip, we show that the same general behaviour is
  seen in all SPB stars. After providing some caveats to contextualize our
  results, we hypothesize that the observed slower surface rotation of SPB
  stars (as compared to other B-type stars) may be the direct consequence
  of the angular momentum transport that our simulations demonstrate.

---------------------------------------------------------
Title: Unveiling the Origin of the Fermi Bubbles
Authors: Yang, H. -Y.; Ruszkowski, Mateusz; Zweibel, Ellen
2018Galax...6...29Y    Altcode: 2018arXiv180203890Y
  The Fermi bubbles, two giant structures above and below the Galactic
  center (GC), are among the most important discoveries of the Fermi
  Gamma-ray Space Telescope. Studying their physical origin has
  been providing valuable insights into cosmic-ray transport, the
  Galactic magnetic field, and past activity at the GC in the Milky
  Way galaxy. Despite their importance, the formation mechanism of the
  bubbles is still elusive. Over the past few years, there have been
  numerous efforts, both observational and theoretical, to uncover
  the nature of the bubbles. In this article, we present an overview
  of the current status of our understanding of the bubbles' origin,
  and discuss possible future directions that will help to distinguish
  different scenarios of bubble formation.

---------------------------------------------------------
Title: Mock X-ray Observations of Localized LMC Outflows
Authors: Tomesh, Teague; Bustard, Chad; Zweibel, Ellen
2018AAS...23125108T    Altcode:
  The Milky Way’s nearest neighbor, the Large Magellanic Cloud (LMC),
  is a perfect testing ground for modeling a variety of astrophysical
  phenomena. Specifically, the LMC provides a unique opportunity for the
  study of possible localized outflows driven by star formation and their
  x-ray signatures. We have developed FLASH simulations of theoretical
  outflows originating in the LMC that we have used to generate predicted
  observations of X-ray luminosity. This X-ray emission can be a useful
  probe of the hot gas in these winds which may couple to the cool gas
  and drive it from the disk. Future observations of the LMC may provide
  us with valuable checks on our model. This work is partially supported
  by the National Science Foundation (NSF) Graduate Research Fellowship
  Program under grant No. DGE-125625 and NSF grant No. AST-1616037.

---------------------------------------------------------
Title: New Perspectives on the Dynamical State of Extraplanar Diffuse
    Ionized Gas Layers
Authors: Boettcher, Erin; Zweibel, Ellen; Gallagher, John S.; Benjamin,
   Robert A.
2018AAS...23142904B    Altcode:
  Gaseous, disk-halo interfaces are an important boundary in the baryon
  cycle in galaxies like the Milky Way, and their structure, support,
  and kinematics carry clues about the star formation feedback and
  accretion processes that produce them. Due to their unexpectedly large
  scale heights, which are often several times greater than their thermal
  scale heights, it is unclear whether they are in dynamical equilibrium,
  or are evidence of a galactic fountain, wind, or accretion flow. In the
  nearby, edge-on disk galaxies NGC 891 and NGC 5775, we test a dynamical
  equilibrium model of the extraplanar diffuse ionized gas (eDIG) layer
  by quantifying the thermal, turbulent, magnetic field, and cosmic ray
  pressure gradients using optical emission-line spectroscopy from the
  SparsePak IFU at the WIYN Observatory and the Robert Stobie Spectrograph
  on the Southern African Large Telescope and radio continuum observations
  from Continuum Halos in Nearby Galaxies - an EVLA Survey. The vertical
  pressure gradients are too shallow to produce the observed scale heights
  at the moderate galactocentric radii where the gas is believed to be
  found (R &lt; 8 kpc). For the low-inclination galaxy M83, we develop a
  Markov Chain Monte Carlo method to decompose the [NII]λλ6548, 6583,
  Hα, and [SII]λλ6717, 6731 emission lines into multiple components,
  and identify eDIG emission based on its rotational velocity lag and
  elevated [NII]/Hα and [SII]/Hα line ratios. The median, line-of-sight
  velocity dispersion of the eDIG layer, σ = 96 km/s, greatly exceeds
  the horizontal velocity dispersions observed in edge-on eDIG layers
  (σ = 20 - 60 km/s), presenting the possibility that these layers
  have anisotropic random motions. The role of an anisotropic velocity
  dispersion in producing eDIG scale heights, as well as the absence
  of evidence for large-scale inflow or outflow, motivates further
  study of eDIG dynamics in face-on galaxies with a range of star
  formation rates. This work was supported by the NSF GRFP under Grant
  No. DGE-1256259.

---------------------------------------------------------
Title: Cosmic Ray Acceleration from Multiple Galactic Wind Shocks
Authors: Cotter, Cory; Bustard, Chad; Zweibel, Ellen
2018AAS...23125104C    Altcode:
  Cosmic rays still have an unknown origin. Many mechanisms have been
  suggested for their acceleration including quasars, pulsars, magnetars,
  supernovae, supernova remnants, and galactic termination shocks. The
  source of acceleration may be a mixture of these and a different
  mixture in different energy regimes. Using numerical simulations, we
  investigate multiple shocks in galactic winds as potential cosmic rays
  sources. By having shocks closer to the parent galaxy, more particles
  may diffuse back to the disk instead of being blown out in the wind,
  as found in Bustard, Zweibel, and Cotter (2017, ApJ) and also Merten,
  Bustard, Zweibel, and Tjus (to be submitted to ApJ). Specifically,
  this flux of cosmic rays could contribute to the unexplained "shin"
  region between the well-known "knee" and "ankle" of the cosmic ray
  spectrum. We would like to acknowledge support from the National
  Science Foundation (NSF) Graduate Research Fellowship Program under
  grant No. DGE-125625 and NSF grant No. AST-1616037.

---------------------------------------------------------
Title: High β effects on cosmic ray streaming in galaxy clusters
Authors: Wiener, Joshua; Zweibel, Ellen G.; Oh, S. Peng
2018MNRAS.473.3095W    Altcode: 2017arXiv170608525W
  Diffuse, extended radio emission in galaxy clusters, commonly referred
  to as radio haloes, indicate the presence of high energy cosmic ray
  (CR) electrons and cluster-wide magnetic fields. We can predict from
  theory the expected surface brightness of a radio halo, given magnetic
  field and CR density profiles. Previous studies have shown that the
  nature of CR transport can radically effect the expected radio halo
  emission from clusters (Wiener, Oh &amp; Guo 2013). Reasonable levels
  of magnetohydrodynamic (MHD) wave damping can lead to significant CR
  streaming speeds. But a careful treatment of MHD waves in a high β
  plasma, as expected in cluster environments, reveals damping rates may
  be enhanced by a factor of β<SUP>1/2</SUP>. This leads to faster CR
  streaming and lower surface brightnesses than without this effect. In
  this work, we re-examine the simplified, 1D Coma cluster simulations
  (with radial magnetic fields) of Wiener et al. (2013) and discuss
  observable consequences of this high β damping. Future work is required
  to study this effect in more realistic simulations.

---------------------------------------------------------
Title: Detection of microgauss coherent magnetic fields in a galaxy
    five billion years ago
Authors: Mao, S. A.; Carilli, C.; Gaensler, B. M.; Wucknitz, O.;
   Keeton, C.; Basu, A.; Beck, R.; Kronberg, P. P.; Zweibel, E.
2017NatAs...1..621M    Altcode: 2017arXiv170807844M
  Magnetic fields play a pivotal role in the physics of
  interstellar medium in galaxies<SUP>1</SUP>, but there are
  few observational constraints on how they evolve across cosmic
  time<SUP>2-7</SUP>. Spatially resolved synchrotron polarization maps
  at radio wavelengths reveal well-ordered large-scale magnetic fields
  in nearby galaxies<SUP>1,8,9</SUP> that are believed to grow from a
  seed field via a dynamo effect<SUP>10,11</SUP>. To directly test and
  characterize this theory requires magnetic field strength and geometry
  measurements in cosmologically distant galaxies, which are challenging
  to obtain due to the limited sensitivity and angular resolution of
  current radio telescopes. Here, we report the cleanest measurements
  yet of magnetic fields in a galaxy beyond the local volume, free of the
  systematics traditional techniques would encounter. By exploiting the
  scenario where the polarized radio emission from a background source
  is gravitationally lensed by a foreground galaxy at z = 0.439 using
  broadband radio polarization data, we detected coherent μG magnetic
  fields in the lensing disk galaxy as seen 4.6 Gyr ago, with similar
  strength and geometry to local volume galaxies. This is the highest
  redshift galaxy whose observed coherent magnetic field property is
  compatible with a mean-field dynamo origin.

---------------------------------------------------------
Title: Gamma-ray puzzle in Cygnus X: Implications for high-energy
    neutrinos
Authors: Yoast-Hull, Tova M.; Gallagher, John S.; Halzen, Francis;
   Kheirandish, Ali; Zweibel, Ellen G.
2017PhRvD..96d3011Y    Altcode: 2017arXiv170302590Y
  The Cygnus X region contains giant molecular cloud complexes and
  populous associates of massive young stars. The discovery of spatially
  extended, hard γ -ray emission in Cygnus X by both Milagro and Fermi
  indicates that Cygnus X is also a potential source of high-energy
  Galactic neutrinos. Here, we adapt our single-zone model for cosmic ray
  interactions in the central molecular zones of starburst galaxies for
  use in Cygnus X. We calculate the potential neutrino flux corresponding
  to the hard γ -ray emission from the "Cygnus Cocoon" and to the soft,
  diffuse interstellar γ -ray emission. We check our results by comparing
  the corresponding γ -ray emission against the Fermi interstellar
  emission model and Milagro, ARGO-YBJ, and HAWC observations. In
  comparing our results against a recent IceCube analysis and the current
  sensitivity limits, we find that neutrino emission from the Cocoon has a
  large enough flux that it could plausibly be detected, provided hadronic
  interactions are occurring at sufficiently high energies. High-energy
  neutrinos from Cygnus X would provide direct evidence for the presence
  of as yet unidentified PeV energy accelerators in the Galactic disk.

---------------------------------------------------------
Title: Detection of Extraplanar Diffuse Ionized Gas in M83
Authors: Boettcher, Erin; Gallagher, J. S., III; Zweibel, Ellen G.
2017ApJ...845..155B    Altcode: 2017arXiv170708126B
  We present the first kinematic study of extraplanar diffuse ionized
  gas (eDIG) in the nearby, face-on disk galaxy M83 using optical
  emission-line spectroscopy from the Robert Stobie Spectrograph on
  the Southern African Large Telescope. We use a Markov Chain Monte
  Carlo method to decompose the [N II]λ λ 6548, 6583, Hα, and
  [S II]λ λ 6717, 6731 emission lines into H II region and diffuse
  ionized gas emission. Extraplanar, diffuse gas is distinguished by
  its emission-line ratios ([N II]λ6583/Hα ≳ 1.0) and its rotational
  velocity lag with respect to the disk ({{Δ }}v=-24 km s<SUP>-1</SUP>
  in projection). With interesting implications for isotropy, the velocity
  dispersion of the diffuse gas, σ =96 km s<SUP>-1</SUP>, is a factor
  of a few higher in M83 than in the Milky Way and nearby, edge-on disk
  galaxies. The turbulent pressure gradient is sufficient to support
  the eDIG layer in dynamical equilibrium at an electron scale height of
  {h}<SUB>z</SUB>=1 kpc. However, this dynamical equilibrium model must
  be finely tuned to reproduce the rotational velocity lag. There is
  evidence of local bulk flows near star-forming regions in the disk,
  suggesting that the dynamical state of the gas may be intermediate
  between a dynamical equilibrium and a galactic fountain flow. As one
  of the first efforts to study eDIG kinematics in a face-on galaxy,
  this study demonstrates the feasibility of characterizing the radial
  distribution, bulk velocities, and vertical velocity dispersions in
  low-inclination systems. <P />Based on observations made with the
  Southern African Large Telescope (SALT) under program 2015-2-SCI-004
  (PI: E. Boettcher).

---------------------------------------------------------
Title: Interaction of cosmic rays with cold clouds in galactic haloes
Authors: Wiener, Joshua; Oh, S. Peng; Zweibel, Ellen G.
2017MNRAS.467..646W    Altcode: 2016arXiv161002041W; 2017MNRAS.tmp..111W
  We investigate the effects of cosmic ray (CR) dynamics on cold,
  dense clouds embedded in a hot, tenuous galactic halo. If the magnetic
  field does not increase too much inside the cloud, the local reduction
  in Alfvén speed imposes a bottleneck on CRs streaming out from the
  star-forming galactic disc. The bottleneck flattens the upstream CR
  gradient in the hot gas, implying that multiphase structure could have
  global effects on CR-driven winds. A large CR pressure gradient can
  also develop on the outward-facing edge of the cloud. This pressure
  gradient has two independent effects. The CRs push the cloud upwards,
  imparting it with momentum. On smaller scales, the CRs pressurize
  cold gas in the fronts, reducing its density, consistent with the low
  densities of cold gas inferred in recent Cosmic Origins Spectrograph
  (COS) observations of local L<SUB>*</SUB> galaxies. They also heat
  the material at the cloud edge, broadening the cloud-halo interface
  and causing an observable change in interface ionic abundances. Due
  to the much weaker temperature dependence of CR heating relative to
  thermal-conductive heating, CR mediated fronts have a higher ratio of
  low-to-high ions compared to conduction fronts, in better agreement
  with observations. We investigate these effects separately using 1D
  simulations and analytic techniques.

---------------------------------------------------------
Title: Using rotation measure to search for magnetic fields around
    galaxies at z ~ 0.5
Authors: Williams, Anna; Lundgren, Britt; Mao, Sui Ann; Wilcots,
   Eric; Zweibel, Ellen
2017IAUS..321..330W    Altcode:
  Magnetic fields are an important component in galaxies, and yet, we
  still do not know how these magnetic fields were originally seeded
  within galaxies, nor how they have grown to the strengths we observe
  today. One way we can unravel this complex problem is by measuring the
  growth of magnetic fields over cosmic time. We present the initial
  results of a rotation measure study to search for the presence of
  coherent magnetic fields around young disk-like galaxies at z ~ 0.5. The
  S-band receiver at the VLA allows us to simultaneously observe Stokes I,
  Q, U, and V from 2-4 GHz. With these broadband polarization observations
  we apply multiple methods for determining the rotation measure of each
  source, improving the fidelity of our results. Beyond magnetogenesis,
  the results of this study also have implications for the life-cycle
  of baryons within galaxies and the composition of galactic haloes.

---------------------------------------------------------
Title: Testing a dynamical equilibrium model of the extraplanar
    diffuse ionized gas in NGC 891.
Authors: Boettcher, E.; Zweibel, E. G.; Gallagher, J. S., III;
   Benjamin, R. A.
2017yCat..18320118B    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Global Simulations of Galactic Winds Including Cosmic-ray
    Streaming
Authors: Ruszkowski, Mateusz; Yang, H. -Y. Karen; Zweibel, Ellen
2017ApJ...834..208R    Altcode: 2016arXiv160204856R
  Galactic outflows play an important role in galactic evolution. Despite
  their importance, a detailed understanding of the physical mechanisms
  responsible for the driving of these winds is lacking. In an effort to
  gain more insight into the nature of these flows, we perform global
  three-dimensional magnetohydrodynamical simulations of an isolated
  Milky Way-size starburst galaxy. We focus on the dynamical role
  of cosmic rays (CRs) injected by supernovae, and specifically on
  the impact of the streaming and anisotropic diffusion of CRs along
  the magnetic fields. We find that these microphysical effects can
  have a significant effect on the wind launching and mass loading
  factors, depending on the details of the plasma physics. Due to the
  CR streaming instability, CRs propagating in the interstellar medium
  scatter on self-excited Alfvén waves and couple to the gas. When the
  wave growth due to the streaming instability is inhibited by some
  damping process, such as turbulent damping, the coupling of CRs to
  the gas is weaker and their effective propagation speed faster than
  the Alfvén speed. Alternatively, CRs could scatter from “extrinsic
  turbulence” that is driven by another mechanism. We demonstrate
  that the presence of moderately super-Alfvénic CR streaming enhances
  the efficiency of galactic wind driving. Cosmic rays stream away from
  denser regions near the galactic disk along partially ordered magnetic
  fields and in the process accelerate more tenuous gas away from the
  galaxy. For CR acceleration efficiencies broadly consistent with the
  observational constraints, CRs reduce the galactic star formation
  rates and significantly aid in launching galactic winds.

---------------------------------------------------------
Title: Detection of an ∼20 kpc coherent magnetic field in the
outskirt of merging spirals: the Antennae galaxies
Authors: Basu, Aritra; Mao, S. A.; Kepley, Amanda A.; Robishaw,
   Timothy; Zweibel, Ellen G.; Gallagher, John. S., III
2017MNRAS.464.1003B    Altcode: 2016MNRAS.tmp.1470B; 2016arXiv160904266B
  We present a study of the magnetic field properties of NGC 4038/9 (the
  `Antennae' galaxies), the closest example of a late stage merger of
  two spiral galaxies. Wideband polarimetric observations were performed
  using the Karl G. Jansky Very Large Array between 2 and 4 GHz. Rotation
  measure synthesis and Faraday depolarization analysis was performed to
  probe the magnetic field strength and structure at spatial resolution
  of ∼1 kpc. Highly polarized emission from the southern tidal tail is
  detected with intrinsic fractional polarization close to the theoretical
  maximum (0.62 ± 0.18), estimated by fitting the Faraday depolarization
  with a volume that is both synchrotron emitting and Faraday rotating
  containing random magnetic fields. Magnetic fields are well aligned
  along the tidal tail and the Faraday depths shows large-scale smooth
  variations preserving its sign. This suggests the field in the plane of
  the sky to be regular up to ∼20 kpc, which is the largest detected
  regular field structure on galactic scales. The equipartition field
  strength of ∼ 8.5 μG of the regular field in the tidal tail is
  reached within a few 100 Myr, likely generated by stretching of the
  galactic disc field by a factor of 4-9 during the tidal interaction. The
  regular field strength is greater than the turbulent fields in the tidal
  tail. Our study comprehensively demonstrates, although the magnetic
  fields within the merging bodies are dominated by strong turbulent
  magnetic fields of ∼ 20 μG in strength, tidal interactions can
  produce large-scale regular field structure in the outskirts.

---------------------------------------------------------
Title: Cosmic Ray Acceleration by a Versatile Family of Galactic
    Wind Termination Shocks
Authors: Bustard, Chad; Zweibel, Ellen G.; Cotter, Cory
2017ApJ...835...72B    Altcode: 2016arXiv161006565B
  There are two distinct breaks in the cosmic ray (CR) spectrum: the
  so-called “knee” around 3 × 10<SUP>15</SUP> eV and the so-called
  “ankle” around 10<SUP>18</SUP> eV. Diffusive shock acceleration
  (DSA) at supernova remnant (SNR) shock fronts is thought to accelerate
  galactic CRs to energies below the knee, while an extragalactic origin
  is presumed for CRs with energies beyond the ankle. CRs with energies
  between 3 × 10<SUP>15</SUP> and 10<SUP>18</SUP> eV, which we dub
  the “shin,” have an unknown origin. It has been proposed that DSA
  at galactic wind termination shocks, rather than at SNR shocks, may
  accelerate CRs to these energies. This paper uses the galactic wind
  model of Bustard et al. to analyze whether galactic wind termination
  shocks may accelerate CRs to shin energies within a reasonable
  acceleration time and whether such CRs can subsequently diffuse back
  to the Galaxy. We argue for acceleration times on the order of 100 Myr
  rather than a few billion years, as assumed in some previous works,
  and we discuss prospects for magnetic field amplification at the shock
  front. Ultimately, we generously assume that the magnetic field is
  amplified to equipartition. This formalism allows us to obtain analytic
  formulae, applicable to any wind model, for CR acceleration. Even
  with generous assumptions, we find that very high wind velocities
  are required to set up the necessary conditions for acceleration
  beyond 10<SUP>17</SUP> eV. We also estimate the luminosities of CRs
  accelerated by outflow termination shocks, including estimates for
  the Milky Way wind.

---------------------------------------------------------
Title: Testing a Dynamical Equilibrium Model of the Extraplanar
    Diffuse Ionized Gas in NGC 891
Authors: Boettcher, Erin; Zweibel, Ellen G.; Gallagher, J. S., III;
   Benjamin, Robert A.
2016ApJ...832..118B    Altcode: 2016arXiv160907491B
  The observed scale heights of extraplanar diffuse ionized gas
  (eDIG) layers exceed their thermal scale heights by a factor of a
  few in the Milky Way and other nearby edge-on disk galaxies. Here,
  we test a dynamical equilibrium model of the eDIG layer in NGC
  891, where we ask whether the thermal, turbulent, magnetic field,
  and cosmic-ray pressure gradients are sufficient to support the
  layer. In optical emission-line spectroscopy from the SparsePak
  integral field unit on the WIYN 3.5 m telescope, the Hα emission
  in position-velocity space suggests that the eDIG is found in a ring
  between galactocentric radii of {R}<SUB>\min </SUB>≤slant R≤slant
  8 {kpc}, where {R}<SUB>\min </SUB>≥slant 2 {kpc}. We find that the
  thermal ({σ }<SUB>{th</SUB>}=11 km s<SUP>-1</SUP>) and turbulent
  ({σ }<SUB>{turb</SUB>}=25 km s<SUP>-1</SUP>) velocity dispersions are
  insufficient to satisfy the hydrostatic equilibrium equation given an
  exponential electron scale height of {h}<SUB>z</SUB>=1.0 {kpc}. Using
  a literature analysis of radio continuum observations from the CHANG-ES
  survey, we demonstrate that the magnetic field and cosmic-ray pressure
  gradients are sufficient to stably support the gas at R≥slant 8
  kpc if the cosmic rays are sufficiently coupled to the system ({γ
  }<SUB>{cr</SUB>}=1.45). Thus, a stable dynamical equilibrium model
  is viable only if the eDIG is found in a thin ring around R = 8 kpc,
  and nonequilibrium models such as a galactic fountain flow are of
  interest for further study.

---------------------------------------------------------
Title: HST imaging of the dusty filaments and nucleus swirl in
    NGC4696 at the centre of the Centaurus Cluster
Authors: Fabian, A. C.; Walker, S. A.; Russell, H. R.; Pinto, C.;
   Canning, R. E. A.; Salome, P.; Sanders, J. S.; Taylor, G. B.; Zweibel,
   E. G.; Conselice, C. J.; Combes, F.; Crawford, C. S.; Ferland, G. J.;
   Gallagher, J. S., III; Hatch, N. A.; Johnstone, R. M.; Reynolds, C. S.
2016MNRAS.461..922F    Altcode: 2016MNRAS.tmp.1005F; 2016arXiv160602436F
  Narrow-band HST imaging has resolved the detailed internal structure of
  the 10 kpc diameter H α+[N II] emission line nebulosity in NGC4696,
  the central galaxy in the nearby Centaurus cluster, showing that
  the dusty, molecular, filaments have a width of about 60 pc. Optical
  morphology and velocity measurements indicate that the filaments are
  dragged out by the bubbling action of the radio source as part of the
  active galactic nucleus feedback cycle. Using the drag force we find
  that the magnetic field in the filaments is in approximate pressure
  equipartition with the hot gas. The filamentary nature of the cold
  gas continues inwards, swirling around and within the Bondi accretion
  radius of the central black hole, revealing the magnetic nature of the
  gas flows in massive elliptical galaxies. HST imaging resolves the
  magnetic, dusty, molecular filaments at the centre of the Centaurus
  cluster to a swirl around and within the Bondi radius.

---------------------------------------------------------
Title: A Versatile Family of Galactic Wind Models
Authors: Bustard, Chad; Zweibel, Ellen G.; D'Onghia, Elena
2016ApJ...819...29B    Altcode: 2015arXiv150907130B
  We present a versatile family of model galactic outflows including
  non-uniform mass and energy source distributions, a gravitational
  potential from an extended mass source, and radiative losses. The model
  easily produces steady-state wind solutions for a range of mass-loading
  factors, energy-loading factors, galaxy mass, and galaxy radius. We find
  that, with radiative losses included, highly mass-loaded winds must be
  driven at high central temperatures, whereas low mass-loaded winds can
  be driven at low temperatures just above the peak of the cooling curve,
  meaning radiative losses can drastically affect the wind solution
  even for low mass-loading factors. By including radiative losses,
  we are able to show that subsonic flows can be ignored as a possible
  mechanism for expelling mass and energy from a galaxy compared to
  the more efficient transonic solutions. Specifically, the transonic
  solutions with low mass loading and high energy loading are the most
  efficient. Our model also produces low-temperature, high-velocity
  winds that could explain the prevalence of low-temperature material
  in observed outflows. Finally, we show that our model, unlike the
  well-known Chevalier &amp; Clegg model, can reproduce the observed
  linear relationship between wind X-ray luminosity and star formation
  rate (SFR) over a large range of SFR from 1-1000 M<SUB>⊙</SUB>
  yr<SUP>-1</SUP> assuming the wind mass-loading factor is higher for
  low-mass, and hence, low-SFR galaxies. We also constrain the allowed
  mass-loading factors that can fit the observed X-ray luminosity versus
  SFR trend, further suggesting an inverse relationship between mass
  loading and SFR as explored in advanced numerical simulations.

---------------------------------------------------------
Title: Equipartition and cosmic ray energy densities in central
    molecular zones of starbursts
Authors: Yoast-Hull, Tova M.; Gallagher, John S.; Zweibel, Ellen G.
2016MNRAS.457L..29Y    Altcode: 2015arXiv151200786Y
  The energy densities in magnetic fields and cosmic rays (CRs) in
  galaxies are often assumed to be in equipartition, allowing for an
  indirect estimate of the magnetic field strength from the observed radio
  synchrotron spectrum. However, both primary and secondary CRs contribute
  to the synchrotron spectrum, and the CR electrons also loose energy
  via bremsstrahlung and inverse Compton. While classical equipartition
  formulae avoid these intricacies, there have been recent revisions that
  account for the extreme conditions in starbursts. Yet, the application
  of the equipartition formula to starburst environments also presupposes
  that time-scales are long enough to reach equilibrium. Here, we test
  equipartition in the central molecular zones (CMZs) of nearby starburst
  galaxies by modelling the observed γ-ray spectra, which provide
  a direct measure of the CR energy density, and the radio spectra,
  which provide a probe of the magnetic field strength. We find that in
  starbursts, the magnetic field energy density is significantly larger
  than the CR energy density, demonstrating that the equipartition
  argument is frequently invalid for CMZs.

---------------------------------------------------------
Title: A search for magnetic fields around disk-like galaxies at z~0.5
Authors: Williams, A.; Lundgren, B.; Mao, S. A.; Wilcots, E.; York,
   D. G.; Zweibel, E.
2015fers.confE..12W    Altcode: 2015PoS...267E..12W
  No abstract at ADS

---------------------------------------------------------
Title: Cosmic rays, γ-rays, and neutrinos in the starburst nuclei
    of Arp 220
Authors: Yoast-Hull, Tova M.; Gallagher, John. S.; Zweibel, Ellen G.
2015MNRAS.453..222Y    Altcode: 2015arXiv150605133Y
  The cores of Arp 220, the closest ultraluminous infrared starburst
  galaxy, provide an opportunity to study interactions of cosmic rays
  under extreme conditions. In this paper, we model the populations of
  cosmic rays produced by supernovae in the central molecular zones of
  both starburst nuclei. We find that ∼65-100 per cent of cosmic rays
  are absorbed in these regions due to their huge molecular gas contents,
  and thus, the nuclei of Arp 220 nearly complete proton calorimeters. As
  the cosmic ray protons collide with the interstellar medium, they
  produce secondary electrons that are also contained within the system
  and radiate synchrotron emission. Using results from χ<SUP>2</SUP>
  tests between the model and the observed radio spectral energy
  distribution, we predict the emergent γ-ray and high-energy neutrino
  spectra and find the magnetic field to be at milligauss levels. Because
  of the extremely intense far-infrared radiation fields, the γ-ray
  spectrum steepens significantly at TeV energies due to γ-γ absorption.

---------------------------------------------------------
Title: The Wisconsin Plasma Astrophysics Laboratory
Authors: Forest, C. B.; Flanagan, K.; Brookhart, M.; Clark, M.; Cooper,
   C. M.; Désangles, V.; Egedal, J.; Endrizzi, D.; Khalzov, I. V.; Li,
   H.; Miesch, M.; Milhone, J.; Nornberg, M.; Olson, J.; Peterson, E.;
   Roesler, F.; Schekochihin, A.; Schmitz, O.; Siller, R.; Spitkovsky,
   A.; Stemo, A.; Wallace, J.; Weisberg, D.; Zweibel, E.
2015JPlPh..81e3401F    Altcode: 2015arXiv150607195F
  &gt; The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a
  flexible user facility designed to study a range of astrophysically
  relevant plasma processes as well as novel geometries that mimic
  astrophysical systems. A multi-cusp magnetic bucket constructed from
  strong samarium cobalt permanent magnets now confines a 3$]]&gt; , fully
  ionized, magnetic-field-free plasma in a spherical geometry. Plasma
  parameters of e\approx 5$]]&gt; to and e\approx 10<SUP>11</SUP>$]]&gt;
  to 12~\text{cm}<SUP>-3</SUP>$]]&gt; provide an ideal testbed for a
  range of astrophysical experiments, including self-exciting dynamos,
  collisionless magnetic reconnection, jet stability, stellar winds
  and more. This article describes the capabilities of WiPAL, along
  with several experiments, in both operating and planning stages,
  that illustrate the range of possibilities for future users.

---------------------------------------------------------
Title: Gamma-Ray and Cosmic Ray Escape in Intensely Star-Forming
    Systems
Authors: Yoast-Hull, T.; Gallagher, J. S.; Zweibel, E.
2015ICRC...34..926Y    Altcode: 2015PoS...236..926Y
  No abstract at ADS

---------------------------------------------------------
Title: Stability properties of phase transition layers in the diffuse
    ISM revisited
Authors: Stone, Jennifer M.; Inutsuka, Shu-Ichiro; Zweibel, Ellen G.
2015HiA....16..611S    Altcode:
  In a thermally bistable medium, cold, dense gas is separated from
  warm, rarified gas by thin phase transition layers, or fronts, in
  which radiative heating/cooling, thermal conduction, and convection
  of material are balanced. While these fronts have received only
  scant attention in the literature, and are not resolved by most
  current numerical simulations, they have been shown to have important
  ramifications for transport processes and structure formation in the
  diffuse interstellar medium. Here, we discuss calculations of their
  hydrodynamic and magnetohydrodynamic stability properties.

---------------------------------------------------------
Title: Properties of the Magneto-ionic Medium in the Halo of M51
    Revealed by Wide-band Polarimetry
Authors: Mao, S. A.; Zweibel, E.; Fletcher, A.; Ott, J.; Tabatabaei, F.
2015ApJ...800...92M    Altcode: 2014arXiv1412.8320M
  We present a study of the magneto-ionic medium in the Whirlpool
  galaxy (M51) using new wide-band multi-configuration polarization
  data at L band (1-2 GHz) obtained at the Karl G. Jansky Very Large
  Array. By fitting the observed diffuse complex polarization Q+iU as
  a function of wavelength directly to various depolarization models,
  we find that polarized emission from M51 at 1-2 GHz originates from
  the top of the synchrotron disk and then experiences Faraday rotation
  in the near-side thermal halo of the galaxy. Thus, the scale height of
  the thermal gas must exceed that of the synchrotron emitting gas at L
  band. The observed Faraday depth distribution at L band is consistent
  with a halo field that is comprised of a plane-parallel bisymmetric
  component and a vertical component that produces a Faraday rotation of
  ~-9 rad m<SUP>-2</SUP>. The derived rotation measure (RM) structure
  functions indicate a characteristic scale of RM fluctuations of less
  than 560 pc in the disk and approximately 1 kpc in the halo. The outer
  scale of turbulence of 1 kpc found in the halo of M51 is consistent
  with superbubbles and the Parker instability being the main energy
  injection mechanisms in galactic halos.

---------------------------------------------------------
Title: 3D Study Of Magnetic Fields In NGC 6946
Authors: Williams, Anna; Heald, George; Wilcots, Eric; Zweibel, Ellen
2015IAUS..309..351W    Altcode:
  Recent advancements in both radio observatories and computing have
  opened a new regime of 3D observations. Not only do these instruments
  measure emission lines and radio continuum over much larger bandpasses,
  but they also simultaneously observe the polarized emission over the
  same large bandpasses with increased sensitivity. This “polarization
  spectrum" can be used to recover information about the 3D structure of
  magnetic fields in the universe. Our combined 3-20~cm observations of
  NGC 6946 taken with the Westerbork Synthesis Radio Telescope provide
  highly sensitive diagnostics of the internal depolarization across
  the galaxy. We use model fitting to determine likely mechanisms
  for depolarization in different regions of the galaxy, and glean
  information about the coherent and turbulent magnetic fields in NGC
  6946. We produce Faraday dispersion maps that illustrate how we can
  probe different depths into the galaxy at different wavelengths and
  display new features of the line of sight magnetic field. This work
  is just a sample of the new 3D studies that are possible with upgraded
  and new radio instruments like the VLA, ATCA, and SKA.

---------------------------------------------------------
Title: Ambipolar Diffusion
Authors: Zweibel, Ellen G.
2015ASSL..407..285Z    Altcode:
  When magnetic forces are present in a partially ionized medium, the
  plasma drifts with respect to the neutrals. This plasma—neutral
  drift, which is known as ambipolar diffusion, occurs in all partially
  ionized astrophysical systems, including portions of the interstellar
  medium, protostellar accretion disks, and the chromosphere of the
  Sun and other cool stars. Ambipolar drift redistributes magnetic
  flux, which can trigger star formation. It affects short wavelength
  interstellar turbulence, the structure of interstellar shocks, flow
  driven instabilities, and the nature of magnetic reconnection. Energy
  dissipated by ion-neutral friction can be an important source of
  heat. This chapter reviews ambipolar drift as a process and discusses
  some of the implications.

---------------------------------------------------------
Title: Onset and Evolution of Magnetic Reconnection in Line-Tied
    Systems
Authors: Daughton, W. S.; Akcay, C.; Billey, Z.; Finn, J.; Zweibel,
   E.; Gekelman, W. N.
2014AGUFMSH22A..04D    Altcode:
  In space and astrophysical plasmas, current sheets arise spontaneously
  from the interaction of large-scale flows or magnetic structures. As
  these current layers approach kinetic scales, they may become unstable
  to the collisionless tearing instability, resulting in the formation
  and interaction of magnetic flux ropes. While theoretical treatments
  of the tearing instability have largely focused on 1D equilibria with
  periodic boundary conditions, current sheets in nature have a finite
  spatial extent and are embedded within larger open systems. In many
  applications, the field boundary conditions are line-tied as in the
  case of flux ropes on the dayside magnetopause where the ionosphere
  acts as a conducting surface. To assess the applicability of existing
  tearing theory to these more realistic configurations, we consider
  a series of 3D kinetic simulations of initially force-free current
  layers with line-tied boundary conditions for the fields, and open
  boundaries for the particles. The geometry and plasma parameters are
  motivated by a new laboratory experiment on the Large Plasma Device at
  UCLA. For sufficiently long systems, we demonstrate that key aspects
  of the theory remain valid, and a threshold condition is derived for
  the onset of reconnection in line-tied systems. To gain additional
  insight into the nonlinear evolution, field-line mapping diagnostics
  are employed to characterize the 3D structure of the magnetic field,
  the nonlinear reconnection rate and the dominant non-ideal terms in
  the generalized Ohm's law.

---------------------------------------------------------
Title: Conduction in Low Mach Number Flows. I. Linear and Weakly
    Nonlinear Regimes
Authors: Lecoanet, Daniel; Brown, Benjamin P.; Zweibel, Ellen G.;
   Burns, Keaton J.; Oishi, Jeffrey S.; Vasil, Geoffrey M.
2014ApJ...797...94L    Altcode: 2014arXiv1410.5424L
  Thermal conduction is an important energy transfer and damping
  mechanism in astrophysical flows. Fourier's law, in which the heat
  flux is proportional to the negative temperature gradient, leading
  to temperature diffusion, is a well-known empirical model of thermal
  conduction. However, entropy diffusion has emerged as an alternative
  thermal conduction model, despite not ensuring the monotonicity of
  entropy. This paper investigates the differences between temperature
  and entropy diffusion for both linear internal gravity waves and
  weakly nonlinear convection. In addition to simulating the two thermal
  conduction models with the fully compressible Navier-Stokes equations,
  we also study their effects in the reduced "soundproof" anelastic
  and pseudoincompressible (PI) equations. We find that in the linear
  and weakly nonlinear regime, temperature and entropy diffusion give
  quantitatively similar results, although there are some larger errors
  in the PI equations with temperature diffusion due to inaccuracies in
  the equation of state. Extrapolating our weakly nonlinear results, we
  speculate that differences between temperature and entropy diffusion
  might become more important for strongly turbulent convection.

---------------------------------------------------------
Title: Proton Calorimetry and Gamma-Rays in Arp 220
Authors: Yoast-Hull, Tova; Gallagher, John S.; Zweibel, Ellen Gould
2014HEAD...1410703Y    Altcode:
  Until recently, it was thought that starburst galaxies were both
  electron and proton calorimeters, making them especially bright in
  gamma-rays. However, with detections of starburst galaxies M82 and NGC
  253 by Fermi, HESS, and VERITAS, we find that such galaxies are only
  partial proton calorimeters due to significant advection by galactic
  winds. Thus, to find cosmic-ray proton calorimeters, we must look for
  much denser systems. Previous models of the cosmic ray interactions in
  Arp 220 (e.g. Torres 2004) suggest it is a proton calorimeter and that
  it should already be detectable by Fermi. The Torres model suggests
  that if Arp 220 is a calorimeter, then it should have been detected in
  gamma-rays by Fermi at levels above current upper limits. We therefore
  must question. whether Arp 220 is a true proton calorimeter, and if
  so what other properties could be responsible for its low gamma ray
  flux. Here, we further explore the observed ranges on environmental
  properties and model the central nuclei to predict both the radio
  and gamma-ray spectra. We test the proton calorimetry hypothesis and
  estimate the observation time needed for a detection by Fermi for a
  range of assumptions about conditions in Arp 220.

---------------------------------------------------------
Title: The Cosmic-Ray Population of the Galactic Central Molecular
    Zone
Authors: Yoast-Hull, Tova M.; Gallagher, J. S., III; Zweibel, Ellen G.
2014ApJ...790...86Y    Altcode: 2014arXiv1405.7059Y
  The conditions in the Galactic Center are often compared with those
  in starburst systems, which contain higher supernova rates, stronger
  magnetic fields, more intense radiation fields, and larger amounts
  of dense molecular gas than in our own Galactic disk. Interactions
  between such an augmented interstellar medium and cosmic rays result
  in brighter radio and γ-ray emission. Here, we test how well the
  comparisons between the Galactic Center and starburst galaxies hold by
  applying a model for cosmic-ray interactions to the Galactic Center
  to predict the resulting γ-ray emission. The model only partially
  explains the observed γ-ray and radio emission. The model for the
  γ-ray spectrum agrees with the data at TeV energies but not at GeV
  energies. Additionally, as the fits of the model to the radio and γ-ray
  spectra require significant differences in the optimal wind speed and
  magnetic field strength, we find that the single-zone model alone cannot
  account for the observed emission from the Galactic Center. Our model
  is improved by including a soft, additional cosmic-ray population. We
  assess such a cosmic-ray population and its potential sources and find
  that a cosmic-ray electron spectrum is energetically favored over a
  cosmic-ray proton spectrum.

---------------------------------------------------------
Title: Magnetic Reconnection Turbulence in Strong Guide Fields:
    Basic Properties and Application to Coronal Heating
Authors: Pueschel, M. J.; Told, D.; Terry, P. W.; Jenko, F.; Zweibel,
   E. G.; Zhdankin, V.; Lesch, H.
2014ApJS..213...30P    Altcode:
  A current sheet susceptible to the tearing instability is used to
  drive reconnection turbulence in the presence of a strong guide
  field. Through nonlinear gyrokinetic simulations, the dependencies
  of central quantities such as the heating rate on parameters like
  collisionality or plasma β are studied, revealing that linear physics
  tends to predict only some aspects of the quasi-saturated state, with
  the nonlinear cascade responsible for additional features. For the solar
  corona, it is demonstrated that the kinetic heating associated with this
  type of turbulence agrees quantitatively with observational volumetric
  heating rates. In the context of short particle acceleration events, the
  self-consistent emergence of plasmoids or flux ropes in the turbulent
  bath is found to be important: ubiquitously occurring merger events of
  these objects cause strong bursts in the heating rate, the timescale
  of which is consistent with nanoflare observations. Furthermore,
  anisotropy of the temperature fluctuations is seen to emerge, hinting
  at a new means of generating coronal ion temperature anisotropy in
  the absence of cyclotron resonances.

---------------------------------------------------------
Title: The Transport of Cosmic Rays Across Magnetic Fieldlines
Authors: Desiati, Paolo; Zweibel, Ellen G.
2014ApJ...791...51D    Altcode: 2014arXiv1402.1475D
  The long residence times and small anisotropies of cosmic rays suggest
  that they are well confined and well scattered by the Galactic magnetic
  field. Due to the disk-like shape of the confinement volume, transport
  in the vertical direction, which is perpendicular to the mean Galactic
  magnetic field, is key to cosmic ray escape. It has long been recognized
  that this vertical transport depends both on the vertical component of
  the field lines themselves and on the extent to which the cosmic rays
  are tied to the field lines. In this paper, we use magnetic fields
  with very simple spatial and temporal structures to isolate some
  important features of cross field line transport. We show that even
  simple magnetic nonuniformities combined with pitch angle scattering
  can enhance cross field line transport by several orders of magnitude,
  while pitch angle scattering is unnecessary for enhanced transport if
  the field is chaotic. Nevertheless, perpendicular transport is much
  less than parallel transport in all the cases we study. We apply the
  results to confinement of cosmic rays in the Fermi bubbles.

---------------------------------------------------------
Title: The Galactic center: a model for cosmic ray interactions in
    starburst galaxies?
Authors: Yoast-Hull, T.; Gallagher, J. S.; Zweibel, E.
2014IAUS..303..153Y    Altcode:
  The Galactic center contains strong magnetic fields, high radiation
  fields, and dense molecular gas, as is also the case in starburst
  galaxies. The close proximity of the Galactic center allows for more and
  better observations of the interstellar medium than for extragalactic
  sources making it an ideal place for testing models for cosmic ray
  interactions. We compare our semi-analytic model for cosmic ray
  interactions to published data for both the Galactic center and the
  starburst galaxy NGC 253. We present the predicted radio and γ-ray
  spectra and compare the results with published measurements. In this
  way we provide a quantitative basis for assessing the degree to which
  the Galactic center resembles a starburst system.

---------------------------------------------------------
Title: Lessons from comparisons between the nuclear region of the
    Milky Way and those in nearby spirals
Authors: Gallagher, John S.; Yoast-Hull, Tova M.; Zweibel, Ellen G.
2014IAUS..303...61G    Altcode: 2013arXiv1312.4433G
  The Milky Way appears as a typical barred spiral, and comparisons can be
  made between its nuclear region and those of structurally similar nearby
  spirals. Maffei 2, M83, IC 342 and NGC 253 are nearby systems whose
  nuclear region properties contrast with those of the Milky Way. Stellar
  masses derived from NIR photometery, molecular gas masses and star
  formation rates allow us to assess the evolutionary states of this set
  of nuclear regions. These data suggest similarities between nuclear
  regions in terms of their stellar content while highlighting significant
  differences in current star formation rates. In particular current
  star formation rates appear to cover a larger range than expected
  based on the molecular gas masses. This behavior is consistent with
  nuclear region star formation experiencing episodic variations. Under
  this hypothesis the Milky Way's nuclear region currently may be in a
  low star formation rate phase.

---------------------------------------------------------
Title: Fermi bubble simulations: black hole feedback in the Milky Way
Authors: Ruszkowski, M.; Yang, H. -Y. K.; Zweibel, E.
2014IAUS..303..390R    Altcode: 2013arXiv1311.6159R
  The Fermi γ-ray telescope discovered a pair of bubbles at the
  Galactic center. These structures are spatially-correlated with the
  microwave emission detected by the WMAP and Planck satellites. These
  bubbles were likely inflated by a jet launched from the vicinity of a
  supermassive black hole in the Galactic center. Using MHD simulations,
  which self-consistently include interactions between cosmic rays and
  magnetic fields, we build models of the supersonic jet propagation,
  cosmic ray transport, and the magnetic field amplification within the
  Fermi bubbles. Our key findings are that: (1) the synthetic Fermi γ-ray
  and WMAP microwave spectra based on our simulations are consistent
  with the observations, suggesting that a single population of cosmic
  ray leptons may simultaneously explain the emission across a range
  of photon energies; (2) the model fits the observed centrally-peaked
  microwave emission if a second, more recent, pair of jets embedded in
  the Fermi bubbles is included in the model. This is consistent with the
  observationally-based suggestion made by Su &amp; Finkbeiner (2012);
  (3) the radio emission from the bubbles is expected to be strongly
  polarized due to the relatively high level of field ordering caused by
  elongated turbulent vortices. This effect is caused by the interaction
  of the shocks driven by the jets with the preexisting interstellar
  medium turbulence; (4) a layer of enhanced rotation measure in the
  shock-compressed region could exist in the bubble vicinity but the level
  of this enhancement depends on the details of the magnetic topology.

---------------------------------------------------------
Title: Wide-band Jansky Very Large Array polarization observations
    of M51
Authors: Mao, Sui Ann; Ott, J.; Zweibel, E. G.
2014AAS...22345308M    Altcode:
  We present new L band (1-2 GHz) multi-configuration Jansky Very Large
  Array polarization observations of M51. Using this new data set,
  we are able to, for the first time, perform direct fits to Stokes
  Q and U of the diffuse polarized emission from external galaxies
  as a function of wavelength to various depolarization models. The
  measured polarized emission as a function of wavelength in L band
  is consistent with Faraday rotation in an external screen in M51's
  near-side halo. The distribution of rotation measure across M51 can
  be explained by the presence of a halo magnetic field, which has
  a bisymmetric plane-parallel component and a coherent perpendicular
  component. Future observations of M51 below 1 GHz and above 2 GHz will
  enable one to model its disk and halo field simultaneously.

---------------------------------------------------------
Title: The Fermi bubbles: gamma-ray, microwave and polarization
    signatures of leptonic AGN jets
Authors: Yang, Hsiang-Yi Karen; Ruszkowski, M.; Zweibel, E. G.
2014AAS...22334634Y    Altcode:
  The origin of the two large bubbles at the Galactic Centre observed
  by the Fermi Gamma-ray Space Telescope and the spatially correlated
  microwave haze emission are yet to be determined. To disentangle
  different models requires detailed comparisons between theoretical
  predictions and multiwavelength observations. Our previous simulations,
  which self-consistently include interactions between cosmic rays (CRs)
  and magnetic fields, have demonstrated that the primary features of the
  Fermi bubbles could be successfully reproduced by recent jet activity
  from the central active galactic nucleus (AGN). In this work, we
  generate gamma-ray and microwave maps and spectra based on the simulated
  properties of CRs and magnetic fields in order to examine whether
  the observed bubble and haze emission could be explained by leptons
  contained in the AGN jets. We also investigate the model predictions
  of the polarization properties of the Fermi bubbles, including the
  polarization fractions and the rotation measures (RMs). We find that
  (1) the same population of leptons can simultaneously explain the bubble
  and haze emission given that the magnetic fields within the bubbles are
  very close to the exponentially distributed ambient field, which can be
  explained by mixing in of the ambient field followed by turbulent field
  amplification. (2) The centrally peaked microwave profile suggests CR
  replenishment, which is consistent with the presence of a more recent
  second jet event. (3) The bubble interior exhibits a high degree of
  polarization because of ordered radial magnetic field lines stretched by
  elongated vortices behind the shocks. (4) Enhancement of RMs could exist
  within the shock-compressed layer because of increased gas density and
  more amplified and ordered magnetic fields. We discuss the possibility
  that the deficient haze emission at b&lt;-35 degrees is due to the
  suppression of magnetic fields, which is consistent with the existence
  of lower energy CRs causing the polarized emission at 2.3 GHz. Possible
  AGN jet composition in the leptonic scenario is also discussed.

---------------------------------------------------------
Title: Do Cosmic Rays Sample the Mean ISM Density of Starburst
    Galaxies?
Authors: Boettcher, Erin; Zweibel, E. G.; Yoast-Hull, T.; Gallagher,
   J. S.
2014AAS...22325208B    Altcode:
  In studies of interacting cosmic rays and the interstellar medium
  (ISM) in starburst galaxies, it is often assumed that cosmic rays
  sample the mean density of the ISM. However, given the very high
  galactic wind speeds and the very small filling factors of fragmented
  molecular clouds, this is far from a foregone conclusion in starburst
  environments. Here, we use Monte Carlo simulations to assess the
  assumption that cosmic rays sample the mean density of a two-phase
  ISM consisting of molecular clouds embedded in a hot, low density
  medium. We simulate cosmic ray propagation in a tangled magnetic
  field with vertical advection and a variety of injection scenarios
  in a medium with properties similar to those of the prototypical
  starburst galaxy M82. The ratio of the sampled density to the mean
  density is calculated by comparing the gamma-ray emissivity from pion
  production in molecular clouds implied by our simulations and by cosmic
  ray sampling of the mean density. This ratio remains close to unity
  over a wide range of conditions on the number of molecular clouds, the
  galactic wind speed, the magnetic field geometry, and the cosmic ray
  injection mechanism. However, this ratio becomes elevated by a factor
  of a few when the cosmic rays are injected close to a small number
  of dense molecular clouds in the presence of a very tangled magnetic
  field. Using the results of our simulations, we evaluate the cosmic
  ray calorimeter model for starburst galaxies, and we argue that our
  simulated starburst region is at best a partial proton calorimeter. We
  acknowledge the support of NSF AST-0907837 and NSF PHY-0821899.

---------------------------------------------------------
Title: Microphysics in Astrophysical Plasmas
Authors: Schwartz, Steven J.; Zweibel, Ellen G.; Goldman, Martin
2014mpcp.book....5S    Altcode: 2014mcp..book....5S
  Although macroscale features dominate astrophysical images and
  energetics, the physics is controlled through microscale transport
  processes (conduction, diffusion) that mediate the flow of mass,
  momentum, energy, and charge. These microphysical processes manifest
  themselves in key (all) boundary layers and also operate within the
  body of the plasma. Crucially, most plasmas of interest are rarefied
  to the extent that classical particle collision length- and time-scales
  are long. Collective plasma kinetic phenomena then serve to scatter or
  otherwise modify the particle distribution functions and in so-doing
  govern the transport at the microscale level. Thus collisionless plasmas
  are capable of supporting thin shocks, current sheets which may be
  prone to magnetic reconnection, and the dissipation of turbulence
  cascades at kinetic scales. This paper lays the foundation for the
  accompanying collection that explores the current state of knowledge
  in this subject. The richness of plasma kinetic phenomena brings with
  it a rich diversity of microphysics that does not always, if ever,
  simply mimic classical collision-dominated transport. This can couple
  the macro- and microscale physics in profound ways, and in ways which
  thus depend on the astrophysical context.

---------------------------------------------------------
Title: Active Galactic Nuclei, Neutrinos, and Interacting Cosmic
    Rays in NGC 253 and NGC 1068
Authors: Yoast-Hull, Tova M.; Gallagher, J. S., III; Zweibel, Ellen
   G.; Everett, John E.
2014ApJ...780..137Y    Altcode: 2013arXiv1311.5586Y
  The galaxies M82, NGC 253, NGC 1068, and NGC 4945 have been detected
  in γ-rays by Fermi. Previously, we developed and tested a model for
  cosmic-ray interactions in the starburst galaxy M82. Now, we aim to
  explore the differences between starburst and active galactic nucleus
  (AGN) environments by applying our self-consistent model to the
  starburst galaxy NGC 253 and the Seyfert galaxy NGC 1068. Assuming a
  constant cosmic-ray acceleration efficiency by supernova remnants with
  Milky Way parameters, we calculate the cosmic-ray proton and primary
  and secondary electron/positron populations, predict the radio and
  γ-ray spectra, and compare with published measurements. We find that
  our models easily fit the observed γ-ray spectrum for NGC 253 while
  constraining the cosmic-ray source spectral index and acceleration
  efficiency. However, we encountered difficultly modeling the observed
  radio data and constraining the speed of the galactic wind and the
  magnetic field strength, unless the gas mass is less than currently
  preferred values. Additionally, our starburst model consistently
  underestimates the observed γ-ray flux and overestimates the radio flux
  for NGC 1068 these issues would be resolved if the AGN is the primary
  source of γ-rays. We discuss the implications of these results and
  make predictions for the neutrino fluxes for both galaxies.

---------------------------------------------------------
Title: Cosmic Ray Sampling of a Clumpy Interstellar Medium
Authors: Boettcher, Erin; Zweibel, Ellen G.; Yoast-Hull, Tova M.;
   Gallagher, J. S., III
2013ApJ...779...12B    Altcode: 2013arXiv1311.0006B
  How cosmic rays sample the multi-phase interstellar medium (ISM) in
  starburst galaxies has important implications for many science goals,
  including evaluating the cosmic ray calorimeter model for these systems,
  predicting their neutrino fluxes, and modeling their winds. Here, we
  use Monte Carlo simulations to study cosmic ray sampling of a simple,
  two-phase ISM under conditions similar to those of the prototypical
  starburst galaxy M82. The assumption that cosmic rays sample the
  mean density of the ISM in the starburst region is assessed over a
  multi-dimensional parameter space where we vary the number of molecular
  clouds, the galactic wind speed, the extent to which the magnetic
  field is tangled, and the cosmic ray injection mechanism. We evaluate
  the ratio of the emissivity from pion production in molecular clouds
  to the emissivity that would be observed if the cosmic rays sampled
  the mean density, and seek areas of parameter space where this ratio
  differs significantly from unity. The assumption that cosmic rays
  sample the mean density holds over much of parameter space; however,
  this assumption begins to break down for high cloud density, injection
  close to the clouds, and a very tangled magnetic field. We conclude by
  evaluating the extent to which our simulated starburst region behaves
  as a proton calorimeter and constructing the time-dependent spectrum
  of a burst of cosmic rays.

---------------------------------------------------------
Title: The Fermi bubbles: gamma-ray, microwave and polarization
    signatures of leptonic AGN jets
Authors: Yang, H. -Y. Karen; Ruszkowski, M.; Zweibel, E.
2013MNRAS.436.2734Y    Altcode: 2013MNRAS.tmp.2432Y; 2013arXiv1307.3551Y
  The origin of the two large bubbles at the Galactic Centre observed
  by the Fermi Gamma-ray Space Telescope and the spatially correlated
  microwave haze emission are yet to be determined. To disentangle
  different models requires detailed comparisons between theoretical
  predictions and multiwavelength observations. Our previous simulations,
  which self-consistently include interactions between cosmic rays
  (CRs) and magnetic fields, have demonstrated that the primary features
  of the Fermi bubbles could be successfully reproduced by recent jet
  activity from the central active galactic nucleus (AGN). In this work,
  we generate gamma-ray and microwave maps and spectra based on the
  simulated properties of CRs and magnetic fields in order to examine
  whether the observed bubble and haze emission could be explained by
  leptons contained in the AGN jets. We also investigate the model
  predictions of the polarization properties of the Fermi bubbles,
  including the polarization fractions and the rotation measures (RMs). We
  find that (1) the same population of leptons can simultaneously explain
  the bubble and haze emission given that the magnetic fields within the
  bubbles are very close to the exponentially distributed ambient field,
  which can be explained by mixing in of the ambient field followed
  by turbulent field amplification. (2) The centrally peaked microwave
  profile suggests CR replenishment, which is consistent with the presence
  of a more recent second jet event. (3) The bubble interior exhibits
  a high degree of polarization because of ordered radial magnetic
  field lines stretched by elongated vortices behind the shocks; highly
  polarized signals could also be observed inside the draping layer. (4)
  Enhancement of RMs could exist within the shock-compressed layer because
  of increased gas density and more amplified and ordered magnetic fields,
  though details depend on projections and the actual field geometry. We
  discuss the possibility that the deficient haze emission at b &lt;
  -35° is due to the suppression of magnetic fields, which is consistent
  with the existence of lower energy CRs causing the polarized emission
  at 2.3 GHz. Possible AGN jet composition in the leptonic scenario is
  also discussed.

---------------------------------------------------------
Title: Using Faraday rotation measure to study the formation of
    galactic-scale coherent magnetic fields
Authors: Williams, Anna; Zweibel, Ellen; Wilcots, Eric; Mao, Sui Ann;
   Lundgren, Britt
2013atnf.prop.5780W    Altcode:
  We propose to use the expanded capabilities of the CABB to obtain
  polarization observations of 50 QSOs from 1-10GHz and determine the
  Faraday rotation measure (RM) of each system. There are two main science
  goals achievable with these observations. First, we are interested
  in determining when, in cosmic time, large-scale coherent magnetic
  fields form in galaxies. We will use the linearly polarized light of
  the QSO to probe the gas in and around intervening disk-like galaxies,
  and use the RM to estimate the strength of the magnetic field. We have
  selected 50 QSOs to observe, of which 25 contain MgII absorption in
  their SDSS spectrum, indicating that there is an intervening galaxy
  along the line of sight. The other half have no such features. We will
  compare the RMs of these two samples as a function of redshift to
  determine when coherent, large-scale magnetic fields form. Second,
  the broadband continuum capabilities will help us determine
  the depolarization of each source. It is known that polarization
  observations at cm wavelengths suffer from depolarization. Many RM
  studies rely on separate observations taken at 20cm and &gt;10cm,
  but the RMs determined at separate wavelengths do not agree. ATCA's
  unique correlator from 1.1-3.1 GHz will allow us to simultaneously
  observe the L-band and part of the S-band, and the 4GHz bandwidth
  available at 4cm will allow us to determine what wavelength regime is
  best suited for future extragalactic RM studies.

---------------------------------------------------------
Title: Microphysics in Astrophysical Plasmas
Authors: Schwartz, Steven J.; Zweibel, Ellen G.; Goldman, Martin
2013SSRv..178...81S    Altcode: 2013SSRv..tmp...50S
  Although macroscale features dominate astrophysical images and
  energetics, the physics is controlled through microscale transport
  processes (conduction, diffusion) that mediate the flow of mass,
  momentum, energy, and charge. These microphysical processes manifest
  themselves in key (all) boundary layers and also operate within the
  body of the plasma. Crucially, most plasmas of interest are rarefied
  to the extent that classical particle collision length- and time-scales
  are long. Collective plasma kinetic phenomena then serve to scatter or
  otherwise modify the particle distribution functions and in so-doing
  govern the transport at the microscale level. Thus collisionless plasmas
  are capable of supporting thin shocks, current sheets which may be
  prone to magnetic reconnection, and the dissipation of turbulence
  cascades at kinetic scales. This paper lays the foundation for the
  accompanying collection that explores the current state of knowledge
  in this subject. The richness of plasma kinetic phenomena brings with
  it a rich diversity of microphysics that does not always, if ever,
  simply mimic classical collision-dominated transport. This can couple
  the macro- and microscale physics in profound ways, and in ways which
  thus depend on the astrophysical context.

---------------------------------------------------------
Title: Astrophysics: Recipe for regularity
Authors: Zweibel, Ellen
2013Natur.502..453Z    Altcode:
  A detailed astrophysical model has been laid out that not only
  reproduces the far-infrared-radio correlation for galaxies that are
  actively forming stars, but also predicts how the correlation is
  modified at high redshift.

---------------------------------------------------------
Title: Energy Conservation and Gravity Waves in Sound-proof Treatments
    of Stellar Interiors. II. Lagrangian Constrained Analysis
Authors: Vasil, Geoffrey M.; Lecoanet, Daniel; Brown, Benjamin P.;
   Wood, Toby S.; Zweibel, Ellen G.
2013ApJ...773..169V    Altcode: 2013arXiv1303.0005V
  The speed of sound greatly exceeds typical flow velocities in many
  stellar and planetary interiors. To follow the slow evolution of
  subsonic motions, various sound-proof models attempt to remove fast
  acoustic waves while retaining stratified convection and buoyancy
  dynamics. In astrophysics, anelastic models typically receive the most
  attention in the class of sound-filtered stratified models. Generally,
  anelastic models remain valid in nearly adiabatically stratified
  regions like stellar convection zones, but may break down in
  strongly sub-adiabatic, stably stratified layers common in stellar
  radiative zones. However, studying stellar rotation, circulation,
  and dynamos requires understanding the complex coupling between
  convection and radiative zones, and this requires robust equations
  valid in both regimes. Here we extend the analysis of equation sets
  begun in Brown et al., which studied anelastic models, to two types
  of pseudo-incompressible models. This class of models has received
  attention in atmospheric applications, and more recently in studies
  of white-dwarf supernova progenitors. We demonstrate that one
  model conserves energy but the other does not. We use Lagrangian
  variational methods to extend the energy conserving model to a
  general equation of state, and dub the resulting equation set the
  generalized pseudo-incompressible (GPI) model. We show that the GPI
  equations suitably capture low-frequency phenomena in both convection
  and radiative zones in stars and other stratified systems, and we
  provide recommendations for converting low-Mach number codes to this
  equation set.

---------------------------------------------------------
Title: Erratum: "Winds, Clumps, and Interacting Cosmic Rays in M82"
    <A href="/abs/2013ApJ...768...53Y">(2013, ApJ, 768, 53)</A>
Authors: Yoast-Hull, Tova M.; Everett, John E.; Gallagher, J. S.,
   III; Zweibel, Ellen G.
2013ApJ...771...73Y    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The microphysics and macrophysics of cosmic rays
Authors: Zweibel, Ellen G.
2013PhPl...20e5501Z    Altcode:
  This review paper commemorates a century of cosmic ray research,
  with emphasis on the plasma physics aspects. Cosmic rays comprise
  only ∼10<SUP>-9</SUP> of interstellar particles by number, but
  collectively their energy density is about equal to that of the thermal
  particles. They are confined by the Galactic magnetic field and well
  scattered by small scale magnetic fluctuations, which couple them to
  the local rest frame of the thermal fluid. Scattering isotropizes the
  cosmic rays and allows them to exchange momentum and energy with the
  background medium. I will review a theory for how the fluctuations
  which scatter the cosmic rays can be generated by the cosmic rays
  themselves through a microinstability excited by their streaming. A
  quasilinear treatment of the cosmic ray-wave interaction then leads
  to a fluid model of cosmic rays with both advection and diffusion by
  the background medium and momentum and energy deposition by the cosmic
  rays. This fluid model admits cosmic ray modified shocks, large scale
  cosmic ray driven instabilities, cosmic ray heating of the thermal
  gas, and cosmic ray driven galactic winds. If the fluctuations were
  extrinsic turbulence driven by some other mechanism, the cosmic ray
  background coupling would be entirely different. Which picture holds
  depends largely on the nature of turbulence in the background medium.

---------------------------------------------------------
Title: Winds, Clumps, and Interacting Cosmic Rays in M82
Authors: Yoast-Hull, Tova M.; Everett, John E.; Gallagher, J. S.,
   III; Zweibel, Ellen G.
2013ApJ...768...53Y    Altcode: 2013arXiv1303.4305Y
  We construct a family of models for the evolution of energetic particles
  in the starburst galaxy M82 and compare them to observations to test
  the calorimeter assumption that all cosmic ray energy is radiated
  in the starburst region. Assuming constant cosmic ray acceleration
  efficiency with Milky Way parameters, we calculate the cosmic-ray
  proton and primary and secondary electron/positron populations as
  a function of energy. Cosmic rays are injected with Galactic energy
  distributions and electron-to-proton ratio via Type II supernovae at
  the observed rate of 0.07 yr<SUP>-1</SUP>. From the cosmic ray spectra,
  we predict the radio synchrotron and γ-ray spectra. To more accurately
  model the radio spectrum, we incorporate a multiphase interstellar
  medium in the starburst region of M82. Our model interstellar medium
  is highly fragmented with compact dense molecular clouds and dense
  photoionized gas, both embedded in a hot, low density medium in overall
  pressure equilibrium. The spectra predicted by this one-zone model are
  compared to the observed radio and γ-ray spectra of M82. χ<SUP>2</SUP>
  tests are used with radio and γ-ray observations and a range of model
  predictions to find the best-fit parameters. The best-fit model yields
  constraints on key parameters in the starburst zone of M82, including
  a magnetic field strength of ~250 μG and a wind advection speed in
  the range of 300-700 km s<SUP>-1</SUP>. We find that M82 is a good
  electron calorimeter but not an ideal cosmic-ray proton calorimeter
  and discuss the implications of our results for the astrophysics of
  the far-infrared-radio correlation in starburst galaxies.

---------------------------------------------------------
Title: The Fermi Bubbles: Possible Nearby Laboratory for AGN Jet
    Activity
Authors: Yang, Hsiang-Yi Karen; Ruszkowski, M.; Zweibel, E. G.;
   Ricker, P. M.
2013HEAD...1330002Y    Altcode:
  The two giant gamma-ray bubbles discovered by the Fermi Gamma-ray Space
  Telescope are nearly symmetric about the Galactic plane, suggesting
  some episode of energy injection from the Galactic center, such as a
  nuclear starburst or active galactic nucleus (AGN) jet activity. Using
  three-dimensional magnetohydrodynamic simulations that self-consistently
  include the dynamical interaction between cosmic rays (CR) and thermal
  gas, and anisotropic CR diffusion along magnetic field lines, we show
  that the key characteristics of the observed bubbles can be successfully
  reproduced by a recent jet activity from the central AGN. This implies
  that the Fermi bubbles could be a unique laboratory for studying AGN
  jet-inflated bubbles. Our simulations allow us to generate maps of the
  distribution of the magnetic field, radio polarization, and synchrotron,
  X-ray, and gamma-ray emission. While the source of pressure support of
  extragalactic AGN bubbles is still poorly known due to observational
  limitations, we are able to derive constraints on the composition of
  the Fermi bubbles by comparing our model predictions with the spatially
  resolved gamma-ray bubble and microwave haze observations.

---------------------------------------------------------
Title: Cosmic Ray Heating of the Warm Ionized Medium
Authors: Wiener, Joshua; Zweibel, Ellen G.; Oh, S. Peng
2013ApJ...767...87W    Altcode: 2013arXiv1301.4445W
  Observations of line ratios in the Milky Way's warm ionized medium
  suggest that photoionization is not the only heating mechanism
  present. For the additional heating to explain the discrepancy, it would
  have to have a weaker dependence on the gas density than the cooling
  rate, \Lambda n_e^2. Reynolds et al. suggested turbulent dissipation
  or magnetic field reconnection as possible heating sources. We
  investigate here the viability of MHD-wave mediated cosmic ray heating
  as a supplemental heating source. This heating rate depends on the gas
  density only through its linear dependence on the Alfvén speed, which
  goes as n_e^{-1/2}. We show that, scaled to appropriate values of cosmic
  ray energy density, cosmic ray heating can be significant. Furthermore,
  this heating is stable to perturbations. These results should also
  apply to warm ionized gas in other galaxies.

---------------------------------------------------------
Title: Improved Sound-proof Treatments of Fluid Dynamics in Stellar
    Interiors
Authors: Brown, Benjamin; Vasil, G. M.; Lecoanet, D.; Zweibel, E. G.
2013AAS...22115801B    Altcode:
  Plasma flows in deep stellar interiors are typically much slower
  than the local speed of sound. Owing to this, simulations of stellar
  convection and dynamo action typically employ various "sound-proof"
  equations, which filter the fast sound waves but can follow the subsonic
  convective flows. These sound-proof equations include the anelastic
  equations, which typically are derived in adiabatically-stratified
  stellar convection zones, and the pseudo-incompressible equations. In
  stars like the Sun, the radiative zone underlying the convection zone
  is a region of stable subadiabatic stratification, where motions remain
  highly subsonic and gravity waves dominate the dynamics. Sound-proof
  equations filter sound waves by imposing additional constraints on
  the fluid equations. If the momentum equation is not consistent with
  the additional constraints, the equations violate energy conservation
  in stratified atmospheres. Using a consistent Lagrangian approach we
  derive energy conserving sound-proof equations and study applications of
  sound-proof equations to dynamics in stellar radiative zones. We find
  that some formulations fail to conserve energy in regions of stable
  stratification and instead conserve a stratification weighted pseudo
  energy. Dynamics in the non-energy-conserving systems are incorrectly
  captured. We provide a mapping to equations that do conserve energy
  and discuss gravity wave dynamics in stably-stratified stellar regions
  in the context of stars more massive than the Sun, where overshooting
  convection drives gravity waves in the overlying radiative envelope.

---------------------------------------------------------
Title: Is NGC 253 A Calorimeter?
Authors: Yoast-Hull, Tova; Gallagher, J. S.; Zweibel, E. G.; Everett,
   J. E.
2013AAS...22115705Y    Altcode:
  Based on our semi-analytic model for M82 (Yoast-Hull et al. 2012),
  we have developed and applied a model for cosmic ray interactions in
  starburst galaxies as a means to test the calorimeter model. We aim to
  determine whether the model is generic and applicable to other starburst
  systems. NGC 253 and M82 are the only starburst galaxies detected
  at GeV and TeV energies. Assuming constant cosmic-ray acceleration
  efficiency with Milky Way parameters, we calculate the cosmic-ray proton
  and primary &amp; secondary electron/positron populations. From the
  cosmic-ray spectra and observed parameters of the interstellar medium
  of the NGC 253 starburst nucleus and its wind, we predict the radio
  and gamma-ray spectra and compare with published measurements.

---------------------------------------------------------
Title: The Fermi Bubbles: Supersonic Active Galactic Nucleus Jets
    with Anisotropic Cosmic-Ray Diffusion
Authors: Yang, H. -Y. K.; Ruszkowski, M.; Ricker, P. M.; Zweibel,
   E.; Lee, D.
2012ApJ...761..185Y    Altcode: 2012arXiv1207.4185Y
  The Fermi Gamma-Ray Space Telescope reveals two large bubbles in the
  Galaxy, which extend nearly symmetrically ~50° above and below the
  Galactic center. Using three-dimensional (3D) magnetohydrodynamic
  simulations that self-consistently include the dynamical interaction
  between cosmic rays (CRs) and thermal gas and anisotropic CR diffusion
  along the magnetic field lines, we show that the key characteristics
  of the observed gamma-ray bubbles and the spatially correlated X-ray
  features in the ROSAT 1.5 keV map can be successfully reproduced by
  recent jet activity from the central active galactic nucleus. We find
  that after taking into account the projection of the 3D bubbles onto
  the sky the physical heights of the bubbles can be much smaller than
  previously thought, greatly reducing the formation time of the bubbles
  to about a Myr. This relatively small bubble age is needed to reconcile
  the simulations with the upper limit of bubble ages estimated from the
  cooling time of high-energy electrons. No additional physical mechanisms
  are required to suppress large-scale hydrodynamic instabilities
  because the evolution time is too short for them to develop. The
  simulated CR bubbles are edge-brightened, which is consistent with the
  observed projected flat surface brightness distribution. Furthermore,
  we demonstrate that the sharp edges of the observed bubbles can be
  due to anisotropic CR diffusion along magnetic field lines that drape
  around the bubbles during their supersonic expansion, with suppressed
  perpendicular diffusion across the bubble surface. Possible causes of
  the slight bends of the Fermi bubbles to the west are also discussed.

---------------------------------------------------------
Title: Cosmic Ray- and Thermal-Pressure Driven Winds: Does the Milky
    Way Host a Kpc-Scale Outflow?
Authors: Everett, J.; Zweibel, E.; Benjamin, B.; McCammon, D.;
   Schiller, Q.; Rocks, L.; Gallagher, J. S.
2012EAS....56...73E    Altcode:
  We show that the X-ray emission observed towards the center of our
  Milky Way Galaxy is consistent with a strong (2.1 M<SUB>☉</SUB>/yr)
  outflow powered by both cosmic-ray pressure and thermal-gas pressure. In
  addition, the inferred launch parameters of such an outflow seem
  consistent with conditions inferred in the central Milky Way and other
  galaxies (although it is not clear if a significant vertical magnetic
  field exists in the center of the Galaxy). We also show that in galaxies
  with cosmic-ray pressure, gas pressure, and a vertical magnetic field
  component, cosmic-ray pressure can yield outflows over a wider range
  of conditions.

---------------------------------------------------------
Title: Energy Conservation and Gravity Waves in Sound-proof Treatments
    of Stellar Interiors. Part I. Anelastic Approximations
Authors: Brown, Benjamin P.; Vasil, Geoffrey M.; Zweibel, Ellen G.
2012ApJ...756..109B    Altcode: 2012arXiv1207.2804B
  Typical flows in stellar interiors are much slower than the speed
  of sound. To follow the slow evolution of subsonic motions, various
  sound-proof equations are in wide use, particularly in stellar
  astrophysical fluid dynamics. These low-Mach number equations include
  the anelastic equations. Generally, these equations are valid in
  nearly adiabatically stratified regions like stellar convection zones,
  but may not be valid in the sub-adiabatic, stably stratified stellar
  radiative interiors. Understanding the coupling between the convection
  zone and the radiative interior is a problem of crucial interest and
  may have strong implications for solar and stellar dynamo theories
  as the interface between the two, called the tachocline in the Sun,
  plays a crucial role in many solar dynamo theories. Here, we study
  the properties of gravity waves in stably stratified atmospheres. In
  particular, we explore how gravity waves are handled in various
  sound-proof equations. We find that some anelastic treatments fail to
  conserve energy in stably stratified atmospheres, instead conserving
  pseudo-energies that depend on the stratification, and we demonstrate
  this numerically. One anelastic equation set does conserve energy in
  all atmospheres and we provide recommendations for converting low-Mach
  number anelastic codes to this set of equations.

---------------------------------------------------------
Title: Cosmic ray production and emission in M82
Authors: Yoast-Hull, Tova; Everett, John; Gallagher, J. S.; Zweibel,
   Ellen
2012IAUS..284..397Y    Altcode:
  Starting from first principles, we construct a simple model for
  the evolution of energetic particles produced by supernovae in the
  starburst galaxy M82. The supernova rate, geometry, and properties
  of the interstellar medium are all well observed in this nearby
  galaxy. Assuming a uniform interstellar medium and constant cosmic-ray
  injection rate, we estimate the cosmic-ray proton and primary &amp;
  secondary electron/positron populations. From these particle spectra,
  we predict the gamma ray flux and the radio synchrotron spectrum. The
  model is then compared to the observed radio and gamma-ray spectra of
  M82 as well as previous models by Torres (2004), Persic et al. (2008),
  and de Cea del Pozo et al. (2009). Through this project, we aim to build
  a better understanding of the calorimeter model, in which energetic
  particle fluxes reflect supernova rates, and a better understanding
  of the radio-FIR correlation in galaxies.

---------------------------------------------------------
Title: Erratum: "Ambipolar Drift Heating in Turbulent Molecular
    Clouds" <A href="/abs/2000ApJ...540..332P">(2000, ApJ, 540, 332)</A>
Authors: Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke
2012ApJ...755..182P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Self-organization of Reconnecting Plasmas to Marginal
    Collisionality in the Solar Corona
Authors: Imada, S.; Zweibel, E. G.
2012ApJ...755...93I    Altcode: 2012arXiv1206.2706I
  We explore the suggestions by Uzdensky and Cassak et al. that coronal
  loops heated by magnetic reconnection should self-organize to a state
  of marginal collisionality. We discuss their model of coronal loop
  dynamics with a one-dimensional hydrodynamic calculation. We assume that
  many current sheets are present, with a distribution of thicknesses,
  but that only current sheets thinner than the ion skin depth can
  rapidly reconnect. This assumption naturally causes a density-dependent
  heating rate which is actively regulated by the plasma. We report nine
  numerical simulation results of coronal loop hydrodynamics in which
  the absolute values of the heating rates are different but their
  density dependences are the same. We find two regimes of behavior,
  depending on the amplitude of the heating rate. In the case that the
  amplitude of heating is below a threshold value, the loop is in stable
  equilibrium. Typically, the upper and less dense part of a coronal
  loop is collisionlessly heated and conductively cooled. When the
  amplitude of heating is above the threshold, the conductive flux to the
  lower atmosphere required to balance collisionless heating drives an
  evaporative flow which quenches fast reconnection, ultimately cooling
  and draining the loop until the cycle begins again. The key elements
  of this cycle are gravity and the density dependence of the heating
  function. Some additional factors are present, including pressure-driven
  flows from the loop top, which carry a large enthalpy flux and play
  an important role in reducing the density. We find that on average the
  density of the system is close to the marginally collisionless value.

---------------------------------------------------------
Title: The Madison Plasma Dynamo Experiment: a Laboratory for
    Astrophysics
Authors: Brown, Benjamin; Nornberg, M. D.; Forest, C. B.; Zweibel,
   E. G.; Wallace, J. B.; Clark, M.; Spence, E. J.; Rahbarnia, K.;
   Kaplan, E. J.; Taylor, N. Z.
2012AAS...22021102B    Altcode:
  Plasma experiments in laboratory settings offer the opportunity to
  address fundamental aspects of the solar dynamo, magnetism in solar and
  stellar atmospheres, and instabilities that may play important roles
  in astrophysical systems. The newly constructed Madison Plasma Dynamo
  Experiment (MPDX) Is a platform for investigating the self-generation
  of magnetic fields and related processes in large, weakly magnetized,
  fast flowing, and hot (conducting) plasmas. Planned experiments will
  probe questions that are of crucial importance to heliophysics in the
  solar interior, atmosphere and wind. These include studying large and
  small scale dynamos, varying between laminar and turbulent regimes,
  studying stratified convection and magnetic buoyancy instabilities, and
  studying dissipation processes in collisionless plasmas. In addition,
  MPDX will allow us to study the basic physical processes underlying
  magnetic reconnection and flares in the solar atmosphere, the nature
  of CMEs, and the interactions between planetary magnetospheres and the
  solar wind. Results from these experiments will create the benchmarks
  necessary for validating heliospheric codes used to mode our Sun and
  forecast solar activity. Laboratory plasma experiments are likely
  to contribute new understanding complementary to the traditional
  observational and modeling approach normally used by space physicists.

---------------------------------------------------------
Title: Cosmic Rays in M82: Testing the Calorimeter Model
Authors: Yoast-Hull, Tova; Everett, J.; Gallagher, J. S., III;
   Zweibel, E.
2012AAS...22043403Y    Altcode:
  From first principles, we construct a simple model for the evolution
  of energetic particles in the starburst galaxy M82. Assuming constant
  cosmic-ray acceleration efficiency with Milky Way parameters,
  we calculate the cosmic-ray proton and primary &amp; secondary
  electron/positron populations. From the cosmic-ray spectra, we can
  predict the radio synchrotron and gamma-ray spectrum. To more accurately
  model the radio spectrum, we incorporate a multiphase interstellar
  medium in the starburst region of M82. The interstellar medium is
  highly fragmented with compact dense molecular clouds and dense ionized
  gas, both embedded in a hot, low density medium in overall pressure
  equilibrium. The spectra for this simple model are compared to the
  observed radio and gamma-ray spectra of M82. Chi-squared tests are used
  to compare with radio observations to find the best-fit parameters. The
  best-fit model yields constraints on the magnetic field strength for
  the starburst zone in M82. Through this project, we aim to check the
  calorimeter model, in which energetic particles lose most of their
  energy within the galaxy, and build a better understanding of the
  radio-FIR correlation in starburst galaxies.

---------------------------------------------------------
Title: Energy Conservation And Gravity Waves In Stellar Interior
    Simulations That Employ Sound-proof Treatments
Authors: Brown, Benjamin; Vasil, G. M.; Lecoanet, D.; Zweibel, E. G.
2012AAS...22032802B    Altcode:
  In the deep stellar interiors, flows are typically much slower than the
  local speed of sound. Owing to this, simulations of stellar convection
  and dynamo action typically employ various "sound-proof" equations,
  which filter the fast sound waves but can follow the subsonic convective
  flows. These sound-proof equations include the anelastic equations,
  which typically are derived in adiabatically-stratified stellar
  convection zones, and the pseudo-incompressible equations. In stars
  like the Sun, the radiative zone underlying the convection zone is
  a region of stable subadiabatic stratification, where motions remain
  highly subsonic and gravity waves dominate the dynamics. We study the
  application of sound-proof equations to dynamics in stellar radiative
  zones. We find that some formulations fail to conserve energy in regions
  of stable stratification and consequently do not correctly capture the
  dynamics of gravity waves. We provide a mapping to equations that do
  conserve energy. We discuss gravity wave dynamics in stably-stratified
  stellar regions in the context of simulations of stars like the Sun,
  and also consider more massive stars, where the radiative envelope
  lies above a convective core.

---------------------------------------------------------
Title: The Magnetic Field of the Supergiant Shell LMC 5 in the Large
    Magellanic Cloud
Authors: Mao, Sui Ann; Mcclure-Griffiths, Naomi; Dawson, Joanne;
   Zweibel, Ellen; Hill, Alex
2012atnf.prop.4662M    Altcode:
  Supergiant shells (SGSs) play an important role in the interstellar
  medium (ISM) by heating, compressing ambient gas and shaping galactic
  disks. While magnetic fields in SGSs affect their dynamical evolution,
  SGSs in turn play a crucial role in the generation of large-scale
  magnetic fields via the dynamo mechanism. Unfortunately, the few
  existing studies of SGS magnetism do not provide a clear picture of
  the field geometry. Hence, we propose a new 16-cm linear polarization
  ATCA survey of a 1.3*1.7 sq degree region covering LMC 5, a SGS in
  the Large Magellanic Cloud. We aim to measure the diffuse synchrotron
  polarized emission and compute Faraday rotation measures to characterize
  its magnetic field structure. Using the available zoom bands, we will
  simultaneously obtain high spectral resolution HI kinematics of the
  shell and conduct a search for OH masers in the shell wall.

---------------------------------------------------------
Title: The Plasma Physics of Cosmic Rays
Authors: Zweibel, Ellen
2012APS..APR.A1002Z    Altcode:
  Cosmic rays produce some of the most energetic emission in the
  Universe. They play a key role in the dynamics and energy balance of
  galactic gas, and allow us to probe galactic and intergalactic magnetic
  fields in the present epoch and over cosmic time. The acceleration and
  propagation of cosmic rays, and the mechanisms by which they interact
  collectively with their environments, are largely plasma physics
  problems. I will briefly review some salient aspects of cosmic ray
  astrophysics and describe recent progress toward understanding cosmic
  ray plasma physics.

---------------------------------------------------------
Title: Modeling magnetic reconnection in partially ionized
    chromospheric plasmas
Authors: Murphy, Nicholas A.; Raymond, John C.; Zweibel, Ellen G.
2012decs.confE..65M    Altcode:
  Simulations of magnetic reconnection generally assume that the plasma
  is fully ionized. However, the ionization fraction in the solar
  chromosphere ranges from 0.005 to 0.5 so we must consider partial
  ionization effects such as ambipolar diffusion. In this poster we report
  on the initial stages of development for a new plasma simulation code
  to model partially ionized chromospheric reconnection. We will model
  ions and neutrals separately and include time-dependent ionization. By
  including elements with both high and low first ionization potentials,
  we will determine the amount of elemental fractionation that should
  be expected during chromospheric reconnection. These simulations will
  provide insight into observations of Type II spicules and chromospheric
  reconnection events by IRIS, SDO/AIA, and Hinode/SOT.

---------------------------------------------------------
Title: The impact of recent advances in laboratory astrophysics on
    our understanding of the cosmos
Authors: Savin, D. W.; Brickhouse, N. S.; Cowan, J. J.; Drake, R. P.;
   Federman, S. R.; Ferland, G. J.; Frank, A.; Gudipati, M. S.; Haxton,
   W. C.; Herbst, E.; Profumo, S.; Salama, F.; Ziurys, L. M.; Zweibel,
   E. G.
2012RPPh...75c6901S    Altcode: 2011arXiv1112.2770S
  An emerging theme in modern astrophysics is the connection between
  astronomical observations and the underlying physical phenomena
  that drive our cosmos. Both the mechanisms responsible for the
  observed astrophysical phenomena and the tools used to probe such
  phenomena—the radiation and particle spectra we observe—have their
  roots in atomic, molecular, condensed matter, plasma, nuclear and
  particle physics. Chemistry is implicitly included in both molecular
  and condensed matter physics. This connection is the theme of the
  present report, which provides a broad, though non-exhaustive,
  overview of progress in our understanding of the cosmos resulting
  from recent theoretical and experimental advances in what is commonly
  called laboratory astrophysics. This work, carried out by a diverse
  community of laboratory astrophysicists, is increasingly important as
  astrophysics transitions into an era of precise measurement and high
  fidelity modeling.

---------------------------------------------------------
Title: One-dimensional Modeling for Temperature-dependent Upflow in
    the Dimming Region Observed by Hinode/EUV Imaging Spectrometer
Authors: Imada, S.; Hara, H.; Watanabe, T.; Murakami, I.; Harra,
   L. K.; Shimizu, T.; Zweibel, E. G.
2011ApJ...743...57I    Altcode: 2011arXiv1108.5031I
  We previously found a temperature-dependent upflow in the dimming region
  following a coronal mass ejection observed by the Hinode EUV Imaging
  Spectrometer (EIS). In this paper, we reanalyzed the observations along
  with previous work on this event and provided boundary conditions for
  modeling. We found that the intensity in the dimming region dramatically
  drops within 30 minutes from the flare onset, and the dimming region
  reaches the equilibrium stage after ~1 hr. The temperature-dependent
  upflows were observed during the equilibrium stage by EIS. The
  cross-sectional area of the flux tube in the dimming region does not
  appear to expand significantly. From the observational constraints,
  we reconstructed the temperature-dependent upflow by using a new method
  that considers the mass and momentum conservation law and demonstrated
  the height variation of plasma conditions in the dimming region. We
  found that a super-radial expansion of the cross-sectional area is
  required to satisfy the mass conservation and momentum equations. There
  is a steep temperature and velocity gradient of around 7 Mm from
  the solar surface. This result may suggest that the strong heating
  occurred above 7 Mm from the solar surface in the dimming region. We
  also showed that the ionization equilibrium assumption in the dimming
  region is violated, especially in the higher temperature range.

---------------------------------------------------------
Title: The Origin and Evolution of Cosmic Magnetic Fields
Authors: Zweibel, Ellen
2011APS..DPPJ10001Z    Altcode:
  Magnetic fields are observed in galaxies, clusters of galaxies,
  and possibly the intergalactic medium. In some cases, the fields
  display coherent structures on scales much larger than the turbulent
  injection scale. I will summarize the evidence for magnetic fields on
  cosmic scales in the contemporary Universe and in the distant past,
  and discuss current theories for their origin and evolution.

---------------------------------------------------------
Title: Onset of Fast Magnetic Reconnection in Partially Ionized Gases
Authors: Malyshkin, Leonid M.; Zweibel, Ellen G.
2011ApJ...739...72M    Altcode: 2011arXiv1105.1559M
  We consider quasi-stationary two-dimensional magnetic reconnection
  in a partially ionized incompressible plasma. We find that when the
  plasma is weakly ionized and the collisions between the ions and the
  neutral particles are significant, the transition to fast collisionless
  reconnection due to the Hall effect in the generalized Ohm's law
  is expected to occur at much lower values of the Lundquist number,
  as compared to a fully ionized plasma case. We estimate that these
  conditions for fast reconnection are satisfied in molecular clouds
  and in protostellar disks.

---------------------------------------------------------
Title: The Interaction of Cosmic Rays with Diffuse Clouds
Authors: Everett, John E.; Zweibel, Ellen G.
2011ApJ...739...60E    Altcode: 2011arXiv1107.1243E
  We study the change in cosmic-ray pressure, the change in cosmic-ray
  density, and the level of cosmic-ray-induced heating via Alfvén-wave
  damping when cosmic rays move from a hot ionized plasma to a cool cloud
  embedded in that plasma. The general analysis method outlined here can
  apply to diffuse clouds in either the ionized interstellar medium or
  in galactic winds. We introduce a general-purpose model of cosmic-ray
  diffusion building upon the hydrodynamic approximation for cosmic rays
  (from McKenzie &amp; Völk and Breitschwerdt and collaborators). Our
  improved method self-consistently derives the cosmic-ray flux and
  diffusivity under the assumption that the streaming instability is the
  dominant mechanism for setting the cosmic-ray flux and diffusion. We
  find that, as expected, cosmic rays do not couple to gas within cool
  clouds (cosmic rays exert no forces inside of cool clouds), that the
  cosmic-ray density does not increase within clouds (it may decrease
  slightly in general, and decrease by an order of magnitude in some
  cases), and that cosmic-ray heating (via Alfvén-wave damping and not
  collisional effects as for ~10 MeV cosmic rays) is only important under
  the conditions of relatively strong (10 μG) magnetic fields or high
  cosmic-ray pressure (~10<SUP>-11</SUP> erg cm<SUP>-3</SUP>).

---------------------------------------------------------
Title: Magnetic Fields in Galaxies
Authors: Zweibel, Ellen G.
2011IAUS..271..135Z    Altcode:
  The origin and evolution of magnetic fields in the Universe is a
  cosmological problem. Although exotic mechanisms for magneotgenesis
  cannot be ruled out, galactic magnetic fields could have been
  seeded by magnetic fields from stars and accretion disks, and must
  be continuously regenerated due to the ongoing replacement of the
  interstellar medium. Unlike stellar dynamos, galactic dynamos operate
  in a multicomponent gas at low collisionality and high magnetic Prandtl
  number. Their background turbulence is highly compressible, the plasma
  β ~ 1, and there has been time for only a few large exponentiation
  times at large scale over cosmic time. Points of similarity include the
  importance of magnetic buoyancy, the large range of turbulent scales
  and tiny microscopic scales, and the coupling between the magnetic
  field and certain properties of the flow. Understanding the origin
  and maintenance of the large scale galactic magnetic field is the most
  challenging aspect of the problem.

---------------------------------------------------------
Title: The Magnetic Field of the Irregular Galaxy NGC 4214
Authors: Kepley, Amanda A.; Zweibel, Ellen G.; Wilcots, Eric M.;
   Johnson, Kelsey E.; Robishaw, Timothy
2011ApJ...736..139K    Altcode: 2011arXiv1105.2820K
  We examine the magnetic field in NGC 4214, a nearby irregular galaxy,
  using multi-wavelength radio continuum polarization data from the
  Very Large Array. We find that the global radio continuum spectrum
  shows signs that free-free absorption and/or synchrotron losses may
  be important. The 3 cm radio continuum morphology is similar to that
  of the Hα while the 20 cm emission is more diffuse. We estimate that
  50% of the radio continuum emission in the center of the galaxy is
  thermal. Our estimate of the magnetic field strength is 30 ± 9.5
  μG in the center and 10 ± 3 μG at the edges. We find that the hot
  gas, magnetic, and the gravitational pressures are all the same order
  of magnitude. Inside the central star-forming regions, we find that
  the thermal and turbulent pressures of the H II regions dominate the
  pressure balance. We do not detect any significant polarization on
  size scales greater than 200 pc. We place an upper limit of 8 μG on
  the uniform field strength in this galaxy. We suggest that the diffuse
  synchrotron region, seen to the north of the main body of emission
  at 20 cm, is elongated due to a uniform magnetic field with a maximum
  field strength of 7.6 μG. We find that, while the shear in NGC 4214
  is comparable to that of the Milky Way, the supernova rate is half
  that of the Milky Way and suggest that the star formation episode in
  NGC 4214 needs additional time to build up enough turbulence to drive
  an α-ω dynamo.

---------------------------------------------------------
Title: Laboratory Dynamo Experiments
Authors: Nornberg, M. D.; Forest, C. B.; Brown, B. P.; Zweibel,
   E. G.; Wallace, J. B.; Clark, M.; Spence, E. J.; Taylor, N. Z.;
   Kaplan, E. J.; Rahbarnia, K.
2011nlaw.confC..47N    Altcode:
  The basic physics of a dynamo have been addressed utilizing
  magnetohydrodynamic (MHD) simulations and liquid metal experiments
  to determine the requirements for the excitation, sustainment, and
  saturation of magnetic fields generated from turbulent flows. The more
  challenging problem of addressing plasma physics effects will require
  experiments with unmagnetized plasmas. An experimental facility
  utilizing cusp field confinement to produce a hot, steadystate
  unmagnetized plasma would provide the necessary conditions for
  studying effects beyond MHD (such as two-fluid and kinetic effects)
  in a turbulent dynamo.

---------------------------------------------------------
Title: Gamma-ray Constraints on Cosmic Rays in Galactic Winds
Authors: Hu, Kaiqi; Everett, J. E.; Zweibel, E. G.
2011AAS...21724119H    Altcode: 2011BAAS...4324119H
  Our group is constructing a hybrid thermal gas and cosmic-ray pressure
  driven wind model. This model is built on past work by Breitschwerdt
  et al. (1991) and Zirakashvili et al. (1996), and was motivated by
  unexplained high latitude Galactic X-ray emission observed by ROSAT,
  and further tested with radio synchrotron observations. In this poster,
  the role of cosmic-ray protons in generating gamma-ray emission in
  a Galactic wind is explored. In interacting with the wind plasma,
  cosmic-ray protons have three mechanisms to generate gamma-rays (pion
  production, Bremsstrahlung, and inverse Compton scattering), which can
  be detected by the Fermi Gamma-ray Space Telescope. To test the model,
  we have calculated the gamma-ray intensity from the wind model of
  Everett et al (2010), and we compare these predictions to the observed
  emission in the central Milky Way. Also, we have recently developed a
  new wind model which includes an azimuthal magnetic field and galactic
  rotation; we compare the driving in this improved model to the previous
  one, and report on the gamma-ray emissivity of this model as well. In
  the future we will apply this model to other galaxies which are observed
  to have a large scale wind, such as M82 and NGC 253. Understanding the
  high latitude gamma-ray emission from relativistic particles in galactic
  winds may help to constrain dark-matter models as well. <P />This work
  has been supported by NASA through grant NNX10AO50G, and by the NSF
  through grants NSF AST-0907837 and NSF PHY-0821899 (to the Center for
  Magnetic Self-Organization in Laboratory and Astrophysical Plasmas).

---------------------------------------------------------
Title: The Physics of the Calorimeter Model in M82
Authors: Yoast-Hull, Tova; Everett, J.; Gallagher, J. S., III;
   Zweibel, E.
2011AAS...21724504Y    Altcode: 2011BAAS...4324504Y
  The striking correlation between far-infrared and radio emission found
  in disk galaxies has led to a calorimetric model for galaxies. In this
  calorimeter model, all the energy input from supernovae is expended
  within the galaxy, so both the far-infrared and radio synchrotron
  emission from cosmic rays are proportional to the supernova rate. To
  determine how broadly the calorimeter model applies, we examine the
  energy balance in the starburst galaxy M82: this galaxy's hyperactive
  star formation rate per unit area (100 times the Milky Way's mean value)
  provides a severe test of the calorimeter model. Using an empirical
  model for the M82 starburst zone, we determine confinement times of
  cosmic rays in the galaxy and the energy losses of primary and secondary
  cosmic rays, specifically accounting for pair production, ionization
  losses, and synchrotron emission. We also consider the role of a wind
  in the propagation and transport of the cosmic rays. The results are
  tested against the observed multi-wavelength characteristics of M82.

---------------------------------------------------------
Title: The Dynamical Role of Cosmic Rays in Galactic Winds
Authors: Everett, J.; Zweibel, E.; Hu, K.
2010ASPC..438...45E    Altcode:
  Cosmic rays are well-known for generating synchrotron radiation, as
  well as for helping to ionize cool gas in the interstellar medium. In
  this paper, we review the dynamical role of cosmic rays, focusing in
  particular on the Streaming Instability and how cosmic-ray pressure
  can help to launch galactic winds. We review progress in modeling soft
  X-ray emission and synchrotron emission, observed towards the center
  of the Milky Way, as a wind driven by both cosmic-ray and thermal-gas
  pressure. We also outline our ongoing work to include driving by
  magnetic pressure and galactic rotation, in a general semi-analytic
  numerical framework.

---------------------------------------------------------
Title: An Experimental Plasma Dynamo Program for Investigations of
    Fundamental Processes in Heliophysics
Authors: Brown, Benjamin; Forest, Cary; Nornberg, Mark; Zweibel,
   Ellen; Cattaneo, Fausto; Cowley, Steven
2011arXiv1101.0176B    Altcode:
  Plasma experiments in laboratory settings offer unique opportunities
  to address fundamental aspects of the solar dynamo and magnetism
  in the solar atmosphere. We argue here that ground-based laboratory
  experiments have direct connections to NASA based missions and NSF
  programs, and that a small investment in laboratory heliophysics may
  have a high payoff. We advocate for broad involvement in community-scale
  plasma experiments.

---------------------------------------------------------
Title: A Plasma Dynamo Experiment For Studying Astrophysically
    Relevant Flow Driven MHD Instabilities
Authors: Forest, C.; Zweibel, E. G.; Katz, N. K.; Spence, E. J.;
   Nornberg, M.; Khalzov, I.; Collins, C.; Weisberg, D.; Wallace, J.;
   Jara-Almonte, J.; Clark, M.
2010AGUFMNG51C..05F    Altcode:
  Many astrophysical objects, like the Sun, are composed of high magnetic
  Reynolds number, turbulent, flowing plasma in which the flow energy
  is much larger than that of magnetic field. Creating such conditions
  in laboratory plasma experiments is challenging since confinement
  is usually required to keep the plasma hot (and conducting) which is
  typically achieved by using strong applied magnetic fields. For this
  reason, laboratory experiments using liquid metals have been addressing
  fundamental plasma processes in this unique parameter regime. This
  talk will begin by reviewing self-generation of a magnetic field of
  energy comparable to the turbulent flow from which it arises--the dynamo
  process. Liquid metal experiments have (1) demonstrated self-excitation
  of magnetic fields, (2) two scale dynamos where a small scale flow
  drives a large scale magnetic field, (3) intermittent self-excitation
  and a variety of time dynamics including field reversals, and (4)
  showed the existence of a turbulent electromotive force (mean-field
  current generation). Liquid metals are, however, not plasmas: dynamos
  may differ in plasmas where the relative importance of viscosity
  and resistivity can be interchanged, and new instability mechanisms,
  outside the scope of incompressible MHD may be critical in plasmas. This
  suggests that the next generation of experiments in this important
  astrophysics regime should be based upon plasmas. The Madison Plasma
  Dynamo experiment (now under construction) will then be described with
  an overview of the concept and show how the dynamos might operate in
  this plasma. Modeling of several experimental scenarios that mimic solar
  processes will also be described, including experiments on rotating,
  compressible convection driven by magnetic buoyancy.

---------------------------------------------------------
Title: Ambipolar Diffusion-mediated Thermal Fronts in the Neutral
    Interstellar Medium
Authors: Stone, Jennifer M.; Zweibel, Ellen G.
2010ApJ...724..131S    Altcode: 2010arXiv1009.3926S
  In a thermally bistable medium, cold, dense gas is separated from
  warm, rarefied gas by thin phase transition layers, or fronts, in
  which heating, radiative cooling, thermal conduction, and convection
  of material are balanced. We calculate the steady-state structure of
  such fronts in the presence of magnetic fields, including the processes
  of ion-neutral drift and ion-neutral frictional heating. We find that
  ambipolar diffusion efficiently transports the magnetic field across
  the fronts, leading to a flat magnetic field strength profile. The
  thermal profiles of such fronts are not significantly different from
  those of unmagnetized fronts. The near uniformity of the magnetic field
  strength across a front is consistent with the flat field strength-gas
  density relation that is observed in diffuse interstellar gas.

---------------------------------------------------------
Title: Star formation in the outer filaments of NGC 1275
Authors: Canning, R. E. A.; Fabian, A. C.; Johnstone, R. M.; Sanders,
   J. S.; Conselice, C. J.; Crawford, C. S.; Gallagher, J. S.; Zweibel, E.
2010MNRAS.405..115C    Altcode: 2010arXiv1002.1056C; 2010MNRAS.tmp..745C
  We present photometry of the outer star clusters in NGC 1275, the
  brightest galaxy in the Perseus cluster. The observations were taken
  using the Hubble Space Telescope Advanced Camera for Surveys. We focus
  on two stellar regions in the south and south-east, far from the nucleus
  of the low-velocity system (~22kpc). These regions of extended star
  formation trace the Hα filaments, drawn out by rising radio bubbles. In
  both regions, bimodal distributions of colour (B - R)<SUB>0</SUB>
  against magnitude are apparent, suggesting two populations of star
  clusters with different ages; most of the Hα filaments show no
  detectable star formation. The younger, bluer population is found to be
  concentrated along the filaments while the older population is dispersed
  evenly about the galaxy. We construct colour-magnitude diagrams and
  derive ages of at most 10<SUP>8</SUP>years for the younger population,
  a factor of 10 younger than the young population of star clusters in
  the inner regions of NGC 1275. We conclude that a formation mechanism
  or event different to that for the young inner population is needed
  to explain the outer star clusters and suggest that formation from the
  filaments, triggered by a buoyant radio bubble rising either above or
  below these filaments, is the most likely mechanism.

---------------------------------------------------------
Title: Effects of line-tying on magnetohydrodynamic instabilities
    and current sheet formation
Authors: Huang, Yi-Min; Bhattacharjee, A.; Zweibel, Ellen G.
2010PhPl...17e5707H    Altcode: 2010arXiv1003.5927H
  An overview of some recent progress on magnetohydrodynamic stability
  and current sheet formation in a line-tied system is given. Key results
  on the linear stability of the ideal internal kink mode and resistive
  tearing mode are summarized. For nonlinear problems, a counterexample
  to the recent demonstration of current sheet formation by Low and
  Janse [Astrophys. J. 696, 821 (2009)] is presented, and the governing
  equations for quasistatic evolution of a boundary driven, line-tied
  magnetic field are derived. Some open questions and possible strategies
  to resolve them are discussed.

---------------------------------------------------------
Title: A Survey of Extragalactic Faraday Rotation at High Galactic
Latitude: The Vertical Magnetic Field of the Milky Way Toward the
    Galactic Poles
Authors: Mao, S. A.; Gaensler, B. M.; Haverkorn, M.; Zweibel, E. G.;
   Madsen, G. J.; McClure-Griffiths, N. M.; Shukurov, A.; Kronberg, P. P.
2010ApJ...714.1170M    Altcode: 2010arXiv1003.4519M
  We present a study of the vertical magnetic field of the Milky Way
  toward the Galactic poles, determined from observations of Faraday
  rotation toward more than 1000 polarized extragalactic radio sources
  at Galactic latitudes |b| &gt;= 77°, using the Westerbork Radio
  Synthesis Telescope and the Australia Telescope Compact Array. We
  find median rotation measures (RMs) of 0.0 ± 0.5 rad m<SUP>-2</SUP>
  and +6.3 ± 0.7 rad m<SUP>-2</SUP> toward the north and south Galactic
  poles, respectively, demonstrating that there is no coherent vertical
  magnetic field in the Milky Way at the Sun's position. If this is a
  global property of the Milky Way's magnetism, then the lack of symmetry
  across the disk rules out pure dipole or quadrupole geometries for the
  Galactic magnetic field. The angular fluctuations in RM seen in our
  data show no preferred scale within the range ≈0fdg1 to ≈25°. The
  observed standard deviation in RM of ~9 rad m<SUP>-2</SUP> then implies
  an upper limit of ~1 μG on the strength of the random magnetic field
  in the warm ionized medium at high Galactic latitudes.

---------------------------------------------------------
Title: Violation of Richardson's Criterion Via Introduction of a
    Magnetic Field
Authors: Lecoanet, Daniel; Zweibel, Ellen G.; Townsend, Richard H. D.;
   Huang, Yi-Min
2010ApJ...712.1116L    Altcode: 2010arXiv1002.3335L
  Shear flow instabilities can profoundly affect the diffusion
  of momentum in jets, stars, and disks. The Richardson criterion
  gives a sufficient condition for instability of a shear flow in a
  stratified medium. The velocity gradient V' can only destabilize a
  stably stratified medium with squared Brunt-Väisälä frequency
  N <SUP> 2</SUP> if V'<SUP>2</SUP>/4&gt;N <SUP>2</SUP>. We find
  this is no longer true when the medium is a magnetized plasma. We
  investigate the effect of stable stratification on the magnetic field
  and velocity profiles unstable to magneto-shear instabilities, i.e.,
  instabilities which require the presence of both magnetic field and
  shear flow. We show that a family of profiles originally studied by
  Tatsuno &amp; Dorland remains unstable even when V'<SUP>2</SUP>/4
  &lt; N <SUP>2</SUP>, violating the Richardson criterion. However,
  not all magnetic fields can result in a violation of the Richardson
  criterion. We consider a class of flows originally considered by Kent,
  which are destabilized by a constant magnetic field, and show that they
  become stable when V'<SUP>2</SUP>/4 &lt; N <SUP>2</SUP>, as predicted
  by the Richardson criterion. This suggests that magnetic free energy
  is required to violate the Richardson criterion. This work implies
  that the Richardson criterion cannot be used when evaluating the ideal
  stability of a sheared, stably stratified, and magnetized plasma. We
  briefly discuss the implications for astrophysical systems.

---------------------------------------------------------
Title: Synchrotron Constraints on a Hybrid Cosmic-ray and Thermally
    Driven Galactic Wind
Authors: Everett, John E.; Schiller, Quintin G.; Zweibel, Ellen G.
2010ApJ...711...13E    Altcode: 2009arXiv0904.1964E
  Cosmic rays and magnetic fields can substantially impact the launching
  of large-scale galactic winds. Many researchers have investigated the
  role of cosmic rays; our group previously showed that a cosmic-ray
  and thermally driven wind could explain soft X-ray emission toward
  the center of the Galaxy. In this paper, we calculate the synchrotron
  emission from our original wind model and compare it to observations;
  the synchrotron data show that earlier assumptions about the launching
  conditions of the wind must be changed: we are required to improve that
  earlier model by restricting the launching region to the domain of the
  inner "Molecular Ring," and by decreasing the magnetic field strength
  from the previously assumed maximum strength. With these physically
  motivated modifications, we find that a wind model can fit both the
  radio synchrotron and the X-ray emission, although that model is
  required to have a higher gas pressure and density than the previous
  model in order to reproduce the observed X-ray emission within the
  smaller "footprint." The drop in magnetic field also decreases the
  effect of cosmic-ray heating, requiring a higher temperature at the
  base of the wind than the previous model.

---------------------------------------------------------
Title: The Role of the Magnetic Field in the Interstellar Medium of
    the Post-Starburst Dwarf Irregular Galaxy NGC 1569
Authors: Kepley, Amanda A.; Mühle, Stefanie; Everett, John; Zweibel,
   Ellen G.; Wilcots, Eric M.; Klein, Uli
2010ApJ...712..536K    Altcode: 2010arXiv1002.2375K
  NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense
  burst of star formation 10-40 Myr ago. We present observations that
  reach surface brightnesses 2-80 times fainter than previous radio
  continuum observations and the first radio continuum polarization
  observations of this galaxy at 20 cm, 13 cm, 6 cm, and 3 cm. These
  observations allow us to probe the relationship of the magnetic field
  of NGC 1569 to the rest of its interstellar medium (ISM). We confirm
  the presence of an extended radio continuum halo at 20 cm and see for
  the first time the radio continuum feature associated with the western
  Hα arm at wavelengths shorter than 20 cm. Although, in general, the
  spectral indices derived for this galaxy steepen as one moves into
  the halo of the galaxy, there are filamentary regions of flat spectral
  indices extending to the edge of the galaxy. The spectral index trends
  in this galaxy support the theory that there is a convective wind at
  work in this galaxy. There is strong polarized emission at 3 cm and
  6 cm and weak polarized emission at 20 cm and 13 cm. We estimate that
  the thermal fraction is 40%-50% in the center of the galaxy and falls
  off rapidly with height above the disk. Using this estimate, we derive
  a total magnetic field strength of 38 μG in the central regions and
  10-15 μG in the halo. The magnetic field is largely random in the
  center of the galaxy; the uniform field is ~3-9 μG and is strongest
  in the halo. Using our total magnetic field strength estimates and
  the results of previous observations of NGC 1569, we find that the
  magnetic pressure is the same order of magnitude but, in general,
  a factor of a few less than the other components of the ISM in this
  galaxy. The uniform magnetic field in NGC 1569 is closely associated
  with the Hα bubbles and filaments. We suggest that a supernova-driven
  dynamo may be operating in this galaxy. Based on our pressure estimates
  and the morphology of the magnetic field, the outflow of hot gas from
  NGC 1569 is clearly shaping the magnetic field, but the magnetic field
  in turn may be aiding the outflow by channeling gas out of the disk
  of the galaxy. Dwarf galaxies with extended radio continuum halos
  like that of NGC 1569 may play an important role in magnetizing the
  intergalactic medium.

---------------------------------------------------------
Title: Environments for Magnetic Field Amplification by Cosmic Rays
Authors: Zweibel, Ellen G.; Everett, John E.
2010ApJ...709.1412Z    Altcode: 2009arXiv0912.3511Z
  We consider a recently discovered class of instabilities, driven by
  cosmic ray streaming, in a variety of environments. We show that
  although these instabilities have been discussed primarily in the
  context of supernova-driven interstellar shocks, they can also operate
  in the intergalactic medium and in galaxies with weak magnetic fields,
  where, as a strong source of helical magnetic fluctuations, they could
  contribute to the overall evolution of the magnetic field. Within
  the Milky Way, these instabilities are strongest in warm ionized gas
  and appear to be weak in hot, low density gas unless the injection
  efficiency of cosmic rays is very high.

---------------------------------------------------------
Title: Cosmic Rays Help Drive Galactic Winds
Authors: Everett, John; Zweibel, Ellen; Schiller, Quintin; Hu, Kaiqi
2010cosp...38.2608E    Altcode: 2010cosp.meet.2608E
  Cosmic rays deposit both momentum and energy into thermal gas
  when magnetic fields are present (Wentzel, 1968; Kulsrud Pearce,
  1969). Since we observe evidence of magnetic fields, cosmic-rays,
  and hot gas in galactic disks, it is plausible that cosmic rays may
  add momentum and energy to gas, and therefore help drive galactic-scale
  outflows. Building on past work by Breitschwerdt, Zirakashvili, Ptuskin,
  and others, our group investigates "hybrid" galactic winds driven by
  cosmic-ray and thermal-gas pressure. We have found that such a wind
  can fit mid-latitude Galactic X-ray emission observed by ROSAT towards
  the center of the Galaxy, and also (with some physically motivated
  modifications) fits radio-synchrotron survey data. We are now building
  estimates of the gamma-ray emission for this wind. So far, we also find
  that the parameters of such a wind are plausible for the conditions
  of the central Milky Way. More generally, we illustrate how cosmic-ray
  driving may increase the prevalance of galactic winds. For this talk,
  I will review the hydrodynamics of cosmic rays, including the work of
  other groups on cosmic-ray driven winds. I will highlight how cosmic ray
  pressure can help drive Galactic winds, how such a wind may work in our
  Galaxy, and include estimates of the impact of cosmic-ray diffusivity
  and the driving of cool clouds (observed within extragalactic winds
  and important to constrain wind velocities) within such a wind.

---------------------------------------------------------
Title: Dusty Gas and New Stars: Disruption of the High Velocity
    Intruder Galaxy Falling Towards NGC 1275
Authors: Gallagher, John S., III; Lee, M.; Canning, R.; Fabian, A.;
   O'Connell, R. W.; Sanders, J.; Zweibel, E.
2010AAS...21536308G    Altcode: 2010BAAS...42..552G
  Among the many fascinating features of NGC1275, the brightest cluster
  galaxy in Perseus, is the high velocity optical emission line system
  (HVS) discovered by Minkowski 50 years ago. While subsequent studies
  show that the HVS arises from a spiral galaxy falling towards NGC1275
  with a relative velocity of 3000 km/s, the details of this situation
  have remained elusive. We obtained a better view of the disrupting
  HVS galaxy by combining x-ray absorption measurements from Chandra
  with optical and far-UV imaging obtained with the Advanced Camera
  for Surveys on the Hubble Space Telescope. These data and comparisons
  with simulations of galaxies experiencing ram pressure stripping in
  clusters of galaxies suggest that: (1) gas is lost from the spiral in
  the form of large clouds or streamers, (2) many clouds support active
  star formation, and (3) star formation, like that within NGC1275,
  often yields luminous, compact star clusters. These phenomena resemble
  optically visible gas stripping events in other galaxy clusters, and
  suggest they define how cluster infall affects spiral galaxies. We
  also consider the implications of a possible eventual collision of
  the stripped HVS gas clouds with the massive warm ISM in NGC1275.

---------------------------------------------------------
Title: Magnetic Reconnection in Astrophysical and Laboratory Plasmas
Authors: Zweibel, Ellen G.; Yamada, Masaaki
2009ARA&A..47..291Z    Altcode:
  Magnetic reconnection is a topological rearrangement of magnetic field
  that converts magnetic energy to plasma energy. Astrophysical flares,
  from the Earth's magnetosphere to γ-ray bursts and sawtooth crashes in
  laboratory plasmas, may all be powered by reconnection. Reconnection
  is essential for dynamos and the large-scale restructuring known as
  magnetic self-organization. We review reconnection theory and evidence
  for it. We emphasize recent developments in two-fluid physics,
  and the experiments, observations, and simulations that verify
  two-fluid effects. We discuss novel environments such as line-tied,
  relativistic, and partially ionized plasmas, focusing on mechanisms
  that make reconnection fast, as observed. Because there is evidence
  that fast reconnection in astrophysics requires small-scale structure,
  we briefly introduce how such structure might develop. Several areas
  merit attention for astrophysical applications: development of a
  kinetic model of reconnection to enable spectroscopic predictions,
  better understanding of the interplay between local and global scales,
  the role of collisionless reconnection in large systems, and the
  effects of flows, including turbulence.

---------------------------------------------------------
Title: Spectroscopic Study of Young Star Clusters in the Outskirts
    of NGC 1275 (Perseus A)
Authors: Gallagher, John S., III; Smith, Linda J.; Trancho, Gelys;
   Westmoquette, Mark; Zweibel, Ellen
2009noao.prop..372G    Altcode:
  NGC 1275, the brightest galaxy in the A426 (Perseus) cluster, offers
  an extraordinary opportunity to explore feedback processes in a nearby
  (75 Mpc) system. This galaxy hosts a powerful AGN that feeds a double-
  lobe radio source in combination with a massive molecular ISM that
  supports extensive star formation. While the existence of a spider web
  array of ionized gas filaments extending R=90 kpc to the north is well
  known, their quiescent kinematics, molecular content, and associated
  spatially extended star formation only now is becoming clear. Our
  ongoing research addresses a variety of interrelated issues ranging
  from understanding what prevents high ICM cooling rates to the origin
  and evolution of the giant ionized filaments and the population of the
  halo with massive young star clusters. A major theme is an exploration
  of the ways in which mechanical luminosity and momentum from the AGN
  and SNe II operate as a feedback mechanism to halt the cooling flow
  and build new stellar content in NGC 1275. This proposal requests
  use of the Gemini-N GMOS IFU system to obtain spectra of young star
  clusters and their associated ionized gas located at a progression of
  galactocentric radii in NGC 1275. The resulting measurements of cluster
  kinematics, conditions in surrounding HII filaments, estimated ages,
  and chemical abundances will add fundamental new constraints on the
  evolution of this amazing galaxy.

---------------------------------------------------------
Title: Do Potential Fields Develop Current Sheets Under Simple
    Compression or Expansion?
Authors: Huang, Yi-Min; Bhattacharjee, A.; Zweibel, Ellen G.
2009ApJ...699L.144H    Altcode: 2009arXiv0904.3140H
  The recent demonstration of current singularity formation by Low et
  al. assumes that potential fields will remain potential under simple
  expansion or compression. An explicit counterexample to their key
  assumption is constructed. Our findings suggest that their results
  may need to be reconsidered.

---------------------------------------------------------
Title: A Cosmic-Ray and Thermally Driven Kiloparsec-scale Outflow
    from the Milky Way
Authors: Everett, John; Schiller, Quintin; Zweibel, Ellen
2009APS..APR.B8007E    Altcode:
  We review the importance of cosmic-ray pressure in helping to drive
  kpc-scale galactic outflows. In particular, we examine the case of
  the Milky Way, and outline a theory that the “Galactic X-ray Bulge”
  discovered by Snowden et al. (1997) is the signature of a large-scale
  outflow driven by combined thermal and cosmic-ray pressure. We confront
  this model with observations of the synchrotron halo from Haslam
  et al. (1981), and discuss the constraints that these observations
  place on the wind model and perhaps any model of the “Galactic X-ray
  Bulge”. We also outline further advances to the model including a
  more detailed cosmic-ray diffusion model, and the possible role of
  clumping and mass loading in the outflow.

---------------------------------------------------------
Title: MHD Stability of Interstellar Medium Phase Transition
    Layers. I. Magnetic Field Orthogonal to Front
Authors: Stone, Jennifer M.; Zweibel, Ellen G.
2009ApJ...696..233S    Altcode: 2009arXiv0902.3664S
  We consider the scenario of a magnetic field orthogonal to a front
  separating two media of different temperatures and densities, such
  as cold and warm neutral interstellar gas, in a two-dimensional
  plane-parallel geometry. A linear stability analysis is performed to
  assess the behavior of both evaporation and condensation fronts when
  subject to incompressible, corrugational perturbations with wavelengths
  larger than the thickness of the front. We discuss the behavior
  of fronts in both super-Alfvénic and sub-Alfvénic flows. Since
  the propagation speed of fronts is slow in the interstellar medium
  (ISM), it is the sub-Alfvénic regime that is relevant, and magnetic
  fields are a significant influence on front dynamics. In this case, we
  find that evaporation fronts, which are unstable in the hydrodynamic
  regime, are stabilized. Condensation fronts are unstable, but for
  parameters typical of the neutral ISM the growth rates are so slow that
  steady-state fronts are effectively stable. However, the instability
  may become important if condensation proceeds at a sufficiently
  fast rate. This paper is the first in a series exploring the linear
  and nonlinear effects of magnetic field strength and orientation on
  the corrugational instability, with the ultimate goal of addressing
  outstanding questions about small-scale ISM structure.

---------------------------------------------------------
Title: Magnetic fields in irregular galaxies
Authors: Kepley, Amanda; Wilcots, Eric; Zweibel, Ellen; Mühle,
   Stefanie; Everett, John; Robishaw, Timothy; Heiles, Carl; Klein, Uli
2009IAUS..259..555K    Altcode:
  The low masses of irregular galaxies change the behavior of their
  interstellar medium (ISM) compared to that of normal spirals, so the
  role of magnetic fields in the ISM in irregulars may be very different
  than in spirals. We present high-resolution and high-sensitivity
  observations of the magnetic fields of two irregular galaxies: NGC
  4214 and NGC 1569.

---------------------------------------------------------
Title: Plasma Astrophysics Problems in Star and Planet Formation
Authors: Zweibel, Ellen; Goodman, Jeremy; Ji, Hantao; Lazarian, Alex
2009astro2010S.334Z    Altcode: 2009arXiv0902.3617Z
  The major questions relevant to star and planet formation are: What
  controls the rate, efficiency, spatial clustering, multiplicity, and
  initial mass function of star formation, now and in the past? What are
  the major feedback mechanisms through which star formation affects
  its environment? What controls the formation and orbital parameters
  of planets, especially terrestrial planets? These questions cannot be
  fully addressed without understanding key magnetohydrodynamics (MHD)
  and plasma physics processes. Although some of these basic problems have
  long been considered intractable, attacking them through a combination
  of laboratory experiment, theory, and numerical simulation is now
  feasible, and would be fruitful. Achieving a better understanding of
  these processes is critical to interpreting observations, and will form
  an important component of astrophysical models. These models in turn
  will serve as inputs to other areas of astrophysics, e.g. cosmology
  and galaxy formation.

---------------------------------------------------------
Title: Engineering Einstein: Astrophysical Black Holes
Authors: Hawley, John; Krolik, Julian; Beloborodov, Andrei; Blues,
   Omer; Campanelli, Manuela; Coppi, Paolo; Garnmie, Charles; Melia,
   Fulvio; Shapiro, Stuart; Stone, Jim; Zweibel, Ellen
2009astro2010S.116H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Plasma Physics Processes of the Interstellar Medium
Authors: Splanger, Steven; Haverkorn, Marijke; Intrator, Thomas;
   Kulsrud, Russell; Lazarian, Alex; Redfield, Seth; Zweibel, Ellen
2009astro2010S.282S    Altcode: 2009arXiv0902.4181S
  We discuss the outstanding issues of the interstellar medium which
  will depend on the application of knowledge from plasma physics. We
  particularly advocate attention to recent developments in experimental
  plasma physics, and urge that the astronomical community consider
  support of these experiments in the next decade.

---------------------------------------------------------
Title: The Role of Magnetic Fields in the Interstellar Medium of
    Irregular Galaxies
Authors: Kepley, Amanda A.; Muehle, S.; Everett, J.; Wilcots, E.;
   Zweibel, E.; Robishaw, T.; Heiles, C.
2009AAS...21344305K    Altcode: 2009BAAS...41..324K
  Irregular galaxies are the present day analogs of the high redshift
  building blocks of galaxies like the Milky Way. The shallow potential
  wells of irregular galaxies makes their interstellar medium a chaotic
  system prone to disruption by star formation, interactions, and
  mergers. An important, but oft-overlooked component of the interstellar
  medium of irregulars is their magnetic field. Previous observations
  suggest that irregulars have a wide range magnetic field strengths
  and properties. To increase the number of irregulars with detailed
  observations of their magnetic fields, we have observed three irregular
  galaxies with the VLA and WSRT: NGC 4214, NGC 1569, and NGC 1156. The
  magnetic field of NGC 1569 is shaped almost entirely by the outflow of
  gas from this galaxy, but the central magnetic field in this galaxy may
  be a dominant source of pressure in this galaxy. The magnetic field of
  NGC 4214 is mostly random and is not a dominant source of pressure. NGC
  1156 is similar in size to NGC 4214, but has a much more extended radio
  continuum envelope. We will place these fields into the context of the
  interstellar medium of these galaxies. Finally, we will draw conclusions
  on the role of magnetic fields in irregular galaxies in general.

---------------------------------------------------------
Title: The Need for Plasma Astrophysics in Understanding Life Cycles
    of Active Galaxies
Authors: Li, H.; Arons, J.; Bellan, P.; Colgate, S.; Forest, C.;
   Fowler, K.; Goodman, J.; Intrator, T.; Kronberg, P.; Lyutikov, M.;
   Zweibel, E.
2009astro2010S.182L    Altcode: 2009arXiv0902.3469L
  In this White Paper, we emphasize the need for and the important role
  of plasma astrophysics in the studies of formation, evolution of, and
  feedback by Active Galaxies. We make three specific recommendations:
  1) We need to significantly increase the resolution of VLA, perhaps
  by building an EVLA-II at a modest cost. This will provide the
  angular resolution to study jets at kpc scales, where, for example,
  detailed Faraday rotation diagnosis can be done at 1GHz transverse
  to jets; 2) We need to build coordinated programs among NSF, NASA,
  and DOE to support laboratory plasma experiments (including liquid
  metal) that are designed to study key astrophysical processes,
  such as magneto-rotational instability (origin of angular momentum
  transport), dynamo (origin of magnetic fields), jet launching and
  stability. Experiments allowing access to relativistic plasma regime
  (perhaps by intense lasers and magnetic fields) will be very helpful
  for understanding the stability and dissipation physics of jets from
  Supermassive Black Holes; 3) Again through the coordinated support
  among the three Agencies, we need to invest in developing comprehensive
  theory and advanced simulation tools to study the accretion disks and
  jets in relativistic plasma physics regime, especially in connecting
  large scale fluid scale phenomena with relativistic kinetic dissipation
  physics through which multi-wavelength radiation is produced.

---------------------------------------------------------
Title: Scaling of magnetic reconnection processes from MRX to
    astrophysical plasmas
Authors: Yamada, M.; Kulsrud, R.; Ji, H.; Uzdensky, D.; Zweibel, E.
2008APS..DPPGP6020Y    Altcode:
  We discuss how the MRX (Magnetic Reconnection Experiment) physics
  results scale to space astrophysical plasmas. When the collisionality is
  reduced to satisfy the relationship c/φ<SUB>pi</SUB>&gt;δ<SUB>SP</SUB>
  between the ion skin depth (c/φ<SUB>pi</SUB>) and the Sweet-Parker
  width δ<SUB>SP</SUB>, a fast reconnection rate is observed in MRX
  [1], and the results are verified by numerical simulations. Since
  (c/φ<SUB>pi</SUB>)/δ<SUB>SP</SUB> is roughly equal to 5
  (λ<SUB>mfp</SUB>/L)^1/2, this relationship suggests that two-fluid
  effects become dominant even when the electron mean free path is one
  order of magnitude smaller than the system size [1]. The reconnection
  rate is found to increase rapidly as the ratio of the electron mean
  free path to the scale length increases. This result is attributed
  primarily to the large Hall electric field in the reconnection layer
  except near the X point where dissipative processes caused by electron
  pressure gradients and high frequency turbulence take place. Finally,
  a fast local reconnection generally leads to an impulsive global
  topology change or global magnetic self-organization phenomena. We also
  discuss how our local analysis can be applied to variety of magnetic
  reconnection phenomena in space astrophysical plasmas [2]. [1]M. Yamada,
  Phys. Plasmas, v. 14, 058102 (2007)[2]D. Uzdensky, Ap. J v.671, 2139
  (2007)

---------------------------------------------------------
Title: Effects of Line-tying on Resistive Tearing Instability in
    Slab Geometry
Authors: Huang, Yi-Min; Zweibel, Ellen G.
2008APS..DPPCP6074H    Altcode:
  The effects of line-tying on magnetohydrodynamic instabilities are an
  important issue for astrophysical plasmas, such as the solar corona or
  astrophysical jets. Recently, several laboratory experiments aimed at
  studying line-tying effects have been initiated. This work studies the
  effect of line-tying on the resistive tearing instability in the slab
  geometry. A strong guide field perpendicular to the conducting boundary
  is assumed, therefore the system is described by the well-known reduced
  magnetohydrodynamic (RMHD) equations. The linearized eigenvalue problem
  is solved numerically. It is found that line-tying has a stabilizing
  effect. The tearing mode is stabilized when the system length L is
  shorter than a critical length L<SUB>c</SUB>, which is independent of
  the resistivity η. When L is not too much longer than L<SUB>c</SUB>,
  the growthrate γ is proportional to η . When L is sufficiently
  long, the tearing mode scaling γ∼&amp;3/5circ; is recovered. The
  transition from γ∼η to γ∼&amp;3/5circ; occurs at a transition
  length L<SUB>t</SUB>∼&amp;-2/5circ;.

---------------------------------------------------------
Title: Fast Dynamos in Weakly Ionized Gases
Authors: Zweibel, Ellen G.; Heitsch, Fabian
2008ApJ...684..373Z    Altcode:
  The turnover of interstellar gas on ~10<SUP>9</SUP> yr timescales argues
  for the continuous operation of a galactic dynamo. The conductivity
  of interstellar gas is so high that the dynamo must be "fast"; i.e.,
  the magnetic field must be amplified at a rate nearly independent
  of the magnetic diffusivity. Yet all the fast dynamos so far known,
  and all direct numerical simulations of interstellar dynamos, yield
  magnetic power spectra that peak at the resistive scale, while galactic
  magnetic fields have substantial power on large scales. In this paper
  we show that in weakly ionized gas the limiting scale may be the
  ion-neutral decoupling scale, which, although still small, is many
  orders of magnitude larger than the resistive scale.

---------------------------------------------------------
Title: Magnetic support of the optical emission line filaments in
    NGC 1275
Authors: Fabian, A. C.; Johnstone, R. M.; Sanders, J. S.; Conselice,
   C. J.; Crawford, C. S.; Gallagher, J. S., III; Zweibel, E.
2008Natur.454..968F    Altcode: 2008arXiv0808.2712F
  The giant elliptical galaxy NGC 1275, at the centre of the Perseus
  cluster, is surrounded by a well-known giant nebulosity of emission-line
  filaments, which are plausibly in excess of 10<SUP>8</SUP>years
  old. The filaments are dragged out from the centre of the galaxy by
  radio-emitting `bubbles' rising buoyantly in the hot intracluster gas,
  before later falling back. They act as markers of the feedback process
  by which energy is transferred from the central massive black hole to
  the surrounding gas. The mechanism by which the filaments are stabilized
  against tidal shear and dissipation into the surrounding extremely hot
  (4×10<SUP>7</SUP>K) gas has been unclear. Here we report observations
  that resolve thread-like structures in the filaments. Some threads
  extend over 6kpc, yet are only 70pc wide. We conclude that magnetic
  fields in the threads, in pressure balance with the surrounding gas,
  stabilize the filaments, so allowing a large mass of cold gas to
  accumulate and delay star formation.

---------------------------------------------------------
Title: MHD Stability of Phase Transition Layers in the Neutral ISM
Authors: Stone, Jennifer M.; Zweibel, E. G.
2008AAS...212.0506S    Altcode: 2008BAAS...40R.195S
  The thermal-type corrugational instability is important in accelerating
  burning in combustion fronts in Type Ia supernova explosions (Dursi
  2004). Recent work has shown that evaporation fronts, analogous to
  combustion fronts, in an unmagnetized, neutral ISM are also unstable
  to wrinkling, whereas condensation fronts are stable (Inoue et
  al. 2006). Whether a front is of the evaporation or condensation
  type depends on the external pressure. <P />We extend this work
  by considering the scenario of a magnetic field perpendicular to a
  front separating the Cold Neutral Medium and Warm Neutral Medium in a
  plane-parallel geometry. A linear stability analysis is performed to
  assess the behavior of both front types when subject to incompressible,
  corrugational perturbations with wavelengths larger than the thickness
  of the front. <P />We demonstrate the existence of a mode that is
  unstable in condensation fronts but stable in evaporation fronts,
  in contrast to the hydrodynamic case. Estimates of the growth rate
  and magnetic field strength threshold of this mode are provided and
  implications for generation of small-scale neutral ISM structure are
  discussed. <P />We acknowledge support from NASA ATP grant NNG051G09G.

---------------------------------------------------------
Title: Evolution of Unmagnetized and Magnetized Shear Layers
Authors: Palotti, M. L.; Heitsch, F.; Zweibel, E. G.; Huang, Y. -M.
2008ApJ...678..234P    Altcode: 2008arXiv0802.2497P
  We present numerical simulations of the growth and saturation of the
  Kelvin-Helmholtz instability in a compressible fluid layer with and
  without a weak magnetic field. In the absence of a magnetic field, the
  instability generates a single eddy that flattens the velocity profile,
  stabilizing it against further perturbations. Adding a weak magnetic
  field—weak in the sense that it has almost no effect on the linear
  instability—leads to a complex flow morphology driven by MHD forces
  and to enhanced broadening of the layer due to Maxwell stresses. We
  corroborate earlier studies, which showed that magnetic fields destroy
  the large-scale eddy structure through periodic cycles of windup and
  resistive decay, but we show that the rate of decay decreases with
  decreasing plasma resistivity η, at least within the range of η
  accessible to our simulations. Magnetization increases the efficiency
  of momentum transport, and the transport increases with decreasing η.

---------------------------------------------------------
Title: On the origin of cosmic magnetic fields
Authors: Kulsrud, Russell M.; Zweibel, Ellen G.
2008RPPh...71d6901K    Altcode: 2007arXiv0707.2783K
  We review the extensive and controversial literature concerning
  how the cosmic magnetic fields pervading nearly all galaxies and
  clusters of galaxies actually got started. Some observational evidence
  supports a hypothesis that the field is already moderately strong
  at the beginning of the life of a galaxy and its disc. One argument
  involves the chemical abundance of the light elements Be and B, while
  a second one is based on the detection of strong magnetic fields
  in very young high red shift galaxies. <P />Since this problem of
  initial amplification of cosmic magnetic fields involves important
  plasma problems it is obvious that one must know the plasma in which
  the amplification occurs. Most of this review is devoted to this basic
  problem and for this it is necessary to devote ourselves to reviewing
  studies that take place in environments in which the plasma properties
  are most clearly understood. For this reason the authors have chosen
  to restrict themselves almost completely to studies of dynamos in our
  Galaxy. It is true that one can get a much better idea of the grand
  scope of galactic fields in extragalactic systems. However, most mature
  galaxies share the same dilemma as ours of overcoming important plasma
  problems. Since the authors are both trained in plasma physics we may
  be biased in pursuing this approach, but we feel it is justified by
  the above argument. In addition we feel we can produce a better review
  by staying close to that which we know best. <P />In addition we have
  chosen not to consider the saturation problem of the galactic magnetic
  field since if the original dynamo amplification fails the saturation
  question does not arise. <P />It is generally accepted that seed fields,
  whose strength is of order 10<SUP>-20</SUP> G, easily spring up in the
  era preceding galaxy formation. Several mechanisms have been proposed
  to amplify these seed magnetic fields to a coherent structure with
  the microgauss strengths of the currently observed galactic magnetic
  fields. <P />The standard and most popular mechanism is the α-Ω
  mean field dynamo theory developed by a number of people in the late
  sixties. This theory and its application to galactic magnetic fields is
  discussed in considerable detail in this review. We point out certain
  difficulties with this theory that make it seem unlikely that this
  is the whole story. The main difficulty with this as the only such
  amplification mechanism is rooted in the fact that, on galactic scales,
  flux is constant and is frozen in the interstellar medium. This implies
  that flux must be removed from the galactic discs, as is well recognized
  by the standard theory. <P />For our Galaxy this turns out to be a major
  problem, since unless the flux and the interstellar mass are somehow
  separated, some interstellar mass must also be removed from the deep
  galactic gravitational well. This is very difficult. It is pointed out
  that unless the field has a substantial field strength, much larger
  than that of the seed fields, this separation can hardly happen. And
  of course, it must if the α-Ω dynamo is to start from the ultra weak
  seed field. (It is our philosophy, expressed in this review, that if an
  origin theory is unable to create the magnetic field in our Galaxy it
  is essentially incomplete.) <P />Thus, it is more reasonable for the
  first and largest amplification to occur before the Galaxy forms, and
  the matter embedded in the field is gravitationally trapped. Two such
  mechanisms are discussed for such a pregalactic origin; (1) they are
  generated in the turbulence of the protogalaxy and (2) the fields come
  from giant radio jets. Several arguments against a primordial origin
  are also discussed, as are ways around them. <P />Our conclusion as
  to the most likely origin of cosmic magnetic fields is that they are
  first produced at moderate field strengths by primordial mechanisms and
  then changed and their strength increased to their present value and
  structure by a galactic disc dynamo. The primordial mechanisms have
  not yet been seriously developed, and this preliminary amplification
  of the magnetic fields is still very open. If a convincing case can
  be made that these primordial mechanisms are necessary, more effort
  will of course be devoted to their study.

---------------------------------------------------------
Title: The Galactic Halo Magnetic field
Authors: Mao, Sui Ann; Gaensler, Bryan; McClure-Griffiths, Naomi;
   Haverkorn, Marijke; Kronberg, Phil; Madsen, Greg; Zweibel, Ellen;
   Shukurov, Anvar
2008camf.confE..10M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Turbulent Ambipolar Diffusion
Authors: Zweibel, Ellen
2008camf.confE..11Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Magnetic Field Structure of NGC 1569
Authors: Kepley, A.; Mühle, Stefanie; Wilcots, Eric; Everett, John;
   Zweibel, Ellen
2008camf.confP...8K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Milky Way's Kiloparsec-Scale Wind: A Hybrid Cosmic-Ray
    and Thermally Driven Outflow
Authors: Everett, John E.; Zweibel, Ellen G.; Benjamin, Robert A.;
   McCammon, Dan; Rocks, Lindsay; Gallagher, John S., III
2008ApJ...674..258E    Altcode: 2007arXiv0710.3712E
  We apply a wind model, driven by combined cosmic-ray and thermal-gas
  pressure, to the Milky Way, and show that the observed Galactic diffuse
  soft X-ray emission can be better explained by a wind than by previous
  static gas models. We find that cosmic-ray pressure is essential to
  driving the observed wind. Having thus defined a "best-fit" model for
  a Galactic wind, we explore variations in the base parameters and
  show how the wind's properties vary with changes in gas pressure,
  cosmic-ray pressure, and density. We demonstrate the importance of
  cosmic rays in launching winds, and the effect cosmic rays have on
  wind dynamics. In addition, this model adds support to the hypothesis
  of Breitschwerdt and collaborators that such a wind may help to explain
  the relatively small gradient observed in γ-ray emission as a function
  of galactocentric radius.

---------------------------------------------------------
Title: Magnetic Fields in Irregular Galaxies
Authors: Kepley, Amanda A.; Mühle, Stefanie; Wilcots, Eric M.;
   Everett, John; Zweibel, Ellen; Robishaw, Timothy; Heiles, Carl
2008ASSP....5...73K    Altcode: 2008glv..book...73K; 2007arXiv0708.3405K
  Magnetic fields are an important component of the interstellar medium,
  especially in low-mass galaxies like irregulars where the magnetic
  pressure may be significant. However, few irregular galaxies have
  observed magnetic field structures. Using the VLA, the GBT, and the
  ATCA, we have observed several irregular galaxies in the radio continuum
  to determine their magnetic field structures. Here we report on our
  results for the galaxies NGC 4214 and NGC 1569.

---------------------------------------------------------
Title: Magnetic Fields in Irregular Galaxies
Authors: Kepley, Amanda A.; Muehle, S.; Robishaw, T.; Everett, J.;
   Wilcots, E.; Zweibel, E.; Heiles, C.
2007AAS...21111305K    Altcode: 2007BAAS...39..943K
  Magnetic fields are an important component of the interstellar medium
  (ISM). They provide a source of pressure support, transfer energy from
  supernovae, are a possible heating mechanism for the ISM, and channel
  gas flows. Despite the importance of magnetic fields in the ISM, what
  generates and sustains galactic magnetic fields or how magnetic fields,
  gas, and stars interact in galaxies is not well understood. The magnetic
  fields may be especially important in low-mass galaxies like irregulars
  where the magnetic pressure may be great enough for the field to be
  dynamically important. Only three irregular galaxies besides the LMC
  and the SMC have previously observed magnetic field structures. NGC
  4449 (Chyzy et al. 2000) and the LMC (Gaensler et al. 2005) both have
  large-scale fields, while IC 10 and NGC 6822 have mostly random fields
  (Chyzy et al. 2003). Our goal is to determine what mechanisms generate
  and sustain large-scale magnetic fields in irregular galaxies and what
  causes the range of magnetic field structure in irregular galaxies. We
  have observed the polarized radio continuum emission of four irregular
  galaxies with the VLA, GBT, and ATCA. Our observations double the number
  of irregular galaxies with observed magnetic field structure. Here we
  present results from two of our galaxies: NGC 4214 and NGC 1569. We find
  that NGC 4214 has a mostly random magnetic field structure, which is
  not surprising given its weak bar, small size, and high star formation
  rate. The magnetic field of NGC 1569 has large-scale structure which has
  been shaped not by a dynamo, but by an outflow generated by the massive
  star formation rate in this galaxy. <P />Support for this research has
  been provided by a GBT Student Support Award, a Wisconsin Space Grant
  Consortium Graduate Fellowship, and an NSF Graduate Research Fellowship.

---------------------------------------------------------
Title: Angular momentum transport at early times
Authors: Zweibel, Ellen
2007APS..DPPJM4001Z    Altcode:
  It is well known that angular momentum must be efficiently removed
  from interstellar clouds as they contract and eventually collapse to
  form stars. At the present epoch, angular momentum is transported
  primarily by magnetic fields. At the time the first stars formed,
  galactic magnetic fields were probably either absent or were much
  weaker than they are now. I will discuss the growth of magnetic fields
  in star forming regions and their role in angular momentum transport
  under primordial conditions.

---------------------------------------------------------
Title: Does the Milky Way launch a large-scale wind?
Authors: Everett, John E.; Zweibel, Ellen G.; Benjamin, Robert A.;
   McCammon, Dan; Rocks, Lindsay; Cox, Donald P.; Gallagher, John S.
2007Ap&SS.311..105E    Altcode: 2007Ap&SS.tmp..280E
  The ROSAT All-Sky Survey revealed soft X-ray emission on kiloparsec
  scales towards the Galactic center. Separately, it has also been
  observed that the cosmic ray intensity (measured via γ-ray emission)
  rises only very slowly towards the center of the Galaxy, counter to
  expectations based on the greater number of cosmic ray sources there. A
  thermal and cosmic-ray driven wind could potentially explain both of
  these observations. We find that a cosmic-ray and thermally driven wind
  fits the X-ray observations well; in fact, a wind fits significantly
  better than an earlier-proposed static-polytrope gas model.

---------------------------------------------------------
Title: Photoionization Rates in Clumpy Molecular Clouds
Authors: Bethell, T. J.; Zweibel, E. G.; Li, Pak Shing
2007ApJ...667..275B    Altcode:
  We present calculations of the continuum ultraviolet radiation field
  (91.2 nm&lt;λ&lt;550 nm) penetrating both uniform and clumpy (3D
  turbulent supersonic magnetohydrodynamic) starless molecular gas
  layers. We find that despite the self-shielding of clumps, pristine
  (i.e., unreddened) radiation penetrates deeply both the cloud's volume
  and its mass, resulting in a brighter and bluer intracloud radiation
  field compared to that in an equivalent uniform cloud. Motivated by
  these results, we construct and test a toy model ray-tracing scheme
  for the radiative transfer that fits the UV-visible spectral range
  with a three-parameter function. We calculate the photoionization
  rates, Γ, of the elements C, Na, Mg, Si, S, and Fe as functions of
  the visual extinction A<SUB>V</SUB> along lines of sight. Typically,
  the difference in Γ(A<SUB>V</SUB>) between the clumpy and uniform
  clouds increases to orders of magnitude at even modest extinctions
  (A<SUB>V</SUB>~2). Photoionization in the clumpy model extends 2-3 times
  deeper than in the uniform case, and it dominates cosmic-ray ionization
  throughout almost the entire volume. We encapsulate these average
  results in a parameterized form appropriate for when an approximate
  treatment of the effects of clumpiness is desired. However, the large
  point-to-point variance in this behavior suggests that uncertainties
  may arise when using mean values to model particular lines of sight in
  detail. Ideally, these new results would be used in conjunction with
  established results for homogeneous clouds in order to span a range of
  behavior that arises due to cloud inhomogeneities. We briefly explore
  the importance of the adopted dust properties, characterized by the
  selective extinction R<SUB>V</SUB> and the scattering parameter g. We
  find that the UV field is considerably less sensitive to these dust
  properties in clumpy clouds, emphasizing the preeminence of geometry.

---------------------------------------------------------
Title: Cosmic Ray and Magnetic Field Histories of Galaxies
Authors: Zweibel, Ellen
2007sftn.confE..41Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Ambipolar Diffusion in a Turbulent Medium
Authors: Zweibel, Ellen
2007AAS...210.6501Z    Altcode: 2007BAAS...39..173Z
  The magnetic diffusivity of the interstellar medium is critical
  to the evolution of interstellar clouds, to star formation, and to
  galactic dynamo processes. There are two types of magnetic diffusion:
  resistive diffusion, which changes the magnetic topology, and ambipolar
  diffusion, which operates in weakly ionized gas and preserves magnetic
  topology but allows the field to slip relative to the neutrals. The
  ambipolar diffusivity is generally the larger of the two, but the
  kinetic theory values of both diffusivities are small enough that the
  magnetic field should be frozen to the gas on large scales. I will show
  that ambipolar diffusion, like other forms of diffusion, is greatly
  accelerated by turbulence, and will discuss the consequences for the
  fieldstrength-density relation in the interstellar medium and for the
  evolution of molecular clouds.

---------------------------------------------------------
Title: A Multiwavelength Study of M17: The Spectral Energy
    Distribution and PAH Emission Morphology of a Massive Star Formation
    Region
Authors: Povich, Matthew S.; Stone, Jennifer M.; Churchwell, Ed;
   Zweibel, Ellen G.; Wolfire, Mark G.; Babler, Brian L.; Indebetouw,
   Rémy; Meade, Marilyn R.; Whitney, Barbara A.
2007ApJ...660..346P    Altcode:
  We combine diffuse emission photometry from GLIMPSE and several
  other Galactic plane surveys covering near-IR through radio
  wavelengths to synthesize a global spectral energy distribution
  (SED) for the M17 complex. By balancing the integrated flux in
  the SED with the total bolometric luminosity of all known O and
  early B stars in the ionizing cluster, we estimate a distance to
  M17 of 1.6<SUP>+0.3</SUP><SUB>-0.1</SUB> kpc. At this distance,
  the observed total flux in the SED corresponds to a luminosity of
  2.4+/-0.3×10<SUP>6</SUP> L<SUB>solar</SUB>. We find that the SED from
  the H II region peaks at shorter wavelengths and has a qualitatively
  different shape than the SED from the photodissociation region
  (PDR). We find that polycyclic aromatic hydrocarbons (PAHs) are
  destroyed over a short distance or edge at the boundary of the H II
  region. We demonstrate that this PAH destruction edge can be located
  easily using GLIMPSE band-ratio images and confirm this using Spitzer
  IRS spectra. We investigate the relative roles of extreme ultraviolet
  (EUV) and X-ray photons in the destruction of PAHs, concluding that
  X-rays are not an important PAH destruction mechanism in M17 or, by
  extension, in any other Galactic H II region. Our results support
  the hypothesis that PAHs are destroyed by EUV photons within H II
  regions. PAHs dominate the mid-IR emission in the neutral PDR beyond
  the ionized gas.

---------------------------------------------------------
Title: The Bipolar Outflow toward G5.89-0.39
Authors: Watson, C.; Churchwell, E.; Zweibel, E. G.; Crutcher, R. M.
2007ApJ...657..318W    Altcode:
  We present high-resolution observations of G5.89-0.39 in
  CO(1--&gt;0), <SUP>13</SUP>CO(1--&gt;0), C<SUP>18</SUP>O(1--&gt;0),
  and HCO<SUP>+</SUP>(1--&gt;0). We characterize the G5.89-0.39 outflow
  using the <SUP>13</SUP>CO emission. The outflow is found to be young,
  massive, and powerful. We conclude that (1) the outflow is nearly along
  the line of sight, (2) there is dynamical evidence for entrainment of
  ambient interstellar material into the outflow, (3) the mass entrainment
  rate is ~4×10<SUP>-3</SUP> M<SUB>solar</SUB> yr<SUP>-1</SUP>, (4)
  in the blue lobe, only ~27% of the outflow mass is due to entrainment,
  and (5) expansion of the outflow lobes perpendicular to the flow axis
  is occurring at ~1-10 v<SUB>sound</SUB>. A neutral and ionized outflow
  tracer are compared. Watson and coworkers predicted that if entrainment
  through the Kelvin-Helmholtz shear instability adds significantly to
  the outflow mass, a difference in the turbulent velocity widths of
  neutral and ionized outflow tracers could be measurable. We cannot
  conclude that the Kelvin-Helmholtz shear instability is the physical
  process causing this entrainment, but it may operate at a level below
  our detection limit.

---------------------------------------------------------
Title: Magnetic Fields in Irregular Galaxies: NGC 4214
Authors: Kepley, Amanda A.; Wilcots, E. M.; Robishaw, T.; Heiles,
   C.; Zweibel, E.
2006AAS...20916701K    Altcode: 2006BAAS...38.1135K
  Magnetic fields are an important component of the interstellar medium
  of galaxies. They provide support, transfer energy from supernovae,
  provide a possible heating mechanism, and channel gas flows (Beck
  2004). Despite the importance of magnetic fields in the ISM, it is
  not well known what generates and sustains galactic magnetic fields or
  how magnetic fields, gas, and stars interact in galaxies. The magnetic
  fields may be especially important in low-mass galaxies like irregulars
  where the magnetic pressure may be great enough for the field to be
  dynamically important. However, only four irregular galaxies besides
  the LMC and the SMC have observed magnetic field structures. The
  goal of our project is to significantly increase the number of
  irregular galaxies with observed magnetic field structure. Here we
  present preliminary results for one of the galaxies in our sample:
  NGC 4214. Using the VLA and the GBT, we have obtained 3cm, 6cm, and
  20cm radio continuum polarization observations of this well-studied
  galaxy. Our observations allow us to investigate the effects of NGC
  4214's high star formation rate, slow rotation rate, and weak bar on
  the structure of its magnetic field. We find that NGC 4214's magnetic
  field has an S-shaped structure, with the central field following the
  bar and the outer edges curving to follow the shape of the arms. The
  mechanism for generating these fields is still uncertain. <P />A. Kepley
  is funded by an NSF Graduate Research Fellowship.

---------------------------------------------------------
Title: Star-Disk Coupling by a Time-varying Magnetic Field
Authors: Zweibel, Ellen G.; Hole, K. Tabetha; Mathieu, Robert D.
2006ApJ...649..879Z    Altcode:
  Observations suggest that stars lose appreciable angular momentum prior
  to reaching the main sequence. Two principal spin-down mechanisms have
  been proposed. One is removal of angular momentum by magnetized winds
  or jets; the other is transfer of angular momentum from the star to its
  accretion disk through the effects of magnetic fields. In the latter
  case, spin evolution occurs due to both mass accretion along field
  lines and torques resulting from coupling of the stellar magnetic
  field to the disk. In this paper we study the latter torques in the
  context of a magnetic field varying in time. We find that magnetic
  variability reduces the efficiency with which the field can wind up,
  somewhat widening the region of magnetic coupling. Nonetheless, the
  steady state result-that magnetic torques can be applied only within
  a thin annulus around the corotation radius-is little changed for
  what we believe to be realistic physical conditions. These results
  are generally applicable to disk accretion onto magnetized bodies.

---------------------------------------------------------
Title: m=1 ideal internal kink modes in a line-tied screw pinch
Authors: Huang, Yi-Min; Zweibel, Ellen G.; Sovinec, Carl R.
2006PhPl...13i2102H    Altcode: 2006astro.ph..7390H
  It is well known that the radial displacement of the m=1 internal kink
  mode in a periodic screw pinch has a steep jump at the resonant surface
  where k.B=0 [Rosenbluth, Dagazian, and Rutherford, Phys. Fluids 16,
  1894 (1973)]. In a line-tied system, relevant to solar and astrophysical
  plasmas, the resonant surface is no longer a valid concept. It is then
  of interest to see how line-tying alters the aforementioned result for a
  periodic system. If the line-tied kink also produces a steep gradient,
  corresponding to a thin current layer, it may lead to strong resistive
  effects even with weak dissipation. Numerical solution of the eigenmode
  equations shows that the fastest growing kink mode in a line-tied
  system still possesses a jump in the radial displacement at the location
  coincident with the resonant surface of the fastest growing mode in the
  periodic counterpart. However, line-tying thickens the inner layer and
  slows down the growth rate. As the system length L approaches infinity,
  both the inner layer thickness and the growth rate approach the periodic
  values. In the limit of small ∊~B<SUB>φ</SUB>/B<SUB>z</SUB>, the
  critical length for instability L<SUB>c</SUB>~∊<SUP>-3</SUP>. The
  relative increase in the inner layer thickness due to line-tying scales
  as ∊<SUP>-1</SUP>(L<SUB>c</SUB>/L)<SUP>2.5</SUP>.

---------------------------------------------------------
Title: The Weak-Field Limit of the Magnetorotational Instability
Authors: Krolik, Julian H.; Zweibel, Ellen G.
2006ApJ...644..651K    Altcode: 2006astro.ph..2317K
  We investigate the behavior of the magnetorotational instability
  in the limit of extremely weak magnetic field, i.e., as the
  ratio of ion cyclotron frequency to orbital frequency (X) becomes
  small. Considered only in terms of cold two-fluid theory, instability
  persists to arbitrarily small values of X, and the maximum growth
  rate is of the order of the orbital frequency except for the range
  m<SUB>e</SUB>/m<SUB>i</SUB>&lt;|X|&lt;1, where it can be rather
  smaller. In this range, field aligned with rotation (X&gt;0) produces
  slower growth than antialigned field (X&lt;0). The maximum growth
  rate is generally achieved at smaller and smaller wavelengths as |X|
  diminishes. When |X|&lt;m<SUB>e</SUB>/m<SUB>i</SUB>, new unstable
  “electromagnetic-rotational” modes appear that do not depend on the
  equilibrium magnetic field. Because the most rapidly growing modes have
  extremely short wavelengths when |X| is small, they are often subject
  to viscous or resistive damping, which can result in suppressing all
  but the longest wavelengths, for which growth is much slower. We find
  that this sort of damping is likely to severely curtail the frequently
  invoked mechanism for cosmological magnetic field growth in which a
  magnetic field seeded by the Biermann battery is then amplified by
  the magnetorotational instability. On the other hand, the small-|X|
  case may introduce interesting effects in weakly ionized disks in
  which dust grains carry most of the electric charge.

---------------------------------------------------------
Title: Evolution of magnetic fields at high redshift
Authors: Zweibel, E. G.
2006AN....327..505Z    Altcode:
  The origin of magnetic fields in the Universe is a cosmology
  problem. The evolution of the field is a plasma physics problem. I
  review these problems and focus on magnetogenesis in accretion
  disks, specifically, the transition from the Biermann battery,
  which creates seed fields, to amplification by turbulence driven
  by magnetorotational instability. In collisional disks, there is a
  gap between the fieldstrength characteristic of the battery and the
  fieldstrength necessary to sustain magnetorotational instability, but in
  collisionless disks the transition occurs at low fieldstrength. Because
  collisionless disks are generally hot, and have short dynamical times,
  they are likely to be small. Thus, in the battery scenario, magnetic
  fields on large scales were built from fields created in many small
  sources. Simple estimates based on turbulent diffusion suggest that
  galaxies and the cores of galaxy clusters can be magnetized in this way,
  but not the intergalactic medium at large. The problem of creating a
  large-scale field remains unsolved.

---------------------------------------------------------
Title: Ideal m=1 internal kink mode in line-tied screw pinch
Authors: Huang, Yi-Min; Zweibel, Ellen G.; Sovinec, Carl R.
2006APS..APR.D1037H    Altcode:
  It is well known that the radial displacement of the m=1 internal
  kink mode in a periodic screw pinch has a steep jump at the resonant
  surface where k .B=0. In a line-tied system, relevant to solar and
  astrophysical plasmas, the resonant surface is no longer a valid
  concept. It is then of interest to see how line-tying alters the
  result for a periodic system. If the line-tied kink also produces
  a steep gradient, it may lead to strong heating even with weak
  dissipation. Numerical solution of the eigenmode equations finds that
  the fastest growing kink mode in a line-tied system still possesses
  a jump in the radial displacement at the location coincident with
  the resonant surface of the fastest growing mode in the periodic
  counterpart. However, line-tying thickens the inner layer and reduces
  the growth rate. As the system length L approaches infinity, both
  the inner layer thickness and the growth rate approach the periodic
  values. In the limit of small ∊∼B<SUB>φ</SUB>/B<SUB>z</SUB>, the
  critical length for instability L<SUB>c</SUB>∼1/3̂. The relative
  increase in the inner layer thickness due to line-tying scales as
  (1/∊)(L<SUB>c</SUB>/L)^2.5. The nonlinear equilibrium after the onset
  of the kink instability is of greater interest. Work is in progress to
  solve the new equilibrium by a magnetofrictional relaxation method. To
  avoid reconnection due to numerical resistivity, we take a Lagrangian
  approach formulated in Clebsch coordinates.

---------------------------------------------------------
Title: Unravelling The High-Latitude Magnetic Field of the Milky Way
Authors: Gaensler, Bryan; McClure-Griffiths, Naomi; Mao, Ann;
   Haverkorn, Marijke; Kronberg, Phil; Zweibel, Ellen; Shukurov, Anvar;
   Madsen, Greg
2006atnf.prop..239G    Altcode:
  The presence of coherent galactic magnetic fields points to a
  powerful process which organises random motions into highly-ordered
  structures. The dynamo is the favoured mechanism, but dynamos are not
  yet well understood and still face theoretical difficulties. The Milky
  Way is an excellent test-bed to address these issues, because it covers
  a sufficiently large solid angle that a huge ensemble of background
  rotation measures (RMs) can be used to probe the three-dimensional
  magnetic field structure. Indeed, RMs for pulsars and for extragalactic
  sources have yielded much information about the strength and orientation
  of fields in the Galactic plane. However, crucial new insights on
  Galactic magnetism can be provided by studying the magnetic field at
  high latitudes: such data can directly discriminate between various
  primordial field and dynamo models, and can clarify the role of the
  vertical field in transporting cosmic rays to the halo. We propose a
  targeted survey of the South Galactic Cap, in which we will obtain RMs
  for 500 highly polarized NVSS sources over 500 square degrees. Combined
  with a study of the North Galactic Cap with Westerbork, these data
  will provide a comprehensive characterisation of the Galactic magnetic
  field at high latitudes.

---------------------------------------------------------
Title: The Ionisation Fraction in Clumpy Molecular Clouds
Authors: Bethell, T. J.; Zweibel, E. G.
2005AAS...20719506B    Altcode: 2005BAAS...37.1493B
  The interstellar medium is clumpy, a property which is often considered
  important when attempting to reconcile differences between models
  and observations. While simple (two-phase, fractal) models have their
  successes and failures, we now have physically motivated turbulently
  driven MHD models. These offer perhaps the most realistic models yet
  of the clumpiness in molecular clouds. <P />We present results of
  radiative transfer (using a Reverse Monte Carlo scheme) in turbulent
  MHD models, and its effects on the abundances of important chemical
  species in a large time-dependent chemical network. <P />In the
  presence of a penetrating radiation field the clumpiness tends to make
  the mass darker, suppressing photochemistry, while the interclump
  medium may be brightly illuminated and undergoes markedly different
  chemistry. <P />Where appropriate we also solve iteratively for the
  case where H<SUB>2</SUB> is self-shielding, following the character
  of the H/H<SUB>2</SUB> reservoir which plays a central role in
  astrochemistry. Clumping tends to enhance self-shielding, causing
  the global distribution of hydrogen to become molecular both sooner
  temporally and at lower A<SUB>V</SUB>, when compared with homogeneous
  models. <P />Particular attention is paid to the ionisation fraction
  which may play an important role in magnetic field transport. This
  work may be extended to the later stages of star formation. <P />This
  research is supported by NSF, NASA and the University of Wisconsin
  NSF grant.

---------------------------------------------------------
Title: Magnetic fields in galaxies
Authors: Zweibel, Ellen G.
2005fdda.conf..115Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Slow Motions in the NGC 1275 (Perseus A) System of Giant
    Ionized Filaments
Authors: Cigan, P.; Gallagher, J.; Zweibel, E.
2004AAS...20511004C    Altcode: 2004BAAS...36.1534C
  Emission line strengths and velocities have been obtained at 5 positions
  in the NGC1275 emission line filaments system covering a range in radial
  distance. These observations taken with the Densepak fiber array at the
  Cassegrain focus of the WIYN 3.5-m telescope provide radial velocities
  with an rms precision of about 50 km/s. The spectra cover the Hα
  spectral region and include the [NII] and [SII] emission lines in all
  but the faintest regions. Our analysis of the filament radial velocity
  distribution of the ionized gas based on these data shows only a small
  velocity spread of about 350 km/s as compared with the sound speed in
  the intracluster medium (ICM) of &gt;1000 km/s. The ionized filaments
  are effectively floating in the ICM. We discuss the implications of
  this result in terms of the origin of the filaments, and find support
  for models where the filaments consist of interstellar gas that was
  pulled out of the main body of NGC1275. <P />Research supported in
  part by NSF grant AST98-03018 to the University of Wisconsin-Madison
  and by our Graduate School through funds supplied by the Wisconsin
  Alumni Research Foundation.

---------------------------------------------------------
Title: Ambipolar Diffusion in a Turbulent Medium
Authors: Zweibel, E. G.
2004ASPC..323...97Z    Altcode:
  The relationship between gas density and magnetic field strength in
  the interstellar medium the nature of magnetohydrodynamic processes
  under interstellar conditions. The B-ρ relation is important
  not only as a probe of interstellar gas dynamics but also for its
  implications for the generation and evolution of the Galactic magnetic
  field. Observations suggest that the B-ρ relation is determined largely
  by diffusion. Ambipolar drift acting in concert with turbulence is
  the most viable candidate for a rapid diffusion mechanism.

---------------------------------------------------------
Title: Galactic magnetic fields
Authors: Zweibel, Ellen
2004APS..DPPQR1001Z    Altcode:
  Magnetic fields in galaxies play an important role in the dynamics
  and energetics of interstellar gas. Their existence - especially their
  spatial coherence - poses a formidable challenge to theory. This talk
  is a review of the observations of galactic magnetic fields and the
  main physical processes which control their generation and evolution.

---------------------------------------------------------
Title: Turbulence in the Star-forming Interstellar Medium: Steps
    toward Constraining Theories with Observations
Authors: Heyer, Mark; Zweibel, Ellen
2004Ap&SS.292....9H    Altcode: 2003astro.ph.10835H
  Increasingly sophisticated observational tools and techniques are now
  being developed for probing the nature of interstellar turbulence. At
  the same time, theoretical advances in understanding the nature of
  turbulence and its effects on the structure of the ISM and on star
  formation are occurring at a rapid pace, aided in part by numerical
  simulations. These increased capabilities on both fronts open new
  opportunities for strengthening the links between observation and
  theory, and for meaningful comparisons between the two.

---------------------------------------------------------
Title: Magnetic Flux Transport in the ISM Through Turbulent Ambipolar
    Diffusion
Authors: Heitsch, Fabian; Zweibel, Ellen G.; Slyz, Adrianne D.;
   Devriendt, Julien E. G.
2004Ap&SS.292...45H    Altcode:
  Under ideal MHD conditions the magnetic field strength should be
  correlated with density in the interstellar medium (ISM). However,
  observations indicate that this correlation is weaker than
  expected. Ambipolar diffusion can decrease the flux-to-mass ratio
  in weakly ionized media; however, it is generally thought to be too
  slow to play a significant role in the ISM except in the densest
  molecular clouds. Turbulence is often invoked in other astrophysical
  problems to increase transport rates above the (very slow) diffusive
  values. Building on analytical studies, we test with numerical models
  whether turbulence can enhance the ambipolar diffusion rate sufficiently
  to explain the observed weak correlations. The numerical method is
  based on a gas-kinetic scheme with very low numerical diffusivity,
  thus allowing us to separate numerical and physical diffusion effects.

---------------------------------------------------------
Title: Dust Heating by the Interstellar Radiation Field in Models
    of Turbulent Molecular Clouds
Authors: Bethell, Thomas J.; Zweibel, Ellen G.; Heitsch, Fabian;
   Mathis, J. S.
2004ApJ...610..801B    Altcode: 2004astro.ph..4056B
  We have calculated the radiation field, dust grain temperatures, and
  far-infrared emissivity of numerical models of turbulent molecular
  clouds. When compared to a uniform cloud of the same mean optical
  depth, most of the volume inside the turbulent cloud is brighter,
  but most of the mass is darker. There is little mean attenuation from
  center to edge, and clumping causes the radiation field to be somewhat
  bluer. There is also a large dispersion, typically by a few orders
  of magnitude, of all quantities relative to their means. However,
  despite the scatter, the 850 μm emission maps are well correlated
  with surface density. The fraction of mass as a function of intensity
  can be reproduced by a simple hierarchical model of density structure.

---------------------------------------------------------
Title: Magnetic Field Evolution in Neutron Star Crusts Due to the
    Hall Effect and Ohmic Decay
Authors: Cumming, Andrew; Arras, Phil; Zweibel, Ellen
2004ApJ...609..999C    Altcode: 2004astro.ph..2392C
  We present calculations of magnetic field evolution by the Hall
  effect and ohmic decay in the crust of neutron stars (NSs). In
  accreting NSs, ohmic decay is always the dominant effect because
  of the large resistivity. In isolated NSs with relatively pure
  crusts, the Hall effect dominates ohmic decay after a time
  t<SUB>switch</SUB>~=10<SUP>4</SUP>yrB<SUP>-3</SUP><SUB>12</SUB>,
  where B<SUB>12</SUB> is the magnetic field strength in units of
  10<SUP>12</SUP>G. We compute the evolution of an initial field
  distribution by ohmic decay and give approximate analytic formulae for
  both the surface and interior fields as a function of time. Because of
  the strong dependence of t<SUB>switch</SUB> on B<SUB>12</SUB>, early
  ohmic decay can alter the currents down to the base of the crust for
  B~10<SUP>11</SUP>G, neutron drip for B~10<SUP>12</SUP>G, and near the
  top of the crust for B&gt;~10<SUP>13</SUP>G. We then discuss magnetic
  field evolution by the Hall effect. Several examples are given to
  illustrate how an initial field configuration evolves. Hall-wave
  eigenfunctions are computed, including the effect of the large density
  change across the crust. We estimate the response of the crust to the
  magnetic stresses induced by Hall waves and give a detailed discussion
  of the boundary conditions at the solid-liquid interface. Finally,
  we discuss the implications for the Hall cascade proposed by Goldreich
  &amp; Reisenegger.

---------------------------------------------------------
Title: Magnetoelliptic Instabilities
Authors: Lebovitz, Norman R.; Zweibel, Ellen
2004ApJ...609..301L    Altcode: 2004astro.ph..3316L
  We consider the stability of a configuration consisting of a vertical
  magnetic field in a planar flow on elliptical streamlines in ideal
  hydromagnetics. In the absence of a magnetic field the elliptical
  flow is universally unstable (the “elliptical instability”). We find
  that this universal instability persists in the presence of magnetic
  fields of arbitrary strength, although the growth rate decreases
  somewhat. We also find further instabilities due to the presence
  of the magnetic field. One of these, a destabilization of Alfvén
  waves, requires the magnetic parameter to exceed a certain critical
  value. A second, involving a mixing of hydrodynamic and magnetic modes,
  occurs for all magnetic field strengths. These instabilities may be
  important in tidally distorted or otherwise elliptical disks. A disk
  of finite thickness is stable if the magnetic field strength exceeds
  a critical value, similar to the field strength that suppresses the
  magnetorotational instability.

---------------------------------------------------------
Title: Kelvin-Helmholtz Instability in a Weakly Ionized Medium
Authors: Watson, C.; Zweibel, E. G.; Heitsch, F.; Churchwell, E.
2004ApJ...608..274W    Altcode: 2004astro.ph..1243W
  Ambient interstellar material may become entrained in outflows from
  massive stars as a result of shear flow instabilities. We study
  the linear theory of the Kelvin-Helmholtz instability, the simplest
  example of shear flow instability, in a partially ionized medium. We
  model the interaction as a two-fluid system (charged and neutral) in a
  planar geometry. Our principal result is that for much of the relevant
  parameter space, neutrals and ions are sufficiently decoupled that the
  neutrals are unstable while the ions are held in place by the magnetic
  field. Thus, we predict that there should be a detectably narrower line
  profile in ionized species tracing the outflow compared with neutral
  species, since ionized species are not participating in the turbulent
  interface with the ambient ISM. Since the magnetic field is frozen to
  the plasma, it is not tangled by the turbulence in the boundary layer.

---------------------------------------------------------
Title: Dust Heating by the Interstellar Radiation Field in Models
    of Turbulent Molecular Clouds
Authors: Bethell, T. J.; Zweibel, E. G.; Heitsch, F.; Mathis, J. S.
2004AAS...204.6101B    Altcode: 2004BAAS...36..768B
  We have calculated the radiation field, dust grain temperatures, and
  far infrared emissivity of numerical models of turbulent molecular
  clouds. When compared to a uniform cloud of the same mean optical
  depth, most of the volume inside the turbulent cloud is brighter,
  but most of the mass is darker. There is little mean attenuation from
  center to edge, and clumping causes the radiation field to be somewhat
  bluer. There is also a large dispersion, typically by a few orders
  of magnitude, of all quantities relative to their means. However,
  despite the scatter, the 850μ m emission maps are well correlated with
  surface density. The fraction of mass as a function of intensity can be
  reproduced by a simple hierarchical model of density structure. <P />We
  have also computed the ionization of the gas by ambient UV radiation
  and low energy cosmic rays. The reduced radiative field in the dense
  material where most of the mass resides has implications for chemistry
  and magnetic field transport.

---------------------------------------------------------
Title: Turbulent Ambipolar Diffusion: Numerical Studies in Two
    Dimensions
Authors: Heitsch, Fabian; Zweibel, Ellen G.; Slyz, Adrianne D.;
   Devriendt, Julien E. G.
2004ApJ...603..165H    Altcode: 2003astro.ph..9306H
  Under ideal MHD conditions the magnetic field strength should be
  correlated with density in the interstellar medium (ISM). However,
  observations indicate that this correlation is weak. Ambipolar diffusion
  can decrease the flux-to-mass ratio in weakly ionized media; however,
  it is generally thought to be too slow to play a significant role in
  the ISM except in the densest molecular clouds. Turbulence is often
  invoked in astrophysical problems to increase transport rates above the
  (very slow) laminar values predicted by kinetic theory. We describe
  a series of numerical experiments addressing the problem of turbulent
  transport of magnetic fields in weakly ionized gases. We show, subject
  to various geometrical and physical restrictions, that turbulence in a
  weakly ionized medium rapidly diffuses the magnetic flux-to-mass ratio
  B/ρ through the buildup of appreciable ion-neutral drifts on small
  scales. These results are applicable to the field strength-density
  correlation in the ISM, as well as the merging of flux systems such as
  protostar and accretion disk fields or protostellar jets with ambient
  matter, and the vertical transport of galactic magnetic fields.

---------------------------------------------------------
Title: A Comparison Between Observations and Simulations of
    Polarization in Star Forming Regions
Authors: Hernandez, A. K.; Williams, J. P.; Zweibel, E. G.
2003AAS...203.0705H    Altcode: 2003BAAS...35.1213H
  Previous research has hinted that magnetic fields play a key role in
  the formation of cores in star forming molecular clouds. However,
  the extent of that role is not fully understood. Here, we compare
  polarization data of observed cores with clumps formed in a simulated
  model of turbulent molecular clouds. Through an investigation of the
  polarized angle dispersion of both sets of data, it was found that
  the simulations were accurate in representing cores formed with either
  a low turbulence or within a relatively strong magnetic field. Also,
  through investigation of data's polarized flux, the observed sources
  are found to have formed in fields that are more ordered than those
  modeled in the simulations. This finding is also supported by the
  calculated magnetic field strengths of the observed cores via the
  Chandrasekhar-Fermi method. This research will aid in understanding
  the conditions of star formation within turbulent molecular clouds.

---------------------------------------------------------
Title: Suppression of Fast Reconnection by Magnetic Shear
Authors: Heitsch, Fabian; Zweibel, Ellen G.
2003ApJ...590..291H    Altcode:
  Magnetic neutral sheets in weakly ionized interstellar gas are rapidly
  annihilated by ohmic diffusion. In this paper we extend the model to
  a sheared magnetic configuration, and show that the magnetic pressure
  associated with even a small nonzero field drastically reduces the
  reconnection rate of the reversing component.

---------------------------------------------------------
Title: Cosmic-Ray History and Its Implications for Galactic Magnetic
    Fields
Authors: Zweibel, Ellen G.
2003ApJ...587..625Z    Altcode: 2002astro.ph.12559Z
  There is evidence that cosmic rays were present in galaxies at
  moderately high redshift. This suggests that magnetic fields were also
  present. If cosmic rays and magnetic fields must always be close to
  equipartition, as they are to an order of magnitude within the local
  universe, this would provide a powerful constraint on theories of
  the origin and evolution of magnetic fields in galaxies. We evaluate
  the role of magnetic field strength in cosmic-ray acceleration and
  confinement. We find that the properties of small-scale hydromagnetic
  turbulence are fundamentally changed in the presence of cosmic rays. As
  a result, magnetic fields several orders of magnitude weaker than
  present galactic fields can accelerate and retain a population of
  relativistic cosmic rays, provided that the fields are coherent over
  length scales greater than a cosmic-ray gyroradius.

---------------------------------------------------------
Title: Magnetic Field Evolution in the Burning Layer of an Accreting
    Neutron Star
Authors: Cumming, A.; Zweibel, E. G.
2003HEAD....7.4201C    Altcode: 2003BAAS...35..654C
  Type I X-ray burst oscillations are a promising probe of the magnetic
  field strength and geometry in accreting neutron stars. Cumming and
  Bildsten showed that a field as weak as 10<SUP>6</SUP>G could play a
  dynamical role during a burst. Key to interpreting the observations
  is understanding the evolution of the magnetic field in the outer
  layers of the neutron star. We show that thermomagnetic drift operates
  strongly in the accumulating layer, leading to growth of the magnetic
  field between bursts. We discuss the saturation and geometry of the
  magnetic field, its relation to the underlying stellar field, and the
  implications for the dynamics of flows during Type I bursts.

---------------------------------------------------------
Title: Chandra Observations of the Guitar Nebula
Authors: Wong, D. S.; Cordes, J. M.; Chatterjee, S.; Zweibel, E. G.;
   Finley, J. P.; Romani, R. W.; Ulmer, M. P.
2003IAUS..214..135W    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Fast Reconnection in a Two-Stage Process
Authors: Heitsch, Fabian; Zweibel, Ellen G.
2003ApJ...583..229H    Altcode: 2002astro.ph..5103H
  Magnetic reconnection plays an essential role in the generation and
  evolution of astrophysical magnetic fields. The best tested and most
  robust reconnection theory is that of Parker and Sweet. According to
  this theory, the reconnection rate scales with magnetic diffusivity
  λ<SUB>Ω</SUB> as λ<SUP>1/2</SUP><SUB>Ω</SUB>. In the interstellar
  medium, the Parker-Sweet reconnection rate is far too slow to be
  of interest. Thus, a mechanism for fast reconnection seems to be
  required. We have studied the magnetic merging of two oppositely
  directed flux systems in weakly ionized, but highly conducting,
  compressible gas. In such systems, ambipolar diffusion steepens
  the magnetic profile, leading to a thin current sheet. If the ion
  pressure is small enough and the recombination of ions is fast enough,
  the resulting rate of magnetic merging is fast and independent
  of λ<SUB>Ω</SUB>. Slow recombination or sufficiently large ion
  pressure leads to slower merging, which scales with λ<SUB>Ω</SUB> as
  λ<SUP>1/2</SUP><SUB>Ω</SUB>. We derive a criterion for distinguishing
  these two regimes and discuss applications to the weakly ionized ISM
  and to protoplanetary accretion disks.

---------------------------------------------------------
Title: Numerical Simulations of Magnetic Fields in Astrophysical
    Turbulence
Authors: Zweibel, E. G.; Heitsch, F.; Fan, Y.
2003LNP...614..101Z    Altcode: 2003tmfa.conf..101Z; 2002astro.ph..2525Z
  The generation and evolution of astrophysical magnetic fields occurs
  largely through the action of turbulence. In many situations, the
  magnetic field is strong enough to influence many important properties
  of turbulence itself. Numerical simulation of magnetized turbulence
  is especially challenging in the astrophysical regime because of the
  high magnetic Reynolds numbers involved, but some aspects of this
  difficulty can be avoided in weakly ionized systems.

---------------------------------------------------------
Title: BIMA Observations of the Massive Bipolar Outflow from
    G5.89-0.39
Authors: Watson, C.; Churchwell, E. B.; Zweibel, E.; Crutcher, R. M.
2002AAS...201.2014W    Altcode: 2002BAAS...34.1135W
  We present CO and HCO+ observations using the BIMA interferometer of
  the outflow from the massive star formation region G5.89-0.39. We
  achieve 3” spatial resolution with 0.3 km/s velocity resolution
  and 0.2 Jy/Beam per channel rms. We measure very broad line wings
  (full-width ~60 km/s) and, based on morphology, this outflow appears
  nearly along the line-of-sight. We analyze how the outflow interacts
  with the ambient interstellar medium. Specifically, we characterize
  the mass entrainment process in this outflow by comparing emission at
  different velocities and comparing the two molecular species. Three
  different entrainment mechanisms are discussed: bow-shock, turbulent and
  wide-angle wind. CGW acknowledges partial support from the Wisconsin
  Space Grant Consortium. EBC acknowledges partial support from an NSF
  grant AST-9986548.

---------------------------------------------------------
Title: Fast Reconnection in a Two-Stage Process
Authors: Heitsch, F.; Zweibel, E. G.
2002AAS...200.7322H    Altcode: 2002BAAS...34R.769H
  Magnetic reconnection plays an essential role in the generation and
  evolution of astrophysical magnetic fields. The best tested and most
  robust reconnection theory is that of Sweet and Parker. According to
  this theory, the reconnection rate scales with magnetic diffusivity
  λ as λ <SUP>1/2</SUP>. In the interstellar medium, the Sweet-Parker
  reconnection rate is far too slow to be of interest. Thus, a mechanism
  for fast reconnection seems to be required. We have studied the magnetic
  merging of two oppositely directed flux systems in weakly ionized,
  but highly conducting, compressible gas. In such systems, ambipolar
  diffusion steepens the magnetic profile, leading to a thin current
  sheet. If the ion pressure is small enough, and the recombination of
  ions is fast enough, the resulting rate of magnetic merging is fast,
  and independent of λ . Slow recombination or sufficiently large
  ion pressure leads to slower merging which scales with λ as λ
  <SUP>1/2</SUP>. We derive a criterion for distinguishing these two
  regimes, and discuss applications to the weakly ionized ISM and to
  protostellar accretion disks. This work was supported in part by the
  Alexander von Humboldt Society, and NSF Grants AST 9800616 and AST
  0098701 to U. Colorado

---------------------------------------------------------
Title: How Fast is Magnetic Reconnection?
Authors: Zweibel, Ellen
2002kas..confE..64Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Ambipolar Drift in a Turbulent Medium
Authors: Zweibel, Ellen G.
2002ApJ...567..962Z    Altcode: 2001astro.ph..7462Z
  The interstellar magnetic field strength and density are observed to
  be correlated, but there is a large dispersion in this relation. In
  particular, the magnetic field is often observed to be weaker than
  expected. At low ionization fraction, ion-neutral drift, or ambipolar
  diffusion, permits slip of the field relative to the neutral gas and
  tends to make the field strength more uniform, but it is thought to
  be too slow to explain the observations. The purpose of this paper is
  to show that ion-neutral drift is significantly faster in a turbulent
  medium than in a quiescent one. We suggest that this fast ambipolar
  diffusion can explain the surprisingly low magnetic field strengths
  sometimes observed in dense interstellar gas.

---------------------------------------------------------
Title: Astronomy: Magnetic bubbles in space
Authors: Zweibel, Ellen G.
2002Natur.415...31Z    Altcode:
  The origin of magnetic fields found in galaxies and galaxy clusters
  is unknown. Both models and observations suggest that extinct radio
  galaxies could be responsible.

---------------------------------------------------------
Title: The Effect of Dust Grains on Reconnection in Molecular Clouds
Authors: Shay, M. A.; Rogers, B. N.; Rudakov, L.; Zweibel, E.
2001AAS...199.5903S    Altcode: 2001BAAS...33R1392S
  Magnetic reconnection enables a plasma system to convert magnetic
  energy into high speed flows and thermal energy. It is thought to play
  an important role in a broad range of astrophysical systems, including
  the interstellar medium, molecular clouds, the solar atmosphere, and
  accretion disks. However, it is widely accepted that reconnection is
  very slow unless effects beyond standard magnetohydrodynamics (MHD) come
  into play. Charged dust grains, because of their large Larmor radii,
  can have an important role in such non-MHD processes. We have derived
  a simple set of fluid equations valid in a plasma where the dust has
  a very small charge density but a large mass density relative to the
  ions. Such a parameter regime is relevant to molecular clouds. We will
  present a linear analysis of the waves in these equations and discuss
  their relation to reconnection in dusty plasmas, especially stressing
  the interplay between dust particles and neutrals. Finally, we will
  present initial fluid simulations of reconnection in dusty plasmas.

---------------------------------------------------------
Title: Magnetic Field Diagnostics Based on Far-Infrared Polarimetry:
    Tests Using Numerical Simulations
Authors: Heitsch, Fabian; Zweibel, Ellen G.; Mac Low, Mordecai-Mark;
   Li, Pakshing; Norman, Michael L.
2001ApJ...561..800H    Altcode: 2001astro.ph..3286H
  The dynamical state of star-forming molecular clouds cannot be
  understood without determining the structure and strength of their
  magnetic fields. Measurements of polarized far-infrared radiation from
  thermally aligned dust grains are used to map the orientation of the
  field and estimate its strength, but the accuracy of the results has
  remained in doubt. In order to assess the reliability of this method,
  we apply it to simulated far-infrared polarization maps derived from
  three-dimensional simulations of supersonic magnetohydrodynamical
  turbulence, and we compare the estimated values to the known magnetic
  field strengths in the simulations. We investigate the effects of
  limited telescope resolution and self-gravity on the structure of the
  maps. Limited observational resolution affects the field structure such
  that small-scale variations can be completely suppressed, thus giving
  the impression of a very homogeneous field. The Chandrasekhar-Fermi
  method of estimating the mean magnetic field in a turbulent medium is
  tested, and we suggest an extension to measure the rms field. Both
  methods yield results within a factor of 2 for field strengths
  typical of molecular clouds, with the modified version returning
  more reliable estimates for slightly weaker fields. However, neither
  method alone works well for very weak fields, missing them by a factor
  of up to 150. Taking the geometric mean of both methods estimates
  even the weakest fields accurately within a factor of 2.5. Limited
  telescope resolution leads to a systematic overestimation of the field
  strengths for all methods. We discuss the effects responsible for this
  overestimation and show how to extract information on the underlying
  (turbulent) power spectrum.

---------------------------------------------------------
Title: Magnetic Screening in Accreting Neutron Stars
Authors: Cumming, Andrew; Zweibel, Ellen; Bildsten, Lars
2001ApJ...557..958C    Altcode: 2001astro.ph..2178C
  We investigate whether the magnetic field of an accreting neutron star
  may be diamagnetically screened by the accreted matter. We assume the
  freshly accumulated material is unmagnetized and calculate the rate at
  which the intrinsic stellar magnetic flux is transported into it via
  Ohmic diffusion from below. For very high accretion rates M (larger
  than the Eddington rate M<SUB>Edd</SUB>), Brown &amp; Bildsten have
  shown that the liquid ocean and outer crust of the neutron star are
  built up on a timescale much shorter than the Ohmic penetration time. We
  extend their work to lower accretion rates and calculate the resulting
  screening of the magnetic field. We find that the Ohmic diffusion and
  accretion timescales are equal for M~0.1 M<SUB>Edd</SUB>. We calculate
  the one-dimensional steady state magnetic field profiles and show
  that the magnetic field strength decreases as one moves up through
  the outer crust and ocean by n orders of magnitude, where n~M/0.02
  M<SUB>Edd</SUB>. We show that these profiles are unstable to buoyancy
  instabilities when B&gt;~10<SUP>10</SUP>-10<SUP>11</SUP> G in the ocean,
  providing a new limit on the strength of the buried field. Our results
  have interesting implications for the weakly magnetic neutron stars in
  low-mass X-ray binaries. We find that magnetic screening is ineffective
  for M&lt;10<SUP>-2</SUP> M<SUB>Edd</SUB>, so that, no matter how the
  accreted material joins onto the star, the underlying stellar field
  should always be evident. This is consistent with the fact that the
  only known persistently pulsing accreting X-ray millisecond pulsar, SAX
  J1808.4-3658, has an unusually low accretion rate of M~10<SUP>-3</SUP>
  M<SUB>Edd</SUB>. Although the simplified magnetic and accretion
  geometry we adopt here does not allow us to definitively say so, we
  are led to suggest that perhaps most of the weakly magnetic neutron
  stars in low-mass X-ray binaries have a screened magnetic field,
  explaining the lack of persistent pulsations from these sources. If
  screened, then the underlying field will emerge after accretion halts,
  on a timescale of only 100-1000 yr, set by the Ohmic diffusion time
  across the outer crust.

---------------------------------------------------------
Title: Condensation of a Self-Gravitating Layer Due to Dissipation
    of Alfvenic Turbulence
Authors: Myers, P. C.; Zweibel, E. G.
2001AAS...198.8705M    Altcode: 2001BAAS...33..915M
  We model the structure and evolution of a turbulent, magnetized,
  flattened, self-gravitating molecular cloud. A uniform mean magnetic
  field threads an infinite horizontal layer, supported against
  self-gravity by the pressure of Alfven waves and thermal motions. The
  equilibrium density decreases asymptotically with height as z**(-2),
  declining more gradually than in the isothermal case. The layer is
  within a factor 2 of magnetically critical. The dominant wave damping
  mechanism is nonlinear steepening into shocks rather than ion-neutral
  friction. Such a layer can condense quasistatically if its initial
  midplane turbulence has Mach number &lt; 2. The layer "settles"
  rather than collapses, with speed proportional to the wave damping
  rate. If the layer has modest nonuniformity in its column density,
  it can develop "differential condensation," where regions of column
  density greater than average by a factor of 2 produce midplane number
  density greater than average by a factor of 10. For field strength,
  temperature, column density, and velocity dispersion observed in
  nearby clouds, this model predicts a temporal increase in midplane
  density by a factor 4, to 2 x 10**(4) cm**(-3), with typical inward
  speed 0.1 km/s, and with line width decreasing to 0.2 km/s, all in
  0.5 Myr. This idealized model of turbulent dissipation matches more
  observed features of star-forming dense cores and their environs than
  do models which assume purely static magnetic fields.

---------------------------------------------------------
Title: The Structure of the Galactic Magnetic Field as Revealed by
    Starlight Polarimetry
Authors: Walawender, J. M.; Zweibel, E. G.; Heiles, C.
2001AAS...198.4105W    Altcode: 2001BAAS...33..841W
  We present the results of an observational study of the local galactic
  magnetic field probed by the polarization of starlight. Using a
  least-squares fitting technique we derive the local direction and
  curvature of the uniform component of the galactic magnetic field. We
  find a local direction of l<SUB><SUP>o</SUP></SUB> = 80.6<SUP>o</SUP>,
  a radius of curvature, R<SUB>cc</SUB> = 7.8 kpc, and the position
  of the center of curvature to be l<SUB>cc</SUB> = -15.4<SUP>o</SUP>,
  which are roughly consistent with the structure of the spiral arms of
  the Galaxy and with the earlier results of Heiles 1996. We also probe
  the structure of the turbulence in the field and attempt to measure
  its amplitude and correlation length. Both density and magnetic field
  fluctuations affect the distribution of polarization. We present
  simulations which demonstrate the extent to which the two can be
  separated and use the observations to place constraints on the nature
  of the magnetic fluctuation spectrum.

---------------------------------------------------------
Title: From Simulations to Observations: Polarization Maps of
    Star-Forming Regions
Authors: Heitsch, F.; Zweibel, E. G.; Li, P. S.; Mac Low, M. -M.;
   Norman, M. L.
2000AAS...197.0513H    Altcode: 2000BAAS...32.1396H; 2000AAS...197..513H
  Far infrared polarimetry can be used to map magnetic fields in dense
  clouds, using the thermal emission from aligned dust grains as a
  tracer. We are making polarimetric maps from numerical simulations
  to calibrate the effects of line of sight and horizontal averaging
  on the apparent field morphology. We use full 3D simulations of
  self-gravitating MHD turbulence run with ZEUS-3D and ZEUS-MP at
  resolution up to 512<SUP>3</SUP> to produce simulated polarization maps
  of molecular clouds. We derive the Stokes parameters U and V by solving
  the radiative transfer problem according to Zweibel (1995), with the
  emissivity proportional to the local density. Smoothing due to finite
  telescope beam size w changes the distribution of polarization angles
  σ (φ ). Doubling w reduces σ (φ ) by ≈ 25%. We find that column
  density structures have arbitrary orientations with respect to apparent
  magnetic field directions. Examples can be found with apparent field
  parallel, intermediate and perpendicular to column density filaments. We
  test the method of Chandrasekhar &amp; Fermi (1953) to estimate the
  field strength in a turbulent medium and find that it yields results
  accurate up to a factor of 2 for field strengths typical for molecular
  clouds. We suggest a modification which increases the accuracy for
  weak magnetic fields. We also study the effect of smoothing on the
  method. This work is funded in parts by an NSF CAREER fellowship to
  M-MML, grant number AST 99-85392. Computations were performed at the
  NCSA and at the Rechenzentrum Garching of the MPG.

---------------------------------------------------------
Title: Magnetic Screening in Accreting Neutron Stars
Authors: Cumming, A.; Zweibel, E. G.; Bildsten, L.
2000HEAD....5.2903C    Altcode: 2000BAAS...32Q1229C
  We investigate the magnetic screening properties of matter accreted
  onto a neutron star. Our calculation includes the unburned atmosphere
  of hydrogen/helium, the liquid layer of ashes and the outer crust. We
  compare the timescales for ohmic diffusion and thermomagnetic drift with
  the radial flow time due to accretion. For the highest accretion rates,
  the diffusion and drift times are always longer than the accretion time,
  making it difficult for magnetic flux to penetrate the freshly accreted
  material. However, for the lowest accretion rates there is adequate time
  for any underlying magnetic field to penetrate the accreted matter. We
  show the one-dimensional steady-state magnetic field profiles, for
  which downwards advection and compression by accretion are balanced
  by upwards ohmic diffusion and thermomagnetic drift, and investigate
  whether these profiles are stable to buoyancy instabilities. We discuss
  possible implications of our results for the observed magnetic fields
  of both steadily and transiently accreting neutron stars.

---------------------------------------------------------
Title: Ambipolar Drift Heating in Turbulent Molecular Clouds
Authors: Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke
2000ApJ...540..332P    Altcode: 1999astro.ph.10147P
  We present calculations of frictional heating by ion-neutral
  drift in three-dimensional simulations of turbulent, magnetized
  molecular clouds. We show that ambipolar drift heating is a strong
  function of position in a turbulent cloud, and its average value
  can be significantly larger than the average cosmic-ray heating
  rate. The heating rate per unit volume due to ambipolar drift,
  H<SUB>AD</SUB>=|JXB|<SUP>2</SUP>/ρ<SUB>i</SUB>ν<SUB>in</SUB>~
  B<SUP>4</SUP>/(16π<SUP>2</SUP>L<SUP>2</SUP><SUB>B</SUB>ρ<SUB>i</SUB>
  ν<SUB>in</SUB>), is found to depend on the rms Alfvénic Mach
  number, M<SUB>A</SUB>, and on the average field strength, as
  H<SUB>AD</SUB>~M<SUP>2</SUP><SUB>A</SUB>&lt;|B|&gt;<SUP>4</SUP>. This
  implies that the typical scale of variation of the magnetic field,
  L<SUB>B</SUB>, is inversely proportional to M<SUB>A</SUB>, which we
  also demonstrate.

---------------------------------------------------------
Title: Generation of the Primordial Magnetic Fields during
    Cosmological Reionization
Authors: Gnedin, Nickolay Y.; Ferrara, Andrea; Zweibel, Ellen G.
2000ApJ...539..505G    Altcode: 2000astro.ph..1066G
  We investigate the generation of magnetic fields by the Biermann
  battery in cosmological ionization fronts, using new simulations of
  the reionization of the universe by stars in protogalaxies. Two
  mechanisms are primarily responsible for magnetogenesis: (1)
  the breakout of ionization fronts from protogalaxies and (2) the
  propagation of ionization fronts through the high-density neutral
  filaments that are part of the cosmic web. The first mechanism is
  dominant prior to overlapping of ionized regions (z~7), whereas
  the second continues to operate even after that epoch. However,
  after overlap the field strength increase is largely due to the gas
  compression occurring as cosmic structures form. As a consequence,
  the magnetic field at z~5 closely traces the gas density, and it is
  highly ordered on megaparsec scales. The mean mass-weighted field
  strength is B<SUB>0</SUB>~10<SUP>-19</SUP> G in the simulation
  box. There is a relatively well-defined, nearly linear correlation
  between B<SUB>0</SUB> and the baryonic mass of virialized objects,
  with B<SUB>0</SUB>~10<SUP>-18</SUP> G in the most massive objects
  (M~10<SUP>9</SUP> M<SUB>solar</SUB>) in our simulations. This is
  a lower limit, as lack of numerical resolution prevents us from
  following small-scale dynamical processes that could amplify the field
  in protogalaxies. Although the field strengths we compute are probably
  adequate as seed fields for a galactic dynamo, the field is too small to
  have had significant effects on galaxy formation, on thermal conduction,
  or on cosmic-ray transport in the intergalactic medium. It could,
  however, be observed in the intergalactic medium through innovative
  methods based on analysis of γ-ray burst photon arrival times.

---------------------------------------------------------
Title: Ambipolar Drift Heating in Turbulent Molecular Clouds
Authors: Padoan, P.; Zweibel, E.; Nordlund, Å.
2000ESASP.445..479P    Altcode: 2000sfsl.conf..479P
  No abstract at ADS

---------------------------------------------------------
Title: Discussion on Dynamos and Magnetic Reconnection
Authors: Moffatt, Keith; Zweibel, Ellen
2000astu.confE..21M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Astrophysical Turbulence
Authors: Ostriker, Eve C.; Zweibel, Ellen G.
2000astu.confE....O    Altcode:
  It is difficult to exaggerate the importance of turbulence in
  astrophysics, or the challenges which it poses. Turbulence is
  responsible for dynamical pressure support, energy transport, angular
  momentum transport, chemical mixing, and magnetic field generation and
  evolution in a host of astrophysical settings. Turbulent astrophysical
  flows differ from terrestrial forms of turbulence which have been
  studied traditionally by virtue of their inherent compressibility,
  strong radiative cooling, self-gravity, and the importance in many
  environments of magnetic fields. <P />Recent years have seen important
  advances in several distinct areas of astrophysical turbulence theory
  -- including modeling of turbulence in stars, accretion disks, and
  the interstellar medium, as well basic studies of MHD turbulence which
  provide the framework for all these applications. In one star, the Sun,
  helioseismologic data are allowing increasingly sophisticated comparison
  of observations with the theory of turbulent stellar interiors. Attempts
  to model the solar differential rotation has shown clearly that
  turbulent angular momentum transport is an essential ingredient,
  and attempts to model it are improving. Solar dynamo calculations
  are only slightly behind. Models of turbulent accretion disks are
  becoming increasingly realistic, with the dynamical role of magnetic
  fields a crucial element, and global, time-dependent modeling now
  feasible. It seems likely that the alpha viscosity parameter will
  be soon be supplanted by ab initio calculations of the accretion
  rate. Although the presence of interstellar turbulence has long been
  recognized, recent theoretical studies have significantly increased
  our understanding of its effects, particularly in the cold ISM where it
  plays a dominant role. Self-consistent dynamical studies will soon be
  able to identify how strong turbulence evolves and shapes the internal
  structure of magnetized interstellar clouds. Basic studies in MHD
  turbulence have made substantial recent progress in such longstanding
  problems as the nature of the turbulent cascade, dynamo generation
  of fields, and the process of magnetic reconnection. <P />In all
  of these studies, a crucial new ingredient has been computational
  advances that now make possible direct hydrodynamic/MHD simulations
  of three-dimensional, time-dependent turbulence with inertial dynamic
  ranges of more than two orders of magnitude. These advances in numerical
  experimentation are inspiring new analytical work, new comparisons
  between models and observations, and advances in observations and data
  analysis themselves. The ITP program on Astrophysical Turbulence will
  provide a forum for intensive interaction among analytical theorists,
  computational physicists, and observers from all of the subspecialties,
  with prospects for major research progress through interdisciplinary
  discussions and collaborations.

---------------------------------------------------------
Title: The Milky Way Magnetic Field Mapping Mission: M4
Authors: Clemens, D. P.; Bookbinder, J.; Goodman, A.; Kristen, H.;
   Myers, P.; Padoan, P.; Wood, K.; Heyer, M. H.; Heiles, C.; Jones,
   T. J.; Dickey, J.; Young, E.; Rieke, G.; Dow, K.; Dowell, C. D.;
   Draine, B.; Greaves, J.; Klaas, U.; Laureijs, R.; Lazarian, A.; Shulz,
   B.; Zweibel, E.
2000AAS...196.2508C    Altcode: 2000BAAS...32..709C
  M4 has been proposed this year as a potential new SMEX mission. The
  central goal of the mission is to measure magnetic field orientations in
  the interstellar medium of the Milky Way Galaxy to assess the importance
  of the field in star formation and other physical processes. The
  measurement technique is far-infrared imaging linear polarimetry,
  which has been extensively proven from both the Kuiper Airborne
  Observatory and the recent Infrared Space Observatory. M4 will conduct
  the first extensive surveys of magnetic field orientations, spanning
  1400 square degrees of the dense interstellar medium of the inner
  Milky Way, 300 square degrees of nearby star-forming dark molecular
  clouds, and 330 square degrees away from the Galactic plane, chosen
  to cover regions with infrared cirrus. The M4 instrument consists of
  a 20 cm cooled telescope, far-infrared light polarization analysis
  optics, and two 32x32 focal plane array detectors operating around 100
  microns wavelength. The M4 spacecraft is a 3-axis stablized pointing
  platform. The nominal launch date is 1 March 2004. The Pegasus XL is
  the baseline launch vehicle. The flight portion will span 3-4 months,
  in a 500 km, Sun-sync orbit. Data will be released in two stages: 6
  months and 12 months after the end of the flight portion of the project.

---------------------------------------------------------
Title: The Origin of Magnetic Fields in Galaxies
Authors: Zweibel, Ellen
2000astu.progE..36Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Astrophysical Turbulence
Authors: Zweibel, Ellen
2000astu.progE..35Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Astrophysical Turbulence
Authors: Zweibel, Ellen
2000kbls.confE..34Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Astrophysical Turbulence
Authors: Ostriker, Eve C.; Zweibel, Ellen G.
2000astu.progE....O    Altcode:
  It is difficult to exaggerate the importance of turbulence in
  astrophysics, or the challenges which it poses. Turbulence is
  responsible for dynamical pressure support, energy transport, angular
  momentum transport, chemical mixing, and magnetic field generation and
  evolution in a host of astrophysical settings. Turbulent astrophysical
  flows differ from terrestrial forms of turbulence which have been
  studied traditionally by virtue of their inherent compressibility,
  strong radiative cooling, self-gravity, and the importance in many
  environments of magnetic fields. <P />Recent years have seen important
  advances in several distinct areas of astrophysical turbulence theory
  -- including modeling of turbulence in stars, accretion disks, and
  the interstellar medium, as well basic studies of MHD turbulence which
  provide the framework for all these applications. In one star, the Sun,
  helioseismologic data are allowing increasingly sophisticated comparison
  of observations with the theory of turbulent stellar interiors. Attempts
  to model the solar differential rotation has shown clearly that
  turbulent angular momentum transport is an essential ingredient,
  and attempts to model it are improving. Solar dynamo calculations
  are only slightly behind. Models of turbulent accretion disks are
  becoming increasingly realistic, with the dynamical role of magnetic
  fields a crucial element, and global, time-dependent modeling now
  feasible. It seems likely that the alpha viscosity parameter will
  be soon be supplanted by ab initio calculations of the accretion
  rate. Although the presence of interstellar turbulence has long been
  recognized, recent theoretical studies have significantly increased
  our understanding of its effects, particularly in the cold ISM where it
  plays a dominant role. Self-consistent dynamical studies will soon be
  able to identify how strong turbulence evolves and shapes the internal
  structure of magnetized interstellar clouds. Basic studies in MHD
  turbulence have made substantial recent progress in such longstanding
  problems as the nature of the turbulent cascade, dynamo generation
  of fields, and the process of magnetic reconnection. <P />In all
  of these studies, a crucial new ingredient has been computational
  advances that now make possible direct hydrodynamic/MHD simulations
  of three-dimensional, time-dependent turbulence with inertial dynamic
  ranges of more than two orders of magnitude. These advances in numerical
  experimentation are inspiring new analytical work, new comparisons
  between models and observations, and advances in observations and data
  analysis themselves. The ITP program on Astrophysical Turbulence will
  provide a forum for intensive interaction among analytical theorists,
  computational physicists, and observers from all of the subspecialties,
  with prospects for major research progress through interdisciplinary
  discussions and collaborations.

---------------------------------------------------------
Title: Fragmentation Instability of Molecular Clouds: Numerical
    Simulations
Authors: Indebetouw, Rémy; Zweibel, Ellen G.
2000ApJ...532..361I    Altcode: 2000astro.ph..2092I
  We simulate fragmentation and gravitational collapse of cold,
  magnetized molecular clouds. We explore the nonlinear development of an
  instability mediated by ambipolar diffusion, in which the collapse rate
  is intermediate to fast gravitational collapse and slow quasistatic
  collapse. Initially uniform stable clouds fragment into elongated
  clumps with masses largely determined by the cloud temperature,
  but substantially larger than the thermal Jeans mass. The clumps are
  asymmetric, with significant rotation and vorticity, and lose magnetic
  flux as they collapse. The clump shapes, intermediate collapse rates,
  and infall profiles may help explain observations not easily fit by
  contemporary slow or rapid collapse models.

---------------------------------------------------------
Title: Magnetic Field Dissipation in the Interstellar Medium
Authors: Zweibel, Ellen
1999APS..DPP.GM101Z    Altcode:
  There is strong phenomenological evidence that magnetic reconnection
  occurs in the interstellar medium. In particular, if there is a galactic
  dynamo there must be magnetic reconnection. But the magnetic Reynolds
  number R<SUB>m</SUB> is very large for most interstellar structures,
  typically 10^15 - 10^20, suggesting that the magnetic reconnection
  rate does not scale as a power of R_m. Furthermore, interstellar
  magnetic fields are near equipartition with the turbulent velocities,
  suggesting that reconnection is dynamical as opposed to kinematic. I
  will discuss physical effects beyond MHD that could enhance the
  interstellar reconnection rate: ion-neutral drift, collisionless
  effects, and charged dust.

---------------------------------------------------------
Title: Alfvén resonances and forced reconnection
Authors: Uberoi, Chanchal; Zweibel, Ellen G.
1999JPlPh..62..345U    Altcode:
  Available from <A
  href="http://journals.cambridge.org/bin/bladerunner?REQUNIQ=1105385370&amp;REQSESS=958582&amp;118000REQEVENT=&amp;REQINT1=18609&amp;REQAUTH=0">http://journals.cambridge.org/bin/bladerunner?REQUNIQ=1105385370&amp;REQSESS=958582&amp;118000REQEVENT=&amp;REQINT1=18609&amp;REQAUTH=0</A>

---------------------------------------------------------
Title: The Rise of Kink-unstable Magnetic Flux Tubes and the Origin
    of δ-Configuration Sunspots
Authors: Fan, Y.; Zweibel, E. G.; Linton, M. G.; Fisher, G. H.
1999ApJ...521..460F    Altcode:
  We perform three-dimensional simulations of the rise of twisted magnetic
  flux tubes in an adiabatically stratified model solar convection
  zone. The initial flux tube in our simulations is a uniformly twisted,
  buoyant, horizontal tube located near the bottom of the stratified
  layer. The twist of the initial flux tube is described by a parameter
  α, which is defined as the angular rate of field-line rotation about
  the tube axis per unit length of the tube. We study the nonlinear
  evolution of the helical kink instability of the flux tube as it rises
  through the stratified layer. We find from our simulations that in order
  for the tube to develop significant kinking during its rise, the initial
  twist of the tube needs to be close to or greater than the critical
  limit (α<SUB>c</SUB>) for the onset of the kink instability. If the
  initial twist is significantly below the critical limit (α below about
  50% of α<SUB>c</SUB>), we find essentially no kink development and
  the evolution is similar to the results from previous two-dimensional
  simulations of the rise of twisted, horizontal flux tubes. On the other
  hand, if the initial twist is sufficiently greater than the critical
  limit such that the e-folding period of the fastest growing kink mode
  is small compared to the rise time of the tube, we find sharp bending
  and distortion of the tube as a result of the nonlinear evolution of
  the kink instability. In this case, we find that due to the effect of
  gravitational stratification, the kinked flux tube arches upward and
  evolves into a buckled loop with a local change of tube orientation
  at the loop apex that exceeds 90° from the original direction of the
  tube. The emergence of this buckled loop can give rise to a compact
  magnetic bipole with polarity order inverted from the Hale polarity
  law, similar to the configuration often seen in δ spots. Furthermore,
  our simulations show that the writhing of the tube axis as a result
  of the kink instability stretches the flux tube and increases its
  buoyancy. Hence, the development of the kink instability can speed up
  the overall rise of the flux tube.

---------------------------------------------------------
Title: The Effects of Large-Scale Convective Structures on Solar
    Eigenfrequencies
Authors: Swisdak, M.; Zweibel, E.
1999AAS...194.2103S    Altcode: 1999BAAS...31Q.858S
  Although the differences between observed p-mode eigenfrequencies
  and those calculated from solar models are small, they are
  significant. Strong evidence supports the contention that convection
  is responsible for much of the discrepancy. In most solar models,
  mixing-length parameterizations are used only to establish the
  mean structure of the convection zone; however, no efforts are
  made to calculate the influence of convective structures on p-mode
  eigenfrequencies. I will review an algorithm we have developed,
  using a method known as adiabatic switching, which allows us to
  determine the eigenfrequencies of p-modes in complex convective
  structures. This method is valid when describing p-modes in the ray
  approximation (not as global modes of oscillation). This requirement
  is equivalent to the familiar WKB approximation and restricts our
  considerations to large-scale convective motions. Our current work
  focuses on two-dimensional plane-parallel convection which includes
  variations in the local sound speed (temperature) as well as advective
  motions of the underlying fluid. I will present results from several
  convective simulations: Rayleigh-Benard cells, thermal plumes (such
  as are found on supergranular boundaries), and turbulent convective
  models. Our investigations show that simple models of convective
  cells produce downshifts which are second-order in the strength of the
  perturbation. More complex simulations, while consistently displaying
  downshifts, exhibit more complicated dependences on the strength of
  the convection. Finally, we demonstrate the dependence of the shift
  on the radial order n and degree l of the modes and show they agree
  with analytic estimates. At the minimum, these results demonstrate
  convective effects are of the proper sign and magnitude to explain
  the observed discrepancies although a complete correspondence with
  data has not yet been established.

---------------------------------------------------------
Title: Magnetohydrodynamics problems in the interstellar medium
Authors: Zweibel, Ellen G.
1999PhPl....6.1725Z    Altcode:
  Many defining features of galaxies and their evolution are controlled
  by their interstellar gas. From 20% to 50% of the interstellar gas
  in the Milky Way is in molecular clouds, which are also the sites of
  star formation. Molecular clouds are turbulent, and magnetic fields are
  strong enough that the turbulence is believed to be magnetohydrodynamic
  in character. Analytical and numerical estimates suggest that the
  turbulence would be dissipated in less than the lifetime of the cloud
  if it were not driven. A small fraction of the mass in molecular clouds
  fails to be supported against self-gravity and collapses to form stars;
  star formation also is strongly influenced by magnetohydrodynamic
  effects.

---------------------------------------------------------
Title: The Emergence of Kink-Unstable Magnetic Flux Tubes and the
    Origin of delta -Configuration Sunspots
Authors: Fan, Y.; Zweibel, E. G.; Linton, M. G.; Fisher, G. H.
1999AAS...194.5903F    Altcode: 1999BAAS...31Q.918F
  The so-called delta -configuration sunspots are an unusual class of
  compact sunspots in which umbrae of opposite polarities are gathered
  closely in a common penumbra, and the polarity order is often inverted
  from Hale's polarity law. One appealing suggestion for the origin of
  the delta -spots, is that they are formed through the emergence of
  flux tubes that have become kinked (or knotted) due to the onset of the
  current driven kink instability. In this talk I present 3D simulations
  of the non-linear evolution of the helical kink instability of twisted
  magnetic flux tubes rising buoyantly through an adiabatically stratified
  layer. We study the kink evolution of buoyant flux tubes with a range
  of different initial twist. We find that in order for the tube to
  develop significant kinking during its rise, the initial twist of the
  tube needs to be close to or greater than the critical limit for the
  onset of the kink instability. If the initial twist is sufficiently
  super-critical such that the e-folding period of the fastest growing
  kink mode is small compared to the rise time of the tube, we find that
  sharp bending and distortion of the tube develop. Due to the effect of
  gravitational stratification, the kinked flux tube arches upward and
  evolves into a buckled loop with a local change of tube orientation
  at the loop apex that exceeds 90(deg) from the original direction of
  the tube. I will discuss the similarities and differences between the
  structure of the buckled emerging flux loop and the magnetic field
  morphology of several delta -spots.

---------------------------------------------------------
Title: Absorption/Emission of Solar p-Modes: Michelson Doppler
    Interferometer/SOHO Observations
Authors: Haber, Deborah; Jain, Rekha; Zweibel, Ellen G.
1999ApJ...515..832H    Altcode:
  We search for sources and sinks of solar p-mode waves by creating
  absorption maps from Dopplergrams taken by the Michelson Doppler
  Interferometer instrument on board SOHO. Although the maps are noisy,
  we present evidence for sources and sinks, of duration ~0.5-1 hour or
  less, based on an autocorrelation analysis of maps made from sequential
  intervals of data. Emission and absorption nearly balance each other
  in the data. This may imply that emission and absorption are inverse
  processes of each other rather than fundamentally different in nature.

---------------------------------------------------------
Title: Effects of Large-Scale Convection on p-Mode Frequencies
Authors: Swisdak, M.; Zweibel, E.
1999ApJ...512..442S    Altcode: 1998astro.ph..9135S
  We describe an approach for finding the eigenfrequencies of
  solar acoustic modes (p-modes) in a convective envelope in the
  Wentzel-Kramers-Brillouin limit. This approximation restricts us to
  examining the effects of fluid motions that are large compared with
  the mode wavelength but allows us to treat the three-dimensional mode
  as a localized ray. The method of adiabatic switching is then used to
  investigate the frequency shifts resulting from simple perturbations
  to a polytropic model of the convection zone as well as from two
  basic models of a convective cell. We find that although solely
  depth-dependent perturbations can give frequency shifts that are
  first order in the strength of the perturbation, models of convective
  cells generate downward frequency shifts that are second order in
  the perturbation strength. These results may have implications for
  resolving the differences between eigenfrequencies derived from solar
  models and those found from helioseismic observations.

---------------------------------------------------------
Title: Turbulence and Magnetic Reconnection in the Interstellar Medium
Authors: Zweibel, Ellen
1999intu.conf..232Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Effects of Convective Structures on P Mode Frequencies
Authors: Swisdak, M.; Zweibel, E. G.
1998AAS...19310003S    Altcode: 1998BAAS...30.1397S
  While the correspondence between observed and predicted p-mode
  frequencies is good, it is clear that the detailed effects of convection
  are not adequately treated by contemporary solar models. Even models
  of simple convective structures which ignore the difficulties inherent
  in realistically modelling solar convection encounter difficulties
  in determining their effects on global oscillations. However,
  under the WKB approximation p modes may be treated as rays and their
  propagation described using the formalism of Hamiltonian systems. This
  is an adequate approximation for large-scale convection and modes
  of large spherical harmonic degree l. Only simple stellar models
  (polytropes, for example) have eigenfrequencies which may be found
  analytically. However, a computer program we have written using a method
  known as adiabatic switching allows us to determine the eigenfrequencies
  of modes in the ray approximation. The motion of a ray is governed by
  a dispersion relation which may account for several effects, including
  variations in the local sound speed as well as advective motions of the
  underlying fluid. Our investigations have shown that simple models of
  convective cells produce downshifts in the eigenfrequencies which are
  of second-order in the strength of the perturbation. At the minimum,
  this result is of the proper sign to explain the observed discrepancy
  although it is unclear if the correction is large enough to account
  for the entire effect. In addition, we demonstrate the dependence of
  the shift on the radial order n and degree l of the mode and show that
  they roughly agree with our analytic estimates predicting a frequency
  shift which varies as a/l + bl where a and b are constants. Finally,
  we consider the case of a ray interacting with a thermal plume such
  as those observed at the edges of granules and supergranules.

---------------------------------------------------------
Title: Gravitational Instability in a Cold Magnetized Molecular
    Cloud with Ambipolar Drift
Authors: Indebetouw, R.; Zweibel, E. G.
1998AAS...193.7104I    Altcode: 1998BAAS...30.1356I
  I present a numerical simulation of gravitational instability in a thin,
  cold, magnetized molecular cloud with ambipolar diffusion. The nonlinear
  evolution of density perturbations shows many of the characteristics
  evident in the linear analytic theory. The presence of ambipolar
  diffusion decreases the stability of clouds, and density enhancements
  grow on a time-scale intermediate to the dynamical collapse and
  diffusive time-scales. This instability can cause the collapse of dense
  cores with significant loss of magnetic flux. The magnetic and velocity
  fields in the simulation become quite complex, and this mechanism is a
  possible source of magnetized turbulence in clouds. Thermal pressure in
  the cloud stabilizes the smallest spatial scales, and the instability
  produces cloud cores or fragments on the order of ~ 1M_☉.

---------------------------------------------------------
Title: Plasma Physics Problems of the Interstellar Medium
Authors: Zweibel, Ellen G.
1998APS..DPPR8TU03Z    Altcode:
  The morphology and evolution of galaxies is determined to a great extent
  by the structure and energy balance of the gas within them. In disk
  galaxies such as the Milky Way, the interstellar medium is turbulent,
  magnetized, and contains a population of relativistic particles (cosmic
  rays) in addition to its thermal component, the latter ranging in
  temperature from 10^circK to more than 10^7circK. Some of the most
  challenging problems of interstellar astrophysics, including star
  formation, the evolution of supernova remnants, and the acceleration
  and propagation of cosmic rays are plasma physics problems which
  involve basic processes such as the nature of MHD turbulence,
  the structure of collisionless shocks, magnetic reconnection, and
  transport phenomena. Not only must these phenomena be understood in
  extreme environments, but effects not always encountered in laboratory
  plasmas, such as radiation, charged dust grains, compressibility,
  and self gravity come into play. And, the interstellar medium is a
  plasma without walls. I will discuss some plasma physics problems
  encountered in the theory of star formation, with emphasis on the
  behavior of magnetic fields in highly conducting but weakly ionized
  media. Partial ionization is of overwhelming importance in removing
  magnetic flux from interstellar clouds, which appears to occur
  during star formation. It also leads to novel features such as the
  local relaxation of the magnetic field to a force free state, to
  the development of current sheets, which may promote rapid magnetic
  reconnection, and to a instability which can fragment clouds into
  smaller units, accelerate the rate of flux removal, and provide a free
  energy source for turbulent motions.

---------------------------------------------------------
Title: The Rise of Kink-Unstable Magnetic Flux Tubes in the Solar
    Convection Zone
Authors: Fan, Y.; Zweibel, E. G.; Linton, M. G.; Fisher, G. H.
1998ApJ...505L..59F    Altcode:
  We report preliminary results of a three-dimensional simulation of the
  buoyant rise of a strongly twisted, kink-unstable magnetic flux tube
  through a gravitationally stratified layer representing the solar
  convection zone. The numerical calculations employ the well-known
  anelastic approximation, which is suitable for studying slow, subsonic
  dynamical processes in the pressure-dominated, high-β plasma of the
  solar interior. This Letter investigates the case in which the initial
  twist of the buoyant flux tube is sufficiently high that the e-folding
  growth times of the unstable kink modes are short in comparison to
  the rise time of the flux tube. Our simulation shows that the flux
  tube becomes kinked and that the top portion of the flux tube evolves
  into a buckled shape with the tube axis being deflected by more than
  90° from its original orientation. We suggest that the emergence
  of this buckled flux tube can give rise to a compact magnetic bipole
  with polarity order inverted from Hale's polarity law, similar to the
  configuration often seen in δ spots.

---------------------------------------------------------
Title: Fragmentation Instability of Molecular Clouds
Authors: Zweibel, Ellen G.
1998ApJ...499..746Z    Altcode:
  Turbulent motions observed in molecular clouds are thought to reflect
  initial conditions associated with cloud formation and may be sustained
  over the cloud lifetime by mechanical energy sources associated with
  star formation. This paper demonstrates that free energy stored in
  the magnetic fields of clouds represents another source of turbulent
  energy, which can be released through an instability driven by ambipolar
  drift. The instability operates even in cases in which the cloud would
  be dynamically stable if the magnetic field were completely frozen to
  the gas. The instability has a weak form, to which clouds are generally
  susceptible, and a strong form, which appears if the cloud is within
  about 30% of critical. In the strong form, the instability grows at
  a rate intermediate between the slow rate of ambipolar drift and the
  more rapid rates associated with dynamical processes. In the weak form
  of the instability the growth rate is close to the ambipolar drift
  rate. The instability drives turbulent motions, both compressive and
  vortical, and may accelerate the fragmentation of a molecular cloud
  into substructures.

---------------------------------------------------------
Title: Fast reconnection of weak magnetic fields
Authors: Zweibel, Ellen G.
1998PhPl....5..247Z    Altcode:
  Fast magnetic reconnection refers to annihilation or topological
  rearrangement of magnetic fields on a timescale that is independent
  (or nearly independent) of the plasma resistivity. The resistivity
  of astrophysical plasmas is so low that reconnection is of little
  practical interest unless it is fast. Yet, the theory of fast
  magnetic reconnection is on uncertain ground, as models must avoid
  the tendency of magnetic fields to pile up at the reconnection layer,
  slowing down the flow. In this paper it is shown that these problems
  can be avoided to some extent if the flow is three dimensional. On
  the other hand, it is shown that in the limited but important case of
  incompressible stagnation point flows, every flow will amplify most
  magnetic fields. Although examples of fast magnetic reconnection abound,
  a weak, disordered magnetic field embedded in stagnation point flow will
  in general be amplified, and should eventually modify the flow. These
  results support recent arguments against the operation of turbulent
  resistivity in highly conducting fluids.

---------------------------------------------------------
Title: Two-dimensional Simulations of Buoyantly Rising, Interacting
    Magnetic Flux Tubes
Authors: Fan, Y.; Zweibel, E. G.; Lantz, S. R.
1998ApJ...493..480F    Altcode:
  We perform two-dimensional simulations of the buoyant rise of twisted
  horizontal magnetic flux tubes through an adiabatically stratified
  layer representing the solar convection zone or other marginally
  stable atmosphere. The numerical calculations employ the anelastic
  approximation to the basic MHD equations. We confirm the results
  of recent compressible simulations by Moreno-Insertis &amp; Emonet
  that the azimuthal component of the tube magnetic field can prevent
  the splitting of the tube into a vortex pair, and that most of the
  flux in the initial tube cross section rises in the form of a rigid
  body that reaches a terminal speed similar to the prediction of the
  often-employed thin-flux-tube model. We also study the interaction
  between a pair of buoyant flux tubes as they rise in proximity. In the
  case of two identical flux tubes that start from the same level, we find
  that the wake behind each tube interacts with the wake of the other,
  prompting mirror-symmetric vortex shedding in each wake. As a result,
  each tube gains around it a net circulation of the opposite sign of
  the most recently shed eddy; this causes a periodic, horizontal lift
  force that makes the tubes oscillate horizontally as they rise. The
  tube interactions in this case differ substantially from the inviscid
  limit studied previously. For two identical flux tubes that start at
  different levels, the resulting interactions depend upon the details of
  the initial configuration of the two tubes and can be very different
  from the interactions seen in the symmetrical case. In the asymmetric
  case, it becomes possible for one flux tube to be drawn into the wake
  of the other, leading eventually to a merger of the tubes.

---------------------------------------------------------
Title: Probing Convective Effects on P modes using the Adiabatic
    Switching Method
Authors: Swisdak, M.; Zweibel, E.
1997AAS...191.7402S    Altcode: 1997BAAS...29.1323S
  While the correspondence between observed and predicted p-mode
  frequencies is generally good, it is clear that the detailed effects of
  the convection zone are not adequately treated by contemporary solar
  models. Indeed, this discrepancy is to be expected since the effects
  of simple convective structures on p-mode frequencies and linewidths
  are not completely understood. While the full problem is difficult to
  treat, under the WKB approximation p modes may be treated as rays and
  their propagation can be described using the formalism of Hamiltonian
  systems. In general, only simple stellar models such as polytropes have
  analytic solutions for the eigenfrequencies. However, I have written
  computer code which uses the method of adiabatic switching (Skodje &amp;
  Cary, 1988) to determine the approximate eigenfrequencies of a ray with
  a given dispersion relation traveling through a medium with prescribed
  variations in the local wave velocity. In this application of the method
  of adiabatic switching, the initial state is a ray propagating in a
  polytropic spherical shell. Variations in the properties of the medium
  (e.g., sound speed perturbations or advective flows) are expressed as
  time-dependent perturbations to the Hamiltonian. These perturbations
  are turned on slowly and, consequently, the ray adiabatically
  adjusts its frequency, eventually yielding the eigenfrequency for the
  final state. The method is well-suited for describing the effects of
  time-dependent convection. This method gives accurate eigenfrequencies
  for a number of trial simulations, including wave speed perturbations
  and simple models of convective cells. By coupling this program to
  snapshots of convective simulations, I can determine both frequency
  shifts and line widths of p modes and compare them to high-quality
  data sets such as those taken by the MDI instrument on SOHO. Skodje,
  R., &amp; Cary, J. 1988, Comp. Phys. Reports, 8, 221.

---------------------------------------------------------
Title: Effects of Convection on Solar $p$ Modes
Authors: Swisdak, M.; Zweibel, E.
1997ASSL..225..177S    Altcode: 1997scor.proc..177S
  We are investigating how advective motions and structures such
  as magnetic flux tubes affect the propagation of $p$ modes in the
  Sun. We do this by means of a computer code which determines the
  eigenfrequencies of a ray with a given dispersion relation traveling
  through a plane-parallel medium with variations in the local wave
  velocity. By coupling this program to a numerical model of the solar
  convection zone, we plan to calculate shifts in eigenfrequencies as
  well as line widths and compare them to new high-quality data from
  the GONG project as well as from the MDI instrument on SOHO.

---------------------------------------------------------
Title: Current Sheet Formation in the Interstellar Medium: Erratum
Authors: Zweibel, Ellen G.; Brandenburg, Axel
1997ApJ...485..920Z    Altcode:
  In the paper "Current Sheet Formation in the Interstellar Medium"
  by Ellen G. Zweibel and Axel Brandenburg (ApJ, 478, 563 [1997]), the
  address given for Dr. Brandenburg is incorrect. It should be Department
  of Mathematics, University of Newcastle, Newcastle upon Tyne, NE1 7RU,
  England, UK. His e-mail address is Axel.Brandenburg@newcastle.ac.uk.

---------------------------------------------------------
Title: Warp Stability of Magnetically Supported Disks
Authors: Lovelace, Richard V. E.; Zweibel, Ellen G.
1997ApJ...485..285L    Altcode:
  Cold, magnetically supported, self-gravitating disks are evolutionary
  precursors of protostars. Magnetic tension balances self-gravity in
  these systems, so the magnetic field lines kink as they cross the
  disk. Previously, we have shown that such disks are marginally stable
  to in-plane perturbations. Here, we show that a sufficient condition
  for stability of such disks to out-of-plane, warping or tilting,
  perturbations is that the azimuthal current in the disk have one
  polarity as a function of radius. This corresponds to the gravitational
  acceleration in the disk pointing radially inward for all radii. This
  criterion is not necessary, however, and can be violated as long as the
  vertical magnetic field exceeds the radial magnetic field (in magnitude)
  everywhere in the disk.

---------------------------------------------------------
Title: Two-dimensional Anelastic MHD Simulations of the Buoyant Rise
    of Magnetic Flux Tubes in the Solar Convection Zone
Authors: Fan, Y.; Zweibel, E. G.; Lantz, S. R.
1997SPD....28.1704F    Altcode: 1997BAAS...29..921F
  We perform two-dimensional simulations of the buoyant rise of
  twisted horizontal magnetic flux tubes through an adiabatically
  stratified layer representing the solar convection zone. The numerical
  calculations employ the anelastic approximation of the basic MHD
  equations. We confirm the results of recent compressible simulations
  by Moreno-Insertis and Emonet that the transverse component of the tube
  magnetic field can prevent the splitting of the tube into a vortex pair,
  and that most of the flux in the initial tube cross-section rises in
  the form of a rigid body and reaches a terminal speed similar to the
  prediction of the thin flux tube model. Furthermore, we studied the
  interaction between a pair of buoyant flux tubes as they rise side by
  side. Our simulations show that the vortices in the wakes of the two
  tubes interact and are continuously shed by the tubes. As a result
  each tube gains around it a net circulation of the opposite sign of
  the shed eddy and experiences a periodic lift force which causes the
  tubes to show an oscillatory horizontal motion as they rise.

---------------------------------------------------------
Title: M4: The Milky Way Magnetic Field Mapping Mission
Authors: Clemens, D.; Jones, T. J.; Goodman, A.; Myers, P.; Field, G.;
   Rieke, G.; Young, E.; McKee, C.; Heiles, C.; Draine, B.; Hildebrand,
   R.; Zweibel, E.; Lord, S.; Ellis, B.
1997AAS...190.2401C    Altcode: 1997BAAS...29..804C
  The roles played by magnetic fields in the star-forming regions
  of the Milky Way Galaxy are largely unknown. To address this
  knowledge gap, we have developed a concept for a new, focused, small
  explorer mission. The Milky Way Magnetic Field Mapping Mission, or
  M4 will consist of a cryogenically cooled 40 cm aperture telescope
  illuminating twin 32 x 32 pixel germanium arrays, already developed for
  SIRTF. Operating in a unique, no-moving-parts linear polarimetry mode,
  M4 will be used to measure the magnetic field structure in the dense
  interstellar medium. For broadband operation at 100mu m, M4 will be
  diffraction limited for a 60 arcsec beam, and pixel sampling of 24
  arcseconds will support detailed model testing and superresolution
  image reconstruction. Since the expected linear polarization is
  around 2%, ensuring that M4 can map field geometry with better than
  10(deg) uncertainty requires operations with S/N beyond 150. This
  challenge will be met by a combination of hardware, software, and
  operations/observing solutions. The minimum cryogen lifetime of four
  months will permit mapping a large piece of the Milky Way (+/- 50(deg)
  of galactic longitude and +/- 5(deg) of latitude) in four weeks, much
  of the nearby Sco/Oph dark cloud region (three weeks), a significant
  piece of infrared cirrus (two weeks), and enable a very robust guest
  investigator program (at least 3 weeks duration) which is expected to
  highlight extragalactic observations.

---------------------------------------------------------
Title: Current Sheet Formation in the Interstellar Medium
Authors: Zweibel, Ellen G.; Brandenburg, Axel
1997ApJ...478..563Z    Altcode:
  There is phenomenological evidence that magnetic reconnection
  operates in the interstellar medium, and magnetic reconnection is
  also necessary for the operation of a galactic dynamo. The extremely
  long ohmic diffusion times of magnetic fields in typical interstellar
  structures suggest that reconnection occurs in two stages, with thin
  current layers that have relatively short resistive decay times forming
  by magnetohydrodynamical processes first, followed by reconnection of
  the fields in the layers. We propose that ambipolar drift can lead to
  the formation of these thin sheets in weakly ionized interstellar gas
  and can delineate the parameter regime in which this occurs by means
  of a numerical model: we find that the magnetic field cannot be too
  large and the medium cannot be too diffusive. Both limits are imposed
  by the requirement that the field be wound up about 1 time by the eddy.

---------------------------------------------------------
Title: The Surface Amplitudes and Frequencies of p-Mode Oscillations
    in Active Regions
Authors: Hindman, Bradley W.; Jain, Rekha; Zweibel, Ellen G.
1997ApJ...476..392H    Altcode:
  It is well established that the surface amplitudes of solar p-mode
  oscillations are reduced in regions of magnetic activity. In this
  paper, we examine the conjecture that this reduction is produced by
  direct modification of the surface values of the p-mode eigenfunctions,
  rather than changes in the mode energies or alterations in the spectral
  line formation process. We calculate the oscillation modes of a solar
  model with a horizontal magnetic field, convection, and radiative
  diffusion. We find that magnetic fields with strengths characteristic of
  solar active regions can produce the observed decrease of surface power.

---------------------------------------------------------
Title: Magnetic fields in galaxies and beyond
Authors: Zweibel, Ellen G.; Heiles, Carl
1997Natur.385..131Z    Altcode: 1996Natur.385..131Z
  Astronomical magnetic fields are generally strong enough to influence
  the dynamics of gas in present-day galaxies, and may have played an
  important role in the formation and early evolution of galaxies. Yet the
  origin of these magnetic fields remains controversial, and observational
  tests that could discriminate between competing theories will challenge
  the capabilities of telescopes now under construction.

---------------------------------------------------------
Title: Stability of Magnetically Supported Disks
Authors: Zweibel, Ellen G.; Lovelace, Richard V. E.
1997ApJ...475..260Z    Altcode:
  Cold, magnetically supported disklike clouds are evolutionary precursors
  of protostars. Magnetic tension balances self-gravity in these systems,
  so the magnetic field lines kink as they cross the disk. Despite this
  strong magnetic curvature, these disks turn out to be marginally stable
  to self-gravitating perturbations which preserve the plane of the disk.

---------------------------------------------------------
Title: The Effects of Ambipolar Drift on Reconnection and Dynamos
Authors: Zweibel, Ellen G.; Strauss, H. R.
1996APS..DPP..8S01Z    Altcode:
  Tenuous, weakly ionized but highly conducting gases display ambipolar
  drift of magnetic fieldlines: the lines are tied to the plasma
  component, but drift with respect to the neutral (or center of mass)
  component. The drift velocity v<SUB>D</SUB> is approximately J×B/ρν
  c, where ρ is the gas density and ν is the neutral - ion collision
  frequency. Many of the effects of ambipolar drift are scaled by an
  ambipolar diffusion coefficient λ_AD≡ V_A^2/ν, from which can be
  formed the ambipolar Reynolds number R_AD≡ VL/λ_AD. Ambipolar drift
  is believed to operate in the cool to cold portions of the interstellar
  gas in galaxies. Ambipolar drift is known to have three properties:
  it sets a minimum scale L_min below which the field is poorly coupled
  to the neutral fluid (defined approximately by R_AD(L_min)≈ 1,
  (2) it acts as a nonlinear diffusion, and mediates the formation of
  current sheets and sharp moving fronts, and (3) it drives the fluid
  to a relaxed state in which J× B≈ 0 to the extent permitted by
  topological constraints. We obtain the RAMHD equations, 3D reduced MHD
  with ambipolar drift, and use them to explore the effects of ambipolar
  drift on magnetic reconnection and on mean field dynamo theory as they
  apply to galaxies.

---------------------------------------------------------
Title: Slow and steady spirals
Authors: Zweibel, Ellen G.
1996Natur.383..758Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Influence of Magnetism on p-Mode Surface Amplitudes
Authors: Jain, Rekha; Hindman, Bradley W.; Zweibel, Ellen G.
1996ApJ...464..476J    Altcode:
  We propose a mechanism to explain the observed suppression of p-mode
  surface velocities in solar active regions. We show that a horizontal
  magnetic field can lower the upper turning point and change the skin
  depth for a simple plane-parallel adiabatically stratified polytrope. In
  addition to power suppression, the magnetic field alters the phase of
  p-modes. Simultaneous measurements of phase as well as amplitude in
  the active and quiet regions would provide an additional diagnostic
  for probing the structure of active region magnetic fields.

---------------------------------------------------------
Title: Effects of Convection on Solar P Modes
Authors: Swisdak, M.; Zweibel, E.
1996AAS...188.6909S    Altcode: 1996BAAS...28Q.937S
  The effects of convection on p mode frequencies and linewidths are
  not completely understood. This paper takes an approach based on
  geometrical optics. We have written computer code which uses EBK
  quantization to determine the eigenfrequencies of a ray with a given
  dispersion relation traveling through a plane-parallel medium with
  variations in the local wave velocity. The path of one ray is calculated
  for many orbits as it comes arbitrarily close to every point in its
  region of propagation. After sampling the entire domain, the path
  itself (specifically, the wavenumber as a function of position) is
  used to find the eigenfrequencies of the modes. This method proved
  to be quite robust in finding frequencies for a number of trial
  simulations, including horizontal advective motions and large wave
  speed perturbations. By coupling my ray-tracing program to snapshots
  of detailed simulations of convection in a plane-parallel atmosphere,
  we can determine both frequency shifts and line widths of p modes.

---------------------------------------------------------
Title: Simulations of Magnetic Field Effects on Solar Oscillation
    Ring Diagrams
Authors: Hill, F.; Zweibel, E.; Haber, D.
1996AAS...188.6905H    Altcode: 1996BAAS...28Q.936H
  Solar oscillation ring diagrams are an effective technique for inferring
  the horizontal velocity in the solar convection zone as a function of
  heliographic position and depth. While the signature of the velocity in
  the diagram is a shift in the central position of the rings, the shape
  of the rings contains information on the subsurface temperature and
  magnetic field. Direct detection of the velocity and magnetic fields
  in the convection zone will provide important input to theories of
  the solar activity cycle, convection, and the dynamo. We present here
  the results of simulating ring shape variations arising from magnetic
  effects. Simple polytropic models containing various magnetic field
  profiles are used to construct artificial oscillation ring diagrams. We
  then determine the altered ring shapes, invert for the magnetic field,
  and compare the results of the inversion to the field of the model to
  place limits on the sensitivity of the technique.

---------------------------------------------------------
Title: The Solar Wind. (Book Reviews: Interplanetary
    Magnetohydrodynamics.)
Authors: Zweibel, Ellen G.
1996Sci...272..495Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Driven Acoustic Oscillations within a Vertical Magnetic Field
Authors: Hindman, Bradley W.; Zweibel, Ellen G.; Cally, P. S.
1996ApJ...459..760H    Altcode:
  We examine the effects of a vertical magnetic field on p-mode
  frequencies, line widths, and eigenfunctions. We employ a simple solar
  model consisting of a neutrally stable polytropic interior matched to
  an isothermal chromosphere. The p-modes are produced by a spatially
  distributed driver. The atmosphere is threaded by a constant vertical
  magnetic field. The frequency shifts due to the vertical magnetic
  field are much smaller than the shifts caused by horizontal fields
  of similar strength. A large vertical field of 2000 G produces shifts
  on the order of 1 muHz while a weak field of 50 G produces very small
  shifts of several nanohertz. We find that the frequency shifts decrease
  with increasing frequency and increase with field strength. The shifts
  are positive, except at high frequency and low field strength, where
  small negative shifts are possible. Coupling of the acoustic fast mode
  to escaping slow modes is extremely inefficient. Constant vertical
  magnetic field models are therefore incapable of explaining the high
  level of absorption observed in sunspots and plage The damping due
  to this mode conversion process produces very narrow line widths. For
  a 2000 G field the line widths are several microhertz and for a 50 G
  field the line widths are several nanohertz.

---------------------------------------------------------
Title: Polarimetry and the Theory of the Galactic Magnetic Field
Authors: Zweibel, E. G.
1996ASPC...97..486Z    Altcode: 1996pim..conf..486Z
  No abstract at ADS

---------------------------------------------------------
Title: Going with the flow?
Authors: Zweibel, Ellen
1996Natur.379...20Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Effects of Pressure and Resistivity on the Ambipolar Diffusion
Singularity: Too Little, Too Late
Authors: Brandenburg, Axel; Zweibel, Ellen G.
1995ApJ...448..734B    Altcode:
  Ambipolar diffusion, or ion-neutral drift, can lead to steepening of the
  magnetic field profile and even to the formation of a singularity in the
  current density. These results are based on an approximate treatment
  of ambipolar drift in which the ion pressure is assumed vanishingly
  small and the frictional coupling is assumed to be very strong, so
  that the medium can be treated as a single fluid. This steepening, if
  it really occurs, must act to facilitate magnetic reconnection in the
  interstellar medium, and so could have important consequences for the
  structure and evolution of the galactic magnetic field on both global
  and local scales. <P />In actuality, the formation of a singularity
  must be prevented by physical effects omitted by the strong coupling
  approximation. In this paper we solve the coupled equations for charged
  and neutral fluids in a simple slab geometry, which was previously shown
  to evolve to a singularity in the strong coupling approximation. We
  show that both ion pressure and resistivity play a role in removing the
  singularity, but that, for parameters characteristic of the interstellar
  medium, the peak current density is nearly independent of ion pressure
  and scales inversely with resistivity. The current gradient length
  scale, however, does depend on ion pressure. In the end, effects
  outside the fluid approximation, such as the finite ion gyroradius,
  impose the strictest limit on the evolution of the magnetic profile.

---------------------------------------------------------
Title: Changes in the Upper Turning Point due to Magnetism
Authors: Jain, R.; Hindman, B. W.; Zweibel, E. G.
1995ESASP.376b..63J    Altcode: 1995soho....2...63J; 1995help.confP..63J
  It has been observed that p-mode power is substantially suppressed in
  magnetic regions. One possible explanation is that the upper turning
  point (acoustic cut-off point) of the solar p-modes is lowered in the
  presence of a magnetic field (Brown 1994). A related possibility is
  that the attenuation length scale in the evanescent region is reduced
  in the presence of a magnetic field. Furthermore, it is likely that the
  observations sample a different position in the evanescent tails of the
  eigenfunctions, in magnetic regions because of different temperature
  structure there. The authors use a simple model to quantify the first
  of these effects.

---------------------------------------------------------
Title: Driven Acoustic Oscillations Within a Vertical Magnetic Field
Authors: Hindman, B. W.; Zweibel, E. G.; Cally, P. S.
1995ESASP.376b..77H    Altcode: 1995help.confP..77H; 1995soho....2...77H
  Examines the effects of a vertical magnetic field on p-mode frequencies,
  line widths, and eigenfunctions. The authors employ a simple solar
  model consisting of a neutrally stable polytropic interior matched to
  an isothermal chromosphere. The p-modes are produced by a spatially
  distributed driver. The atmosphere is threaded by a constant vertical
  magnetic field. The frequency shifts due to the vertical magnetic
  field are much smaller than the shifts caused by horizontal fields of
  similar strength. A large vertical field of 2000 G produces shifts
  on the order of a μHz while a weak field of 50 G produces very
  small shifts of several nHz. The authors find that the frequency
  shifts decrease with increasing frequency and increase with field
  strength. Coupling of the acoustic fast mode to escaping slow modes
  is extremely inefficient. Constant vertical magnetic field models are
  therefore incapable of explaining the high level of absorption observed
  in sunspots and plages.

---------------------------------------------------------
Title: Soi/mdi Studies of Active-Region Seismology and Evolution
Authors: Tarbell, T. D.; Title, A.; Hoeksema, J. T.; Scherrer, P.;
   Zweibel, E.
1995ESASP.376b..99T    Altcode: 1995help.confP..99T; 1995soho....2...99T
  The Solar Oscillations Investigation (SOI) will study active regions
  in many ways using both helioseismic and conventional observing
  techniques. The Michelson Doppler Imager (MDI) instrument can
  make Doppler, continuum and line depth images and also longitudinal
  magnetograms, showing either the full disk or a high resolution field of
  view. There will be a Dynamics Program of continuous full disk Doppler
  observations for two months per year, many Campaign Programs of 8 hours
  continuous observing per day, and a synoptic Magnetic Program of about
  15 full disk magnetograms per day. This paper gives a brief description
  of some of the scientific plans, measurements, and observing programs.

---------------------------------------------------------
Title: Is There a Seismic Signature of the Sun's Magnetic Field?
Authors: Zweibel, E. G.; Gough, D.
1995ESASP.376b..73Z    Altcode: 1995soho....2...73Z; 1995help.confP..73Z
  Both weak magnetic fields and latitudinally dependent acoustic
  perturbations remove the degeneracy with respect to the azimuthal
  quantum number m of acoustic modes of an otherwise spherically
  symmetrical solar model. In the case of acoustic perturbations,
  the degeneracy is removed because the range of latitude in which a
  mode propagates depends on m, and therefore modes of like principal
  order n and degree l sample the aspherical (scalar) sound-speed
  distribution differently. In the magnetic case too, the removal of the
  degeneracy is caused by the same geometrical effect, and it is also
  influenced by the anisotropy of the Lorentz forces. Given any set of
  frequency splittings arising from a perturbation to the equilibrium
  configuration, it is possible to invert them to solve for a purely
  acoustic perturbation that might have produced them. In particular,
  if those splittings were actually produced by a magnetic field, there
  is thus an acoustic perturbation that mimics the field. The authors
  use asymptotic analysis to show that the frequency splittings cannot
  be unambiguously attributed to the direct effect of a magnetic field.

---------------------------------------------------------
Title: Non-Local Transport in Turbulent MHD Convection
Authors: Miesch, M.; Brandenburg, A.; Zweibel, E.; Toomre, J.
1995ESASP.376b.253M    Altcode: 1995help.confP.253M; 1995soho....2..253M
  No abstract at ADS

---------------------------------------------------------
Title: Astrophysical Discontinuities. (Book Reviews: Spontaneous
    Current Sheets in Magnetic Fields. With Applications to Stellar
    X-Rays.)
Authors: Zweibel, Ellen G.
1995Sci...267.1367Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Astrophysical Discontinuities. (Book Reviews: Spontaneous
    Current Sheets in Magnetic Fields. With Applications to Stellar
    X-Rays.)
Authors: Zweibel, Ellen G.
1995Sci...267.1367P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfven Waves in Interstellar Gasdynamics
Authors: McKee, Christopher F.; Zweibel, Ellen G.
1995ApJ...440..686M    Altcode:
  Magnetohydrodynamic (MHD) waves contribute a significant pressure in
  both the diffuse interstellar medium and in molecular clouds. Alfvén
  waves are subject to less damping than compressive MHD waves and
  are therefore likely to be the dominant mode in astrophysical
  environments. Provided that the medium in which the waves are
  propagating is slowly varying, the dynamical effects of ideal MHD waves
  are governed by equations derived by Dewar. We show that these equations
  are similar in form to the equations of radiation hydrodynamics to
  order υ/c, provided that the radiation is nearly isotropic. For the
  case of Alfvén waves, the pressure due the waves, P<SUB>w</SUB>,
  is isotropic. Furthermore, P<SUB>w</SUB> is directly observable
  through the non- thermal line width σ<SUB>nt</SUB>; for a randomly
  oriented field, P<SUB>w</SUB> = (3/2)ρσ<SUP>2</SUP><SUB>nt</SUB>. In
  several simple cases, including that in which the Alfvén waves are
  isotropic, that in which the density is spatially uniform, and that in
  which the medium undergoes a self-similar contraction or expansion,
  undamped Alfvén waves behave like a gas with a ratio of specific
  heats of 3/2; i.e., pressure variations are related to density
  variations by ∆ ln P<SUB>w</SUB> = γ<SUB>w</SUB>∆ ln ρ with
  γ<SUB>w</SUB> = 3/2. In a spatially nonuniform cloud, γ<SUB>w</SUB>
  generally depends on position; an explicit expression is given. In
  the opposite limit of rapid variations, such as in a strong shock,
  the wave magnetic field behaves like a static field and the wave
  pressure can increase as fast as ρ<SUP>2</SUP>, depending on the
  orientation of the shock and the polarization of the waves. The jump
  conditions for a shock in a medium containing MHD waves are given. For
  strong nonradiative shocks, neither the wave pressure nor the static
  magnetic field pressure is significant downstream, but for radiative
  shocks these two pressures can become dominant. <P />Alfvén waves
  are essential in supporting molecular clouds against gravitational
  collapse. In a static cloud with a nonuniform density ρ(r), the
  spatial variation of the wave pressure is given by the polytropic
  relation P<SUB>w</SUB>(r) ∝ ρ(r)<SUP>γ<SUB>ρ</SUB></SUP>) with
  γ<SUB>ρ</SUB> = 1/2. This generalizes the result obtained by Fatuzzo
  &amp; Adams and is consistent with observations showing that molecular
  clouds have velocity dispersions that increase outward. The polytropic
  index γ<SUB>ρ</SUB> for Alfvén waves differs substantially from
  the adiabatic index γ<SUB>w</SUB> which has implications for the
  gravitational stability of molecular clouds.

---------------------------------------------------------
Title: Equipartition of Energy for Turbulent Astrophysical Fluids:
    Accounting for the Unseen Energy in Molecular Clouds
Authors: Zweibel, Ellen G.; McKee, Christopher F.
1995ApJ...439..779Z    Altcode:
  Molecular clouds are observed to be partially supported by turbulent
  pressure. The kinetic energy of the turbulence is directly measurable,
  but the potential energy, which consists of magnetic, thermal, and
  gravitational potential energy, is largly unseen. We have extended
  previous results on equipartition between kinetic and potential
  energy to show that it is likely to be a very good approximation in
  molecular clouds. We have used two separate approaches to demonstrate
  this result: For small-amplitude perturbations of a static equilibrium,
  we have used the energy principle analysis of Bernstein et al. (1958);
  this derivation applies to perturbations of arbitary wavelength. To
  treat perturbations of a nonstatic equilibrium, we have used the
  Lagrangian analysis of Dewar (1970); this analysis applies only to
  short-wavelength perturbations. Both analysis assume conservation of
  energy. Wave damping has only a small effect on equipartition if the
  wave frequency is small compared to the neutral-ion collision frequency;
  for the particular case we considered, radiative losses have no effect
  on equipartition. These results are then incorporated in a simple way
  into analyses of cloud equilibrium and global stability. We discuss
  the effect of Alfvenic turbulence on the Jeans mass and show that it
  has little effect on the magnetic critical mass.

---------------------------------------------------------
Title: Magnetic Merging in Colliding Flux Tubes
Authors: Zweibel, Ellen G.; Rhoads, James E.
1995ApJ...440..407Z    Altcode: 1994astro.ph..8041Z
  We develop an analytical theory of reconnection between colliding,
  twisted magnetic flux tubes. Our analysis is restricted to direct
  collisions between parallel tubes and is based on the collision
  dynamics worked out by Bogdan (1984). We show that there is a range
  of collision velocities for which neutral point reconnection of the
  Parker-Sweet type can occur, and a smaller range for which reconnection
  leads to coalescence. Mean velocities within the solar convection
  zone are probably significantly greater than the upper limit for
  coalescence. This suggests that the majority of flux tube collisions
  do not result in merging, unless the frictional coupling of the tubes
  to the background flow is extremely strong.

---------------------------------------------------------
Title: The Effects of a Hot Outer Atmosphere on Acoustic-Gravity Waves
Authors: Hindman, B. W.; Zweibel, E. G.
1995ASPC...76..366H    Altcode: 1995gong.conf..366H
  No abstract at ADS

---------------------------------------------------------
Title: The Theory of the Galactic Magnetic Field
Authors: Zweibel, E. G.
1995ASPC...80..524Z    Altcode: 1995pimi.conf..524Z
  No abstract at ADS

---------------------------------------------------------
Title: The Effects of a Hot Outer Atmosphere on Acoustic-Gravity Waves
Authors: Hindman, Bradley W.; Zweibel, Ellen G.
1994ApJ...436..929H    Altcode:
  We examine the effects of a hot chromosphere and corona on
  acoustic-gravity waves in the Sun. We use a simple solar model
  consisting of a neutrally stable polytrope smoothly matched
  to an isothermal chromosphere or corona. The temperature of
  the isothermal region is higher than the minimum temperature of
  the model. We ignore sphericity, magnetic fields, changes in the
  gravitational potential, and nonadiabatic effects. We find a family
  of atmospheric g-modes whose cavity is formed by the extremum in
  the buoyancy frequency at the transition region. The f-mode is the
  zero-order member of this family. For large values of the harmonic
  degree l, f-mode frequencies are below the classic f-mode frequency,
  mu=(gk)<SUP>1/2</SUP>, whereas at small values of l, the f-mode is
  identical to the classical f-mode solution. We also find a family
  of g-modes residing in the low chromosphere. Frequency shifts of
  p-modes can be positive or negative. When the frequency is less than
  the acoustic cutoff frequency of the upper isothermal atmsophere, the
  frequency of the upper isothermal atmosphere, the frequency shift is
  negative, but when the frequency is above this cutoff, the shifts can
  be positive. High-frequency acoustic waves which are not reflected by
  the photospheric cutoff are reflected at the corona by the high sound
  speed for moderate values of l and v. This result is independent of the
  solar model as long as the corona is very hot. The data are inconsistent
  with this result, and reasons for this discrepancy are discussed.

---------------------------------------------------------
Title: Umbral Oscillations in Sunspots: Absorption of p-Modes and
    Active Region Heating by Mode Conversion
Authors: Cally, P. S.; Bogdan, T. J.; Zweibel, E. G.
1994ApJ...437..505C    Altcode:
  The linear adiabatic oscillations of an infinite, stratified,
  perfectly conducting atmosphere pervaded by a uniform vertical magnetic
  field are computed. The stratification is chosen to approximate the
  conditions appropriate for a sunspot umbra where strong reflection
  of the fast magnetoacoustic-gravity waves from the rapid increase
  of the Alfven speed with height and the sound speed with depth is
  anticipated. Since the umbral oscillations are presumably excited by
  external p-mode forcing, the angular frequency omega is prescribed --
  being set by the p-modes -- and it is required to solve for the allowed
  eigenvalues assumed by the horizontal wavenumber k and the corresponding
  displacement eigenfunctions. We term these allowed solutions pi-modes
  in recognition of the crucial influence of the imposed magnetic
  field and to distinguish them from their p-mode cousins present in
  the surrounding nonmagnetic quiet Sun. The wavenumber eigenvalues are
  complex and are consistent with the spatial decay of the pi-modes inward
  toward the center of the sunspot from the umbral boundary. This spatial
  attenuation is a consequence of the slow magnetoacoustic-gravity waves
  that propagate along the magnetic field lines and extract energy from
  the trapped fast waves through mode coupling in the layers where the
  sound and Alfven speeds are comparable. Through the consideration of
  several ancillary computations we argue that this salient attribute
  of the pi-modes may be essential both in explaining the observed
  absorption of (the forcing) p-modes by sunspots and in providing a
  source of mechanical energy for the overlying active regions. However,
  more realistic calculations are clearly called for before these notions
  may be confirmed.

---------------------------------------------------------
Title: Shock Propagation and the Generation of Magnetohydrodynamic
    Wave Fields in Inhomogeneous Molecular Clouds
Authors: Miesch, Mark S.; Zweibel, Ellen G.
1994ApJ...432..622M    Altcode:
  We develop a simple one-dimensional model for the interaction of a
  steady, thin, planar shock wave with a nonrigid cloud which may be in
  motion relative to the surrounding medium, and we apply the model to
  shocks impinging on, and propagating through, molecular clouds. Both
  'adiabatic' (gamma = 5/3) and radiative (gamma = 1) shocks are
  considered and we allow for the presence of a uniform magnetic field
  directed either parallel or perpendicular to the shock normal. The
  former field orientation is equivalent to the hydrodynamic case, and
  the latter involves only fast MHD shocks. We focus on the manner in
  which such shocks can generate internal kinetic motions in the cloud
  on a range of size and density scales through the direct acceleration
  of cores and clumps by shocks transmitted into them and through the
  generation of an MHD wavefield via the reflection of the incident shock
  at clump boundaries. We find that stronger incident Mach numbers and
  smaller density contrasts lead to more efficient cloud acceleration,
  as do isothermal intercloud shocks and small intercloud magnetic field
  strengths. The acceleration efficiency is insensitive to the adiabatic
  index and the magnetic field strength in the cloud itself. For typical
  parameter choices, the direct acceleration of clouds and clumps by
  strong shocks is found to be substantial and could at least in part
  account for their observed velocity dispersions. If the shocks are
  moderately weak, the final velocity of the cloud is linearly related to
  its initial velocity, with higher acceleration giving shallower slopes
  (i.e., final velocity distributions which are less sensitive to the
  initial distribution). Compared to the kinetic energy of the postshock
  cloud, the energy given to the wavefield at each encounter is small,
  and the heating of the interclump medium by the dissipation of this
  wavefield is found to be insufficient to balance the cooling rate in the
  cloud as a whole (although it may be important in particular regions),
  even if this medium is warm, unless it is also extremely tenuous (n
  approximately less than 0.1/cu cm). Nevertheless, the correction for
  the velocity imparted to the cloud leads to a substantial increase
  in the critical incident Mach number for wave emission over that
  reported by Spitzer for the rigid case. The implications of our model
  for shock-induced star formation are discussed briefly.

---------------------------------------------------------
Title: Acoustic Instability Driven by Cosmic-Ray Streaming
Authors: Begelman, Mitchell C.; Zweibel, Ellen G.
1994ApJ...431..689B    Altcode:
  We study the linear stability of compressional waves in a medium through
  which cosmic rays stream at the Alfven speed due to strong coupling with
  Alfven waves. Acoustic waves can be driven unstable by the cosmic-ray
  drift, provided that the streaming speed is sufficiently large compared
  to the thermal sound speed. Two effects can cause instability: (1) the
  heating of the thermal gas due to the damping of Alfven waves driven
  unstable by cosmic-ray streaming; and (2) phase shifts in the cosmic-ray
  pressure perturbation caused by the combination of cosmic-ray streaming
  and diffusion. The instability does not depend on the magnitude of
  the background cosmic-ray pressure gradient, and occurs whether or
  not cosmic-ray diffusion is important relative to streaming. When
  the cosmic-ray pressure is small compared to the gas pressure, or
  cosmic-ray diffusion is strong, the instability manifests itself as
  a weak overstability of slow magnetosonic waves. Larger cosmic-ray
  pressure gives rise to new hybrid modes, which can be strongly unstable
  in the limits of both weak and strong cosmic-ray diffusion and in the
  presence of thermal conduction. Parts of our analysis parallel earlier
  work by McKenzie &amp; Webb (which were brought to our attention
  after this paper was accepted for publication), but our treatment of
  diffusive effects, thermal conduction, and nonlinearities represent
  significant extensions. Although the linear growth rate of instability
  is independent of the background cosmic-ray pressure gradient, the onset
  of nonlinear eff ects does depend on absolute value of DEL (vector
  differential operator) P<SUB>c</SUB>. At the onset of nonlinearity
  the fractional amplitude of cosmic-ray pressure perturbations is delta
  P<SUB>C</SUB>/P<SUB>C</SUB> approximately (kL) <SUP>-1</SUP> much less
  than 1, where k is the wavenumber and L is the pressure scale height
  of the unperturbed cosmic rays. We speculate that the instability may
  lead to a mode of cosmic-ray transport in which plateaus of uniform
  cosmic-ray pressure are separated by either laminar or turbulent jumps
  in which the thermal gas is subject to intense heating.

---------------------------------------------------------
Title: The Formation of Sharp Structures by Ambipolar Diffusion
Authors: Brandenburg, Axel; Zweibel, Ellen G.
1994ApJ...427L..91B    Altcode:
  The effect of ambipolar diffusion is investigated using simple
  numerical models. Examples are shown where sharp structures develop
  around magnetic nulls. In contrast to the case of ordinary diffusion,
  the magnetic field topology is conserved by ambipolar diffusion. This
  is demonstrated in an example where differential rotation winds up an
  initially uniform magnetic field and brings oppositely oriented field
  lines close together. It is argued that ambipolar diffusion produces
  structures of scales small enough for reconnection to occur.

---------------------------------------------------------
Title: Magnetic Fields and Star Formation
Authors: Zweibel, E. G.
1994ASIC..422...73Z    Altcode: 1994coma.conf...73Z
  No abstract at ADS

---------------------------------------------------------
Title: The tangled web of magnetism
Authors: Zweibel, Ellen
1993Natur.362..591Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic Fields in Star-Forming Regions - Theory
Authors: McKee, C. F.; Zweibel, E. G.; Goodman, A. A.; Heiles, C.
1993prpl.conf..327M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamos with ambipolar diffusion.
Authors: Zweibel, E. G.; Proctor, M. R. E.
1993spd..conf..355Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic Fields in Star-Forming Regions - Observations
Authors: Heiles, Carl; Goodman, Alyssa A.; McKee, Christopher F.;
   Zweibel, Ellen G.
1993prpl.conf..279H    Altcode:
  We review the observational aspects of magnetic fields in dense,
  star-forming regions. First we discuss ways to observe the field. These
  include direct methods, which consist of the measurement of both linear
  and circular polarization of spectral line and continuum radiation;
  and indirect methods, consisting of the angular distribution of H2O
  masers on the sky and the measurement of ambipolar diffusion. Next
  we discuss selected observational results, focusing on detailed
  discussions of a small number of points rather than a generalized
  discussion that covers the waterfront. We discuss the Orion/BN-KL
  region in detail, both on the small and large scales. Next we discuss
  the derivation of the complete magnetic vector, including both the
  systematic and fluctuating component, from a large sample of Zeeman
  and linear polarization measurements for the L204 dark cloud. We
  examine the virial theorem as it applies to dark clouds in general
  and one dark cloud, Barnard 1, in particular. We critically discuss
  the numerous claims for alignment of cloud structural features with
  the plane-of-the-sky component of the magnetic field, and find that
  many of these have not been definitively established.

---------------------------------------------------------
Title: The Effect of Line Tying on Parker's Instability
Authors: Zweibel, Ellen G.; Bruhwiler, David L.
1992ApJ...399..318Z    Altcode:
  We consider the effect of line tying on the magnetic Rayleigh-Taylor
  or Parker instability. We find that stabilization occurs if the field
  lines are sufficiently short that the Alfven travel time between the
  boundaries is less than 2 pi times the instability growth time in an
  unbounded medium. In a magnetically dominated system, the critical
  field line length scales linearly with magnetic field strength. This
  criterion may be revelant to the stabilization of solar prominences.

---------------------------------------------------------
Title: On the Virial Theorem for Turbulent Molecular Clouds
Authors: McKee, Christopher F.; Zweibel, Ellen G.
1992ApJ...399..551M    Altcode:
  An Eulerian, rather than Lagrangian, form of the virial theorem is
  derived for a turbulent, magnetized cloud embedded in a steady,
  turbulent, low-density intercloud medium. The role of turbulent
  pressure in cloud confinement is clarified, and it is shown that, in
  the absence of a magnetic field, a cloud can be at a somewhat lower
  pressure than the intercloud medium. Simple forms for the magnetic term
  in the virial equation are obtained. Radiation pressure is considered;
  its effects are relatively small under average conditions in the
  interstellar medium. Under typical conditions, external pressure and
  magnetic fields are shown to have a relatively small effect on virial
  estimates of the mass of self-gravitating clouds.

---------------------------------------------------------
Title: Energy spectrum of particles accelerated near a magnetic x line
Authors: Bruhwiler, D. L.; Zweibel, Ellen G.
1992JGR....9710825B    Altcode:
  We study the acceleration of test particles near a static magnetic
  x line with a uniform electric field and a strong component of
  the magnetic field, B<SUB>∥</SUB>, parallel to the x line. The
  energy spectrum of the accelerated particles is found analytically
  in the nonrelativistic limit, showing good agreement with numerical
  simulations. At high energies, the spectrum decays exponentially with
  a characteristic energy very different from that found in studies
  assuming B<SUB>∥</SUB>=0.

---------------------------------------------------------
Title: Magnetohydrodynamic Waves in Sharply and Smoothly Bounded
    Cylinders
Authors: Shulman, Bonnie J.; Zweibel, Ellen G.
1992ApJ...389..428S    Altcode:
  Wave propagation in solar coronal loops are studied by finding the
  modes of a magnetized cylinder which joins smoothly to an exterior
  medium across a transition region. The density inhomogeneity was
  treated first as a step function and next as a continuous function
  with a transition region joining the internal and external value
  across the cylinder's radius. The spectra for both models were then
  computed employing analytical and numerical procedures. The spectra
  for both models were then computed employing analytical and numerical
  procedures. The sharp-boundary case consisted of a purely discrete
  spectrum. A continuous profile was found to introduce a continuous
  spectrum. For the specific continuous profile chosen, the complex
  discrete spectrum was perturbed from the sharp boundary case.

---------------------------------------------------------
Title: The Loop Paradigm. (Book Reviews: Plasma Loops in the Solar
    Corona.)
Authors: Zweibel, Ellen
1992Sci...255..857B    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Book Review: Plasma loops in the solar corona / Cambridge U
    Press, 1991
Authors: Cram, L. E.; Currant, C. J.; Loughhead, R. E.; Zweibel, E.
1992Sci...255..857C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamos with ambipolar diffusion drifts
Authors: Proctor, Michael R. E.; Zweibel, Ellen G.
1992GApFD..64..145P    Altcode:
  In a weakly ionized medium, there is diffusive transport of magnetic
  field relative to the neutral gas. The effective diffusion coefficient
  is quadratic in the magnetic field strength so that the diffusion
  is nonlinear. We have included this nonlinear diffusion in a simple
  model of an - dynamo in a slab. A rich variety of solutions, including
  steady states and steady travelling waves, are found. Our results may
  be relevant to the generation of magnetic fields in galaxies and in
  accretion disks around young starts.

---------------------------------------------------------
Title: Magnetized Supernova Remnants with Cosmic Rays
Authors: Ferriere, Katia M.; Zweibel, Ellen G.
1991ApJ...383..602F    Altcode:
  The effects of interstellar magnetic fields and cosmic rays on the
  dynamics of an SNR expanding into a warm H I gas are examined. As
  long as the shock wave driven by the SN explosion propagates faster
  than 110 km/s, the vicinity of the shock front is fully ionized, and
  cosmic rays are well coupled to the thermal fluid. They are first
  accelerated at the adiabatic front, and further compressed in the
  postshock cooling zone. When the shock velocity drops below 110 km/s,
  ion-neutral collisions in the vicinity of the shock dissipate the waves
  which couple cosmic rays to the thermal gas, and impede cosmic-ray
  acceleration. It is found that magnetic and cosmic-ray pressures
  together dominate over thermal pressure away from the magnetic poles. As
  a result, most of the shell becomes considerably thicker, and the shock
  wave propagates somewhat faster than in the nonmagnetic case. At late
  times, the transverse mass motions which take place from the poles to
  the equator create H I holes at the polar caps. This theory leads to
  a simple interpretation of the 'barrel-shaped' distribution of radio
  emission observed in some SNRs.

---------------------------------------------------------
Title: Cosmic-Ray Heating of Cooling Flows: A Critical Analysis
Authors: Loewenstein, Michael; Zweibel, Ellen G.; Begelman, Mitchell C.
1991ApJ...377..392L    Altcode:
  We present a detailed investigation of the hypothesis that a combination
  of magnetohydrodynamic-wave mediated cosmic-ray heating and thermal
  conduction might serve to balance cooling in intracluster media
  and substantially reduce the rate of inflow. We show that this is a
  particularly promising way to heat intracluster media (ICM) for a number
  of reasons. Because of the form of the cosmic-ray heating, a nearly
  static ICM with a positive temperature gradient can exist. Because
  of the origin of the cosmic rays-presumed to be an active nucleus
  in the central galaxy fueled by residual inflow-a globally stable
  feedback mechanism is at work, and, in contrast to the case where
  conduction alone operates, no unphysical fine-tuning of parameters is
  required. Since, while cosmic rays dominate the heating at small cluster
  radii, thermal conduction dominates at large radii, an (undetectably)
  small cosmic-ray luminosity is required. we derive and solve the
  appropriate system of steady state equations that include a new,
  self-consistent formulation for the cosmic- ray diffusivity. Models
  successful in producing substantial positive temperature gradients
  in static configurations are indeed found, but only if conduction
  is reduced by a factor of 10 or more. Unfortunately, these models
  become cosmic-ray pressure-dominated and as a result have too-flat
  thermal pressure profiles when compared with the observations. This
  negative result is confirmed by semiempirical models which solve,
  simultaneously, for the required cosmic-ray pressure and wave (Alfven)
  speed distributions. The cosmic-ray pressure gradient can be reduced
  to acceptable levels only for central values of the Alfven speed
  considerably in excess of the local thermal sound speed. In such a case
  the magnetic pressure can no longer be justifiably neglected. in the
  radial force equation, since the required reduction in conduction is
  presumably a result of tangled magnetic fields; this pressure would
  again lead to a thermal gas distribution flatter than observed. The
  effect of cosmic rays on the thermal stability of the ICM is also
  investigated, as is the role of cosmic rays in heating emission-line
  filaments. We find that cosmic-ray heating is unlikely either to
  stabilize positive- density perturbations against condensation or to
  contribute appreciably to the powering of the optical filaments.

---------------------------------------------------------
Title: Evolution to Nonequilibrium in Simple Models of Prominence
    Filaments
Authors: Zweibel, Ellen G.
1991ApJ...376..761Z    Altcode:
  The support of cold plasma against gravity by a line-tied magnetic field
  is considered. The normal component of the field at the boundaries,
  the mass/unit flux, and the magnetic shear are all specified. Fixing
  the first two of these quantities, and varying the third leads
  to evolutionary sequences. When the magnetic shear is less than
  a critical value, an infinite number of solutions exist. When the
  magnetic shear is sufficiently large, there are no solutions with
  the desired topology. These results support the idea that prominences
  erupt as a result of evolution to magnetic nonequilibrium.

---------------------------------------------------------
Title: Spinning a tangled web
Authors: Zweibel, Ellen
1991Natur.352..755Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Expansion of a Superbubble in a Uniform Magnetic Field
Authors: Ferriere, Katia M.; Mac Low, Mordecai-Mark; Zweibel, Ellen G.
1991ApJ...375..239F    Altcode:
  The effects of interstellar magnetic fields on the evolution and
  structure of superbubbles are investigated. First, the governing
  equations for propagation of a radiative shock are derived and solved
  analytically in the limit of high expansion velocity. A numerical
  code able to solve them in the general case and discuss the numerical
  results is then presented. It is found that magnetic fields of the
  strength present in the Galactic disk do not significantly modify the
  overall shape and dimensions of a superbubble. They reduce the volume
  of the interior cavity by one-third on average during the expansion
  phase. The shell elongates slightly in the direction of the external
  field. Magnetic pressure dominates over gas pressure in most of the
  shell and forces it to thicken substantially. Finally, because of the
  transverse mass motions which take place from the magnetic poles to
  the equator, the column density at the poles is reduced by typically
  a factor of 10.

---------------------------------------------------------
Title: Magnetic Fields in Dense Regions
Authors: Heiles, C.; Goodman, A. A.; McKee, C. F.; Zweibel, E. G.
1991IAUS..147...43H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Electron acceleration by magnetosonic waves in solar flares
Authors: Zweibel, Ellen G.; de La Beaujardiere, Jean-Francois
1990GeoRL..17.2051Z    Altcode:
  Results of a simulation of electron acceleration by magnetosonic
  waves in a solar coronal loop are presented. The dependence of the
  energization rate on the strength and shape of the wave spectrum is
  discussed. Comparisons are made with the predictions of quasilinear
  theory and of a model describing stochastic acceleration by waves as
  isotropic diffusion in momentum space.

---------------------------------------------------------
Title: Magnetic Field Line Tangling and Polarization Measurements
    in Clumpy Molecular Gas
Authors: Zweibel, Ellen G.
1990ApJ...362..545Z    Altcode:
  Magnetic field lines entrained by randomly moving clumps of molecular
  gas tend to acquire a fluctuating or tangled component, which can
  be detected by measuring the dispersion of polarization angle about
  the mean. It is shown how the size of this component depends on the
  relative kinetic and magnetic energy densities of the medium, the
  covering factor of the clumps, and the degree of ionization.

---------------------------------------------------------
Title: Test-Particle Simulations of Electron Acceleration by
    Magnetosonic Waves
Authors: de La Beaujardiére, J. -F.; Zweibel, E. G.
1990BAAS...22.1226D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Virial Theorem Analysis of the Structure and Stability of
    Magnetized Clouds
Authors: Zweibel, Ellen G.
1990NASCP3084..252Z    Altcode: 1990imeg.conf..252Z
  The tensor virial theorem is used to analyze the structure and stability
  of self-gravitating, magnetized spheroids surrounded by a low-density
  medium with pressure and magnetic field. Analytical expressions
  are developed for the effect of a weak field and calculate critical
  states when the effect of the field is arbitrarily strong, comparing
  the results with full magnetohydrostatic calculations. This analysis
  suggests that a magnetic field may prevent gravitational collapse but
  may also be destabilizing, depending on its degree of concentration
  within the cloud.

---------------------------------------------------------
Title: Evolution of a Superbubble Blastwave in a Magnetized Medium
Authors: Ferriere, Katia M.; Zweibel, Ellen G.; Mac Low, Mordecai-Mark
1990NASCP3084..227F    Altcode: 1990imeg.conf..227F
  Researchers investigate the effects of interstellar magnetic fields on
  the evolution and structure of interstellar superbubbles, using both
  analytic and numerical magnetohydrodynamic (MHD) calculations. These
  cavities of hot gas, surrounded by shells of cold dense material
  preceded by a shock wave result from the combined action of stellar
  winds and supernova explosions in OB associations. If the medium in
  which a superbubble goes off is homogeneous and unmagnetized, the blast
  wave expands isotropically. As the interstellar gas flows through the
  shock, it cools significantly and gets strongly compressed such that
  thermal pressure remains approximately equal to ram pressure. Hence,
  the swept up material is confined to a very thin shell. However,
  if the ambient medium is permeated by a uniform magnetic field
  B<SUB>o</SUB> approx. 3 mu G (typical value for the interstellar
  matter (ISM)), the configuration loses its spherical symmetry, and,
  due to magnetic pressure, the shell of swept up material does not
  remain thin. Researchers found the following qualitative differences:
  (1) Except in the immediate vicinity of the magnetic poles, the shell
  is supported by magnetic pressure. (2) The refraction of field lines
  at the shock and the thermal pressure gradient along the shell both
  contribute to accelerating the gas toward the equator. The resulting
  mass flux considerably decreases the column density at the magnetic
  poles. (3) Away from the poles, magnetic tension in the shell causes
  the field lines (particularly the inner boundary) to elongate in
  the direction of B<SUB>o.</SUB> In contrast, the shock wave radius
  increases with increasing theta. (4) The reduced inertia of a parcel in
  the polar neighborhood makes it easier to decelerate, and accounts for
  the dimple which appears at the poles in numerical simulations. This
  dimple also results from the necessity to call on intermediate shocks
  in order to insure a smooth transition between a purely thermal shock
  at the poles and a magnetic shock in the rest of the shell. (5) The
  shock wave propagates faster than in the absence of magnetic field,
  except near the poles where the reduced mass of the shell allows it
  to be more efficiently decelerated.

---------------------------------------------------------
Title: Evolution of a supernova blastwave in a magnetized medium
Authors: Ferriere, K. M.; Zweibel, E. G.; Mac Low, M. -M.
1990BAAS...22..750F    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Virial Theorem Analysis of the Structure and Stability of
    Magnetized Clouds
Authors: Zweibel, Ellen G.
1990ApJ...348..186Z    Altcode:
  The tensor virial theorem is used to analyze the structure and stability
  of self-gravitating, magnetized spheroids surrounded by a low-density
  medium with pressure and magnetic field. Analytical expressions
  are developed for the effect of a weak field and calculate critical
  states when the effect of the field is arbitrarily strong, comparing
  the results with full magnetohydrostatic calculations. This analysis
  suggests that a magnetic field may prevent gravitational collapse but
  may also be destabilizing, depending on its degree of concentration
  within the cloud.

---------------------------------------------------------
Title: Electron Acceleration in a Solar Coronal Loop
Authors: de La Beaujardiere, J. -F.; Zweibel, E. G.
1989BAAS...21.1150D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Effects of Magnetic Fibrils on Solar Oscillation Frequencies:
    Mean Field Theory
Authors: Zweibel, Ellen G.; Daeppen, Werner
1989ApJ...343..994Z    Altcode:
  It is quite likely that the solar interior magnetic field retains
  the fibril structure observed at photospheric levels down to some
  significant depth in the convection zone. Furthermore, previous work
  has shown that the effect of magnetic fibrils on p-mode oscillation
  frequencies is enhanced by 1/f, the reciprocal of the fibril
  filling factor, over the effect of an equal amount of flux in diffuse
  form. Previous calculations have used multiple scattering methods which
  effectively preclude the inclusion of stratification or consideration
  of realistic geometries. In this paper an alternative simpler method
  of treating fibrils is developed on the basis of mean-field theory
  originally developed by Parker (1982).

---------------------------------------------------------
Title: Magnetic Reconnection in Partially Ionized Gases
Authors: Zweibel, Ellen G.
1989ApJ...340..550Z    Altcode:
  Magnetic field lines in a plasma reconnect at a rate scaled by the
  Alfven speed. In a partially ionized gas there are two natural Alfven
  speeds: one determined by the ionized mass density alone, which
  applies when ion-neutral friction is negligible, and one determined
  by the total mass density, which applies when ion-neutral friction is
  strong. When the ionization fraction is low, as in a dense molecular
  cloud, these two speeds differ by several orders of magnitude. Both
  time-dependent tearing modes and steady-state magnetic reconnection
  in partially ionized gas are considered, and the regimes in which
  the charged and neutral components are strongly, intermediately, and
  weakly coupled are delineated. Molecular clouds are probably in the
  intermediate regime, while reconnection in solar prominences probably
  has strong ion-neutral coupling. Reconnection proceeds more rapidly
  when coupling is not strong.

---------------------------------------------------------
Title: Stability of deformed coronal current tubes and energy storage
    in solar corona.
Authors: Zweibel, E. G.; Tzihong, Chiueh
1989BAPS...34.1288Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Stability of Stressed Current Tubes and Energy Storage in
    the Solar Corona
Authors: Chiueh, Tzihong; Zweibel, Ellen G.
1989ApJ...338.1158C    Altcode:
  Ideal MHD stability in the solar corona is studied in order to
  understand the issue of magnetic energy storage. It is shown that
  circular flux tubes confined by a potential field can store a large
  fraction of their energy in nonpotential form. Such tubes are stabilized
  by their circularity, by line tying, and by their external field. In
  contrast, interacting, noncircular tubes can be far less stable,
  despite line tying and an external field. If a substantial fraction
  of the magnetic energy in an active region is to be stably stored in
  nonpotential form, only a very simple magnetic geometry is probably
  required.

---------------------------------------------------------
Title: Particle acceleration by magnetosonic waves in a coronal loop.
Authors: de La Beaujardière, J. -F.; Zweibel, E. G.
1989sasf.confP.317D    Altcode: 1988sasf.conf..317D; 1989IAUCo.104P.317D
  A model for the acceleration of electrons in a flaring coronal loop
  is described. The mechanism is stochastic acceleration by resonant
  interactions with a spectrum of compressive magnetosonic waves. Current
  results of test particle calculations examining the feasibility of
  this model are presented.

---------------------------------------------------------
Title: Magnetohydrodynamic Waves and Particle Acceleration in a
    Coronal Loop
Authors: de La Beaujardiere, Jean-Francois; Zweibel, Ellen G.
1989ApJ...336.1059D    Altcode:
  A model is presented for the acceleration of electrons in a flaring
  solar coronal loop. It is suggested that the primary energy release
  event in the flare establishes a spectrum of MHD waves within the loop
  which accelerate electrons by wave-particle resonant interactions. The
  nature of the MHD oscillations are described and numerical and analytic
  solution of the dispersion relation are examined. It is found that the
  behavior of a particle depends on whether or not it is in resonance
  with the wave spectrum. Resonant particles follow chaotic trajectories
  and can diffuse in velocity space between wave effective potentials,
  thereby gaining energy.

---------------------------------------------------------
Title: Hydromagnetic wave heating of low density interstellar gas
Authors: Zweibel, E. G.; Ferriere, K. M.; Shull, J. M.
1988AIPC..174...70Z    Altcode: 1988rwsi.conf...70Z
  The origin of the observed wave spectrum for hot gas in the ISM is
  considered theoretically. The governing equations for the generation,
  propagation, and dissipation of compressive waves are reviewed, and
  particular attention is given to the heating of warm neutral gas and
  the implications for radio-wave scattering. It is shown that little
  power from interactions between SN shocks and hot coronal gas reaches
  short wavelengths, and that scintillation probably does not originate
  in a warm weakly ionized gas.

---------------------------------------------------------
Title: Acceleration of Coronal Loop Electrons by Magnetosonic Waves
Authors: de La Beaujardiere, J. -F.; Zweibel, E. G.
1988BAAS...20..978D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Hydromagnetic Wave Heating of the Low-Density Interstellar
    Medium
Authors: Ferriere, Katia M.; Zweibel, Ellen G.; Shull, J. Michael
1988ApJ...332..984F    Altcode:
  Using a simple model for supernova remnant sources of MHD waves, the
  authors calculate the energy spectrum of waves in the intercloud medium
  and the heating rate resulting from their dissipation. The authors then
  construct models of thermal phases of interstellar gas in ionization
  and thermal equilibrium and demonstrate that wave dissipation can be
  an important heating mechanism which can account for the observed high
  H I temperatures in low-density (intercloud) neutral gas.

---------------------------------------------------------
Title: The Growth of Magnetic Fields Prior to Galaxy Formation
Authors: Zweibel, Ellen G.
1988ApJ...329L...1Z    Altcode:
  We apply hydromagnetic dynamo theory to the problem of magnetic
  field amplification in the epoch after recombination but prior to
  galaxy formation. Large-scale density fluctuations acquire angular
  momentum from tidal torques exerted by neighboring fluctuations, and
  small-scale subfluctuations possess helicity due to the Coriolis forces
  associated with the rotation of the large- scale host fluctuations. We
  take into account cosmic expansion and the interaction of matter with
  the ambient radiation field. Despite many uncertainties, we conclude
  that significant dynamo amplification may have occurred.

---------------------------------------------------------
Title: Ambipolar Diffusion Drifts and Dynamos in Turbulent Gas
Authors: Zweibel, Ellen G.
1988ApJ...329..384Z    Altcode:
  The author considers ambipolar drift in turbulent fluids. Using
  mean-field electrodynamics, a two-scale theory originally used to
  study hydromagnetic dynamos, it is shown that magnetic fields can
  be advected by small-scale magnetosonic (compressional) turbulence
  or generated by Alfvénic (helical) turbulence. The author makes a
  simple dynamo theory and compares it with standard theories in which
  dissipation is caused by turbulent diffusion. The redistribution of
  magnetic flux in interstellar clouds is also discussed.

---------------------------------------------------------
Title: The Scattering of Alfven Waves by Density Fluctuations
Authors: Li, He-Sheng; Zweibel, Ellen G.
1987ApJ...322..248L    Altcode:
  The authors consider the propagation of an Alfvén wave packet through
  a medium containing time-dependent random density fluctuations. The
  Alfvén wave interaction with these density irregularities causes
  the transfer of wave power to both shear (Alfvénic) and compressive
  (magnetosonic) disturbances. The latter are dissipated and heat the
  plasma. The authors compute the Alfvén wave decay rate in the limit
  of short-wavelength density fluctuations and apply the results to the
  solar corona and interstellar medium.

---------------------------------------------------------
Title: Hydromagnetic Wave Heating of the Low Density Interstellar
    Medium
Authors: Ferriere, K. M.; Zweibel, E. G.; Shull, J. M.
1987BAAS...19.1057F    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Structure and Dissipation of Forced Current Sheets in
    the Solar Atmosphere
Authors: Chiueh, Tzihong; Zweibel, Ellen G.
1987ApJ...317..900C    Altcode:
  The equilibrium structure and magnetic-reconnection (MR) processes
  in current sheets produced by global MHD forces are investigated
  theoretically, considering the case (complementary to that studied by
  Parker, 1979 and 1983) where the magnetic field near the sheet lies
  on good flux surfaces, with substantial accumulation of magnetic
  stress. The derivation and application of the model, based on the
  approximate MHD equations developed by Strauss (1976) for tokamak
  plasmas, are presented in detail. Three main stages of MR are
  characterized: a linear sheet-tearing mode which grows more rapidly
  than the magnetic-tearing mode for diffuse profiles, acceleration of
  MR by small shear-flow-driven MHD fluctuations acting as an anomalous
  resistivity, and rapid MR in a broad turbulent layer. This mechanism
  is shown to provide the rapid current dissipation required by some
  models of solar flares and coronal heating.

---------------------------------------------------------
Title: Particle Acceleration by MHD Waves in Solar Flares
Authors: de La Beaujardiere, J. -F.; Zweibel, E. G.
1987BAAS...19..750D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Formation of Current Sheets in the Solar Atmosphere
Authors: Zweibel, Ellen G.; Li, He-Sheng
1987ApJ...312..423Z    Altcode:
  A simple model of the effect of slow photospheric motions on the coronal
  magnetic field is examined. In this model, a magnetic field is stretched
  between two conducting plates and slow tangential displacements of
  the plates cause the field to evolve quasi-statically. If the field
  is treated as a continuum, the interior magnetic topology is uniquely
  specified by the displacement of the boundaries, while the topology of
  a field made up of discrete flux tubes is not determined by the boundary
  displacement. Treating the field as a continuum, it is found that small
  perturbations of a constant vertical or weakly sheared field can come
  to an equilibrium free of singularities if the separation between the
  plates is finite.

---------------------------------------------------------
Title: Propagation of Compressive Waves through Fibril Magnetic Fields
Authors: Bogdan, T. J.; Zweibel, Ellen G.
1987ApJ...312..444B    Altcode:
  The surface effects of interactions between the solar 5-min p-modes and
  the large-scale fibril magnetic field are discussed using a multiple
  scattering approach. Attention is given to the propagation of linear
  disturbances in a two-dimensional, highly conducting magnetized
  plasma with many parallel flux tubes in pressure equilibrium with a
  surrounding stationary field-free plasma. Multiple scattering in the
  fibril half-space is shown to generate acoustic waves that cascade to
  ever-smaller length scales. The scale reduction, proportional to the
  depth into the fibril magnetic field, is responsible for the damping
  of p-mode oscillations observed in plages.

---------------------------------------------------------
Title: The Theory of the Galactic Magnetic Field
Authors: Zweibel, Ellen G.
1987ASSL..134..195Z    Altcode: 1987ip...symp..195Z
  The author discusses the role of the magnetic field in determining
  the large scale structure and dynamics of the interstellar medium. She
  then discusses the origin and maintenance of the galactic field. The
  two major competing theories are that the field is primordial and
  connected to an intergalactic field or that the field is removed from
  and regenerated within the galaxy. The theoretical and observational
  basis for these theories are reviewed. Finally, the author discusses
  cosmic ray acceleration and confinement in the interstellar medium.

---------------------------------------------------------
Title: Is Nonthermal Bremsstrahlung Important in Young Supernova
    Remnants?
Authors: Hamilton, A. J. S.; Zweibel, E. G.
1986BAAS...18R1052H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Effects of Fibril Magnetic Fields on Solar
    p-Modes. II. Calculation of Mode Frequency Shifts
Authors: Zweibel, E. G.; Bogdan, T. J.
1986ApJ...308..401Z    Altcode:
  The influence of magnetic flux tubes embedded in the solar convection
  zone on the observed p-mode oscillation frequencies are estimated
  by WKB ray tracing in a model of the solar interior. For randomly
  distributed but parallel magnetic fibrils with radii small compared
  to the wavelength of the mode, a local correction to the acoustic
  dispersion relation can be calculated exactly. The results show that
  if the observed photospheric flux remains vertically oriented deep into
  the convection zone, then only modes which are confined to within 0.5%
  R_sun; of the solar surface are shifted in frequency by as much as 0.1%.

---------------------------------------------------------
Title: Oscillation Spectra of Neutron Stars with Strong Magnetic
    Fields
Authors: Carroll, B. W.; Zweibel, E. G.; Hansen, C. J.; McDermott,
   P. N.; Savedoff, M. P.; Thomas, J. H.; van Horn, H. M.
1986ApJ...305..767C    Altcode:
  The authors have investigated the effects of a strong vertical
  magnetic field on the oscillation spectrum of a cylindrical slab
  model for the surface layers of a neutron star. In particular, they
  have considered the effects on those modes known to be concentrated
  in the surface layers: the pseudo-toroidal t- and a-modes and the
  pseudo-spheroidal s-, i-, and m/g-modes. The authors have derived
  and solved the Newtonian pulsation equations for the cylindrical
  model. The electromagnetic boundary conditions at the neutron star
  surface match the magnetohydrodynamic motions within the star to
  outgoing electromagnetic radiation in the evacuated waveguide above the
  surface; pseudo-toroidal modes couple to TM electromagnetic modes in
  the waveguide, while pseudo-spheroidal modes match to TE radiation. The
  periods of the t- and s-modes are similar to the quasi-periodicities
  observed in some pulsars.

---------------------------------------------------------
Title: The acceleration and propagation of solar flare energetic
    particles.
Authors: Forman, M. A.; Ramaty, R.; Zweibel, E. G.
1986psun....2..249F    Altcode:
  Contents: Energetic particles in solar flares (electromagnetic
  radiations: radio emissions, hard X-rays, gamma rays; energetic
  particles: energy spectra and electron-proton correlations, chemical
  compositions, isotopic and ionic compositions). Mechanisms of
  solar flare particle acceleration (stochastic acceleration, shock
  acceleration, acceleration in direct electric fields). Solar flare
  particle spectra in interplanetary space. Summary and outlook.

---------------------------------------------------------
Title: Erratum - the Propagation of Energetic Ions in Magnetic Loops
    and Gamma-Ray Emission from Solar Flares
Authors: Zweibel, E. G.; Haber, D. A.
1985ApJ...299..574Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Effect of a fibril magnetic field on solar p-modes
Authors: Bogdan, T. J.; Zweibel, E. G.
1985ApJ...298..867B    Altcode:
  The dispersion relation is obtained for acoustic plane waves that
  scatter coherently from an ensemble of parallel magnetic flux tubes
  when the wave vector is perpendicular to the flux-tube axis. When the
  magnetic flux tubes are distributed uniformly and possess radii that
  are small compared with the wavelength, the frequency can be calculated
  exactly. The waves are damped slightly due to a loss of coherence and
  are shifted downward or upward in frequency relative to a medium devoid
  of magnetic fibrils, depending primarily on whether the flux tubes are
  more or less dense than their surroundings. It is suggested that the
  influence of the fibril magnetic fields observed at the solar surface
  cannot be ignored in the interpretation of high-1 surface p-mode data.

---------------------------------------------------------
Title: A Model of Electromagnetic Damping Mechanisms for Neutron
    Star Oscillations
Authors: Carroll, B. W.; Zweibel, E. G.; Hansen, C. J.; McDermott,
   P. N.; Savedoff, M. P.; Thomas, J. H.; van Horn, H. M.
1985BAAS...17..855C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Evolution of twisted magnetic fields
Authors: Zweibel, E. G.; Boozer, A. H.
1985ApJ...295..642Z    Altcode:
  The magnetic field of the solar corona evolves quasi-statically in
  response to slowly changing photospheric boundary conditions. The
  magnetic topology is preserved by the low resistivity of the solar
  atmosphere. It is shown that a magnetic flux coordinate system
  simplifies the problem of calculating field evolution with invariant
  topology. As an example, the equilibrium of a thin magnetic flux tube
  with small twist per unit length is calculated.

---------------------------------------------------------
Title: Application of the MHD energy principle to magnetostatic
    atmospheres
Authors: Zweibel, E. G.
1985GApFD..32..317Z    Altcode:
  We apply the MHD energy principle to the stability of a magnetized
  atmosphere which is bounded below by much denser fluid, as is the
  solar corona. We treat the two fluids as ideal; the approximation
  which is consistent with the energy principle, and use the dynamical
  conditions that must hold at a fluid-fluid interface to show that if
  vertical displacements of the lower boundary are permitted, then the
  lower atmosphere must be perturbed as well. However, displacements
  which do not perturb the coronal boundary can be properly treated as
  isolated perturbations of the corona alone.

---------------------------------------------------------
Title: The effect of a vertical magnetic field on neutron star
    oscillations.
Authors: Carroll, B. W.; McDermott, P. N.; Savedoff, M. P.; Thomas,
   J. H.; van Horn, H. M.; Zweibel, E. G.; Morrow, C. A.; Hansen, C. J.
1985ASNYN...2...27C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Neutron Star Oscillations in the Presence of a Vertical
    Magnetic Field
Authors: Carroll, B. W.; McDermott, P. N.; Savedoff, M. P.; Thomas,
   J. H.; van Horn, H. M.; Zweibel, E. G.; Morrom, C. A.; Hansen, C. J.
1984BAAS...16..943C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Electromagnetic damping of neutron star oscillations
Authors: McDermott, P. N.; Savedoff, M. P.; van Horn, H. M.; Zweibel,
   E. G.; Hansen, C. J.
1984ApJ...281..746M    Altcode:
  A simple model of magnetic field perturbations driven by neutron star
  oscillations is used to estimate the electromagnetic power radiated by
  g-modes and torsional oscillations. The calculation assumes that the
  neutron star has a frozen-in magnetic field which is perturbed by the
  oscillatory motions of the surface. The disturbances propagate into the
  vacuum as outgoing electromagnetic waves. The relative effectiveness
  of Joule heating of the neutron star crust by pulsation-induced
  electric currents is estimated. It is concluded that electromagnetic
  damping is the dominant energy dissipation mechanism for quadrupole
  g-mode oscillations of neutron stars. For dipole spheroidal modes,
  both electromagnetic radiation and Joule heating are important, and
  there is no gravitational radiation emitted by these modes.

---------------------------------------------------------
Title: Effect of Magnetic Fields on Neutron Star Oscillations
Authors: Morrow, C. A.; Zweibel, E. G.
1984BAAS...16R.542M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Particle acceleration.
Authors: Rosner, R.; Chupp, E. L.; Gloeckler, G.; Gorney, D. J.;
   Krimigis, S. M.; Mok, Y.; Ramaty, R.; Swift, D. W.; Vlahos, L.;
   Zweibel, E. G.
1984NASRP1120....2R    Altcode:
  Contents: 1. Introduction. 2. Phenomenology: Solar flares. Acceleration
  processes in the interplanetary medium. Magnetospheric and
  ionospheric observations. Particle acceleration outside the solar
  system. 3. Theoretical particle acceleration mechanisms: Adiabatic
  compression, magnetic pumping, and diffusion. Acceleration in
  direct electric fields. Stochastic acceleration. Shock particle
  acceleration. Coherent wave acceleration. Injection. 4. The remaining
  questions.

---------------------------------------------------------
Title: Nonlinear periodic solutions for the isothermal magnetostatic
    atmosphere
Authors: Low, B. C.; Hundhausen, A. J.; Zweibel, E. G.
1983PhFl...26.2731L    Altcode:
  Zweibel and Hundhausen (1982) have obtained analytically a family
  of isothermal, horizontally periodic, magnetostatic atmospheres
  in a uniform gravitational field. The present investigation is
  concerned with another set of period analytic solutions, taking into
  account the equilibrium configuration of plasma condensations in an
  otherwise everywhere uniform field. The physics of the support of
  the condensations by the embedded magnetic field is of interest to
  the study of solar prominences and interstellar clouds. Attention is
  given to the nonlinear problem, the general results, models for plasma
  condensations, and questions of stability.

---------------------------------------------------------
Title: Hydromagnetic wave dissipation in molecular clouds
Authors: Zweibel, E. G.; Josafatsson, K.
1983ApJ...270..511Z    Altcode:
  The damping of long wavelength, hydromagnetic waves in molecular
  cloud environments is studied with the aim of determining whether
  the supersonic motions observed in such clouds are likely to be due
  to the waves. It is found that Alfven waves propagating parallel to
  the average magnetic field are the longest lived wave modes. Such
  waves can typically survive for as long as one-million years if the
  wavelength is as long as a few tenths of a pc and the magnetic field
  is 0.1-1 milligauss. Nonlinear steepening of the waves followed by
  ion-neutral friction in the steepened wave profiles appears to be the
  most effective damping mechanism.

---------------------------------------------------------
Title: The propagation of energetic ions in magnetic loops and
    gamma-ray emission from solar flares
Authors: Zweibel, E. G.; Haber, D. A.
1983ApJ...264..648Z    Altcode:
  Gamma-ray emission from solar flares is generally believed to be
  produced by nuclear reactions between energetic ions and ambient
  material in the lower solar atmosphere. We consider the propagation of
  flare accelerated ions from the corona to the lower atmosphere, taking
  into account the mirror force on the particles due to the increase of
  the magnetic field strength with depth. We show that for reasonable
  models of the magnetic field structure, the mirror force constrains
  all but a narrow loss cone of particles from penetrating promptly
  to the lower atmosphere. This has observation consequences for the
  intensity and time profile of the γ-ray emission, and it could affect
  inferences about the timing and nature of the acceleration mechanism
  of flare accelerated particles.

---------------------------------------------------------
Title: Plasma astrophysics at Santa Barbara
Authors: Rosner, R.; Zweibel, E.; Trimble, V.
1982Natur.299..579R    Altcode:
  Discussions at the Space and Astrophysical Plasmas workshop held
  in Santa Barbara, CA in the summer of 1982 are reviewed. Attention
  was given mainly to hydromagnetic shocks and particle acceleration,
  interactions between cold and hot plasmas, and hydromagnetic flows. The
  structure of collisionless shocks was examined, with attention given
  to the terrestrial bowshock. Numerical simulations of collisionless
  shocks were presented, including details of ion dynamics in studies
  of internal shock structure. Shock acceleration was explored on a
  basis of a steady-state model showing cosmic rays to diffuse near a
  high Mach number quasi-parallel shock approximated a discontinuity in
  the flow speed. Problems needing further study were indicated, e.g.,
  the efficiency of a shock wave, energy transfer between plasmas of
  differing temperatures, and heat transport in laser-irradiated plasmas.

---------------------------------------------------------
Title: The acceleration and propagation of solar flare energetic
    particles
Authors: Forman, M. A.; Ramaty, R.; Zweibel, E. G.; Holzer, T. E.;
   Mihalas, D.; Sturrock, P. A.; Ulrich, R. K.
1982STIN...8329162F    Altcode:
  Observations and theories of particle acceleration in solar flares
  are reviewed. The most direct signatures of particle acceleration
  in flares are gamma rays, X-rays and radio emissions produced by the
  energetic particles in the solar atmosphere and energetic particles
  detected in interplanetary space and in the Earth's atmosphere. The
  implication of these observations are discussed. Stochastic and shock
  acceleration as well as acceleration in direct electric fields are
  considered. Interplanetary particle propagation is discussed and
  an overview of the highlights of both current and promising future
  research is presented.

---------------------------------------------------------
Title: Confinement of cosmic rays in molecular clouds
Authors: Zweibel, E. G.; Shull, J. M.
1982ApJ...259..859Z    Altcode:
  (Previously announced in STAR as N82-20073)

---------------------------------------------------------
Title: A sufficient condition for the stability of atmospheres with
    magnetic fields
Authors: Zweibel, E. G.
1982ApJ...258L..53Z    Altcode:
  Using the MHD energy principle, it is shown that P + B sq/8 x pi =
  constant is a sufficient condition for the stability of magnetized fluid
  systems with the following properties: the gravitational acceleration
  g is uniform, the magnetic field lines lie in parallel planes aligned
  with g, and all quantities are uniform in the direction perpendicular to
  the plane of the field lines. An example of a stable system is given,
  consisting of a vertical sheet of plasma supported against gravity by
  bowed field lines.

---------------------------------------------------------
Title: Magnetostatic Atmospheres - a Family of Isothermal Solutions
Authors: Zweibel, E. G.; Hundhausen, A. J.
1982SoPh...76..261Z    Altcode:
  Most models of large scale solar magnetic fields assume either that
  the fields are potential or that they are force free. We present a
  new, analytic, two parameter family of magnetic fields in equilibrium
  with isothermal plasma in a gravitational field. We discuss these
  models from the viewpoint of the insight into the balance of magnetic
  pressure gradient, and gravitational forces that they provide. We show
  that substantial deviations from the potential field configuration are
  obtained for plasma β of order unity, and we emphasize the variety
  of possible relationships between isobars and magnetic fieldlines.

---------------------------------------------------------
Title: Magnetostatic atmospheres in a spherical geometry and their
    application to the solar corona
Authors: Rohrer Hundhausen, J.; Hundhausen, A. J.; Zweibel, Ellen G.
1981JGR....8611117R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Confinement of cosmic rays in molecular clouds
Authors: Zweibel, E. G.; Shull, J. M.
1981ESASP.161..371Z    Altcode: 1981plas.work..371Z
  No abstract at ADS

---------------------------------------------------------
Title: Confinement of cosmic rays in molecular clouds
Authors: Zweibel, E. G.; Shull, J. M.
1981plap.rept..371Z    Altcode:
  The consequences of cosmic ray production by a supernova in a molecular
  cloud are discussed. Self-trapping problems for a higher flux of
  cosmic rays in a molecular cloud are focused on. The column density
  of molecular clouds is probably too high to explain the majority of
  galactic cosmic ray sources, even allowing for fortuitous asymmetry in
  the placement of the supernova in the cloud, however, measurements of
  antiproton flux suggest that some cosmic ray sources do have a high
  column density. The large predicted gamma-ray luminosity of such a
  cloud invites comparison with COS-B sources.

---------------------------------------------------------
Title: MHD instabilities of atmospheres with magnetic fields
Authors: Zweibel, E. G.
1981ApJ...249..731Z    Altcode:
  The MHD energy principle is used to derive a stability criterion for
  two-dimensional equilibrium systems consisting of magnetized plasma in
  a gravitational field. The interplay between magnetic and gravitational
  forces in determining stability is studied, and the criterion is applied
  to a family of two-dimensional magnetostatic atmospheres described
  by Zweibel and Hundhausen (1980). All members of the family except
  the potential field are found to be locally unstable to horizontal
  displacements, and the instabilities are explained in terms of pinching,
  interchange, and Rayleigh-Taylor effects.

---------------------------------------------------------
Title: Thermal Stability of a Corona Heated by Fast Mode Waves
Authors: Zweibel, E.
1980SoPh...66..305Z    Altcode:
  It has been proposed that dissipation of hydromagnetic waves is an
  important heat source for the solar corona. We consider damping by
  collisionless processes and by electron thermal conduction and ion
  viscosity, and calculate the wave energy density such that heating
  balances the energy radiated by the plasma. We then analyze the thermal
  stability of the wave heated medium. The fastest growing instabilities
  are condensations perpendicular to the fieldlines. The instability
  may be important for producing coronal fine structure, and in loops
  and streamers.

---------------------------------------------------------
Title: Thermal Instability of the Solar Corona
Authors: Zweibel, E. G.
1979BAAS...11..697Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Energetic particle trapping by Alfven wave instabilities
Authors: Zweibel, E. G.
1979AIPC...56..319Z    Altcode: 1979pama.work..319Z
  A dispersion relation is derived for Alfven waves which interact
  resonantly with energetic particles in a plasma. It is argued
  that for the particle fluxes expected for cosmic ray production in
  supernova remnants, the waves are unstable and scatter the particles
  strongly. The particles remain in the expanding plasma and lose energy
  adiabatically. These energy losses pose a problem for theories of
  cosmic ray acceleration in supernova remnants. Observations of solar
  energetic particle propagation are found to be consistent with the
  properties of the instability.

---------------------------------------------------------
Title: Convective instability of thin flux tubes.
Authors: Spruit, H. C.; Zweibel, E. G.
1979SoPh...62...15S    Altcode:
  The stability of magnetic flux tubes embedded vertically in a convection
  zone is investigated. For thin tubes, the dominant instability is of the
  convective type, i.e. it is driven by buoyancy forces associated with
  displacements along the tube. The stability is determined by β = 8πP/B
  <SUP>2</SUP>; if β ≤ β <SUB> c </SUB> the tube is convectively
  stable, otherwise it is unstable, where the critical value β <SUB>
  c </SUB> depends on the stratification of the convection zone. For a
  solar convection zone model, β <SUB> c </SUB> = 1.83, corresponding
  to a magnetic field strength of 1350 G at the surface of the Sun. It
  is concluded that the flux tubes making up the small scale field of
  the Sun are probably hydrodynamically stable. In tubes with β &gt;
  β <SUB>c</SUB>, the instability is expected to transform the tube
  either into a state of vanishing surface field strength (in the case of
  an upward flow), or one with a field strength higher than the original
  value (if the instability sets in as a downward flow). Following Parker,
  we suggest that this effect is related to the concentrated nature of
  the observed solar fields.

---------------------------------------------------------
Title: Radial modes of oscillation of cool stellar disks.
Authors: Zweibel, E.
1978ApJ...222..110Z    Altcode:
  Equations describing long wavelength radial modes of oscillation
  for stellar disks are derived. The approach is applied to determine
  the density response of a cool stellar disk to radial potential
  perturbations and is exact for disks with zero velocity dispersion. The
  formulation may be applied to finite or infinite disks of arbitrary
  structure, so long as the length scale of the perturbation is larger
  than the epicyclic amplitude. Three functions - the unperturbed surface
  density, the epicyclic frequency, and the radial velocity dispersion
  - are involved in the expression of the equilibrium structure. The
  effects of differential rotation, halos, and velocity dispersion on
  axisymmetric stability are studied for some disk models.

---------------------------------------------------------
Title: The equilibrium of cool stellar disks.
Authors: Zweibel, E.
1978ApJ...222..103Z    Altcode:
  A system of finite infinitesimally thin self-gravitating disks is
  constructed. The sequence begins with the uniformly rotating disk and
  approaches the isothermal disk, and the latter members of the sequence
  resemble real disk galaxies. A mathematical description of cool stellar
  disks is presented for application to model galaxies in which the ratio
  of random to mean velocity is small, except near the center. Phase-space
  moments of the modified Schwarzschild stellar distribution function
  are shown to have an expansion in powers of the ratio of the star's
  random kinetic energy to gravitational potential energy

---------------------------------------------------------
Title: The Equilibrium and Radial Oscillations of Cool Stellar Disks.
Authors: Zweibel, E. G.
1977PhDT.........1Z    Altcode:
  Observed stars which lie in axisymmetric disks in spiral galaxies were
  considered. Equations used to study the radial modes of oscillation
  of these cool stellar disks of arbitrary structure were derived. Cool
  disk equilibrium was studied along with the response of a cool stellar
  disk to an axisymmetric potential perturbation of longer wavelength
  than the radial average stellar orbit. The linearized Vlasov equation
  and Poisson's equation were used in the later calculations.

---------------------------------------------------------
Title: Radiative processes affecting the abundance of interstellar OH.
Authors: Smith, W. H.; Zweibel, E. G.
1976ApJ...207..758S    Altcode:
  Photodissociation of OH dominates its destruction for a significant
  range of density and shielding in H I regions. Although inverse
  predissociation may provide an observable flux of ultraviolet
  recombination photons, it can provide only a small fraction of the
  total observed OH in most H I regions.

---------------------------------------------------------
Title: The stabilizing effects of cloud reacceleration;
    microturbulence, and rotation on Parker's instability.
Authors: Zweibel, E. G.; Kulsrud, R. M.
1975ApJ...201...63Z    Altcode:
  Parker's instability is reconsidered with the inclusion of a model
  equation of state for the gas of interstellar clouds, a tangled
  interstellar field, and galactic rotation. It is found that the
  instability is stabilized for most choices of interstellar parameters
  consistent with observation. For the remaining choices which can lead
  to instability, the growth time is longer than 100 million years. It
  is concluded that the instability can have little to do with cosmic-ray
  escape and probably cannot destroy a primordial galactic field.

---------------------------------------------------------
Title: The trapping of cosmic rays around supernovae by plasma
    instabilities
Authors: Kulsrud, R.; Zweibel, E.
1975prnc.reptS....K    Altcode:
  Cosmic rays escaping from supernova as sources set up hydromagnetic
  instabilities. The instabilities can trap the cosmic rays in
  the neighborhood of the supernova until they are adiabatically
  decompressed. The importance of these instabilities for theories of
  supernova origin of cosmic rays are discussed.

---------------------------------------------------------
Title: On the Question of the Super-nova Origin of Cosmic Rays
Authors: Zweibel, E. G.; Kulsrud, R. M.
1975BAAS....7..553Z    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Trapping of Cosmic Rays Around Super-novae by Plasma
    Instabilities
Authors: Kulsrud, R.; Zweibel, E.
1975ICRC....2..465K    Altcode: 1975ICRC...14..465K
  No abstract at ADS

---------------------------------------------------------
Title: On the Theory of H_{2} Rotational Excitation
Authors: Spitzer, Lyman, Jr.; Zweibel, Ellen Gould
1974ApJ...191L.127S    Altcode:
  The equilibrium populations of the H2 rotational levels are calculated
  and compared with the excitation observed in interstellar clouds. The
  excitation processes considered include collisions with H atoms and
  electrons and downward radiative cascading following photon excitation
  or molecule formation. If the probability, p, of ultraviolet photon
  absorption is less than the mean value of 5 X tOlo s- , the computed
  excitation temperature T2,4, corresponding to the relative population of
  the levels J = 2 and 4, is between 200 and 40O K, for a gas temperature
  of 800 K, and n(H i) &lt; 102 this is the range of observed excitation
  temperatures for clouds with strong H2 lines, for which p is reduced
  by shielding. To explain the excitation temperatures of about 10000 K
  observed in some weak-line clouds requires either that p is some 10
  times its mean value, corresponding to a cloud position relatively
  close to an earlytype star, or that the cloud is dense [n(H I) &gt;
  102] and possibly also hot (T 102). Snbject headings: interstellar
  matter - molecules, interstellar