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 &gt; 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 $&gt;5$ 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.

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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.

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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.

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

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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

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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

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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

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