Talk Abstracts

I. Basic Plasma Processes and Numerical Methods

THE NONLINEAR ALPHA-OMEGA DYNAMO

Ethan Vishniac *

Johns Hopkins University, USA

The kinematic dynamo is limited by "alpha-quenching", in which the accumulation of magnetic helicity in small-scale structures turns off the dynamo process. Continued dynamo activity depends on the small-scale magnetic helicity current. I will discuss how this works for disk systems, concentrating in particular on systems where the disk turbulence is driven by the magnetorotational instability. This leads to strong large-scale magnetic fields in accretion disks systems, and possibly galactic disks.

LABORATORY STUDY OF MAGNETIC RECONNECTION: RECENT PROGRESS

M. Yamada *

Princeton University, USA

MAGNETIC RECONNECTION

A. Lazarian *

University of Wisconsin, USA

Collisionless magnetic reconnection

Franco Porcelli*

Burning Plasma Research Group, Politecnico di Torino, Italy

Recent progress in magnetic reconnection theory in weakly collisional and collisionless regimes is reviewed. Magnetic reconnection can often be considered as a two-dimensional phenomenon in which oppositely directed field lines spontaneously merge together. In many cases of interest, there is also an essentially uniform magnetic guide field, directed perpendicular to the merging field lines. Most fluid treatments of the reconnection process adopt reduced MHD models, where the compressional Alfven wave is decoupled from the reconnective dynamics. This standard reduction procedure is normally valid in the limit of a strong guide field. A new set of reduced equations governing 2D, two-fluid, collisionless magnetic reconnection is presented. These equations are valid for arbitrary values of the magnetic guide field. This represents a significant advance in magnetic reconnection theory, as it allows to bridge the limiting regimes where collisionless reconnection is mediated by either whistler waves at low guide fields (where the Hall term in the generalized Ohm law plays an important role) or by kinetic Alfven waves (where electron compressibility along the field lines becomes important). Indeed, the model exhibits a single scale length, which we denote by , where , is the plasma beta parameter (kinetic pressure/magnetic pressure) based on the magnetic guide field (i.e., is large when the guide field is weak) and is the ion skin depth. In the strong guide field limit, and , the ion sound Larmor radius. In the opposite limit of weak guide field, . On the basis of this model, basic questions on the theory of collisionless reconnection are revisited. These questions include the scaling of the reconnection rate with microscopic parameters, the non-dissipative transfer of magnetic energy through a phase mixing process, and the difference in reconnection with and without a guide field.

II. Basic Plasma Processes and Numerical Methods

THERMAL INSTABILITIES IN PLASMAS

Shu-ichiro Inutsuka(1)*, Hiroshi Koyama (2)

(1) Kyoto University, Japan

(2) Kobe University

The role of thermal instability in the generation and maintenance of tubulence in magnetized interstellar medium is investigated. The analysis of the propagation of a shock wave into atomic interstellar medium shows that the thermal instability in the post-shock gas in the interstellar medium produces high-density molecular cloudlets embedded in warm neutral medium. The molecular cloudlets have velocity dispersion which is supersonic with respect to the sound speed of the cold medium but is subsonic with respect to the warm medium. The stability of the interface between warm medium and cold medium is studied in detail, and the dissipation processes of the turbulence in interstellar medium are analyzed.

FLUID DESCRIPTION FOR DISPERSIVE MHD WAVES IN A COLLISIONLESS PLASMA

T. Passot* and P.L. Sulem

Observatoire de la Cote d'Azur, France

The origin and nature of the small-scale fluctuations observed by radio-wave scintillation mostly in the warm and hot (intercloud) ionized phases of the interstellar medium, as well as in HII regions, is still a matter of debate. The connection between this small-scale turbulence and the cascade originating from the large-scale motions, the role of these fluctuations on cosmic ray scattering and acceleration or vice-versa the impact of cosmic ray streaming instabilities in generating such fluctuations, are still open issues. Theirs scales extend beyond the ion Larmor radius and well below the ion-neutral and Coulomb mean free paths. In order to investigate this regime of collisionless plasma turbulence, we have developed a dispersive Landau fluid model that extends usual MHD in presence of a strong ambient magnetic field by retaining main kinetic processes such as Landau damping and finite Larmor radius corrections. After briefly recalling the main steps and assumptions involved in the derivation of this monofluid model, we present recent numerical simulations that address, in a slab geometry, the damping of fast magnetosonic waves, the formation of solitonic structures, and the parametric instabilities of parallel propagating Alfven waves. Large-scale mirror instability in anisotropic plasma is also correctly reproduced. Validations are made by comparisons with analytic predictions of kinetic theory and with hybrid simulations. New results will be presented concerning the nonlinear phase of Alfven decay instability leading to a dominant ion heating, but also to a non-neglibible electron temperature increase associated with magnetosonic Landau damping involved in the saturation of this instability. In contrast with particle methods, the Landau-fluid approach, which captures electron heating, also permits the simulation of turbulent regimes involving extended range of scales and long-time evolution. Further developments concern the modelization of particle trapping and small-scale stabilization of mirror modes.

ENERGETICS OF FORCED MAGNETIC RECONNECTION

Grigory Vekstein

Department of Physics and Astronomy, The University of Manchester, UK

Unlike spontaneous magnetic reconnection via MHD instability such as the tearing mode, forced reconnection occurs in an MHD-stable magnetic field subjected to external perturbation. This brings about relaxation of the system to a state of lower magnetic energy, which has important implications for many astrophysical objects (for example, the solar corona). Here we discuss various aspects of the magnetic energy release and plasma heating by forced reconnection in a force-free magnetic field.

ATHENA - A NEW CT-GODUNOV MHD CODE

Thomas Gardiner, James Stone

Princeton University, USA

In recent years there has been an increased emphasis on applying high order Godunov-type algorithms to the system of ideal MHD. This is motivated by their strong shock capturing and their conservation properties which make them ideally suited for use in combination with adaptive mesh refinement. Such efforts, however, have traditionally met with difficulty owing to the divergence free constraint on the magnetic field. Applying the method of Constrained Transport, we have constructed a second order accurate, conservative, unsplit, Godunov-type integration algorithm for MHD. This integration algorithm includes two novel features, 1) the incorporation of MHD source terms in the PPM-type reconstruction procedure and 2) an upwind CT-algorithm for combining the Godunov fluxes to calculate the electric fields needed for CT. We present a detailed description of this integration algorithm and a variety of test calculations to demonstrate its effectiveness. Along the way we highlight recent results obtained by the application of this algorithm to problems of astrophysical interest. Following the Zeus prototype, the source code and documentation will be made freely available to the astrophysics community. http://www.astro.princeton.edu/ ~jstone/athena.html

III. Space and Solar Plasmas

ALFVEN INTERMITTENT TURBULENCE IN SPACE PLASMAS

Abraham C.-L. Chian(1,2)*

Erico L. Rempel(1,2)

Félix A. Borotto (2,3)

(1) National Institute for Space Research (INPE), P.O. Box 515, S.J. Campos-SP 12227-010, Brazil

(2) World Institute for Space Environment Research (WISER), www.cea.inpe.br/wiser

(3) Universidad de Concepción, Concepción, Chile

Alfven intermittent turbulence is ubiquitous in the solar system and play an important role in the dynamics and structure of solar atmosphere, solar wind and planetary magnestospheres. For example, large-amplitude interplanetary Alfven waves are relevant for space weather since they can cause intense auroral and geomagnetic activities. Dynamical systems approach provides a powerful tool to understand the origin and nature of nonlinear phenomena such as Alfven intermittent turbulence in space plasmas. We show how Alfven intermittent chaos and turbulence appear due to local and global bifurcations in the numerical solutions of nonlinear Alfven wave equations. The origin of Alfven intermittent turbulence is elucidated by introducing the concept of chaotic saddles embedded in a chaotic attractor. Our theory can account for the power-law spectra observed in Alfven intermittent turbulence in space plasmas and improve the prediction of space weather.

- Chian, A.C.-L., Borotto, F.A., Gonzalez, W.D., Alfven intermittent turbulence driven by temporal chaos, Astrophys. J. 505, 993 (1998).

- Chian, Borotto, F.A., Rempel, E.L., Alfven boundary crisis, Int. J. Bifurcation Chaos 12, 1653 (2002).

- Chian, A.C.-L. et al., High-dimensional interior crisis in the Kuramoto-Sivashinsky equation, Phys. Rev. E 65, 035203 (2002).

- Rempel, E.L., Chian, A.C.-L., High-dimensional chaotic saddles in the Kuramoto-Sivashinsky equation, Phys. Lett. A 319, 104 (2003).

DISPERSIVE AND NONLINEAR ALFVEN WAVES IN SPACE PLASMAS

K. Stasiewicz

Swedish Institute of Space Physics, Uppsala, Sweden

We present a review of observations and theories of low frequency Alfvén and magnetosonic waves in media of a wide range of plasma parameter: from cold ionospheric plasma, intermediate magnetospheric plasma to high- plasmas of the magnetosheath and the bow shock. We discuss dispersion effects caused by the electron inertia and finite ion gyroradius on the wave propagation, interactions with ions and electrons, and emergence of strongly nonlinear structures as solitary waves, solitons, shocklets and oscillitons. We provide detailed comparison of theoretical models and the observed nonlinear structures, which can reach amplitudes of ten times the ambient field values in a high- regime. We provide evidence on the electron energisation and ion heating related to dispersive and nonlinear Alfvén waves in the terrestrial environment and discuss possible applications of these mechanisms in solar physics and in astrophysical context.

SOLAR RADIO ASTRONOMY

Adriana V. R. Silva*

CRAAM - Universidade Presbiteriana Mackenzie, SP, Brazil

According to their timescale, emission from the Sun can be divided into three types. There is the slowly varying emission which changes on a timescale of 11 years and follows the solar cycle. A more rapidly varying radiation on timescale of weeks, known as quiescent emission, is related to the occurrence of active regions. Last but not least is the very abrupt emission produced by solar activity such as flares and coronal mass ejections on timescales of seconds to hours. All three kinds of behavior can also be observed at radio wavelengths, each being produced mostly by different mechanisms. At microwave and millimeter wavelengths, the quiet Sun emission, which varies on 11 years timescale, is due to thermal bremsstrahlung. Nevertheless, the quiescent radio emission has a strong contribution from thermal gyroresonance especially at microwaves, whereas thermal bremsstrahlung dominates the millimetric and submillimetric waves. On the other hand, gyrosynchrotron radiation from nonthermal electrons is the main mechanism producing the flare emission at wavelengths shorter than cm, whereas the metric and decimetric emission from radio bursts are due to coherent plasma radiation. Especial emphasis will be given to recent results from the Brazilian solar radio groups at the various wavelengths.

SOLAR CORONAL LOOPS AND CORONAL HEATING: WHAT CAN OBSERVATIONS TELL US ABOUT CORONAL HEATING?

Cristina H. Mandrini*

IAFE, CC. 67, Suc. 28, 1428 Buenos Aires, Argentina

Ever since it was realized, some 65 years ago, that the solar corona is 3 orders of magnitude hotter than the underlying photosphere, scientist have puzzled over the reason for these conditions. A number of plausible ideas have been proposed, including the dissipation of magnetohydrodynamic (MHD) waves and the dissipation of stressed, current-carrying magnetic fields. Unfortunately, the conversion of magnetic to thermal energy occurs on spatial scales that are far too small to be observed directly by present-day solar instruments, and it has been proven extremely difficult to identify the exact cause of the heating. We will describe how we can combine coronal observations with other indirect means (e.g. the investigation of the height dependence of the heating rate, the determination of how the heating rate correlates with observable physical parameters) to discriminate between the models that have so far been proposed to explain the coronal heating problem.

SOLAR CORONAL LOOP OSCILLATIONS: THEORY OF RESONANTLY DAMPED OSCILLATIONS AND COMPARISON WITH OBSERVATIONS

Marcel Goossens

Centre Plasma Astrophysics K.U.Leuven, Belgium

One of the proposed damping mechanisms of coronal transverse loop oscillations in the kink mode is resonant absorption as a result of the spatial variation of the Alfvén velocity in the equilibrium configuration. Analytical expressions for the period and the damping time exist for 1-D cylindrical equilibrium models with thin nonuniform transitional layers. Comparison with observations indicates that the assumption of thin nonuniform transitional layers is not a very accurate approximation of reality. This contributions starts with a short review of observations on transverse oscillations in solar coronal loops. Then it presents results on periods and damping times of resonantly damped kink mode oscillations for (i) fully non- uniform 1-D cylindrical equilibrium models in which the equilibrium quantities vary in the radial direction across the magnetic field from the centre of the loop up to its boundary and (ii) non-uniform 2-D cylindrical equilibrium models in which the equilibrium quantities vary both in the radial direction across the magnetic field and in the axial direction along the magnetic field. An important point is that the periods and damping times obtained for these fully non-unform models can differ substantially from those obtained for thin nonuniform transitional layers. This contribution then reports on a consistency test between theory and observations showing that there is a very good agreement within the observational inaccuracies.

IV. Space and Solar Plasmas, Basic Plasma Processes and Numerical Methods

MHD FLARES AND JETS IN THE SUN, STARS, AND ACCRETION DISKS

K. Shibata*

Kyoto University, Japan

Recent space observations of the Sun with Yohkoh, SOHO, and TRACE have revealed that the solar corona and underlying atmosphere are much more dynamic than had been thought; they are full of dynamic phenomena such as jets, plasmoid ejections, microflares, nanoflares, and so on. It has also been found that they are more or less caused by magnetic reconnection so that a unified view has emerged on the origin of solar flare-like phenomena including nanoflares, microflares, flares, and coronal mass ejections. In this talk, I will first discuss how these solar flares and flare-like phenomena are modeled by MHD simulations, and then discuss the application of unified reconnection model of solar flares to stellar flares, especially to flares in pre-main sequence stars. These studies revealed that the mass ejection and jet are closely associated with magnetic reconnection (i.e., flares). Finally, a discussion is given on the application of the same physical processes to accretion disks and jets in active galactic nuclei and gamma ray bursts.

THE RELATIONSHIP BETWEEN SYNCHROTRON AND BREMSSTRAHLUNG EMISSION OF NONTHERMAL ELECTRONS, DURING A SOLAR FLARE. A

DETAILED STUDY DURING THE AUGUST 30, 2002 X1.5 EVENT

Carlos G. Giménez de Castro (1), Adriana V.R Silva (1), Gerard Trottet (2),

Säm Krucker (3), Joaquim E.R. Costa (4), Pierre Kaufmann (1,5), Emilia Correia (4),

Thomas Lüthi ,(6) Andreas Magun (6), Hugo Levato (7)

(1) Centro de Rádio Astronomia e Astrofísica Mackenzie, Univ. Presbiteriana Mackenzie. Brazil.

(2) Observatoire de Paris, Section de Meudon. France.

(3) Space Sciences Laboratory, University of California. USA.

(4) Centro de Radio Astronomia e Astrofísica Mackenzie, Inst. Nac. de Pesquisas Espaciais, Brazil

(5) Centro de Componentes Semicondutores, Universidade Estadual de Campinas. Brazil.

(6) Institute of Applied Physics, University of Bern. Switzerland.

(7) Complejo Astronomico El Leoncito. Consejo Nac. de Investigaciones Cientif. y Tec., Argentina.

Synchrotron emission from non-thermal electrons has a strong dependence on the magnetic field of the medium. On the contrary, Bremsstrahlung emission does not depend on the magnetic field. The simultaneous observations of both forms of radiation may give clues about local magnetic field configuration. In this report we use the optically thin part of the radio spectrum during the microwave maximum of the flare occurred on August 30, 2002, at 1328 UT to determine different mean magnetic field intensities and nonthermal electron density distributions compatibles with the observed data. Assuming that the same electrons emit by coulomb interactions, the obtained distributions are used to compute the photon spectrum of the X-Ray emission by Bremsstrahlung and the spectra are compared with observations obtained by instruments on board the RHESSI satellite. We discuss the effects of the trapping on the Bremsstrahlung emitted radiation, giving constraints on both magnetic field intensity and trapping time.

MAGNETIC FIELDS IN PROTOPLANETARY DISKS

Mark Wardle(1)

Raquel Salmeron (2)

(1) Macquarie University, Australia

(2) University of Sydney, Australia

Magnetic fields may play several roles in astrophysical discs through coupling to the disc rotation and shear. They may produce MHD turbulence and transport angular momentum, drive winds or jets from the disc surfaces, or participate in a disc dynamo. The role of magnetic fields in protoplanetary discs is unclear because of the low level of ionisation may limit the ability of magnetic fields to couple to the disc fluid. It has recently been realised that the presence of Hall diffusion and an x-ray flux from the central young star substantially expands the range of conditions under which the field can couple to the rotation and shear of protoplanetary discs. We present calculations of the resistivity tensor in protostellar discs based on ionisation equilibrium with and without dust grains, and the initial linear growth of the magnetorotational instability under these conditions. In the absence of grains the instability can grow at all heights for field strengths up to several Gauss at 1AU. If a substantial grain population of is present - i.e. before settling of grains to the disc midplane has occurred - magnetic coupling is limited to the region above about three scale heights and the MRI operates for field strengths of about 20 mG. The implications for magnetic activity in protoplanetary discs are discussed.

RADIO WAVE PROPAGATION IN THE NON-GAUSSIAN INTERSTELLAR MEDIUM

Stanislav Boldyrev(1), Carl Gwinn(2), Arieh Konigl(1)

(1) University of Chicago, USA

(2) University of California, Santa Barbara, USA

Radio waves propagating from distant pulsars in the magnetized interstellar medium, are refracted by electron density inhomogeneities, so that the intensity of observed pulses fluctuates with time. The theory relating the observed pulse time-shapes to the electron-density correlation function has developed for 30 years, however, several puzzles have remained. First, observational scaling of pulse broadening with the pulsar distance is anomalously strong; it is consistent with the standard model only when non-uniform statistics of electron fluctuations along the line of sight are assumed. Second, the observed pulse shapes are consistent with the standard model only when the scattering material is concentrated in a thin slab between the pulsar and the Earth. Third, observations require the presence of electron-density fluctuations at very small scales, 10⁷-10⁸ cm., much smaller than the Coulomb mean free path in ionized (HII) regions. We propose that these paradoxes are resolved at once if one assumes stationary and uniform, but non-Gaussian statistics of the electron-density distribution. Such statistics must be of Levy type, and the propagating ray should exhibit a Levy flight. We propose that a natural realization of such statistics is provided by the interstellar medium with random electron-density discontinuities. We develop a theory of wave propagation in such a non-Gaussian random medium, and demonstrate its good agreement with observations.

REF.: S. Boldyrev & C. R. Gwinn, Phys. Rev. Lett. 91, 131101 (2003); ApJ 584, 791 (2003)

SELF-CONSISTENT MEAN FIELD DYNAMO WITH HALL EFFECT

Alejandra Kandus, Maria Jaqueline Vasconcelos, Adriano Hoth Cerqueira

LATO - DCET, Universidade Estadual de Santa Cruz, Brazil

We investigate the influence of Hall effect on the mean field dyanmo. As a starting point we consider the theory developed by E. Blackman and G. Field (PRL 89, 265007 (2002)), where the generation of a mean magnetic field is treated consistently, by considering the evolution equation for the stochastic electromotive force (instead of prescribing its form) and for the magnetic helicity. The existence of Hall effect, namely, of the fact that the magnetic field is freezed to the electron flux instead of to the bulk flux, introduces substantial changes in the evolution equations for the electromotive force and for the magnetic helicity. In the former, Hall effect influences the evolution of the e.m.f. through cross-helicity like terms that modify the a term. For the magnetic helicity, the effect exists as long as the small scale magnetic field is not force-free. We show some results of numerical integrations of the corresponding equations and compare and discuss the differences with the results in abscense of Hall effect.

V. Magnetic Fields in Star Formation and Evolution

THE ROLE OF MAGNETIC FIELDS IN STAR FORMATION

Frank H. Shu*

National Tsing Hua University, Taiwan, ROC

We review the role of magnetic fields in the formation of individual low-mass stars, from the generation of supercritical molecular-cloud cores by ambipolar diffusion from turbulent subcritical envelopes, to pseudodisk creation during inside-out core collapse, to angular-momentum transport within magnetically flattened pseudodisks and centrifugally flattened disks, to magnetospheric interactions between the rapidly rotating inner-edge of the disk and a slowly rotating (compared to break-up) central star, to the driving of collimated winds, jets, and bipolar outflows. We end with a derivation of the Salpeter initial-mass-function for newborn stars in embedded clusters.

OBSERVATIONS OF MAGNETIC FIELDS IN STAR FORMATION REGIONS

Richard M. Crutcher*

Astronomy Department, University of Illinois, USA

I will briefly discuss observational polarization techniques for studying magnetic fields in star formation regions, present very recent observational results, and review all of the results currently available. I will then use the observational results to test predictions of two extreme-case theoretical paradigms for what drives star formation: (1) magnetic support of molecular clouds against gravity with ambipolar diffusion leading to core collapse, and (2) compressible turbulence in the interstellar medium that sometimes produces self-gravitating cores, which then collapse once turbulent support dissipates. Finally, I will summarize results and briefly discuss future observational work needed before the role of magnetic fields in star formation can be fully understood.

STAR FORMATION HISTORIES

Thibaut Lery

Dublin Institute for Advanced Studies, Ireland

The present work addresses the effects of magnetic field, rotation and opacity on the formation of both low and high mass stars. We will show how the competing factors drastically change the behaviours of proto-stellar sources and outflows.

EVOLUTION OF MAGNETIC FIELDS IN STARS ACROSS

THE UPPER MAIN SEQUENCE

S. Hubrig, P. North, T. Szeifert, M. Schoeller

(1) ESO, Chile

(2) Laboratoire d'Astrophysique de l'Ecole Polytechnique Fédérale de Lausanne, Observatoire

(3) ESO

(4) ESO

We have used FORS1 at the VLT to carry out a systematic search for magnetic fields in normal stars, chemically peculiar stars and a few Herbig Ae stars whose magnetic field has never been studied before. Our recent intriguing discovery of longitudinal magnetic fields of the order of a few hundred Gauss in a sample of so-called "non-magnetic" stars rises a fundamental question about the possible ubiquitous presence of a magnetic field in upper main sequence stars. Further, we rediscuss the evolutionary state of upper main sequence magnetic stars using a large sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields.

SiO MASER POLARIZATION IN EVOLVED STARS: MAGNETIC FIELD

F. Herpin (1), A. Baudry (1), C. Thum (2), D. Morris (2)

(1) Observatoire de Bordeaux, L3AB, France

(2) IRAM, Grenoble, France

The SiO maser theory still needs to be improved, in particular in terms of polarization. The study of the maser geometry inside the circumstellar envelopes can also be achieved through polarization studies (e.g., VLBI observations). However, the most exciting point is the determination of the magnetic field that can be made from polarization measurements: this is definitively a new field of investigation for these evolved objects. The magnetic field probably plays an important role in the AGB star's life and can be a major factor (magnetic rotator theory) on the origin of the high mass loss rates observed in evolved objects. Measurement of the magnetic field is thus essential to study the mass loss mechanisms and the Alfvén waves. During its transition, most quasi-spherical AGB stars (i.e. envelopes) become complicated aspherical objects. This shaping is well explained by the Interacting Stellar Winds theory (Kwok works), but the ISW model fails to reproduce very complicated structures with jets and ansae. A new model (Magnetized Wind Blown Bubble theory) was thus developed by Blackman et al. (2001) and A. Franck: a weak toroidal magnetic field, embedded in the stellar wind, acts as a collimating agent (cf. García-Segura 1997) and can produce such structures. Three molecules can show polarized maser emission in the circumstellar envelopes:

- OH traces the envelope far from the central star (1000-10000 AU),

- HO at intermediate distances (a few 100 AU);

- SiO in the inner circumstellar layers (5-10 AU)

Measurement of the polarization rate of the maser radiation emitted by these molecules can give us the averaged value B_// of the magnetic field along the line of sight (for a single dish observation). We present here the first complete study of the SiO maser polarization in a large sample of evolved stars (more than 100). The four Stokes parameters I, U, Q, V were simultaneously measured with the polarimeter on the IRAM-30m telescope. From the Stokes parameters values we derive the linear (p_L) and circular (p_C) polarization rates and polarization angle. The circular polarization rate gives us directly the magnetic field B_//: B_// varies from 1 to 32 Gauss depending on the source, with an average value of 5 Gauss.

VI. Compact Objects, Accretion Disks and Outflows

Accretion Physics

Andrew King*

Dept. of Physics and Astronomy, University of Leicester, UK

Magnetic fields are important in accretion physics as they offer ways of extracting energy, and also potentially of directing it into jets rather than dissipation. I discuss examples of both processes.

SIMULATIONS OF ACCRETION DISKS WITH A NEW GODUNOV SCHEME FOR MHD

J. Stone(1)*, T. Gardiner (1), J. Hawley (2)

(1) Princeton University, USA

(2) University of Virginia, USA

We describe results from a new high-order Godunov code for astrophysical MHD for several problems, including the nonlinear regime of the magneto-rotational instability in the shearing box, and the decay of supersonic, MHD turbulence in cold interstellar clouds. Results from the new code are directly compared to previous studies using the ZEUS code. Future extensions and applications of the code will be described.

LOCAL BEHAVIOR OF MAGNETOROTATIONAL INSTABILITY IN ACCRETION DISKS

Takayoshi Sano

Osaka University, Japan

A promising physical mechanism for anomalous viscosity in accretion disks is turbulence. The presence of a weak magnetic field leads to magnetorotational instability (MRI), which initiates and sustains MHD turbulence. We investigated the nonlinear evolution of MRI in accretion disks using 3D MHD simulations. The local shearing box approximation is adopted and the vertical component of gravity is ignored, so that the evolution of MRI is followed in a small local part of the disk. Our goal is to understand the saturation mechanism of MRI and derive a predictor function of the saturation amplitude of the Maxwell stress, which determines the size of the angular momentum transport. Ohmic dissipation term is included explicitly in our simulations. Our numerical results are qualitatively consistent with an idea that the saturation level of the MRI is determined by a balance between the growth of the MRI and the dissipation of the field through reconnection. The quantitative interpretation, however, may require advances in the theoretical understanding of non-steady magnetic reconnection. We apply our results to the evolution of protoplanetary disks and discuss the influence of MHD turbulence on planet formation.

TRANSONIC INSTABILITIES IN ACCRETION DISKS

Hans Goedbloed and Rony Keppens

FOM-Institute for Plasma Physics 'Rijnhuizen', Nieuwegein & Astronomical Institute, Utrecht University, the Netherlands

In two previous publications^1,2, we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous number of different MHD instabilities, with the well-known magneto-rotational instability as just one of them. We here concentrate on the new instabilities found that are driven by transonic transitions of the poloidal flow. A particularly promising class of instabilities, from the point of view of MHD turbulence in accretion disks, is the class of trans-slow Alfvén continuum modes that occur when the poloidal flow exceeds a critical value of the slow magnetosonic speed. When this happens, virtually every magnetic/flow surface of the disk becomes unstable with respect to highly localized modes of the continuous spectrum. The mode structures rotate, in turn, about the rotating disk. These structure lock and become explosively unstable when the mass of the central object is increased beyond a certain critical value. Their growth rates then become huge, of the order of the Alfvén transit time. These instabilities appear to have all requisite properties to facilitate accretion flows across magnetic surfaces and jet formation.

R. Keppens, F. Casse, J.P. Goedbloed, "Waves and instabilities in accretion disks: Magnetohydrodynamic spectroscopic analysis'', Astrophys. J. 569, L121-L126 (2002).

J.P. Goedbloed, A.J.C. Beliën, B. van der Holst, R. Keppens, "Unstable continuous spectra of transonic axisymmetric plasmas'', Phys. Plasmas 11, 28-54 (2004).

MAPPING THE DISK FLICKERING AND VISCOSITY PARAMETER IN THE

DWARF NOVA V2051 OPHIUCHI

Raymundo Baptista, Alexandre Bortoletto

Depto. Física, UFSC, Campus Trindade, Florianópolis - SC, Brazil

We report on the eclipse mapping analysis of an ensemble of light curves of the dwarf nova V2051 Oph to study the spatial distribution of its steady-light and flickering sources. The data are combined to derive the orbital dependency of the steady-light and the flickering components at two different brightness levels, named the 'faint' and 'bright' states. The ability to separate the orbital dependency of the low- and high-frequency flickering components allowed us to identify the existence of two different and independent sources of flickering in V2051 Oph, and provided a novel way to estimate the disk viscosity parameter a. The low frequency flickering arises mainly in the overflowing gas stream and is associated to the mass transfer process. High frequency flickering originates in the accretion disk, showing a radial distribution similar to that of the steady-light maps and no evidence of emission from the hot spot, gas stream or white dwarf. This disk flickering component has relative amplitude of about 3 per cent of the steady disk light, independent of disk radius and brightness state. If the disk flickering is caused by fluctuations in the energy dissipation rate induced by magneto-hydrodynamic turbulence, its relative amplitude lead to a viscosity parameter a_cool ⋍ 0.1 - 0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the dwarf novae disk instability model.

VII. Compact Objetcs, Accretion Disks and Outflows

GAMMA RAY BURSTS

T. Piran*

Hebrew University, Israel

ASTROPHYSICAL JETS AND OUTFLOWS

Elisabete M. de Gouveia Dal Pino*

Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

Highly collimated supersonic jets and less collimated outflows are observed to emerge from a wide variety of astrophysical objects. They are seen in young stellar objects (YSOs), proto-planetary nebulae, compact objects (like galactic black holes or microquasars, and X-ray binary stars), and in the nuclei of active galaxies (AGNs). Despite their different physical scales (in size, velocity, and amount of energy transported), they have strong morphological similarities. What physics do they share? These systems are either hydrodynamic or magnetohydrodynamic (MHD) in nature and are, as such, governed by non-linear equations. While theoretical models have helped us to understand the basic physics of these objects, numerical simulations have been allowing us to go beyond the one-dimensional, steady-state approach extracting vital information. In this lecture, the formation, structure, and evolution of the jets are reviewed with the help of observational information and purely hydrodynamical and MHD numerical modeling.

THE CONNECTION BETWEEN LABORATORY AND ASTROPHYSICAL JETS

A. Raga*

Universidad Autononoma de Mexico

THE ORIGIN OF MHD JETS IN T TAURI STARS

Sylvie Cabrit (1), Jonathan Ferreira (2), Catherine Dougados (2), Paulo Garcia (3)

(1) LERMA - Obs. Paris, France

(2) Laboratoire Astrophysique Grenoble, France (3) Centro de Astrofisica do Porto, Portugal

We review the constraints on steady MHD ejection models in young stars set by recent sub-arcsecond observations of T Tauri jets, including measurements of (1) intrinsic collimation, (2) terminal speed, (3) jet rotation, and (4) jet density and accretion/ejection ratio. We show that "cold'' MHD disk winds (with negligible enthalpy) are definitely excluded by observations, consistent with the presence of efficient stellar X-ray heating at the disk surface. On the other hand, a new class of "warm'' MHD disk winds, with smaller magnetic lever arms ⋍ 10, is shown to reproduce all observations very well. Stellar and magnetospheric MHD winds are found unable to explain the reported jet rotation signatures, although they may still be present as a separate hot axial component inside a disk wind. We outline a few implications of the warm disk-wind hypothesis, e.g. on the origin of velocity asymmetries in bipolar jets. We also examine observational discriminants between the two possible configurations of magnetospheric MHD ejection that may coexist with the disk wind (with the stellar magnetic moment either parallel or antiparallel to the disk field).

VIII. Compact Objects, Accretion Disks and Outflows

Magnetic “Springs” and the Creation of Collimated Outflows:

Simulations and Laboratory Experiments

Adam Frank(1)*, Sean Matt (2), Eric Blackman (1) and Sergey Lebedev (3)

(1) University of Rochester, USA

(2) MacMaster University

(3) Imperial College, London

We present new results from astrophysical theory/simulations and laboratory astrophysical experiments which explore the creation of outflows via strong toroidal magnetic fields. Our results are relevant to a variety of astrophysical environments which exhibit outflows from a central source including YSOs, PNe and perhaps GRBs/SNe. Our simulations are designed to explore the generation of toroidal fields from initially poloidal configurations via field line wrapping. We begin with a rapidly rotating magnetized “core” lying below an extended plasma “atmosphere”. This simplified model allows us to isolate the processes by which toroidal field gradients (a “spring”) are generated and how the plasma atmosphere is impulsively expelled. We explore models with both dipolar and split monopolar initial conditions finding strong outflows develop in both cases and that a significant fraction of the rotational energy of the rotating ball can be converted into bulk motions of the overlying atmosphere. We describe scaling relations for the model as well as review its application to a number of environments focusing on Planetary Nebulae. We also present results of laboratory astrophysics experiments using the MAGPIE pulsed power machine at Imperial Collage. Experiments with radial wire arrays show the creation of high velocity outflows driven by strong gradients in toroidal field. These expanding “magnetic bubbles” exhibit jet-like columns on axis which eventually go unstable in manner indicative of kink modes. We present these initial results and discuss their relevance through scaling arguments to astrophysical outflows.

THE STRUCTURING OF PLANETARY NEBULAE

Guillermo García-Segura*

IA-UNAM, México

In this paper, we focus on some features that are addressed by several works on MHD. We review different scenarios for the origin of magnetized winds, either for Protoplanetary Nebulae or Planetary Nebulae, and discusses the production of axisymmetric flows, the confinement of flows and the production of jets and ansae, point-symmetric nebulae and the periodic shells around PNe and proto-PNe.

MHD SIMULATION OF RADIATIVELY DRIVEN STELLAR WINDS FROM

MAGNETIC HOT STARS

Stan Owocki (1), Asif ud-Doula (2), Rich Townsend (1)

(1) Bartol Research Institute, U. of Delaware, USA

(2) Dept. of Physics and Astrononomy, North Carolina State Univ.

Massive, hot, luminous stars have strong stellar winds driven by line-scattering of star's continuum radiation. Recent observations have detected substantial large-scale dipole magnetic fields for several hot stars. This talk will discuss our recent efforts to carry out MHD simulations of the effect of magnetic fields in channeling and confining the wind outflow, with particular emphasis on the "Magnetically Confined Wind Shock" (MCWS) paradigm for explaining the relatively hard X-ray emission observed by the Chandra X-ray observatory for magnetic hot stars like Theta 1 Ori C. I will also describe a new "Rigidly Rotating Magnetosphere" (RRM) model that has proven highly successful in reproducing the rotational modulated Balmer emission seen in magnetic Bp stars like sigma Ori C. The talk will conclude with an outlook for the general role of magnetic fields in structuring hot-star mass loss and circumstellar matter.

LABORATORY SIMULATIONS OF SUPERNOVA SHOCKWAVE PROPAGATION

J. F. Hansen, M. J. Edwards

Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

Supernovae launch spherical shocks into the circumstellar medium (CSM). These shocks may interact with both the intergalactic magnetic field (IGM) and local mass accumulations (possibly with their own local magnetic fields). The latter interaction may trigger star formation. The shocks have high Mach numbers and may be radiative. We have created similar shocks in the laboratory by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas; ablated material from the pin rapidly expands and launches a shock through the surrounding gas. The shock may then be allowed to interact with (a) mass accumulations, (b) magnetic fields, or (c) allowed to expand freely. We will present examples of each type of experiment, but mainly discuss a new phenomena observed first in (c); at the edge of the radiatively heated gas ahead of the shock, a second shock forms. The two expanding shocks are simultaneously visible for a time, until the original shock stalls from running into the heated gas. The second shock remains visible and continues to expand. A minimum condition for the formation of the second shock is that the original shock is super- critical, i.e., the temperature distribution ahead of the original shock has an inflexion point. In a non-radiative control experiment the second shock does not form.

This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.

THE HIGHEST MAGNETIC FIELD IN THE UNIVERSE: EVIDENCE,

IMPLICATIONS & PROSPECTS

Alaa Ibrahim (1,2)

(1) Cairo University, Egypt

(2) NASA GSFC & George Washington Univ.

Magnetars are a new class of neutron stars that possess the highest magnetic field yet observed in the Universe. They are the only cosmic objects whose radiation and particle emission are powered exclusively by the magnetic field energy, which exceeds all other forms of free energy associated with the star system. A direct measurement of the magnetar field strength was recently obtained via the discovery of spectral line features, consistent with proton cyclotron resonance in B=10¹⁵ G. In such a regime of field strength, exotic quantum effects take place and manifestations of otherwise negligible QED processes can be studied, observed, and tested. This gives us new possibilities for leaning about the behavior of radiation and matter under extreme conditions that cannot be found elsewhere in the Universe. We will briefly review the properties of magnetars then discuss their potential in testing the fundamental laws of physics.

IX. Compact Objects, Accretion Disks and Outflows

X-RAY OBSERVATIONS OF ISOLATED NEUTRON STARS AND NEUTRON

STARS IN BINARY SYSTEMS

Mariano Mendez*

SRON - National Institute for Space Research, The Netherlands

Magnetic fields are central to the phenomenology observed in isolated neutron stars (radio and X-ray pulsars, magnetars, etc.), as well as in neutron stars accreting from a secondary star in binary systems, the so-called X-ray binaries. I will discuss the general characteristics of the X-ray emission of some of these objects, both their spectra and variability, with emphasis on some of the most recent results obtained from observations with the X-ray observatories Chandra and XMM-Newton, plus INTEGRAL and the Rossi X-ray Timing Explorer.

NEUTRON STAR MAGNETIC FIELDS

Andreas Reisenegger(1)*, Joaquin Prieto (1), Dong Lai (2), Rafael Benguria (1) Pablo Araya (1,3)

(1) Pontificia Universidad Catolica de Chile

(2) Cornell University

(3) University of Groningen

This talk will start with an up-to-date overview of the rich phenomenology of neutron star magnetic fields (encompassing classical and millisecond radio pulsars, X-ray binaries, magnetars, and purely thermal sources), which suggests that magnetic fields on neutron stars span at least the range 10^8-15 G, corresponding to a range of magnetic fluxes similar to that found in white dwarfs and upper main sequence stars. Observations suggest that this field may decay, at least on accreting neutron stars or for very strong fields. Several physical processes (ambipolar diffusion, Hall drift, resistive dissipation) may contribute to this decay, but their roles still have to be clearly disentangled. Special attention will be given to recent, semi-analytical calculations aiming to understand Hall drift and its interaction with resistive dissipation of the magnetic energy.

PULSAR DYNAMICS: THE MAGNETIC DIPOLE MODEL REVISITED

Germán Lugones* and Ignazio Bombaci

Dipartimento di Fisica, Università di Pisa, Italia

More than 35 years after the discovery of pulsars there is still a very schematic understanding of pulsar dynamics in terms of uniformly rotating magnetic dipoles. In this talk we present the non-uniformly rotating version of the magnetic dipole model and show that this leads to a significant modification of the dynamical behavior of these objects. The popular period vs. period-derivative diagram shows new relevant features that entangle the interpretation currently given within the uniformly rotating version of the model. In particular, the curves of constant surface magnetic field strength B_S crucially depend on the largely unknown initial rotation period, and may be shifted downwards up to a factor 16 with respect to the uniform rotating model. This means that the surface magnetic field strength inferred from timing observations can be substantially larger than currently expected. As a consequence, many apparently radio-like pulsars can now have inferred B_S above the quantum limit B_Q = 4.4 ´ 10¹³ G. We discuss some other implications of the results.

PULSAR MAGNETOSPHERES: MODELS WITH NON-STATIONARY

PAIR PRODUCTION

Don Melrose*

School of Physics, University of Sydney, Australia

Conventional models for the polar cap regions of pulsar magnetospheres are based on steady-state pair creation leading to neutralization of a large-scale parallel (to the magnetic field) electric field except in so-called gap regions. After pointing out difficulties with conventional models, it is argued that pair creation is likely to be highly localized and non-stationary, with the parallel electric field oscillating locally about a non-zero mean. Possible observational consequences of models based on non-stationary pair creation are explored.

X. Magnetized Ism

MHD TURBULENCE

J. Cho* & A. Lazarian*

University of Wisconsin, USA

SUPER-ALFVENIC TURBULENCE IN THE ISM

Å. Nordlund(1)* and P. Padoan (2)

(1) University of Copenhagen, Denmark

(2) University of California, San Diego

Super-Alfvenic turbulence fragments molecular clouds in the interstellar medium into dense sheets, filaments, cores and large low density ''voids'', via the action of highly radiative shocks. The distribution of core masses depends primarily on the power spectrum of the turbulent flow, and on the jump conditions for isothermal shocks in a magnetized gas. For a power spectrum index b=-1.8, consistent with results of numerical experiments of super-sonic turbulence as well as with Larson's velocity-size relation one obtains, by scaling arguments, a power law mass distribution of dense cores with a slope equal to 3/(4+b) ~ 1.35, consistent with the slope of the Salpeter stellar IMF. Results from numerical simulations confirm this scaling. At small masses, only those cores that are dense enough to have smaller Bonnor-Ebert masses than their own mass collapse, while other low-mass cores re-expand. This is the main reason for the roll-over of the Initial Mass Function towards sub-stellar masses. Other effects, such as subfragmentation, may modify the low mass side of the IMF as well.

STATISTICS OF TURBULENCE FROM SELF-ABSORBING INTERSTELLAR GAS

D. Pogosian*

University of Alberta, Canada

I will discuss Velocity Channel Analysis (VCA) technique, including its extension to absorbing medium, relevant to study turbulence in molecular clouds through optically thick lines.

Linear and Nonlinear Theories of Cosmic Ray Transport

Andreas Shalchi*

Theoretische Physik IV, NB 7/30 Ruhr-Universität Bochum 44780 Bochum, Germany

The transport of charged cosmic rays in plasma wave turbulence is a modern and interesting field of research. We are mainly interested in spatial diffusion parallel and perpendicular to a large scale magnetic field. During the last decades quasilinear theory was the standard tool for the calculation of diffusion coefficients. Through comparison with numerical simulations we found several major problems of transport theory. I will demonstrate that new nonlinear theories which were proposed recently can solve at least some of these problems.

TURBULENCE IN DIFFERENT REGIMES OF THE IONIZED INTERSTELLAR MEDIUM

Marijke Haverkorn (1), Bryan M. Gaensler (1), Jo-Anne C. Brown (2), Naomi M. McClure-Griffiths (3), John M. Dickey (4), Anne J. Green (5)

(1) Harvard-Smithsonian Center for Astrophysics, USA

(2) University of Calgary, Canada

(3) ATNF-CSIRO, Australia

(4) University of Tasmania, Australia

(5) University of Sydney, Australia

Observations of turbulence in the Galactic magneto-ionic medium show a wide variety of results in values of the spectral index, amplitude and injection scales, depending on resolution, method used, and position in the sky. The Southern Galactic Plane Survey (SGPS), a polarized radio continuum survey of a large part of the inner Galactic plane, provides an excellent resource to study this dependence of interstellar turbulence on position and regime in the ISM. Using Rotation Measures from the polarized synchrotron background and from extragalactic point sources, we determined a difference in turbulent amplitude and outer scale between spiral arms and interarm regions. Furthermore, we discuss how structure in the ionized gas varies in concrete objects such as HII regions and supernova remnants.

MAGNETIC FIELDS IN MASER REGIONS

Jim Cohen

Jodrell Bank Observatory, The University of Manchester, UK

Masers provide a means of directly measuring cosmic magnetic fields at subarcsecond resolution. Maser lines of OH, H₂O, SiO and other species together probe a range of densities from ~10⁴ to ~10¹⁰ cm^-3, in diverse regions ranging from comets, star-forming clouds and supernova remnants to active galactic nuclei. Different masers are excited under different conditions, allowing measurements of magnetic fields in different zones. Measurments of Zeeman splitting give the magnetic field strength directly if the maser lines are split by more than their thermal linewidth, and in some cases it is possible to deduce the three-dimensional magnetic field vector from linear polarization. Recent observations of magnetic fields in circumstellar envelopes and proto-planetary nebulae, and in regions of star-formation will be reviewed, with emphasis on the possible link between the magnetic field configuration and the bipolar outflows that are often observed. Prospects for measuring magnetic fields in extragalactic megamasers will also be discussed.

XI. Magnetized ISM and the Galactic Center

PROBING MAGNETIC FIELDS IN SUPERNOVA REMNANTS AND PULSAR-DRIVEN NEBULAE WITH HARD X-RAYS AND HIGH ENERGY GAMMA-RAYS

Felix Aharonian*

Max-Planck-Institut fuer Kernphysik, Germany

The sharp morphological structures discovered recently in X-ray images of several young supernova remnants (SNRs) indicate the existence of strong magnetic fields in the shells of SNRs. This allows effective acceleration of protons to energies up to 10¹⁵ eV and beyond, and consequently copious production of very high energy gamma-rays and hard X-rays associated with the secondary products of interactions of accelerated protons with the ambient gas. I will show that spectral and morphological studies of young SNRs in hard X-rays and high energy gamma-rays provide a unique tool for extraction of unambiguous information about the strength and spatial distribution of magnetic fields in young SNRs. I will also discuss the possibility of probing magnetic fields in pulsar-driven nebulae (plerions) based on characteristics of synchrotron X-rays and inverse Compton gamma-rays. I will demonstrate the potential of these methods using the exciting results of recent spectral and morphological studies of several distinct representatives of both source populations in X-rays and TeV gamma-rays.

INTERMITTENCY OF INTERSTELLAR TURBULENCE

E. Falgarone(1)*, P. Hily-Blant (1), J. Pety (2), G. Pineau des Forêts (3), L. Verstraete (3)

(1) ENS & Observatoire de Paris, France

(2) Observatoire de Paris & IRAM, Grenoble, France

(3) Institut d'Astrophysique Spatiale, Orsay, France

Several properties of the interstellar medium can be interpreted as signatures of the intermittency of turbulence. These are non-Gaussian statistics of the velocity field, plus ubiquitous traces of warm gas within the cold medium. The existence of the warm gas is attested to by observations of highly excited molecular hydrogen, and by manifestations of a specific chemistry. Small-scale coherent magnetized vortices and magneto-hydrodynamical shocks are able to reproduce some of these properties. The observations will be presented and the models discussed.

MAGNETIC FIELDS IN THE GALACTIC CENTER

Giles Novak*

Northwestern University, USA

Within the central few hundred parsecs of the Galaxy we find a magnetosphere with a rich and complex filamentary structure. In some ways this structure is reminiscent of what we see in the Solar magnetosphere, but the size scale is a million times larger. Advances in the technology for far-infrared/submillimeter polarimetry are enabling us to acquire new data on interstellar magnetic fields in the Galactic center. We will review recent results in this area and attempt a synthesis with information obtained from non-thermal radio filament morphology, radio Faraday rotation, and Zeeman splitting.

MAGNETIC FIELDS AND TURBULENCE IN THE INNER MILKY WAY

Bryan M. Gaensler (1), Marijke Haverkorn (1), Jo-Anne Brown (2), Naomi McClure-Griffiths. (3) John Dickey (4), Anne Green (5)

(1) Harvard University, USA

(2) University of Calgary, Canada

(3) CSIRO Australia Telescope National Facility, Australia

(4) University of Tasmania, Australia

(5) University of Sydney, Australia

As part of the Southern Galactic Plane Survey (SGPS), we have surveyed the linearly polarized radio emission from 210 square degrees of the inner Galaxy at a frequency of 1.4 GHz and at a spatial resolution of ~1.5 arcmin. The spectropolarimetric capability of the data allows us to extract the rotation measure (RM) at every pixel for which we detect polarization. The SGPS correspondingly represents a comprehensive resource for m apping magnetic fields in the Milky Way, on scales ranging from sub-parsec turbulence up to the global structure of the spiral arms and disc. In this talk, I will present results on the Galaxy's global magnetic field as traced by the RMs of extragalactic background sources, will show how the magnetic fields of individual supernova remnants and HII regions are revealed by their depolarizing effects on the diffuse polarized background, and will explain how RMs towards this diffuse background yield the turbulent properties of the magneto-ionized interstellar medium.

This work is supported by the National Science Foundation through grant AST 03-07358.

XII. Magnetic Fields in Galaxies and the IGM

OBSERVATIONS OF MAGNETIC FIELDS IN GALAXIES

Rainer Beck*

Max Planck Institut fuer Radioastronomie, Germany

The strength and structure of interstellar magnetic fields can be studied by observations of radio continuum emission, its polarization and its Faraday rotation. Fields with a well-ordered spiral structure exist in grand-design, flocculent and even irregular galaxies. Total field strengths in spiral arms galaxies are 10 - 30 MicroGauss. In grand-design galaxies the fields are aligned parallel to the optical spiral arms, but the strongest regular fields are found in inter-arm regions, sometimes forming ` `magnetic spiral arms'' between the optical ones. Within spiral arms with massive star formation, field lines are tangled so that very little polarization is observed. Faraday rotation of the polarization vectors reveals patterns, which are signatures of coherent large-scale fields in galactic disks generated by dynamos. The majority of field structures in galaxies require a superposition of dynamo modes. In barred galaxies the magnetic field is mostly aligned with the gas flow and compressed in the shock. However, the field is already strong in the upstream region and is deflected by shear. Within circumnuclear starburst regions the field is of spiral shape, indicating dynamo action. Magnetic stress may drive inflow of gas towards the nucleus. Magnetic fields were detected in galactic halos at large distances from the disks. The largest halos and strongest fields result from galactic winds, from interaction or from ram pressure in an intercluster medium. Present-day radio polarization observations are limited by sensitivity. Next-generation radio telescopes (e.g. LOFAR and the Square Kilometer Array) will be able to reveal the full wealth of magnetic structures in galaxies.

EXTRAPLANAR GAS IN GALAXIES

R.-J. Dettmar*

Ruhr-University Bochum, Germany

MAGNETIC FIELDS ON DIFFERENT SCALES IN ACTIVE GALAXIES

Geoffrey Bicknell*

Research School of Astronomy & Astrophysics Australian National University, Australia

Magnetic fields are important on all scales in active galaxies: in accretion disks where they are responsible for mass accretion and energy generation, in the launching of jets, in jet propagation and dissipation at hot spots and in the lobes of young radio galaxies where Faraday dispersion can lead to significant depolarization. In this talk I shall attempt to synthesize what we know about magnetic fields on these different scales and how the physics of the different phenomena are inter-related.

VARIABLE ORIENTATION OF THE MAGNETIC FIELD IN PC-SCALE JETS

OF AGNS: EVIDENCES OF PRECESSION?

Zulema Abraham, Anderson Caproni

Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

VLBI polarization maps allow us to study the direction of the magnetic field in the parsec-scale jets of Active Galactic Nuclei. If the bright regions detected with interferometric techniques are due to shocks propagating in a jet with relativistic bulk motion, the magnetic field at the shock position should be enhanced in the direction perpendicular to the shock propagation. Observations show that in most of the studied objects the situation is much more complex, with the direction of the magnetic field changing along the jet over very short distances and varying also with time. We interpreted these differences in orientation as due to jet precession. We discuss some of the objects for which a well defined precession period was determined, like 3C273 (Abraham & Romero 1999, A&A, 344, 61), 3C279 (Abraham & Carrara 1996, A&AS, 115, 543) and 3C345 (Caproni & Abraham 2004, ApJ, 602, 625). We compare the direction of the magnetic field in each of the bright components with the perpendicular to the jet direction, predicted by the precession model at the epoch at which the component was formed and found a very good agreement. Moreover, the components for which large variations in the direction of the magnetic field are seen in short spatial scales were formed at epochs in which the jet formed a very small angle with the line of sight, and the position angle of the jet in the plane of the sky changed by large amounts in very short time scales.

XIII. Magnetic Fields in Galaxies and the IGM

PARTICLE ACCELERATION: FROM GALAXIES TO LARGE SCALE STRUCTURES

Peter L. Biermann*

Max Planck Institute for Radioastronomy, and Dept for Physics and Astronomy

University of Bonn Bonn, Germany

Energetic particles exist almost everywhere in the Universe, usually in cohabitation with magnetic fields; and although we have some ideas where these energetic particles and magnetic fields come from, we are not certain at all. Here we discuss on both Galactic and extragalactic cosmic ray particles, and focus on the simplest models in debate. Standard shock acceleration in the Interstellar Medium leads to energies below the feature in the cosmic ray spectrum usually referred to as the knee, at 3 10¹⁵ eV, where the spectrum bends down by about 1/3. There is one proposal published that links this feature to the explosion of very massive stars, especially Wolf Rayet stars. The observed features, at knee and ankle are so well defined that the critical E/Z ratio must be very well defined in Nature, and be common to all sites contributing to our cosmic ray population. This then leads back to the argument by Bisnovatyi-Kogan (1970) that supernovae are powered by potential energy, and that the energy is transmitted from rotation via magnetic fields to the outside; this argument can be made quantitative. Considering then the abundances of cosmic rays this proposal requires that very massive stars explode with 10⁵² ergs, just what the hypernova model of Paczynski suggested, providing a clear connection in the physical model to Gamma Ray Bursts. These explosions may provide a new standard candle in cosmology, if we found a way to correct for non-sphericity. The origin of Galactic magnetic fields may also lie with the powerful winds of Wolf Rayet stars; however, we still do not know the origin of the large scale order of the magnetic fields. Radiogalaxies and Gamma Ray Bursts provide possible sources for the ultrahigh energy cosmic rays. The topology of the large scale magnetic fields, first in a putative Galactic magnetic wind, and then in the supergalactic plane, are key to check on all such proposals. The prominent shock waves in the large scale structure itself are yet another source for energetic particles, albeit probably not for the highest energies. The most conservative candidate for the locally observed ultra high energy cosmic ray particles is the Virgo cluster radio galaxy M87.

MAGNETIC FIELDS AND ULTRA HIGH ENERGY COSMIC RAYS

Angela V. Olinto*

University of Chicago, USA

The observation of cosmic rays at the highest energies will help to determine cosmic magnetic fields on the largest scales. We review the state of ultra-high energy cosmic ray observations and the ability of next generation observatories to test cosmic magnetic fields.

ACCELERATION AND PROPAGATION OF UHECRS

P. Blasi*

Observatory of Arcetri, Italy

MAGNETIC LENSING AND CLUSTERING OF ULTRA HIGH ENERGY COSMIC RAYS

Diego Harari, Silvia Mollerach, Esteban Roulet

CONICET - Centro Atomico Bariloche, Argentina

Ultra high energy cosmic ray (UHECR) astronomy is inevitably tied to the magnetic fields along the line of sight towards the most powerful sources if the bulk of their emission is in the form of charged particles. The behaviour of the transition from a diffusive regime towards quasirectilinear propagation at the highest energies is a very rich source of information, both on the source properties as well as on the magnetic fields. We summarize strategies to reconstruct the parameters of intervening magnetic fields based on the lensing phenomena that take place around a characteristic energy at which it is likely to observe strongly magnified multiple images of single UHECR sources. We discuss properties of the clustering of arrival directions that may be expected from lensing effects. We identify tools that may help to characterize the nature of cosmic ray sources based on the clustering properties of observed events.

EVOLUTION OF THE ISM OF STARBURST GALAXIES AND THE FORMATION

OF GALACTIC SUPERWINDS

Claudio Melioli, Elisabete M. de Gouveia Dal Pino

Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

The interstellar medium heated by supernova explosions (SN) may acquire an expansion velocity larger than the escape velocity and leave the galaxy through a supersonic wind. Galactic winds are effectively observed in many local starburst galaxies. SN ejecta are transported out of the galaxies by such winds, therefore affecting the chemical evolution of those galaxies. The effectiveness of the processes mentioned above depends on the heating efficiency (HE) of the SNe, i.e., on the fraction of the SNe explosion energy that is effectively stored in the ISM gas, in the form of kinetic and internal energy to produce the wind, and that is not radiated away. The value of HE, in particular in starburst (SB) galaxies, is still a matter of debate. Considering the essential ingredients of a SB environment we have developed a semi-analytic model, which is able to qualitatively trace the thermalisation history of the ISM in a SB region and determine the HE evolution. Our study has been also accompanied by full 3-D radiative cooling hydrodynamical simulations of SNR-SNR and SNR- clouds interactions. We have found that the heating efficiency of the SNe is very sensitive to the amount of ambient gas and clouds of the SB, and may remain very small at least during part of the SB lifetime, therefore preventing or postponing the formation of a superwind (Melioli & de Gouveia Dal Pino 2004, Melioli & de Gouveia Dal Pino, Raga 2004). As long as the efficiency remains small, the cooled gas remains confined to the system and can promote new generations of star-formation, or increase the gas in-fall to the central regions of the SB. As the ambient density decreases, the gas can finally heat and expand very rapidly and abandon the galaxy as a superwind. A magneto-centrifugal mechanism to accelerate and collimate these superwinds as a function of the SNe heating efficiency is also discussed.

- C. Melioli & E. M. de Gouveia Dal Pino, A&A, 424, 817 (2004)

- C. Melioli, E. M. de Gouveia Dal Pino, & A. Raga (2004), in prep.

XIV. Magnetic Fields in Galaxies and the IGM

MHD GAS FLOW IN THE GALAXY: MODELING THE BAR AND

THE SPIRAL PATTERNS

M. Martos*

University of California & Universidad Autonoma de Mexico

Collisions of High Velocity Clouds with a Magnetized Galactic Disk

Alfredo Santillán*

Cómputo Aplicado-DGSCA, UNAM, México

High-velocity clouds are large flows of neutral hydrogen, located at high galactic latitudes, with large velocities (| V_LSR | m 100 km/s) that do not match a simple model of circular rotation for our Galaxy. Numerical simulations have been performed during many years to study the details of their evolution, and their possible interaction with the interstellar disk. Here we present a brief review of the models that have been already published, and describe three-dimensional magneto hydrodynamical simulations of the HVC-Magnetized Galactic Disk interaction.

DETECTION OF NUMEROUS LINEAR FILAMENTS IN THE GALACTIC

CENTER REGION

F. Yusef-Zadeh

Northwestern University, USA

Based on our recent 20cm survey of the Galactic center region with the VLA, we show the evidence for an order of magnitude increase in the number of faint linear filaments with typical lengths of few arcminutes. Many of the filaments show morphological characteristics similar to the Galactic center magnetized radio filaments. The linear filaments are not isolated but are generally clustered in star forming regions where prominent nonthermal radio filaments had been detected previously. The extensions of many of these linear filaments appear to terminate at either a compact source or a resolved shell-like thermal source. We discuss that a relationship between the filaments, the compact and extended thermal sources as well as a lack of preferred orientation for many radio filaments should constrain models that are proposed to explain the origin of nonthermal radio filaments in the Galactic center.

THE SOUTHERN OPTICAL/INFRARED SURVEY OF INTERSTELLAR

POLARIZATION IN THE GALAXY

Antonio Mário Magalhães, Antonio Pereyra, Rocío Melgarejo, Luciana de Matos, Flaviane F. C. Benedito, Rodolfo Valentim and Viviana S. Gabriel

Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

We describe the status of the on-going Optical/IR Survey of Interstellar Polarization in the Galaxy. The aim is to improve the knowledge of the magnetic field structure of the diffuse Interstellar Medium, the ratio between the random and uniform components of the field and the scale length of the field fluctuations. The Survey has been gathering high accuracy optical imaging polarimetry data of selected directions and regions along the Southern Galactic Plane. Data towards clouds at high galactic latitudes and across open clusters have been also obtained. These observations allow mapping of the magnetic field on small scales. We present both an account of the directions observed and representative data and their interpretation. We have started an IR extension of the Survey, concentrated on the Galactic Plane. The IR data will include data on both point source and extended objects. Plans include IR imaging polarimetry with the Spartan Camera on SOAR. The Survey is being carried out with the IAG-Univ. São Paulo 60-cm telescope at the LNA observatory.

This research is supported by Brazilian agencies FAPESP, CAPES and CNPq.

STAR FORMATION IN THE LOCAL SPIRAL ARM

Jacques R.D. Lépine (1), Wilton S. Dias (1,2)

(1)Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

(2) Instituto de Física de Sao Carlos, University of Sao Paulo, Brazil

We analyze the dynamics of the young stellar clusters belonging to the local spiral arm, based on the proper motions, radial velocities and ages of the clusters, which are compiled in our Catalog of Open Clusters. By computing the orbits of the clusters, we obtain their initial velocity components at the instant of formation. In general, the clusters present systematic perturbations (non-circular orbits) that can be understood in the frame of the classical theory of spiral arms. This theory predicts that the galactic material is decelerated by the spiral shock waves inside corotation radius, and accelerated outside corotation radius (the corotation radius being the radius which the interstellar gas and the spiral patterns have the same rotation velocity). While the velocity components (U, V), within the galactic plane are easily understood, the W velocity components (perpendicular to the plane) of the clusters also present systematic trends, which would require 3D models of the spiral arms to be understood.

XV. Magnetic Fields in Galaxies, the IGM,

and the Early Universe

PRIMORDIAL MAGNETIC FIELDS

M. Shaposhnikov*

Inst. Theor. Phys., Swiss Federal Institute of Technology, Switzerland

PRIMORDIAL MAGNETIC FIELDS FROM COSMOLOGICAL PHASE TRANSITIONS

H. J. de Vega*

LPTHE-Univ. Paris VI, France

Different mechanisms may be responsible of the generation of large scale primordial magnetic fields. We study cosmological phase transition during the radiation dominated era where charged scalar fields undergo a phase transition. During this phase transition the fields are out of equilibrium and a large number of charged particles are produced. They emit an abundant number of photons which may be the seed for large scale magnetic fields. The dynamics after the transition features two distinct stages: a spinodal regime dominated by linear long-wavelength instabilities, and a scaling stage in which the non-linearities and backreaction of the scalar fields are dominant. This second stage describes the growth of horizon sized domains. The non-equilibrium Schwinger-Dyson equations are used to obtain the spectrum of magnetic fields that includes the dissipative effects of the plasma. We find that large scale magnetogenesis is efficient during the scaling regime. Charged scalar field fluctuations with wavelengths of the order of the Hubble radius induce large scale magnetogenesis via loop effects. The leading processes are: pair production, pair annihilation and low energy bremsstrahlung, these processes while forbidden in equilibrium are allowed strongly out of equilibrium. The ratio between the energy density on scales larger than L and that in the background radiation r(L,T)= r_B(L,T)/ r_cmb(T) is r(L,T) ~ 10^-34 at the Electroweak scale and r(L,T) sim 10^-14 at the QCD scale for L sim 1 Mpc.The resulting spectrum is insensitive to the magnetic diffusion length and equipartition between electric and magnetic fields does not hold. We conjecture that a similar mechanism could be operative after the QCD chiral phase transition.

MAGNETIC FIELDS AND THE FIRST OBJECTS

Reuven Opher*

Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

Magnetic Fields are important in star formation at low redshifts. Thus, we may assume that a primordial magnetic field was important in the formation of the first objects. We discuss our work on the origin of the primordial magnetic field due to nonminimal gravitational-electromagnetic coupling, primordial density fluctuations, and primordial supernovae explosions. Our work on the amplification of the primordial magnetic field by helical turbulence and the magnetization of the intergalactic medium as well as the effects of the magnetized intergalactic medium on the formation of the first objects is also discussed.

MAGNETIC FIELDS AND CMB ANISOTROPIES

Eduardo Battaner, Estrella Florido, Beatriz Ruiz

University of Granada, Spain

Assuming that magnetic fields were produced before recombination they could constitute a source of anisotropies actually present in the Cosmic Microwave background. We integrate the relativistic linear perturbed equations of fluids, Maxwell and Einstein to follow the evolution of a primordial magnetic filament. Magnetic fields equivalent to present 10^-9 Gauss should have observable effects on CMB anisotropies. The identification of magnetic anisotropies will be discussed.

GRAVITATIONAL WAVES DRIVEN BY BARDEEN-PETTERSON EFFECT AND SUSPENDED ACCRETION IN AGN AS TARGETS FOR LISA

Herman Mosquera Cuesta (1), Anderson Caproni (2) and Zulema Abraham (2)

(1) CBPF-ICRA-Brazil

(2)Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil

We have shown that the Bardeen-Peterson (B-P) effect may be responsible for the precession of the jet inlets in a sample of active galactic nuclei (AGN). We show here that for the dynamical conditions in those systems a suspended accretion state could have developed, too, in the region between the supermassive black hole innermost stable orbit and the B-P radius, where a massive torus orbits. The strong coupling of the magnetic field; generated in the accretion torus, to the supermassive black hole (SMBH) angular momentum makes that most of the SMBH rotational energy to be given off as gravitational radiation rather than electromagnetic waves. Thus, any AGN driven by the Lense-Thirring effect (or spin-induced precession) and a suspended accretion state turn out to be a powerful source of gravitational waves (GWs) that could be detected by LISA. The concommitant detection of GWs together with optical and radio emissions from these AGN may decisively help in picturing a consistent scenario of those cosmic sources.