Since the first discoveries of strong, large-scale organised, and stable magnetic fields in chemically peculiar stars, the Ap/Bp stars, the origin of magnetic fields in these primarily radiative stars have been the subject of intense debate. Today, our favored hypotheses is a fossil origin. This theory implies that the magnetic fields observed in the main sequence A/B stars are remnants of fields either present in molecular clouds from which these stars formed, or that they were generated by a dynamo during the early stages of star formation. In this talk I will review our general knowledge of magnetism in intermediate- and high-mass stars, and I will present our recent and current spectropolarimetric studies in order to understand the origin of magnetic fields in these stars.
Studying the nature and origin of the intergalactic magnetic field (IGMF) is an outstanding problem of cosmology. Measuring Faraday rotation would be a promising method to explore the IGMF in the large-scale structure of the universe. In this talk, we present the characteristics of Faraday rotation measure (RM) due to the IGMF in filaments of galaxies. For the IGMF, we applied the one based on a turbulence dynamo model. We found that the RM through filaments is dominantly contributed by the density peak along line of sight. We predicted that the probability distribution function of |RM| follows the log-normal distribution, and the power spectrum of RM in the local universe peaks at a scale of ~1 h-1 Mpc. The rms of RM through filaments at the present universe was predicted to be ~1 rad m-2, which could be detected and tested by future radio observatories such as the Square Kilometer Array (SKA). We also mention the galactic foreground RM, and demonstrate how we can remove it, using a filtering technique.
Magnetic field is one of the key factors affecting stellar evolution, together with the mass, chemical composition, and rotation. Yet it is the most obscure one, largely due to a lack of sensitive polarimetric instrumentation and challenging theory of polarized radiative transfer. In this talk I will give an overview of our current knowledge on magnetic fields in various types of stars and substellar objects across the HR diagram and of the progress in magnetic diagnostic techniques.
Deep polarimetric imaging of a large sample of nearby galaxies has allowed well-resolved detection of polarized synchrotron emission between 1.3 and 1.8 GHz from the extended disks. Analysis of the systematic variation of both polarized intensity and Faraday depth with galactic azimuth has allowed unambiguous determination of the global magnetic field topology that prevails. The field topology is in all cases consistent with a toroidal axi-symmetric spiral combined with a quadrupole poloidal component near the disk plane, as might be expected from an alpha-omega dynamo process. The extended halo field that is probed in the most edge-on systems requires an additional field component, with a radial toroidal plus dipole poloidal topology.
We present the first cross-correlation between large-scale structure (LSS) and diffuse radio surface-brightness distribution to constrain the intergalactic synchrotron emissivity and equipartition magnetic field. We use the Australia Telescope Large Area Survey (ATLAS) and Spitzer-SWIRE survey as synchrotron and LSS tracers, respectively, over a redshift range of z=0.1-0.6. The average strength of filament magnetic fields relative to known cluster fields is a sensitive discriminator of models of the cosmological origin of large-scale magnetic fields (Donnert et al. 2009), and cross-correlation is a powerful method for circumventing foreground confusion and reaching the needed sensitivity to probe the weak fields in the low-density Warm-Hot Intergalactic Medium (WHIM). We present our initial findings, and outline the potential of this technique for upcoming sensitive radio continuum surveys using the next generation of SKA pathfinders.
We discuss growth of magnetic field induced by turbulent motions. We focus on small-scale turbulence dynamo, in which magnetic field grows by stretching of field lines. When we start off turbulence simulations with weak mean magnetic field only (or with small scale random field with zero imposed field), we observe that there are 3 stages - exponential, linear, and saturation stages. At the exponential stage, stretching is most active near the dissipation scale of turbulence. At the linear stage, the characteristic scale of turbulence increases and magnetic energy density grows linearly with time. Runs with different numerical resolutions and/or different simulation parameters show consistent results for the growth rate at the linear stage. When magnetic energy becomes comparable to the kinetic energy density, growth of magnetic field stops and the saturation stage begins. We also discuss various length scales associated with MHD turbulence.
Cosmic rays with energies below 10**15 eV have gyroradii in likely local magnetic fields which are well below 1 pc. This means that their diffusive flow will be appreciably influenced by the structure and magnitude of those fields. It appears to be possible to use data from the Pierre Auger Observatory to study particles at those energies with high statistics, although limited angular resolution. This should provide data covering a previously sparsely studied region of the sky thus completing an all-sky picture begun by Milagro, the Tibet AS array and IceCube.
An introduction is given to magnetic merging in hot coronal plasmas. Scaling laws are derived for the rate of energy release associated with magnetic reconnection solutions in two and three dimensions.
The amplitude of the magnetic field surrounding the Galactic Centre (GC) on large scales (> 100pc) has been uncertain by two orders of magnitude for several decades: different analyses report fields as weak as ~6 uG on the one hand and ~1 mG on the other. Here I report on our recent work (Crocker et al Nature 2010) which shows that the field on 400 pc scales has a firm lower limit of about 50 uG. To obtain this result we compiled existing (mostly single dish) radio data to construct the spectrum of the GC region on these size scales. This spectrum is a broken power law with a down-break (most conservatively) attributable to a transition from bremsstrahlung to synchrotron cooling of the in-situ cosmic-ray electron population. The lower limit on the magnetic field arises through the consideration that the synchrotron-emitting electrons should not produce too much gamma-ray emission given existing constraints from the EGRET instrument.
TBD
The strength and geometry of magnetic fields in our own galaxy as well as in clusters of galaxies are crucial for many phenomena, but at the same time are still poorly known. I report on progress and plans how to infer their statistical properties. In particular, new Faraday rotation based measurements of the magnetic power spectrum in the Hydra A cool core regions are presented as well as methods to extract information on magnetic force and helicity spectra from radio polarimetry are presented.
It has been known for some time that the highest magnetic fluxes seen on the main sequence are similar to the highest magnetic fluxes in the white dwarfs. This correspondence has been recently shown to extend also to the magnetars and has led to a renewed interest in the fossil field model for compact star magnetic fields. However, it still remains unclear how a large scale fossil field can survive the various stages of stellar evolution, and to what extent such a field would be modified by dynamo generated fields in the convective regions that develop during stellar evolution. Thus alternative scenarios for field generation have recently been proposed and will be presented in this talk.
Field burial and decay in accreting neutron stars are also discussed by drawing parallels with observations of accreting magnetic white dwarfs.
This talk will be largely a review of current theories which attempt to explain how magnetic fields are generated by fluid motions in the conducting gas of which stars like the Sun are comprised. The most widely used theories are based on the ideas of mean field electrodynamics, but there have been a number of (controversial) assertions that these are not applicable to the situations prevailing in the Sun's convection zone. Recently a new class of dynamos has come to light where the magnetic and velocity fields can end up with comparable energy levels within the generation region. The talk will conclude by addressing the prospects of such mechanisms for explaining the magnetic fields actually observed.
Accretion discs are an ubiquitous phenomenon in astrophysics. They can be found in various classes of objects spanning huge ranges in size. Despite the enormous variation in their actual parameters, the underlying physical mechanisms of disc dynamos are universal enough to warrant a common description.
At the lower edge of the main sequence, we enter the regime of the ultracool dwarf, populated by the lowest mass stars and brown dwarfs. In recent years, our understanding of the nature of magnetic activity in this regime has changed dramatically. Current evidence suggests that such objects truly bridge the gap between solar-type stars that exhibit coronal activity and jovian-type planets possessing large-scale magnetospheres and neutral atmospheres. This transition is manifest in every tracer conventionally associated with magnetic activity, with examples including 1) a sharp drop in H-alpha and X-ray luminosities indicating a reduction in chromospheric and coronal activity 2) the ubiquitous presence of rapid rotation highlighting a dearth of stellar wind assisted magnetic braking and 3) a transition to large-scale, stable magnetic field configurations.
Perhaps the most startling manifestation of planet-like behaviour has been observed in the radio. A number of ultracool dwarfs have been found to produce periodic radio pulses, with the resulting light curves very similar to those of pulsars. The radio emission is thought to be produced in the same fashion as that detected at kHz and MHz frequencies from the magnetized planets in our solar system and has allowed very accurate measurement of magnetic field strengths for ultracool dwarfs. These 'auroral emissions' confirm quasi-stable current configurations within the large-scale magnetospheres of these dwarfs. I will discuss these results and the possibility of detecting further auroral emissions across the electromagnetic spectrum.
I will review the methods and updated results on the Galactic magnetic fields revealed by rotation measures of pulsars and radio sources. Pulsars are most efficient probes for magnetic fields in the Galactic disk, and radio sources are good for halo magnetic fields.
As polarised radio waves pass through a magnetised, ionised medium the polarization angle is rotated by an angle that is proportional to the square of the wavelength. This is called the Faraday effect. Therefore, broadband observations of polarized point sources such as pulsars and radio galaxies may be used to probe the magnetic fields of foreground regions. Here I present the results of a study of magnetic field strengths in 5 HII regions in our Galaxy using Faraday rotation of background polarized radio sources. I will discuss the physical origins of the magnetic fields in the HII regions and relationship between density and magnetic field strength.
Cosmic rays and magnetic fields are important ingredients in the interstellar medium. Cosmic-ray electrons gyrate around magnet ic field lines and emit highly linear polarized synchrotron emis sion. This allows us to study large-scale magnetic fields in gal axies with radio continuum polarimetry. Edge-on galaxies possess radio halos indicating a vertical transport of young accelerate d cosmic-ray electrons from the disc into the halo. The magnetic field structure in many edge-on galaxies is X-shaped for yet un known reasons. The observational results can be tested with mode ls for galactic winds and/or dynamos
Establishing the role of the interstellar magnetic field in the dynamics of molecular clouds remains a critical goal in studies of star formation. I will discuss the interplay between the turbulent gas motions and the magnetic field from an observational perspective.
The importance of magnetic fields in the star formation process is extremely hard to quantify due to the difficulty in making the relevant measurements. While Submillimetre polarimetry of dust emission is arguably the most important observational tool to probe magnetic fields in molecular clouds, it has mainly only been used so far to provide a measure of the geometry of the field. Accordingly, I will discuss a promising new method introduced by Houde et al. (2009) and Hildebrand et al. (2009) for characterizing magnetic fields and turbulence from such observations. More precisely, I will describe how an analysis of the difference in the orientation of pairs of polarization vectors as a function of their separation (i.e., the structure function of the polarization angle) leads to a direct determination of the magnetized turbulent power spectrum. This type of analysis thus provides a way to test potential turbulence theories. As an example, I will present an application of this technique to high spatial resolution Submillimeter Array (SMA) polarization data obtained for Orion KL, IRAS 16293 (Rao et al. 2009), and NGC 1333 IRAS 4A (Girart et al 2006). For Orion KL we determine that in the inertial range the spectrum can be approximately fitted with a power law k-1.6 and we obtain an upper limit of 5.5 mpc for $\\delta_{\\mathrm{AD}}$, the high spatial frequency cutoff presumably due to turbulent ambipolar diffusion. For the same parameters we have ~k-1.5 and $\\delta_{\\mathrm{AD}}\\simeq2.0$ mpc for IRAS 16293, and ~k-1.2 and $\\delta_{\\mathrm{AD}}\\simeq2.2$ mpc for NGC 1333 IRAS 4A. We thus have a clear determination of the turbulent ambipolar diffusion scale directly from the magnetized turbulent power spectrum for two sources. I will also discuss the applicability of this technique to other astrophysical system of different scales.
References:
Houde, M., Vaillancourt, J. E., Hildebrand, R. H., Chitsazzadeh, S., and Kirby, L. 2009, ApJ, 706, 1504
Hildebrand, R. H., Kirby, L., Dotson, J. L., Houde, M., and Vaillancourt, J. E. 2009, ApJ, 696, 567
Rao, R. Girart, J. M., Marrone, D. P., Lai, S.-P., and Schnee, S. 2009, ApJ, 707, 921
Girart, J. M., Rao, R., and Marrone, D. P. 2006, Science, 313, 812
The structure and dynamics of cosmic magnetic fields have traditionally been inferred from observations in specific wavelength ranges. We have learned a great deal about magnetic fields on scales from planets to galaxies from radio and X-ray observations, while studies of stellar magnetic fields have benefitted from optical data also. These wavelength ranges, however, may be probing not only magnetic structures that evolve on very different scales, but also separate particle populations each governed by different physics. A complete view of the physics of magnetic fields often demands observations from many wavelength regimes, coupled with modelling that can "knit together" the various length scales and topological structures. Using stellar magnetic fields as examples, I will illustrate some of the revolutions in understanding that this approach has brought and also some of the puzzles that have emerged to challenge future modelling and observing efforts.
Magnetic fields produce activity on the Sun and cool stars. Activity itself involves both the dynamo production of magnetic fields and the magnetic heating of the outer atmosphere of stars, and in some cases brown dwarfs, which produces the radiative signatures of activity. In the past several years, measurements of the photospheric magnetic field strength have become available for a number of low mass stars and brown dwarfs, allowing us to separate the study of field production from the study of non-radiative heating. Much of the recent observational effort has centered on T Tauri stars, due to the importance of strong stellar fields in explaining this stage of star formation, and on very low mass stars and brown dwarfs where measurements of traditional activity indicators suggest sharp differences in field production and/or heating efficiency compared to what is observed in slightly higer mass stars. Somewhat surprisingly, measurements on these two classes of fully convective objects reveals mean magnetic field strengths of 2-3 kG in many cases. Such strong, pervasive fields on fully convective stars challenges some models for field generation in these objects. Here, I will review the techniques for measuring the photospheric magnetic field by looking for effects caused by Zeeman broadening of spectral lines. I will discuss the implications of the latest field measurements for dynamo theories. Finally, I will suggest some avenues likely to be fruitful for future research in the field.
Magnetic reconnection is the most basic process which governs magnetic fields in conducting fluids. If the necessary requirement for the fast reconnection is the fluid being collisionless, then most of MHD simulations, including those of interstellar medium, do not represent astrophysical reality, as high numerical diffusivity makes reconnection fast for present-day simulations. Fortunately, the model of fast reconnection proposed in Lazarian & Vishniac (1999) does not have these restrictive constraints as it allows for fast reconnection in MHD limit provided that magnetic field is weakly stochastic. As turbulence is ubiquitous in astrophysical environments, weak stochasticity of magnetic field lines is a default state for most of astrophysical magnetic fields. This still leaves a question of what is happening when the magnetic fields are rather laminar. Our research shows that the reconnection itself may increase the level of turbulence resulting in reconnection instability or bursts of reconnection. The most evident application of the model is related to explaining of Solar flares. The predictions of the model above include First Order Fermi acceleration of energetic particles within the extended reconnection layers predicted in the model as well as the removal of magnetic flux during star formation. I shall discuss the cases when the new understanding of magnetic reconnection solves the long standing astrophysical puzzles.
Because of the sensitivities of current instruments, our knowledge of cloud magnetic fields has been mostly concentrated on cloud cores, using e.g. field morphologies, Chandrasekhar-Fermi method and Zeeman measurements. Knowing field strength from cloud cores, however, tells us nothing about the global property of a cloud. Nevertheless, field strength compared to turbulence in clouds is a crucial initial condition that significantly influences the efficiency and rate of star formation. We compared magnetic field directions in high-density, small-scale cores (pc to sub-pc scale) with those in low-density, large-scale inter cloud media (several hundreds of pc), and found a significant correlation. This implies that the Galactic field orientations are well reserved inside molecular clouds, and that cloud fields are dynamically important compared to turbulence.
The competition between galactic shear and magnetic fields in cloud formation is even more difficult to observe. Because a face-on galaxy is needed to reveal the effects from galactic shear. Coriolis forces tend to transfer shearing motions and gravitational collapses into epicyclic motions. Will magnetic fields just follow the epicyclic mass flows? Or chock off the development of epicyclic motions? I will present the first polarimetry observation of extragalactic molecular clouds to shed some light on this question.
The interaction of a stellar magnetic field and the accretion disk surrounding a young star may generate a jet flow that is powered by the gradient in the toroidal field above (or below) the inner accretion disk. For the case of axisymmetric flow, it can be shown that the final total force and speed of the gas flow from the jet acceleration region is independent of the z-dependent strength of the radial currents that drive the flow. An analytic flow solution is obtained for the case where the height of the jet acceleration region is small relative to the radial distance from the star.
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. We demonstrate 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 cyclic magnetic activity on the Sun today presents clear evidence of a self-regenerating interface-layer dynamo. However, observations of the spots and magnetic fields of active young Sun-like stars suggests that a different dynamo may be operating in such stars. As the magnetic field in these stars plays a crucial role in the star's evolution, the question is how do these stars actually generate magnetic fields?
We have mapped the magnetic field topology of a small sample of young Sun-like stars over the course of several years, using the Anglo-Australian Telescope and the technique of Zeeman Doppler Imaging. This presentation outlines the techniques used as well as the results from these observations, and discusses their implications for magnetic field generation in young Sun-like stars.
There are three established forms of coherent emission in radio astronomy: plasma emission, notably in solar radio bursts, electron cyclotron emission (ECME), notably in planetary emissions and from flare stars, and pulsar emission, which is the least understood of the three. All three are usually bursty, with the bursts typically obeying log-normal statistics, well described by a maser operating near marginal stability. There are also examples of fine structures that are seemingly inconsistent with maser theory. For ECME it has been suggested these may be explained in terms of a phase-coherent, feedback model, analogous to that of Helliwell (JGR 72, 4773, 1967) for triggered VLF bursts in the terrestrial magnetosphere. I explore the suggestion that giant pulses in pulsars are also due to such a phase-coherent, feedback model.
A model in which intergalactic magnetic fields are generated by cosmic-ray particles produced by the first generation of galaxies and stars that are also responsible for reionization of the universe is presented. Such cosmic-ray particles escape from the parent galaxy into the intergalactic medium where they induce return currents. Given the finite resistivity of the intergalactic plasma, an electric field is required to sustain such return currents. The resistivity takes Spitzer's value and depends only on the plasma temperature. Temperature inhomogeneities due to cosmological structure formation cause resistivity inhomogeneities. Under general conditions this in turn causes a non-zero curl of the electric field and, hence, magnetic field generation. The magnetic field is generated at a rate of 10-16 G / Gyr.
Supersonic MHD turbulence in molecular clouds (MCs) plays an important role in the process of star formation. The effect of the turbulence on the cloud fragmentation process depends on the magnetic field strength. I discuss the idea that the turbulence is super-Alfvenic, at least with respect to the cloud mean magnetic field and support this scenario using a simulation of large-scale turbulence in the warm interstellar medium. I then address the question of the amplification of the magnetic field by the turbulence, and show that MCs may remain super-Alfvenic even with respect to their rms magnetic field strength. Finally, I discuss the comparison with the observations, showing that super-Alfvenic turbulence successfully reproduces the Zeeman measurements of the magnetic field strength in dense MC cores.
I will discuss the role of magnetic fields in the dynamics of the interstellar medium, from galaxies down to the formation of stars. On the galactic scale I will discuss the dynamics of magnetic fields in spiral arm shocks, highlighting the competing roles of cold and warm gas in disordering/maintaining order in the large scale magnetic field in spiral galaxies as well as the role of magnetic field in controlling the degree of spiral arm substructure in the gas. On smaller scales, magnetic fields were once thought to provide the controlling factor in the star formation process. More recently this role has been assigned to supersonic turbulence which can naturally explain many observational aspects of star formation in nearby molecular clouds. However, the role of magnetic fields in the turbulent picture is less clear - particularly given that observations suggest that the field strengths are significant. I will summarise some of our current understanding of magnetic fields in the turbulent star formation picture: providing a means of explaining the observed very low star formation efficiencies and relatively slow star formation rates. Magnetic fields also present theoretical challenges in explaining the formation of circumstellar discs and binaries from collapsing cores, since even relatively low field strengths can completely prevent disc formation and have a strong effect on fragmentation.
We present a high resolution full-polarisation study of the historical supernova remnant SN 1006 which combines observations performed with the Australia Telescope Compact Array and the Very Large Array. We analyse variations of the rotation measure (RM) along the shell. We find high polarised fractions at the SE, where the radio emission of the shell is faintest in total intensity.
Theoretical models of jet-launching in a wide range of astrophysical objects from protostars and microquasars to gamma-ray bursts and AGN universally invoke magnetic fields as the most plausible mechanism for the generation, acceleration and collimation of the observed jets. In the case of AGN, differential rotation of a magnetised accretion disk and/or black hole magnetosphere can generate a stiff helical magnetic field that enables efficient jet production and propagation.
I will show evidence for the presence of these helical fields in AGN jets on parsec-scales in both the synchrotron emitting region and Faraday rotating material surrounding the jet from high-resolution, multi-frequency VLBI polarization observations. Furthermore, I will explain how these observations can be used to calculate the jet magnetic field strength and, by extrapolation, estimate the field strength at the accretion disk and black hole jet-launching regions.
Finally, no jet simulation to date has shown how a random field configuration can evolve into a field structure favourable for jet launching. I will describe a promising mechanism for the generation of an ordered magnetic field in the inner accretion disk that has a unique observational signature which can be tested by current techniques.
The interaction of photons with electrons and atoms is drastically affected by the magnetic field strength of the medium in which they travel. As a result, the spectrum and the polarization of emitted radiation carry distinct signatures of the magnetic field of the source. Conversely, the spectra of magnetized cosmic sources can be used to directly probe their magnetic field strengths. In this talk, I will discuss some methods for inferring the magnetic field strengths and topologies of a variety of compact cosmic sources from magnetars to accretion disks using their spectra. I will then compare the spectroscopically measured field strengths with other probes of magnetic fields.
The electrodynamics of pulsars involves pair creation in the polar cap regions. A recent model involves oscillatory pair production, with the oscillations associated with a large amplitude electrostatic wave (LAEW). Electrons and positrons are accelerated to very high Lorentz factors in the LAEW, and as a result they emit linear acceleration emission (LAE). We present analytical investigations of radiation emitted by charged particles subject to the LAEW. We demonstrate that LAE is a valid candidate for explaining pulsar emissions.
We report detection of strong circularly polarized emission from the transient bursting source GCRT J1745-3009 based on new analysis of 325 MHz Giant Metrewave Radio Telescope observations. Implied high brightness temperature required for an object beyond 1 pc and the high fraction of circular polarization firmly establish the emission as coherent. This is interpreted as Electron cyclotron or plasma emission from a highly subsolar magnetically dominated dwarf located <=4 kpc away (Roy et al. 2010, ApJL, 712, L5).
I will briefly review the technique of using rotation measures to determine the structure of magnetic fields and their relationship to thermal materials, including the recent introduction of the powerful "rotation measure synthesis" tool. I will show that the techniques that we have been using and even rotation measure synthesis are subject to ambiguities (in addition to the well-known n*pi issue) that can actually yield the wrong answers when there is more than a single component. Most of the RM measurements in the literature are subject to these ambiguities, and are only reliable if some a priori assumptions are made. I will briefly introduce some ideas by M. Brentjens about how these ambiguities might be resolved for the next generation of radio telescopes.
Intergalactic shock waves result from the supersonic flow motions induced by hierarchical formation of nonlinear structures in the universe, and vorticity is generated at curved surfaces of the shocks. Via cascade vorticity then develops to turbulent flow motions, which in turn amplify magnetic fields and produce the intergalactic magnetic field (IGMF). In this talk, we present the estimate of the strength and coherence length of the resulting IGMF based on a turbulence dynamo model. We then discuss quantitatively the effect of the IGMF on the propagation of ultrahigh energy cosmic rays through the intergalactic space, and the Faraday rotation measure induced by the IGMF.
Interstellar medium is an ideal site for various types of dynamo action, most importantly turbulent dynamos. The fluctuation dynamo produces intermittent random magnetic fields at scales smaller than the integral scale of interstellar tubulence. These fields have the form of magnetic filaments whose volume filling factor is, probably, of order 10-30%. The mean-field dynamo generates magnetic fields at scales larger than the turbulent scale. The mean magnetic field is accompanied by a volume-filling random magnetic field.
We discuss magnetic structures produced by various dynamo mechanisms in the multi-phase interstellar medium, and the effects of the spiral pattern on galactic magnetic fields and cosmic rays.
Polarized radio emission provides valuable clues to magnetic fields in distant galaxies, while Faraday rotation of polarized sources reveals magnetic field structure in objects along the line of sight. The polarized signal is typically only a few percent of the total flux density of a source, so we know very little of the polarization properties of faint radio sources seen in deep total-intensity surveys of the sky. Several studies have found a steady increase increase in the degree of polarization of faint radio sources in the flux density regime where the radio source counts disply a shift in balance from AGN-powered radio sources above the FRI/FRII luminosity boundary to sources below the FRI/FRII luminosity boundary. I will review the evidence for this change in polarization properties with flux density and discuss the relation between polarization and spectral index from a stacking analysis of NVSS polarized intensity. Below a flux density ~1 mJy, star forming galaxies become an increasing fraction of the total number of radio sources. While many of the most luminous star forming galaxies are mergers and massive starbursts,polarization of normal disk galaxies will become an emerging population of polarized radio sources with properties unlike the bright AGN-powered polarized sources.
There are limited observational results of the magnetic (B) fields in dense star forming regions due to the availabilities of observation tools and methods. One method to probe the morphology of the B field projected in the plane of sky is through the polarization of the thermal dust continuum. The dust grains are believed to align with B fields with their minor axes in most of the case. By observing the polarized emission, we are able to derive the B fields. In this talk, I will present our observational results in two massive cores with angular resolution up to 1" in the mm/submm regime. In the closest massive star forming core Orion BN/KL, we find that the B field lines has an azimuthal symmetry. The origin of the azimuth is found to be 2.5" west to the origin of the large scale (~ 0.2 pc) explosive outflows, suggesting that the B field lines in Orion BN/KL might be correlated with the explosive event. On a larger scale of 0.5 pc, the B field was found to be! uniform and dominant over turbulence. Such difference in the B field morphologies between the small and large scale are also found in another massive core W51 e2/e8, where we find that B field lines are shaped locally in dense core e2 and e8 but remains uniform on 0.5 pc. These results seem to suggest an evolving role of B fields in different scales.
An unusual property of lower hybrid (LH) waves is that they can interact resonantly with both electrons and ions, thereby acting to transfer energy between the motions of electrons parallel to the magnetic field and the motions of the ions perpendicular to the magnetic field. There are many examples of plasmas in which LH waves may play an important role. These include the outer heliosphere, where an ion ring beam is observed, and magnetic reconnection regions, where both bulk flows of ions across the magnetic field and electrons accelerated along the magnetic field are observed. We consider regions where bulk flow of electrons along the magnetic field results in a current and may drive waves near the LH frequency. For cases with a sufficiently fast bulk flow of electrons our new numerical and analytic results show a beam mode at frequencies near the LH frequency which sometimes exhibits an instability, transferring energy from the particles to the waves.
The escape of radio emission from a pulsar is not well understood. One model of the pulsar magnetosphere is a time-dependent, oscillating model. The escape of radiation in such a model involves time-dependent coupling between radiation generated near the plasma frequency and outward-propagating electromagnetic modes. This process differs from other proposed emission mechanisms. I outline here some of the proposed mechanisms, then the oscillating model and its implications.
Outside sunspots and solar active regions, there appear network, intranetwork magnetic elements and ephemeral (active) regions everywhere on the quiet Sun. Moreover, these components of solar magnetism have much higher rate of flux emergence than that of sunpots. We first make a brief summary on the early ground-based and space-borne observations of solar small-scale magnetic fields, then present some of our recent studies of quiet Sun magnetism based on the analysis of Hinode Stokes polarimeter and narrow-band filtergraph data. The emphasis will be put on the flux appearance and distribution of tiny interanetwrok magnetic elements, their lifestory and dynamics, and vector magnetic fields of solar granules. Finally, we examine a solar-cycle-long database of Sun's magnetic flux and address the question on whether or not the quiet Sun magnetism follows the sunspot cycle.
Direct measurements of the strong magnetic field of 1012 -1013 G in neuotron stars are still difficult. Detection of the cyclotron resonant absorption features in the X-ray spectra of neutron star systems is a powerful tool to directly measure the magnetic field. INTEGRAL/IBIS is a hard X-ray imager with a good angular resolution and spectral resolution from 18 - 200 keV. INTEGRAL/IBIS hard X-ray surveys have confirmed previously reported cyclotron lines in Vela X-1 (25/50 keV) and A 0535+26 (45/100 keV). In addition, new discoveries were also done: pair cyclotron lines (30/60 keV) in a neutron star binary candidate 4U 2206+54 and cyclotron line at 35 keV in a transient X-ray source IGR J01583+6713. The ongoing INTEGRAL/IBIS hard X-ray surveys would discover more cyclotron absorption features, then identify more magnetic neutron stars in the Galaxy.
Surface magnetic field values may be inferred for a variety of stellar objects. The most detailed information is obtained for the Sun, where the vector magnetic field (all three components of the magnetic field) is routinely determined from spectro-polarimetric observations of photospheric lines. A brief review is presented of the status of magnetic field modelling based on the boundary information provided by the observations, with emphasis on recent advances in solar coronal magnetic field modelling.
Ultra high energy cosmic rays, above energies of roughly 1019.5 eV, present the possibility of probing large scale magnetic fields both within the Milky Way and in local intergalactic space. Assuming that particles at these energies retain information about their point of origin, ultra high energy cosmic ray data may be studied to infer properties of cosmic magnetic fields in regions around their arrival directions.
A technique for analysing arrival directions of ultra high energy cosmic rays will be presented and results from Monte Carlo simulations discussed.
Magnetism is an essential element in the formation of stars and planets. But the influence of magnetic fields still suffers from a lack of observational constraints, specifically in the near environs of young stars, say from several tens to hundreds of AU, and especially the region connecting the disk and outflow. Open questions include: What is the configuration of magnetic fields in the disks and outflows of young stars, and how does it influence the initial collapse phase, outflow generation and collimation, and/or disk viscosity?
In this context, polarimetry in the mid-IR ¬C defined as 5-30 um ¬C is a unique and powerful technique. The polarization arises via a dichroic effect of the dusty ISM. Cosmic dust grains rotate due to collisions with gas and other grains, and through an interaction between an external magnetic field and internal magnetic moments, the rotation component along the grain long axis is damped, bringing the short axis and angular momentum vector into alignment with the magnetic field. Such preferential alignment means that radiation absorbed or emitted by the ensemble of grains becomes partially polarized, and the polarization angle can be related to the magnetic field direction projected onto the plane-of-the-sky. In dichroic absorption the position angle of polarization is parallel to the field, whilst for dichroic (thermal) emission it is perpendicular.
A unique feature of mid-IR polarimetry is that both emissive and absorptive components of polarization can be detected in a single observation. These are indicative of cold (Á† 100 K) and warm (Á? 200 K) dusty regions respectively. Because of their unique spectral form the two components can be readily separated, thus providing spatial information on the magnetic field.
We will present a review of the polarization - and hence magnetic field - studies carried out by the UNSW@ADFA group over the last few decades, spanning single-beam and long-slit spectropolarimetry, plus imaging polarimetry, on 4-m class telescopes. Relevant targets include embedded young stellar objects, HII regions, the Galactic Centre, dust shells of evolved stars and an AGN.
But mid-IR polarimetry has been a photon starved field, requiring long integrations on 4-m telescopes and stable atmospheric conditions to detect the relatively small degrees of polarization of less than 10%. There is thus a large potential discovery space awaiting the routine coupling of a sensitive mid-IR imaging- and spectro-polarimeter on a 10-m class telescope. The CanariCam instrument ¬C built by the University of Florida - heralds such an opportunity. It will be used with the Gran Telescopio Canarias (GTC), at 10.4-m the largest diameter optical/IR telescope currently operating in the world. CanariCam employs a new technique to measure the polarization in the mid-IR, using a Wollaston prism to simultaneously detect the orthogonal polarization states necessary to calculate the Stokes parameters Q and U, and thus the degree of linear polarization.
CanariCam will thus be an order of magnitude more sensitive to polarization than previously possible, not to mention providing sub-arcsecond spatial resolution. This will make much fainter sources amenable to polarization detection, including those similar in mass to our own Sun. CanariCam is scheduled to be commissioned on the GTC in May 2010. We will discuss the promise it holds for magnetic field studies toward a variety of astrophysical objects, and with luck will present some of the first data.
There are evidences that the hot gases in galaxy clusters are permeated by magnetic fields. In particular, some clusters, e.g. the halo and relic sources, are found to be strong radio sources. The questions are now: what is the origin of the magnetic field in galaxy clusters or superclusters? and what is the morphological structures of the magnetic fields in the intracluster and intercluster media? I will address the latter issue and discuss how the polarization of radio emission can be used to infer the structural properties of magnetic fields in galaxy clusters and larger-scale structures. I will show results of some of my recent numerical polarized radiative transfer calculations of radio emission from model galaxy clusters and larger-scale superclusters (obtained by semi-analytic calculations as well as from numerical hydrodynamics simulations) and show how the spatial correlation of polarizations at various radio frequencies depend on the structures of the magnetic fields in the intracluster and intercluster media.
I would like to present a new technique of studying magnetic fields in interstellar and intergalactic gas/plasma. This technique is based on the alignment (in terms of their angular momentum in the ground state) of atoms and ions with fine or hyperfine splitting of the ground state. A unique feature of this technique is that the properties of the polarized radiation (both absorption and emission) depend on the 3D geometry of the magnetic field as well as the direction and anisotropy of incident radiation. I shall outline the prospects of the technique and its possible application to studies magnetic fields within circumstellar regions, interplanetary medium, interstellar medium, intergalactic medium. An example of an ongoing test observation will also be provided to illustrate the practical procedure of measurement.