Author name code: zweibel ADS astronomy entries on 2022-09-14 author:"Zweibel, Ellen G." ------------------------------------------------------------------------ Title: Near-cancellation of up- and down-gradient momentum transport in forced magnetized shear-flow turbulence Authors: Tripathi, B.; Fraser, A. E.; Terry, P. W.; Zweibel, E. G.; Pueschel, M. J. Bibcode: 2022PhPl...29i2301T Altcode: 2022arXiv220803342T Visco-resistive magnetohydrodynamic turbulence, driven by a two-dimensional unstable shear layer that is maintained by an imposed body force, is examined by decomposing it into dissipationless linear eigenmodes of the initial profiles. The down-gradient momentum flux, as expected, originates from the large-scale instability. However, continual up-gradient momentum transport by large-scale linearly stable but nonlinearly excited eigenmodes is identified and found to nearly cancel the down-gradient transport by unstable modes. The stable modes effectuate this by depleting the large-scale turbulent fluctuations via energy transfer to the mean flow. This establishes a physical mechanism underlying the long-known observation that coherent vortices formed from nonlinear saturation of the instability reduce turbulent transport and fluctuations, as such vortices are composed of both the stable and unstable modes, which are nearly equal in their amplitudes. The impact of magnetic fields on the nonlinearly excited stable modes is then quantified. Even when imposing a strong magnetic field that almost completely suppresses the instability, the up-gradient transport by the stable modes is at least two-thirds of the down-gradient transport by the unstable modes, whereas for weaker fields, this fraction reaches up to 98%. These effects are persistent with variations in magnetic Prandtl number and forcing strength. Finally, continuum modes are shown to be energetically less important, but essential for capturing the magnetic fluctuations and Maxwell stress. A simple analytical scaling law is derived for their saturated turbulent amplitudes. It predicts the falloff rate as the inverse of the Fourier wavenumber, a property which is confirmed in numerical simulations. Title: Extragalactic Magnetism with SOFIA (SALSA Legacy Program). IV. Program Overview and First Results on the Polarization Fraction Authors: Lopez-Rodriguez, Enrique; Mao, Sui Ann; Beck, Rainer; Borlaff, Alejandro S.; Ntormousi, Evangelia; Tassis, Konstantinos; Dale, Daniel A.; Roman-Duval, Julia; Subramanian, Kandaswamy; Martin-Alvarez, Sergio; Marcum, Pamela M.; Clark, Susan E.; Reach, William T.; Harper, Doyal A.; Zweibel, Ellen G. Bibcode: 2022ApJ...936...92L Altcode: 2022arXiv220501105L We present the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program) with a set of 14 nearby (<20 Mpc) galaxies with resolved imaging polarimetric observations using HAWC+ from 53 to 214 μm at a resolution of 5″-18″ (90 pc-1 kpc). We introduce the definitions of and background on extragalactic magnetism and present the scientific motivation and sample selection of the program. Here we focus on the general trends in the emissive polarization fraction. Far-infrared polarimetric observations trace the thermal polarized emission of magnetically aligned dust grains across the galaxy disks with polarization fractions of P = 0%-15% in the cold, T d = [19, 48] K, and dense, ${\mathrm{log}}_{10}({N}_{{\rm{H}}{\rm\small{I}}+{{\rm{H}}}_{2}}[{\mathrm{cm}}^{-2}])=[19.96,22.91]$ , interstellar medium. The spiral galaxies show a median <P 154 μm> = 3.3% ± 0.9% across the disks. We report the first polarized spectrum of starburst galaxies showing a minimum within 89-154 μm. The falling 53-154 μm polarized spectrum may be due to a decrease in the dust grain alignment efficiency produced by variations in dust temperatures along the line of sight in the galactic outflow. We find that the starburst galaxies and the star-forming regions within normal galaxies have the lowest polarization fractions. We find that 50% (seven out of 14) of the galaxies require a broken power law in the P - ${N}_{{\rm{H}}{\rm\small{I}}+{{\rm{H}}}_{2}}$ and P - T d relations with three different trends. Group 1 has a relative increase of anisotropic random B-fields produced by compression or shear of B-fields in the galactic outflows, starburst rings, and inner bars of galaxies, and groups 2 and 3 have a relative increase of isotropic random B-fields driven by star-forming regions in the spiral arms and/or an increase of dust grain alignment efficiency caused by shock-driven regions or evolutionary stages of a galaxy. *SALSA provides a software repository at https://github.com/galmagfields/hawc and publicly available data at http://galmagfields.com/. Title: Anisotropic cosmic ray diffusion in isotropic Kolmogorov turbulence Authors: Reichherzer, P.; Becker Tjus, J.; Zweibel, E. G.; Merten, L.; Pueschel, M. J. Bibcode: 2022MNRAS.514.2658R Altcode: 2022MNRAS.tmp.1394R; 2021arXiv211211827R; 2022MNRAS.tmp.1386R Understanding the time-scales for diffusive processes and their degree of anisotropy is essential for modelling cosmic ray transport in turbulent magnetic fields. We show that the diffusion time-scales are isotropic over a large range of energy and turbulence levels, notwithstanding the high degree of anisotropy exhibited by the components of the diffusion tensor for cases with an ordered magnetic field component. The predictive power of the classical scattering relation as a description for the relation between the parallel and perpendicular diffusion coefficients is discussed and compared to numerical simulations. Very good agreement for a large parameter space is found, transforming classical scattering relation predictions into a computational prescription for the perpendicular component. We discuss and compare these findings, in particular, the time-scales to become diffusive with the time-scales that particles reside in astronomical environments, the so-called escape time-scales. The results show that, especially at high energies, the escape times obtained from diffusion coefficients may exceed the time-scales required for diffusion. In these cases, the escape time cannot be determined by the diffusion coefficients. Title: A Heating Mechanism via Magnetic Pumping in the Intracluster Medium Authors: Ley, Francisco; Zweibel, Ellen G.; Riquelme, Mario; Sironi, Lorenzo; Miller, Drake; Tran, Aaron Bibcode: 2022arXiv220900019L Altcode: Turbulence driven by AGN activity, cluster mergers and galaxy motion constitutes an attractive energy source for heating the intracluster medium (ICM). How this energy dissipates into the ICM plasma remains unclear, given its low collisionality and high magnetization (precluding viscous heating by Coulomb processes). Kunz et al. 2011 proposed a viable heating mechanism based on the anisotropy of the plasma pressure (gyroviscous heating) under ICM conditions. The present paper builds upon that work and shows that particles can be gyroviscously heated by large-scale turbulent fluctuations via magnetic pumping. We study how the anisotropy evolves under a range of forcing frequencies, what waves and instabilities are generated and demonstrate that the particle distribution function acquires a high energy tail. For this, we perform particle-in-cell simulations where we periodically vary the mean magnetic field $\textbf{B}(t)$. When $\textbf{B}(t)$ grows (dwindles), a pressure anisotropy $P_{\perp}>P_{\parallel}$ ($P_{\perp}< P_{\parallel}$) builds up ($P_{\perp}$ and $P_{\parallel}$ are, respectively, the pressures perpendicular and parallel to $\textbf{B}(t)$). These pressure anisotropies excite mirror ($P_{\perp}>P_{\parallel}$) and oblique firehose ($P_{\parallel}>P_{\perp}$) instabilities, which trap and scatter the particles, limiting the anisotropy and providing a channel to heat the plasma. The efficiency of this mechanism depends on the frequency of the large-scale turbulent fluctuations and the efficiency of the scattering the instabilities provide in their nonlinear stage. We provide a simplified analytical heating model that captures the phenomenology involved. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM. Title: Mechanism for sequestering magnetic energy at large scales in shear-flow turbulence Authors: Tripathi, B.; Fraser, A. E.; Terry, P. W.; Zweibel, E. G.; Pueschel, M. J. Bibcode: 2022PhPl...29g0701T Altcode: 2022arXiv220501298T Straining of magnetic fields by large-scale shear flow, which is generally assumed to lead to intensification and generation of small scales, is reexamined in light of the persistent observation of large-scale magnetic fields in astrophysics. It is shown that, in magnetohydrodynamic turbulence, unstable shear flows have the unexpected effect of sequestering magnetic energy at large scales due to counteracting straining motion of nonlinearly excited large-scale stable eigenmodes. This effect is quantified via dissipation rates, energy transfer rates, and visualizations of magnetic field evolution by artificially removing the stable modes. These analyses show that predictions based upon physics of the linear instability alone miss substantial dynamics, including those of magnetic fluctuations. Title: Fourier analysis of small-scale plasma instabilities within the ICM Authors: Miller, Drake; Ley, Francisco; Zweibel, Ellen Bibcode: 2022AAS...24013916M Altcode: It is known that roughly 99.9% of the matter in the Universe is in the plasma state. The intracluster medium (ICM) is primarily composed of plasma and is continuously heated over time. This heating can arise due to large-scale phenomena such as active galactic nuclei, in addition to small-scale plasma instabilities such as the mirror and firehose instabilities. It is known that these instabilities persist under the production of pressure anisotropy. In this research, two particle-in-cell simulations with different magnetization values are constructed in order to analyze the effects of these instabilities for a particular, sheared magnetic field configuration. A detailed Fourier analysis is conducted for each simulation in order to document relevant plasma wave characteristics in the ICM. The resulting power spectra suggest that the radiated power is dominated by low frequency waves. Furthermore, the results display ion-cyclotron resonances for both instabilities and additional peaks that require a deeper analysis. These plots are compared with the average magnetic field energy fluctuations in order to match the phases of the sheared magnetic field over time with the corresponding plasma instabilities. A discussion regarding the implications for plasma heating is also provided. A proper understanding of these small-scale plasma instabilities can then be directly extended to the context of galaxy clusters and their properties. The ICM plasma has important connections to events such as galaxy mergers, galactic outflows, and relativistic jets. Title: Cosmic Ray and Magnetically Driven Bubbles in Galaxies Authors: Wong, Sherry; Habegger, Roark; Heintz, Evan; Bustard, Chad; Zweibel, Ellen Bibcode: 2022AAS...24030218W Altcode: The Parker instability impacts the evolution of galaxies and gas clouds. Driven by buoyancy, it describes a feedback loop for warping magnetic fields. As the Parker instability evolves, it can cause magnetic field lines to cross and reconnect, accelerating gas bubbles into or away from the disk. How these bubbles form and evolve is significant to understanding how magnetic reconnection can affect the larger galactic system.

This study investigates the behavior of hot, dense gas bubbles accelerated by magnetic reconnection and heated by cosmic rays (CRs). The results shed light on the effects of reconnection and the effectiveness of the code that simulates it. Similar hot bubbles have been observed in the intergalactic medium.

I examine the evolution of these bubbles through a combination of numerical simulations in the magnetohydrodynamic code FLASH and Athena++ using data processing methods in Python. I find evidence confirming the idea that these bubbles are formed by reconnection. I find bubble velocities over time to identify the relevant forces accelerating the bubble against gravity.

The FLASH code does not rigorously model magnetic reconnection, but it is possible to learn about the bubble's behavior after the reconnection event. Findings will inform future development of magnetohydrodynamic simulations and offer nuance to observed reconnection events. I am working to run the simulations with an improved CR package and resolution in Athena++.

Acknowledgements: This research is generously supported by NSF Grant AST 2007323, the L&S STEM Summer of Excellence in Research (LASER), and the Sophomore Research Scholarship at UW-Madison. Title: Galaxies at a Cosmic-Ray Eddington Limit Authors: Heintz, Evan; Zweibel, Ellen Bibcode: 2022arXiv220604082H Altcode: Cosmic rays have been shown to be extremely important in the dynamics of diffuse gas in galaxies, helping to maintain hydrostatic equilibrium, and serving as a regulating force in star formation. In this paper, we address the influence of cosmic rays on galaxies by re-examining the theory of a cosmic ray Eddington limit, first proposed by Socrates et al. (2008) and elaborated upon by Crocker et al. (2021a) and Huang & Davis (2022). A cosmic ray Eddington limit represents a maximum cosmic ray energy density above which the interstellar gas cannot be in hydrostatic equilibrium, resulting in a wind. In this paper, we continue to explore the idea of a cosmic ray Eddington limit by introducing a general framework that accounts for the circumgalactic environment and applying it to five galaxies that we believe to be a good representative sample of the star forming galaxy population, using different cosmic ray transport models to determine what gives each galaxy the best chance to reach this limit. We show that while an Eddington limit for cosmic rays does exist, for our five galaxies, the limit either falls at star formation rates that are much larger or gas densities that are much lower than each galaxy's measured values. This suggests that cosmic ray pressure is not the main factor limiting the luminosity of starburst galaxies. Title: Fermi and eROSITA bubbles as relics of the past activity of the Galaxy's central black hole Authors: Yang, H. -Y. Karen; Ruszkowski, Mateusz; Zweibel, Ellen G. Bibcode: 2022NatAs...6..584Y Altcode: 2022arXiv220302526Y; 2022NatAs.tmp...52Y The newly launched X-ray satellite, eROSITA, has recently revealed two gigantic bubbles extending to ~80° above and below the Galactic Centre. The morphology of these `eROSITA bubbles' bears a remarkable resemblance to the Fermi bubbles previously discovered by the Fermi Gamma-ray Space Telescope and its counterpart, the microwave haze. The physical origin of these striking structures has been intensely debated; however, because of their symmetry about the Galactic Centre, they probably originate from some energetic outbursts from the Galactic Centre in the past. Here we propose a theoretical model in which the eROSITA bubbles, Fermi bubbles and the microwave haze could be simultaneously explained by a single event of jet activity from the central supermassive black hole a few million years ago. Using numerical simulations, we show that this model could successfully reproduce the morphology and multi-wavelength spectra of the observed bubbles and haze, which allows us to derive critical constraints on the energetics and timescales of the outburst. This study serves as an important step forward in our understanding of the past Galactic Centre activity of our own Galaxy and may bring valuable insights into the broader picture of supermassive-black-hole-galaxy co-evolution in the context of galaxy formation. Title: Research Opportunities in Plasma Astrophysics Authors: Bale, Stuart; Bhattacharjee, Amitava; Cattaneo, Fausto; Drake, Jemes; Ji, Hantao; Lee, Marty; Li, Hui; Liang, Edison; Pound, Marc; Prager, Stewart; Quataert, Eliot; Remington, Bruce; Rosner, Robert; Ryutov, Dmitri; Thomas, Edward, Jr; Zweibel, Ellen Bibcode: 2022arXiv220302406B Altcode: Major scientific questions and research opportunities are described on 10 unprioritized plasma astrophysics topics: (1) magnetic reconnection, (2) collisionless shocks and particle acceleration, (3) waves and turbulence, (4) magnetic dynamos, (5) interface and shear instabilities, (6) angular momentum transport, (7) dusty plasmas, (8) radiative hydrodynamics, (9) relativistic, pair-dominated and strongly magnetized plasmas, (10) jets and outflows. Note that this is a conference report from a Workshop on Opportunities in Plasma Astrophysics (WOPA, https://w3.pppl.gov/conferences/2010/WOPA/) in January 2010, that attracted broad representation from the community and was supported by the U.S. Department of Energy, National Aeronautics and Space Administration, National Science Foundation, American Physical Society's Topical Group for Plasma Astrophysics and Division of Plasma Physics, and Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas. Although there has been much planning and many developments in both science and infrastructure since the report was written, most of the motivation, priorities, problems and technical challenges discussed therein remain unaddressed and are relevant at the time of posting. Title: Implications of turbulence dependent diffusion on cosmic ray spectra Authors: Dörner, J.; Reichherzer, P.; Merten, L.; Becker Tjus, J.; Fichtner, H.; Pueschel, M. J.; Zweibel, E. G. Bibcode: 2022epsc.confE..90D Altcode: 2022PoS...398E..90D; 2021arXiv211006676D The propagation of cosmic rays can be described as a diffusive motion in most galactic environments. High-energy gamma-rays measured by Fermi have allowed inference of a gradient in the cosmic-ray density and spectral energy behavior in the Milky Way, which is not predicted by models. Here, a turbulence-dependent diffusion model is used to probe different types of cosmic-ray diffusion tensors. Crucially, it is demonstrated that the observed gradients can be explained through turbulence-dependent energy-scaling of the diffusion tensor. 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. Bibcode: 2022SNAS....4...15R Altcode: 2021arXiv210413093R 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, which is two-fold turbulent mixing in that the particle motion is stochastic with respect to the field lines, needs to be understood in order to properly model cosmic-ray signatures. One of the most important aspects in the modeling of cosmic-ray diffusion is that fully resonant scattering, the most effective such process, is only possible if the wave spectrum covers the entire range of propagation angles. By taking the wave spectrum boundaries into account, we quantify cosmic-ray diffusion parallel and perpendicular to the guide field direction at turbulence levels above 5% of the total magnetic field. We apply our results of the parallel and perpendicular diffusion coefficient to the Milky Way. We show that simple purely diffusive transport is in conflict with observations of the inner Galaxy, but that just by taking a Galactic wind into account, data can be matched in the central 5 kpc zone. Further comparison shows that the outer Galaxy at $>5\,$kpc, on the other hand, should be dominated by perpendicular diffusion, likely changing to parallel diffusion at the outermost radii of the Milky Way. Title: Extragalactic Magnetism with SOFIA (Legacy Program). I. The Magnetic Field in the Multiphase 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.; Dale, Daniel; Legacy Team Bibcode: 2021ApJ...921..128B Altcode: 2021arXiv210509315B 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 whether any significant structural differences are detectable in the kiloparsec-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 154 μm than at 3 and 6 cm; statistically significant lower values for the magnetic pitch angle are measured at FIR in the outskirts (R ≥ 7 kpc) of the galaxy. This difference is not detected in the interarm region. We find strong correlations of the polarization fraction and total intensity at FIR and radio with the gas column density and 12CO(1-0) velocity dispersion. We conclude that the arms show a relative increase of small-scale turbulent B-fields at regions with increasing column density and dispersion velocities of the molecular gas. No correlations are found with H I 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 multiwavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multiphase ISM. * The SOFIA Legacy Group for Magnetic Fields in Galaxies software repository is available at https://github.com/galmagfields/hawc via the official project website, http://galmagfields.com/, and Zenodo/GitHub, https://doi.org/10.5281/zenodo.5116134. Title: VizieR Online Data Catalog: Best-fit emission-line properties in NGC 5775 (Boettcher+, 2019) Authors: Boettcher, E.; Gallagher, J. S.; Zweibel, E. G. Bibcode: 2021yCat..18850160B Altcode: We used the optical and NUV capabilities of Robert Stobie Spectrograph (RSS) in longslit mode on Southern African Large Telescope (SALT). We used a 1.25" width for the 8' longslits and the pg2300 grating at an angle of 48.875°. This produced a dispersion of 0.26Å/pixel, a spectral resolution of R=4830 (σ=26km/s) at Hα, and wavelength coverage from 6100Å to 6900Å. We obtained these data between 2017 February 23 and 2017 March 4.

(4 data files). Title: Cosmic-Ray Transport, Energy Loss, and Influence in the Multiphase Interstellar Medium Authors: Bustard, Chad; Zweibel, Ellen G. Bibcode: 2021ApJ...913..106B Altcode: 2020arXiv201206585B 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 underresolved 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 stairstep 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. 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. Bibcode: 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. Title: The Kinetic Plasma Physics of Cosmic Ray Streaming Instabilities: Hybrid Simulations of the Nonlinear Growth Authors: Haggerty, Colby; Caprioli, Damiano; Zweibel, Ellen Bibcode: 2021APS..APRE10007H Altcode: Cosmic Rays (CRs) are believed to amplify magnetic fields and heat thermal plasma throughout the galaxy via streaming instability. Both theoretical and numerical models of galaxy formation are sensitive to small changes in these plasma parameters, however most of the scientific understanding of the effects of CR streaming instabilities comes from analytical linear theory. We detail the linear and nonlinear effects of both the resonant and nonresonant (Bell) streaming instability using the relativistic kinetic hybrid code, dHybridR. ``Undriven'' simulations (i.e., where CRs are not continuously supplied) agree well with linear theory for a range of wave numbers but with several novel nonlinear features. Additionally, we examine the ``driven'' case (sustained CR injection) in which nonlinear effects are important as the background plasma begins to be heated and pushed, leading to the saturation of the instability. Finally, we extract from the simulations heating rates and self-generated diffusion coefficients, which can be implemented into galaxy formation models. 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. Bibcode: 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 (~2×1011 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. Title: A heating mechanism for high-β plasmas in galaxy clusters Authors: Ley, Francisco; Zweibel, Ellen; Riquelme, Mario; Sironi, Lorenzo Bibcode: 2021APS..DPPJP1005L Altcode: Turbulence driven by supermassive black hole activity, gravitational infall, and galaxy motions is an attractive energy source for heating the intracluster plasma (ICM) in galaxy clusters. However, how this energy dissipates into heat is unclear, since the ICM is collisionless. In this work, we perform particle-in-cell (PIC) simulations of a plasma subject to a periodic variation of the mean magnetic field, B(t), to show that particles can be heated by gyroviscosity via magnetic pumping. When B(t) grows (dwindles), a pressure anisotropy P >P (P >P) builds up due to the adiabatic invariance of the particle's magnetic moment. When initially β=20, the plasma self-regulates its anisotropy by exciting the Mirror (P >P) and Firehose (P >P) instabilities. In this process, both instabilities pitch-angle scatter particles, breaking their adiabatic invariance and providing a channel to efficiently retain some energy in the plasma after one pump cycle, therefore effectively heating the system. The efficiency at which this mechanism acts depends on the level of macroscopic turbulence and how fast the instabilities can be excited and saturate. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM.

FL acknowledges partial support from NSF Grant 2010189. Title: Buoyancy of Cosmic Ray Loaded Magnetic Flux Tubes in the Galactic Disk Authors: Habegger, Roark; Zweibel, Ellen Bibcode: 2021APS..DPPJP1015H Altcode: Interstellar gas in disk galaxies is vertically supported against gravity by the pressure of thermal gas, magnetic fields, and cosmic rays. When nonthermal pressure support exceeds a threshold, the Parker instability can appear. Like the Rayleigh-Taylor instability, over-dense regions sink, and under-dense regions rise. This produces peaks and valleys in the magnetic field. Gravitational energy provides the free energy necessary to compress the interstellar gas into the valleys. Since cosmic rays are unaffected by the galaxy's gravity, they increase the buoyancy of the ISM. However, the cosmic ray fluid has a finite compressibility, increasing the energy required to form valleys. Linear theory suggests this compressibility dominates buoyancy, suppressing the instability. To address this counterintuitive result, we run local simulations of injections of cosmic ray pressure in the galactic disk. This assumes a supernova as the source. If this physically motivated perturbation creates buoyant magnetic flux tubes, then it is likely the Parker instability can develop in the ISM even if instability criteria from linear theory are not met. Simulation results will be presented, along with implications for vertical stability in the ISM.

Funded by NSF Grant AST-2007323. Title: Turbulence-level dependence of cosmic ray parallel diffusion Authors: Reichherzer, P.; Becker Tjus, J.; Zweibel, E. G.; Merten, L.; Pueschel, M. J. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. Title: Cosmic Ray Transport and Calorimetry in the Simulated LMC Authors: Gardipee, J.; Bustard, C.; Zweibel, E. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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 >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. 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. Bibcode: 2020arXiv200400079J Altcode: This white paper summarizes major scientific challenges and opportunities in understanding magnetic reconnection and related explosive phenomena as a fundamental plasma process. Title: The Role of the Parker Instability in Structuring the Interstellar Medium Authors: Heintz, Evan; Bustard, Chad; Zweibel, Ellen G. Bibcode: 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 (γc = 0), locked to the thermal gas (γc = 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. Title: The Role of Pressure Anisotropy in Cosmic-Ray Hydrodynamics Authors: Zweibel, Ellen G. Bibcode: 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. Title: Cosmic Ray Transport and the Galaxy Gas Cycle Authors: Bustard, C.; Zweibel, E.; D'Onghia, E.; Gallagher, J. Bibcode: 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: Heating and acceleration processes in galaxy cluster plasmas Authors: Ley, Francisco; Zweibel, Ellen; Riquelme, Mario; Sironi, Lorenzo Bibcode: 2020APS..DPPP11001L Altcode: Galaxy clusters are the most massive gravitationally bound structures in the Universe. Space among galaxies is filled with hot (β >> 1), weakly collisional plasma, the Intracluster Medium (ICM). The nature of kinetic processes in these plasmas, such as particle energization and heating mechanisms and transport, and the interplay between them and the large-scale dynamics of galaxy clusters are not well understood. We perform Particle in Cell (PIC) simulations of a plasma with an oscillating magnetic field B that is periodically amplified and decreased in magnitude to study the heating and acceleration of particles. Both the amplification and dwindle of B can generate a pressure anisotropy Δ =p -p that heats the plasma by gyroviscosity and is self-regulated by triggering kinetic microinstabilities. When β = 10 initially, both mirror (Δ > 1) and firehose (Δ < 1) instabilities arise, limiting Δ and creating a nonthermal tail. This acceleration mechanism is mediated by the instabilities, but ultimately it acts by extracting energy from the thermal pool and giving it to the nonthermal population of particles. These results are compared with a similar study of a steadily growing magnetic field at lower β (Ley at al. 2019)

FL acknowledges partial support from the National Science Foundation. MR acknowledges partial support from Grant Fondecyt Regular 1191673. 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 Bibcode: 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}-2.8+2.7\circ 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 (<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 (<1%) at both 53 and 89 μm. Title: Cosmic Ray Transport and the Galaxy Gas Cycle Authors: Bustard, C.; Zweibel, E.; D'Onghia, E.; Gallagher, J. Bibcode: 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: 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. Bibcode: 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. Bibcode: 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}z,e=0.6,7.5 kpc) and southwest ({h}z,e=0.8,3.6 kpc) sides of the galaxy at R < 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.

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 Bibcode: 2019MNRAS.489..205W Altcode: 2019MNRAS.tmp.1948W; 2019arXiv190301471W We investigate a mechanism for accelerating cool (104 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-1. Title: Some Recent Results on Cosmic-Ray Feedback Authors: Zweibel, Ellen Bibcode: 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. Bibcode: 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-1 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 1031-1032 cm2 s-1. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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: 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 Bibcode: 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: 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 Bibcode: 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: 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. Bibcode: 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: 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 Bibcode: 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 ($T>1$ 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: [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. Bibcode: 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: Probing the magnetoionized intragroup medium of NGC 2563 Authors: Williams, Anna; Wilcots, Eric; Zweibel, Ellen Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 ∼104 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 Bibcode: 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. Bibcode: 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 c = 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 Bibcode: 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 max = 1016 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 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 Bibcode: 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: High β effects on cosmic ray streaming in galaxy clusters Authors: Wiener, Joshua; Zweibel, Ellen G.; Oh, S. Peng Bibcode: 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 & 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 β1/2. 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: Mock X-ray Observations of Localized LMC Outflows Authors: Tomesh, Teague; Bustard, Chad; Zweibel, Ellen Bibcode: 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: Cosmic Ray Acceleration from Multiple Galactic Wind Shocks Authors: Cotter, Cory; Bustard, Chad; Zweibel, Ellen Bibcode: 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: New Perspectives on the Dynamical State of Extraplanar Diffuse Ionized Gas Layers Authors: Boettcher, Erin; Zweibel, Ellen; Gallagher, John S.; Benjamin, Robert A. Bibcode: 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 < 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: 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. Bibcode: 2017NatAs...1..621M Altcode: 2017arXiv170807844M Magnetic fields play a pivotal role in the physics of interstellar medium in galaxies1, but there are few observational constraints on how they evolve across cosmic time2-7. Spatially resolved synchrotron polarization maps at radio wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies1,8,9 that are believed to grow from a seed field via a dynamo effect10,11. 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: Detection of Extraplanar Diffuse Ionized Gas in M83 Authors: Boettcher, Erin; Gallagher, J. S., III; Zweibel, Ellen G. Bibcode: 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-1 in projection). With interesting implications for isotropy, the velocity dispersion of the diffuse gas, σ =96 km s-1, 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}z=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.

Based on observations made with the Southern African Large Telescope (SALT) under program 2015-2-SCI-004 (PI: E. Boettcher). 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. Bibcode: 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: Interaction of cosmic rays with cold clouds in galactic haloes Authors: Wiener, Joshua; Oh, S. Peng; Zweibel, Ellen G. Bibcode: 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* 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 Bibcode: 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. Bibcode: 2017yCat..18320118B Altcode: No abstract at ADS Title: Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks Authors: Bustard, Chad; Zweibel, Ellen G.; Cotter, Cory Bibcode: 2017ApJ...835...72B Altcode: 2016arXiv161006565B There are two distinct breaks in the cosmic ray (CR) spectrum: the so-called “knee” around 3 × 1015 eV and the so-called “ankle” around 1018 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 × 1015 and 1018 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 1017 eV. We also estimate the luminosities of CRs accelerated by outflow termination shocks, including estimates for the Milky Way wind. 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 Bibcode: 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: Global Simulations of Galactic Winds Including Cosmic-ray Streaming Authors: Ruszkowski, Mateusz; Yang, H. -Y. Karen; Zweibel, Ellen Bibcode: 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: 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. Bibcode: 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}\min ≤slant R≤slant 8 {kpc}, where {R}\min ≥slant 2 {kpc}. We find that the thermal ({σ }{th}=11 km s-1) and turbulent ({σ }{turb}=25 km s-1) velocity dispersions are insufficient to satisfy the hydrostatic equilibrium equation given an exponential electron scale height of {h}z=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 }{cr}=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. Bibcode: 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 Bibcode: 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 & 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 yr-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 χ2 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. Bibcode: 2015JPlPh..81e3401F Altcode: 2015arXiv150607195F > 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$]]> , fully ionized, magnetic-field-free plasma in a spherical geometry. Plasma parameters of e\approx 5$]]> to and e\approx 1011$]]> to 12~\text{cm}-3$]]> 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. Bibcode: 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. Bibcode: 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: 3D Study Of Magnetic Fields In NGC 6946 Authors: Williams, Anna; Heald, George; Wilcots, Eric; Zweibel, Ellen Bibcode: 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: 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. Bibcode: 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-2. 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: Ambipolar Diffusion Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 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. Bibcode: 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: 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. Bibcode: 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. Bibcode: 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: Proton Calorimetry and Gamma-Rays in Arp 220 Authors: Yoast-Hull, Tova; Gallagher, John S.; Zweibel, Ellen Gould Bibcode: 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. Bibcode: 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: The Galactic center: a model for cosmic ray interactions in starburst galaxies? Authors: Yoast-Hull, T.; Gallagher, J. S.; Zweibel, E. Bibcode: 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: Fermi bubble simulations: black hole feedback in the Milky Way Authors: Ruszkowski, M.; Yang, H. -Y. K.; Zweibel, E. Bibcode: 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 & 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: 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. Bibcode: 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: Cosmic Ray Feedback Authors: Zweibel, Ellen Bibcode: 2014glop.progE..14Z Altcode: No abstract at ADS Title: Wide-band Jansky Very Large Array polarization observations of M51 Authors: Mao, Sui Ann; Ott, J.; Zweibel, E. G. Bibcode: 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. Bibcode: 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<-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: Microphysics in Astrophysical Plasmas Authors: Schwartz, Steven J.; Zweibel, Ellen G.; Goldman, Martin Bibcode: 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: Do Cosmic Rays Sample the Mean ISM Density of Starburst Galaxies? Authors: Boettcher, Erin; Zweibel, E. G.; Yoast-Hull, T.; Gallagher, J. S. Bibcode: 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: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 < -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: Microphysics in Astrophysical Plasmas Authors: Schwartz, Steven J.; Zweibel, Ellen G.; Goldman, Martin Bibcode: 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: 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 Bibcode: 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 >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: Astrophysics: Recipe for regularity Authors: Zweibel, Ellen Bibcode: 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. Bibcode: 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" (2013, ApJ, 768, 53) Authors: Yoast-Hull, Tova M.; Everett, John E.; Gallagher, J. S., III; Zweibel, Ellen G. Bibcode: 2013ApJ...771...73Y Altcode: No abstract at ADS Title: Winds, Clumps, and Interacting Cosmic Rays in M82 Authors: Yoast-Hull, Tova M.; Everett, John E.; Gallagher, J. S., III; Zweibel, Ellen G. Bibcode: 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-1. 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. χ2 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-1. 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 microphysics and macrophysics of cosmic rays Authors: Zweibel, Ellen G. Bibcode: 2013PhPl...20e5501Z Altcode: This review paper commemorates a century of cosmic ray research, with emphasis on the plasma physics aspects. Cosmic rays comprise only ∼10-9 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: The Fermi Bubbles: Possible Nearby Laboratory for AGN Jet Activity Authors: Yang, Hsiang-Yi Karen; Ruszkowski, M.; Zweibel, E. G.; Ricker, P. M. Bibcode: 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 Bibcode: 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: Is NGC 253 A Calorimeter? Authors: Yoast-Hull, Tova; Gallagher, J. S.; Zweibel, E. G.; Everett, J. E. Bibcode: 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 & 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: Improved Sound-proof Treatments of Fluid Dynamics in Stellar Interiors Authors: Brown, Benjamin; Vasil, G. M.; Lecoanet, D.; Zweibel, E. G. Bibcode: 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: 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. Bibcode: 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. Bibcode: 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/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. Bibcode: 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: Self-organization of Reconnecting Plasmas to Marginal Collisionality in the Solar Corona Authors: Imada, S.; Zweibel, E. G. Bibcode: 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: Erratum: "Ambipolar Drift Heating in Turbulent Molecular Clouds" (2000, ApJ, 540, 332) Authors: Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke Bibcode: 2012ApJ...755..182P Altcode: No abstract at ADS Title: Cosmic ray production and emission in M82 Authors: Yoast-Hull, Tova; Everett, John; Gallagher, J. S.; Zweibel, Ellen Bibcode: 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 & 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: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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 Bibcode: 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: Modeling magnetic reconnection in partially ionized chromospheric plasmas Authors: Murphy, Nicholas A.; Raymond, John C.; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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: The Plasma Physics of Cosmic Rays Authors: Zweibel, Ellen Bibcode: 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: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 (~10-11 erg cm-3). Title: Magnetic Fields in Galaxies Authors: Zweibel, Ellen G. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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: 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 Bibcode: 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. Bibcode: 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: The Dynamical Role of Cosmic Rays in Galactic Winds Authors: Everett, J.; Zweibel, E.; Hu, K. Bibcode: 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: Ambipolar Diffusion-mediated Thermal Fronts in the Neutral Interstellar Medium Authors: Stone, Jennifer M.; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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)0 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 108years 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: 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. Bibcode: 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| >= 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-2 and +6.3 ± 0.7 rad m-2 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-2 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: Effects of line-tying on magnetohydrodynamic instabilities and current sheet formation Authors: Huang, Yi-Min; Bhattacharjee, A.; Zweibel, Ellen G. Bibcode: 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: Violation of Richardson's Criterion Via Introduction of a Magnetic Field Authors: Lecoanet, Daniel; Zweibel, Ellen G.; Townsend, Richard H. D.; Huang, Yi-Min Bibcode: 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 2 if V'2/4>N 2. 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 & Dorland remains unstable even when V'2/4 < N 2, 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'2/4 < N 2, 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: 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 Bibcode: 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: Synchrotron Constraints on a Hybrid Cosmic-ray and Thermally Driven Galactic Wind Authors: Everett, John E.; Schiller, Quintin G.; Zweibel, Ellen G. Bibcode: 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: Environments for Magnetic Field Amplification by Cosmic Rays Authors: Zweibel, Ellen G.; Everett, John E. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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: 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. Bibcode: 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: Plasma Astrophysics Problems in Star and Planet Formation Authors: Zweibel, Ellen; Goodman, Jeremy; Ji, Hantao; Lazarian, Alex Bibcode: 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: Plasma Physics Processes of the Interstellar Medium Authors: Splanger, Steven; Haverkorn, Marijke; Intrator, Thomas; Kulsrud, Russell; Lazarian, Alex; Redfield, Seth; Zweibel, Ellen Bibcode: 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: 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 Bibcode: 2009astro2010S.116H Altcode: No abstract at ADS 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. Bibcode: 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. Bibcode: 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/φpiSP between the ion skin depth (c/φpi) and the Sweet-Parker width δSP, a fast reconnection rate is observed in MRX [1], and the results are verified by numerical simulations. Since (c/φpi)/δSP is roughly equal to 5 (λmfp/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. Bibcode: 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 Lc, which is independent of the resistivity η. When L is not too much longer than Lc, the growthrate γ is proportional to η . When L is sufficiently long, the tearing mode scaling γ∼&3/5circ; is recovered. The transition from γ∼η to γ∼&3/5circ; occurs at a transition length Lt∼&-2/5circ;. Title: Fast Dynamos in Weakly Ionized Gases Authors: Zweibel, Ellen G.; Heitsch, Fabian Bibcode: 2008ApJ...684..373Z Altcode: The turnover of interstellar gas on ~109 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. Bibcode: 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 108years 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 (4×107K) 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: Evolution of Unmagnetized and Magnetized Shear Layers Authors: Palotti, M. L.; Heitsch, F.; Zweibel, E. G.; Huang, Y. -M. Bibcode: 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: MHD Stability of Phase Transition Layers in the Neutral ISM Authors: Stone, Jennifer M.; Zweibel, E. G. Bibcode: 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.

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.

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.

We acknowledge support from NASA ATP grant NNG051G09G. Title: On the origin of cosmic magnetic fields Authors: Kulsrud, Russell M.; Zweibel, Ellen G. Bibcode: 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.

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.

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.

It is generally accepted that seed fields, whose strength is of order 10-20 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.

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.

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.)

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.

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 Bibcode: 2008camf.confE..10M Altcode: No abstract at ADS Title: Turbulent Ambipolar Diffusion Authors: Zweibel, Ellen Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.

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 Bibcode: 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. Bibcode: 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 Bibcode: 2007ApJ...667..275B Altcode: We present calculations of the continuum ultraviolet radiation field (91.2 nm<λ<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 AV along lines of sight. Typically, the difference in Γ(AV) between the clumpy and uniform clouds increases to orders of magnitude at even modest extinctions (AV~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 RV 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 Bibcode: 2007sftn.confE..41Z Altcode: No abstract at ADS Title: Ambipolar Diffusion in a Turbulent Medium Authors: Zweibel, Ellen Bibcode: 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. Bibcode: 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+0.3-0.1 kpc. At this distance, the observed total flux in the SED corresponds to a luminosity of 2.4+/-0.3×106 Lsolar. 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. Bibcode: 2007ApJ...657..318W Altcode: We present high-resolution observations of G5.89-0.39 in CO(1-->0), 13CO(1-->0), C18O(1-->0), and HCO+(1-->0). We characterize the G5.89-0.39 outflow using the 13CO 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 ~4×10-3 Msolar yr-1, (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 vsound. 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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 ɛ~Bφ/Bz, the critical length for instability Lc-3. The relative increase in the inner layer thickness due to line-tying scales as ɛ-1(Lc/L)2.5. Title: The Weak-Field Limit of the Magnetorotational Instability Authors: Krolik, Julian H.; Zweibel, Ellen G. Bibcode: 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 me/mi<|X|<1, where it can be rather smaller. In this range, field aligned with rotation (X>0) produces slower growth than antialigned field (X<0). The maximum growth rate is generally achieved at smaller and smaller wavelengths as |X| diminishes. When |X|<me/mi, 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. Bibcode: 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: 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 Bibcode: 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: Ideal m=1 internal kink mode in line-tied screw pinch Authors: Huang, Yi-Min; Zweibel, Ellen G.; Sovinec, Carl R. Bibcode: 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 ɛ∼Bφ/Bz, the critical length for instability Lc∼1/3̂. The relative increase in the inner layer thickness due to line-tying scales as (1/ɛ)(Lc/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: The Ionisation Fraction in Clumpy Molecular Clouds Authors: Bethell, T. J.; Zweibel, E. G. Bibcode: 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.

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.

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.

Where appropriate we also solve iteratively for the case where H2 is self-shielding, following the character of the H/H2 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 AV, when compared with homogeneous models.

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.

This research is supported by NSF, NASA and the University of Wisconsin NSF grant. Title: Magnetic fields in galaxies Authors: Zweibel, Ellen G. Bibcode: 2005fdda.conf..115Z Altcode: No abstract at ADS Title: Ambipolar Diffusion in a Turbulent Medium Authors: Zweibel, E. G. Bibcode: 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: Slow Motions in the NGC 1275 (Perseus A) System of Giant Ionized Filaments Authors: Cigan, P.; Gallagher, J.; Zweibel, E. Bibcode: 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 >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.

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: Galactic magnetic fields Authors: Zweibel, Ellen Bibcode: 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: Magnetic Flux Transport in the ISM Through Turbulent Ambipolar Diffusion Authors: Heitsch, Fabian; Zweibel, Ellen G.; Slyz, Adrianne D.; Devriendt, Julien E. G. Bibcode: 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. Bibcode: 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: Turbulence in the Star-forming Interstellar Medium: Steps toward Constraining Theories with Observations Authors: Heyer, Mark; Zweibel, Ellen Bibcode: 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: Magnetoelliptic Instabilities Authors: Lebovitz, Norman R.; Zweibel, Ellen Bibcode: 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: Magnetic Field Evolution in Neutron Star Crusts Due to the Hall Effect and Ohmic Decay Authors: Cumming, Andrew; Arras, Phil; Zweibel, Ellen Bibcode: 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 tswitch~=104yrB-312, where B12 is the magnetic field strength in units of 1012G. 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 tswitch on B12, early ohmic decay can alter the currents down to the base of the crust for B~1011G, neutron drip for B~1012G, and near the top of the crust for B>~1013G. 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 & Reisenegger. Title: Kelvin-Helmholtz Instability in a Weakly Ionized Medium Authors: Watson, C.; Zweibel, E. G.; Heitsch, F.; Churchwell, E. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 106G 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: Fast Reconnection in a Two-Stage Process Authors: Heitsch, Fabian; Zweibel, Ellen G. Bibcode: 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 λΩ as λ1/2Ω. 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 λΩ. Slow recombination or sufficiently large ion pressure leads to slower merging, which scales with λΩ as λ1/2Ω. We derive a criterion for distinguishing these two regimes and discuss applications to the weakly ionized ISM and to protoplanetary accretion disks. 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. Bibcode: 2003IAUS..214..135W Altcode: No abstract at ADS Title: Numerical Simulations of Magnetic Fields in Astrophysical Turbulence Authors: Zweibel, E. G.; Heitsch, F.; Fan, Y. Bibcode: 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. Bibcode: 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. Bibcode: 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 1/2. 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 1/2. 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 Bibcode: 2002kas..confE..64Z Altcode: No abstract at ADS Title: Ambipolar Drift in a Turbulent Medium Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 MEdd), Brown & 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 MEdd. 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 MEdd. We show that these profiles are unstable to buoyancy instabilities when B>~1010-1011 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<10-2 MEdd, 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~10-3 MEdd. 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. Bibcode: 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 < 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. Bibcode: 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 lo = 80.6o, a radius of curvature, Rcc = 7.8 kpc, and the position of the center of curvature to be lcc = -15.4o, 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. Bibcode: 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 5123 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 & 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. Bibcode: 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 Bibcode: 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, HAD=|JXB|2iνin~ B4/(16π2L2Bρi νin), is found to depend on the rms Alfvénic Mach number, MA, and on the average field strength, as HAD~M2A<|B|>4. This implies that the typical scale of variation of the magnetic field, LB, is inversely proportional to MA, which we also demonstrate. Title: Generation of the Primordial Magnetic Fields during Cosmological Reionization Authors: Gnedin, Nickolay Y.; Ferrara, Andrea; Zweibel, Ellen G. Bibcode: 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 B0~10-19 G in the simulation box. There is a relatively well-defined, nearly linear correlation between B0 and the baryonic mass of virialized objects, with B0~10-18 G in the most massive objects (M~109 Msolar) 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. Bibcode: 2000ESASP.445..479P Altcode: 2000sfsl.conf..479P No abstract at ADS 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. Bibcode: 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: Astrophysical Turbulence Authors: Ostriker, Eve C.; Zweibel, Ellen G. Bibcode: 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.

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.

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: Discussion on Dynamos and Magnetic Reconnection Authors: Moffatt, Keith; Zweibel, Ellen Bibcode: 2000astu.confE..21M Altcode: No abstract at ADS Title: Astrophysical Turbulence Authors: Zweibel, Ellen Bibcode: 2000kbls.confE..34Z Altcode: No abstract at ADS Title: Astrophysical Turbulence Authors: Ostriker, Eve C.; Zweibel, Ellen G. Bibcode: 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.

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.

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 Origin of Magnetic Fields in Galaxies Authors: Zweibel, Ellen Bibcode: 2000astu.progE..36Z Altcode: No abstract at ADS Title: Astrophysical Turbulence Authors: Zweibel, Ellen Bibcode: 2000astu.progE..35Z Altcode: No abstract at ADS Title: Fragmentation Instability of Molecular Clouds: Numerical Simulations Authors: Indebetouw, Rémy; Zweibel, Ellen G. Bibcode: 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 Bibcode: 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 Rm 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. Bibcode: 1999JPlPh..62..345U Altcode: Available from http://journals.cambridge.org/bin/bladerunner?REQUNIQ=1105385370&REQSESS=958582&118000REQEVENT=&REQINT1=18609&REQAUTH=0 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. Bibcode: 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 (αc) for the onset of the kink instability. If the initial twist is significantly below the critical limit (α below about 50% of αc), 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: Magnetohydrodynamics problems in the interstellar medium Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 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: The Effects of Large-Scale Convective Structures on Solar Eigenfrequencies Authors: Swisdak, M.; Zweibel, E. Bibcode: 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: Absorption/Emission of Solar p-Modes: Michelson Doppler Interferometer/SOHO Observations Authors: Haber, Deborah; Jain, Rekha; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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 Bibcode: 1999intu.conf..232Z Altcode: No abstract at ADS Title: Gravitational Instability in a Cold Magnetized Molecular Cloud with Ambipolar Drift Authors: Indebetouw, R.; Zweibel, E. G. Bibcode: 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: The Effects of Convective Structures on P Mode Frequencies Authors: Swisdak, M.; Zweibel, E. G. Bibcode: 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: Plasma Physics Problems of the Interstellar Medium Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 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. Bibcode: 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: Two-dimensional Simulations of Buoyantly Rising, Interacting Magnetic Flux Tubes Authors: Fan, Y.; Zweibel, E. G.; Lantz, S. R. Bibcode: 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 & 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: Fast reconnection of weak magnetic fields Authors: Zweibel, Ellen G. Bibcode: 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: Effects of Convection on Solar $p$ Modes Authors: Swisdak, M.; Zweibel, E. Bibcode: 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: Probing Convective Effects on P modes using the Adiabatic Switching Method Authors: Swisdak, M.; Zweibel, E. Bibcode: 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 & 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., & Cary, J. 1988, Comp. Phys. Reports, 8, 221. Title: Current Sheet Formation in the Interstellar Medium: Erratum Authors: Zweibel, Ellen G.; Brandenburg, Axel Bibcode: 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. Bibcode: 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: 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. Bibcode: 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: 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. Bibcode: 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: Current Sheet Formation in the Interstellar Medium Authors: Zweibel, Ellen G.; Brandenburg, Axel Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 vD 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. Bibcode: 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. Bibcode: 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: Simulations of Magnetic Field Effects on Solar Oscillation Ring Diagrams Authors: Hill, F.; Zweibel, E.; Haber, D. Bibcode: 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: Effects of Convection on Solar P Modes Authors: Swisdak, M.; Zweibel, E. Bibcode: 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: The Solar Wind. (Book Reviews: Interplanetary Magnetohydrodynamics.) Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 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. Bibcode: 1996ASPC...97..486Z Altcode: 1996pim..conf..486Z No abstract at ADS Title: Going with the flow? Authors: Zweibel, Ellen Bibcode: 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. Bibcode: 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.

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: Is There a Seismic Signature of the Sun's Magnetic Field? Authors: Zweibel, E. G.; Gough, D. Bibcode: 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: Changes in the Upper Turning Point due to Magnetism Authors: Jain, R.; Hindman, B. W.; Zweibel, E. G. Bibcode: 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: Non-Local Transport in Turbulent MHD Convection Authors: Miesch, M.; Brandenburg, A.; Zweibel, E.; Toomre, J. Bibcode: 1995ESASP.376b.253M Altcode: 1995help.confP.253M; 1995soho....2..253M No abstract at ADS Title: Driven Acoustic Oscillations Within a Vertical Magnetic Field Authors: Hindman, B. W.; Zweibel, E. G.; Cally, P. S. Bibcode: 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. Bibcode: 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: Astrophysical Discontinuities. (Book Reviews: Spontaneous Current Sheets in Magnetic Fields. With Applications to Stellar X-Rays.) Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 1995Sci...267.1367P Altcode: No abstract at ADS Title: Equipartition of Energy for Turbulent Astrophysical Fluids: Accounting for the Unseen Energy in Molecular Clouds Authors: Zweibel, Ellen G.; McKee, Christopher F. Bibcode: 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. Bibcode: 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: Alfven Waves in Interstellar Gasdynamics Authors: McKee, Christopher F.; Zweibel, Ellen G. Bibcode: 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, Pw, is isotropic. Furthermore, Pw is directly observable through the non- thermal line width σnt; for a randomly oriented field, Pw = (3/2)ρσ2nt. 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 Δ ln Pw = γwΔ ln ρ with γw = 3/2. In a spatially nonuniform cloud, γw 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 ρ2, 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.

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 Pw(r) ∝ ρ(r)γρ) with γρ = 1/2. This generalizes the result obtained by Fatuzzo & Adams and is consistent with observations showing that molecular clouds have velocity dispersions that increase outward. The polytropic index γρ for Alfvén waves differs substantially from the adiabatic index γw which has implications for the gravitational stability of molecular clouds. Title: The Theory of the Galactic Magnetic Field Authors: Zweibel, E. G. Bibcode: 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, B. W.; Zweibel, E. G. Bibcode: 1995ASPC...76..366H Altcode: 1995gong.conf..366H No abstract at ADS 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. Bibcode: 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: The Effects of a Hot Outer Atmosphere on Acoustic-Gravity Waves Authors: Hindman, Bradley W.; Zweibel, Ellen G. Bibcode: 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)1/2, 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: Shock Propagation and the Generation of Magnetohydrodynamic Wave Fields in Inhomogeneous Molecular Clouds Authors: Miesch, Mark S.; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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 & 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) Pc. At the onset of nonlinearity the fractional amplitude of cosmic-ray pressure perturbations is delta PC/PC approximately (kL) -1 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. Bibcode: 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. Bibcode: 1994ASIC..422...73Z Altcode: 1994coma.conf...73Z No abstract at ADS Title: The tangled web of magnetism Authors: Zweibel, Ellen Bibcode: 1993Natur.362..591Z 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. Bibcode: 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: Magnetic Fields in Star-Forming Regions - Theory Authors: McKee, C. F.; Zweibel, E. G.; Goodman, A. A.; Heiles, C. Bibcode: 1993prpl.conf..327M Altcode: No abstract at ADS Title: Dynamos with ambipolar diffusion. Authors: Zweibel, E. G.; Proctor, M. R. E. Bibcode: 1993spd..conf..355Z Altcode: No abstract at ADS Title: On the Virial Theorem for Turbulent Molecular Clouds Authors: McKee, Christopher F.; Zweibel, Ellen G. Bibcode: 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: The Effect of Line Tying on Parker's Instability Authors: Zweibel, Ellen G.; Bruhwiler, David L. Bibcode: 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: Energy spectrum of particles accelerated near a magnetic x line Authors: Bruhwiler, D. L.; Zweibel, Ellen G. Bibcode: 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, 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=0. Title: Magnetohydrodynamic Waves in Sharply and Smoothly Bounded Cylinders Authors: Shulman, Bonnie J.; Zweibel, Ellen G. Bibcode: 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: Book Review: Plasma loops in the solar corona / Cambridge U Press, 1991 Authors: Cram, L. E.; Currant, C. J.; Loughhead, R. E.; Zweibel, E. Bibcode: 1992Sci...255..857C Altcode: No abstract at ADS Title: The Loop Paradigm. (Book Reviews: Plasma Loops in the Solar Corona.) Authors: Zweibel, Ellen Bibcode: 1992Sci...255..857B Altcode: No abstract at ADS Title: Dynamos with ambipolar diffusion drifts Authors: Proctor, Michael R. E.; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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: Spinning a tangled web Authors: Zweibel, Ellen Bibcode: 1991Natur.352..755Z Altcode: No abstract at ADS Title: Cosmic-Ray Heating of Cooling Flows: A Critical Analysis Authors: Loewenstein, Michael; Zweibel, Ellen G.; Begelman, Mitchell C. Bibcode: 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. Bibcode: 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: Expansion of a Superbubble in a Uniform Magnetic Field Authors: Ferriere, Katia M.; Mac Low, Mordecai-Mark; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 1990BAAS...22.1226D Altcode: No abstract at ADS Title: Virial Theorem Analysis of the Structure and Stability of Magnetized Clouds Authors: Zweibel, Ellen G. Bibcode: 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 Bibcode: 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 Bo 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 Bo. 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. Bibcode: 1990BAAS...22..750F Altcode: No abstract at ADS Title: Virial Theorem Analysis of the Structure and Stability of Magnetized Clouds Authors: Zweibel, Ellen G. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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: Magnetohydrodynamic Waves and Particle Acceleration in a Coronal Loop Authors: de La Beaujardiere, Jean-Francois; Zweibel, Ellen G. Bibcode: 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: Particle acceleration by magnetosonic waves in a coronal loop. Authors: de La Beaujardière, J. -F.; Zweibel, E. G. Bibcode: 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: Hydromagnetic wave heating of low density interstellar gas Authors: Zweibel, E. G.; Ferriere, K. M.; Shull, J. M. Bibcode: 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. Bibcode: 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 Bibcode: 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: Ambipolar Diffusion Drifts and Dynamos in Turbulent Gas Authors: Zweibel, Ellen G. Bibcode: 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 Growth of Magnetic Fields Prior to Galaxy Formation Authors: Zweibel, Ellen G. Bibcode: 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: The Scattering of Alfven Waves by Density Fluctuations Authors: Li, He-Sheng; Zweibel, Ellen G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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: The Theory of the Galactic Magnetic Field Authors: Zweibel, Ellen G. Bibcode: 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: Propagation of Compressive Waves through Fibril Magnetic Fields Authors: Bogdan, T. J.; Zweibel, Ellen G. Bibcode: 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: Is Nonthermal Bremsstrahlung Important in Young Supernova Remnants? Authors: Hamilton, A. J. S.; Zweibel, E. G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1985BAAS...17..855C Altcode: No abstract at ADS Title: Evolution of twisted magnetic fields Authors: Zweibel, E. G.; Boozer, A. H. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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: Confinement of cosmic rays in molecular clouds Authors: Zweibel, E. G.; Shull, J. M. Bibcode: 1982ApJ...259..859Z Altcode: (Previously announced in STAR as N82-20073) 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. Bibcode: 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: A sufficient condition for the stability of atmospheres with magnetic fields Authors: Zweibel, E. G. Bibcode: 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. Bibcode: 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. Bibcode: 1981JGR....8611117R Altcode: No abstract at ADS Title: Confinement of cosmic rays in molecular clouds Authors: Zweibel, E. G.; Shull, J. M. Bibcode: 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. Bibcode: 1981plap.rept..371Z Altcode: The consequences of cosmic ray production by a supernova in a molecular cloud are discussed. Self-trapping problems for a higher flux of cosmic rays in a molecular cloud are focused on. The column density of molecular clouds is probably too high to explain the majority of galactic cosmic ray sources, even allowing for fortuitous asymmetry in the placement of the supernova in the cloud, however, measurements of antiproton flux suggest that some cosmic ray sources do have a high column density. The large predicted gamma-ray luminosity of such a cloud invites comparison with COS-B sources. Title: MHD instabilities of atmospheres with magnetic fields Authors: Zweibel, E. G. Bibcode: 1981ApJ...249..731Z Altcode: The MHD energy principle is used to derive a stability criterion for two-dimensional equilibrium systems consisting of magnetized plasma in a gravitational field. The interplay between magnetic and gravitational forces in determining stability is studied, and the criterion is applied to a family of two-dimensional magnetostatic atmospheres described by Zweibel and Hundhausen (1980). All members of the family except the potential field are found to be locally unstable to horizontal displacements, and the instabilities are explained in terms of pinching, interchange, and Rayleigh-Taylor effects. Title: Thermal Stability of a Corona Heated by Fast Mode Waves Authors: Zweibel, E. Bibcode: 1980SoPh...66..305Z Altcode: It has been proposed that dissipation of hydromagnetic waves is an important heat source for the solar corona. We consider damping by collisionless processes and by electron thermal conduction and ion viscosity, and calculate the wave energy density such that heating balances the energy radiated by the plasma. We then analyze the thermal stability of the wave heated medium. The fastest growing instabilities are condensations perpendicular to the fieldlines. The instability may be important for producing coronal fine structure, and in loops and streamers. Title: Thermal Instability of the Solar Corona Authors: Zweibel, E. G. Bibcode: 1979BAAS...11..697Z Altcode: No abstract at ADS Title: Energetic particle trapping by Alfven wave instabilities Authors: Zweibel, E. G. Bibcode: 1979AIPC...56..319Z Altcode: 1979pama.work..319Z A dispersion relation is derived for Alfven waves which interact resonantly with energetic particles in a plasma. It is argued that for the particle fluxes expected for cosmic ray production in supernova remnants, the waves are unstable and scatter the particles strongly. The particles remain in the expanding plasma and lose energy adiabatically. These energy losses pose a problem for theories of cosmic ray acceleration in supernova remnants. Observations of solar energetic particle propagation are found to be consistent with the properties of the instability. Title: Convective instability of thin flux tubes. Authors: Spruit, H. C.; Zweibel, E. G. Bibcode: 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/B2; if β ≤ βc the tube is convectively stable, otherwise it is unstable, where the critical value βc depends on the stratification of the convection zone. For a solar convection zone model, βc = 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. Title: The equilibrium of cool stellar disks. Authors: Zweibel, E. Bibcode: 1978ApJ...222..103Z Altcode: A system of finite infinitesimally thin self-gravitating disks is constructed. The sequence begins with the uniformly rotating disk and approaches the isothermal disk, and the latter members of the sequence resemble real disk galaxies. A mathematical description of cool stellar disks is presented for application to model galaxies in which the ratio of random to mean velocity is small, except near the center. Phase-space moments of the modified Schwarzschild stellar distribution function are shown to have an expansion in powers of the ratio of the star's random kinetic energy to gravitational potential energy Title: Radial modes of oscillation of cool stellar disks. Authors: Zweibel, E. Bibcode: 1978ApJ...222..110Z Altcode: Equations describing long wavelength radial modes of oscillation for stellar disks are derived. The approach is applied to determine the density response of a cool stellar disk to radial potential perturbations and is exact for disks with zero velocity dispersion. The formulation may be applied to finite or infinite disks of arbitrary structure, so long as the length scale of the perturbation is larger than the epicyclic amplitude. Three functions - the unperturbed surface density, the epicyclic frequency, and the radial velocity dispersion - are involved in the expression of the equilibrium structure. The effects of differential rotation, halos, and velocity dispersion on axisymmetric stability are studied for some disk models. Title: The Equilibrium and Radial Oscillations of Cool Stellar Disks. Authors: Zweibel, E. G. Bibcode: 1977PhDT.........1Z Altcode: Observed stars which lie in axisymmetric disks in spiral galaxies were considered. Equations used to study the radial modes of oscillation of these cool stellar disks of arbitrary structure were derived. Cool disk equilibrium was studied along with the response of a cool stellar disk to an axisymmetric potential perturbation of longer wavelength than the radial average stellar orbit. The linearized Vlasov equation and Poisson's equation were used in the later calculations. Title: The equilibrium and radial oscillations of cool stellar disks Authors: Zweibel, Ellen Gould Bibcode: 1977PhDT........21Z Altcode: No abstract at ADS Title: Radiative processes affecting the abundance of interstellar OH. Authors: Smith, W. H.; Zweibel, E. G. Bibcode: 1976ApJ...207..758S Altcode: Photodissociation of OH dominates its destruction for a significant range of density and shielding in H I regions. Although inverse predissociation may provide an observable flux of ultraviolet recombination photons, it can provide only a small fraction of the total observed OH in most H I regions. Title: The stabilizing effects of cloud reacceleration; microturbulence, and rotation on Parker's instability. Authors: Zweibel, E. G.; Kulsrud, R. M. Bibcode: 1975ApJ...201...63Z Altcode: Parker's instability is reconsidered with the inclusion of a model equation of state for the gas of interstellar clouds, a tangled interstellar field, and galactic rotation. It is found that the instability is stabilized for most choices of interstellar parameters consistent with observation. For the remaining choices which can lead to instability, the growth time is longer than 100 million years. It is concluded that the instability can have little to do with cosmic-ray escape and probably cannot destroy a primordial galactic field. Title: The trapping of cosmic rays around supernovae by plasma instabilities Authors: Kulsrud, R.; Zweibel, E. Bibcode: 1975prnc.reptS....K Altcode: Cosmic rays escaping from supernova as sources set up hydromagnetic instabilities. The instabilities can trap the cosmic rays in the neighborhood of the supernova until they are adiabatically decompressed. The importance of these instabilities for theories of supernova origin of cosmic rays are discussed. Title: On the Question of the Super-nova Origin of Cosmic Rays Authors: Zweibel, E. G.; Kulsrud, R. M. Bibcode: 1975BAAS....7..553Z Altcode: No abstract at ADS Title: The Trapping of Cosmic Rays Around Super-novae by Plasma Instabilities Authors: Kulsrud, R.; Zweibel, E. Bibcode: 1975ICRC....2..465K Altcode: 1975ICRC...14..465K No abstract at ADS Title: On the Theory of H_{2} Rotational Excitation Authors: Spitzer, Lyman, Jr.; Zweibel, Ellen Gould Bibcode: 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) < 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) > 102] and possibly also hot (T 102). Snbject headings: interstellar matter - molecules, interstellar