Continuum observations of the southern extension of the radio Arc located near 1~0.2° have been carried out at λ20 and 6cm using the VLA in its hybrid B/C and C/D array configurations. A number of long and narrow filaments have been identified on the negative latitude side of the plane. Some of the filaments appear to extend continuously into the radio continuum Arc and suggesting strongly that they are associated physically with the Arc. Other filaments appear isolated and thus have characteristics similar to those of the radio “threads” which have been seen near the Galactic center. These new threads and filaments are highly polarized at λ6cm and show rotation measures which vary between 300 and 3000 rad m−2. The details present in the high-resolution images of this region strengthen the hypotheses that the large field strength is dynamically important and that the large-scale geometry of the magnetic field is poloidal near the Galactic center.

A number of unique non-thermal filamentary structures, which are all roughly perpendicular to the galactic plane have been discovered in the vicinity of the Galactic centre (see Yusef-Zadeh 1989 and references there in). On the basis of their uniformity, polarization angles, rigidity, and pressure balance with the surrounding medium, it has been hypothesized that a strong polodial field of milligauss strength pervades the inner 50 pc of the Galaxy (Morris and Yusef-Zadeh 1989 and Morris in this volume). We present here a single wide-field image, at λ90cm, which shows all the filamentary structures in this region and show that these observations are consistent with the above hypothesis.

As a mechanism to generate asymmetric radio features in the central 50 pc of the Galaxy (Sofue and Fujimoto 1987), we consider a gaseous jet from the tilted accretion disk at the center and the interaction of the jet with ambient gas on the galactic plane. It is shown schematically in figure 1 that the magnetic torques N1,2,3 exert on a gaseous element ρhΔsΔr of a ring to change its tilted orbital plane, and where γ1,2,3 are constants of a factor of unity and e1,2,3 unit vectors. The azimuthal angle ϕ is measured along the rotation from the lowest part of the ring below the galactic plane. The orbital plane is tilted against the galactic plane by the angle θ The change of the angular momentum of the disk element L = ρhΔsΔrr × v is, where 〈〉 means the average over one rotation period T or over ϕ = 0 to 2π in equations (1) to (3).

Magnetohydrodynamic (MHD) mechanisms producing radio lobes, shells, and filaments in the Galactic center as well as in the gas disk of the Galaxy are studied by using two-dimensional MHD code: (a) the explosion in a magnetized disk, (b) the interaction of a rotating disk with vertical fields, and (c) the nonlinear Parker instability in toroidal magnetic fields in a disk. In all cases, dense shells or filaments are created along magnetic field lines in a transient state, in contrast to the quasi-equilibrium filaments perpendicular to magnetic fields.

We have made observations of the Galactic center region in the CS J=1–0 and J=2–1 lines with the 45-m telescope at Nobeyama Radio Observatory in order to depict the fine structure and the activity of this region. We propose that several expanding molecular structures in this region are originated by successive supernova explosions at least of 102–3 times within 106 years. These may be responsible for strong magnetic fields in the Galactic center region. The interaction between one of the expanding molecular structures and the “vertical” filaments is found. From the appearance of the interaction, the magnetic field in the “vertical” filaments is estimated to be about 3–5 mGauss.

Mid and far IR measurements of polarized dust continuum emission from within the central 5 pc of the Galaxy have been cited as evidence for magnetic alignment of dust grains. Indirect arguments have been used to infer a B-field strength of ~ 10 mG. The presence of such high B-field would play a significant role in the hydrodynamics of the ionized and neutral gas within the region. To obtain a direct measurement of the line-of-sight B-field strength, we conducted VLA observations to try and detect the Zeeman splitting in the 1667 MHz OH absorption line against the SgrA radio source. We observed with A/B configuration, 256 1.1 km s−1 channels, and 3 arcsec resolution. In addition, both left and right circular polarizations were acquired simultaneously. We used the maximum likelihood statistical analysis technique for low signal-to-noise ratio Zeeman data discussed by Sault, Killeen, Loushin, and Zmuidzinas (1989). Analysis of the resulting absorption line spectra is complicated by the many foreground clouds. Excluding the foreground clouds by judicious selection of spectral windows, the strongest absorption by the molecular material closest (in projection) to SgrA∗ is in what Güsten et al. (1987) refer to as the redshifted cloud (it does not appear to be partaking in the general rotation of the circum-nuclear disk but is thought to be in the local vicinity of SgrA∗). Preliminary analysis reveals no detections, and 3-σ upper limits to the line-of-sight magnetic field throughout the red-shifted cloud of approximately 4 mG. By careful simulation of the lines (Gaussians are quite good approximations to these broad [40 km s−1] lines) in select spatial regions of the redshifted cloud, we find that a line-of-sight field as strong as 10 mG should be clearly detectable at about the 10-σ level if it is present. Our data do not show fields at this level.

Direct measurements of the magnetic field in the neutral ring at the Galactic centre were made by observing the Zeeman splitting in HI- absorption around ± 100 kms−1 and give a field of (–0.5mG ± 0.17mG); significantly less than inferred from IR polarisation observations (Werner et al., 1988).

Extragalactic radio sources associated with elliptical galaxies and quasars exhibit a rich variety of structures on scales ranging from < few parsecs to over a Mpc. The sources with extended (> tens of kpc) structure are often classified into the low–luminosity FRI sources which usually have diffuse outer lobes, and the high luminosity FRII sources which tend to have bright hotspots at the outer edges. (Miley 1980; Bridle & Perley 1984). The FRI and II types differ in a number of other aspects as well such as the structure and magnetic field orientation in the radio jets, morphology of the parent optical galaxies and their environments (Bridle & Perley 1984; Lilly & Prestage 1987; Owen and Laing 1989). It therefore seems unlikely that the two FR classes either represent different phases in an evolutionary sequence or are caused by differences in orientation or the supply of energy to the outer lobes.

We discuss the origin of the observed strong poloidal fields Bz in the central regions of galaxies which have gaseous rings. In the context of galactic disk dynamo models only weak poloidal fields but strong toroidal fields result (Bϕ > Bz). Therefore we tie the strength of the poloidal fields to the central activity and apply known and tested ideas rigorously. A battery process on galactic scales is discussed which ensures the existence of a large scale magnetic field in the inner galactic region. The frozen-in field may be amplified by compression and turbulent stretching; the resulting field is poloidal. The central activity provides a nonaxisymmetric flow field which, just as the α – ω dynamo produces mostly Bϕ > Bz, can produce Bz ≥ Bϕ. This model explains the structure and strength of the magnetic field in star-burst galaxies like M82 (Lesch et al., 1989).

The simplest case of the nonlinear turbulent dynamo mechanism is proposed. It is shown that under certain conditions the generated mean magnetic field can become stronger than the small-scale one. Some manifestations (the model of Cyg X-1 bimodal behaviour, asymmetric accretion onto the magnetized rotating compact star) of this mean field are discussed.

Shock acceleration of protons in the central region of active galaxies can energize them to Lorentz factors as high as 108 (Sikora et al., 1987), and these can subsequently produce a host of other relativistic particles including pions, e+e– pairs and neutrons. The luminosities of each species are expected to be of the same order of magnitude. Rapid decay of the pions leads to the secondary production of photons and pairs with energies of around 109 − 1011 MeV. The electrons and positrons can escape the compact central region and interact with the microwave background forming a pair cascade, and can also emit synchrotron radiation in the magnetic field. The neutrons do not interact with the field, and a significant fraction of them can escape the central region of a galaxy (Kirk and Mastichiadis, 1989). They can travel until they decay, producing protons and electrons in outer regions of the galaxy. Their decay time of γnτn gives a typical length for decay of about 1 kpc for the most energetic neutrons. The synchrotron radiation of these decay product electrons is examined in Baring (1989, in preparation), and it produces definite signatures of galactic magnetic fields. Magnetic fields of 1μG imply synchrotron emission in the X-ray and soft gamma-ray range for maximum Lorentz factors of γe = 1010, with a continuum extending down to much lower energies. It is observed that cooler neutrons deposit electrons at smaller radii, and these electrons are cooler (in a decay γe ~ γn). Hence the radiation would be cooler at smaller radii. This provides a diagnostic for the magnetic field: estimates of the field strength are possible from cut-offs that are expected in spectra from galactic halos. The injection of energetic electrons via neutron decay is found to yield a sharp cut-off in the injection distribution at γm = γe ~ r/τnc at radius r. Below this, no electrons are injected since they are produced in decays at smaller radii. This implies a low energy cutoff of ωm = γ2mBmec2 in the spectrum at given radius. Typically for r = 10 pc and a field of 1μG, the cutoff is at 10−8mec2 in the far infra-red. At larger radii, this low energy cutoff rapidly increases to X-ray energies. This cut-off provides a good way to measure the magnetic field strength and obtain its spatial dependence. In practice the situation is complicated by the superposition of different regions within the galaxy along the line of sight. Ways in which the observations could be deconvolved are discussed in Baring (1989, in preparation). A real possibility exists for detecting these signatures of energetic neutron emission from central regions of nearby galaxies and using the electron synchrotron spectra to spatially map galactic fields.

As an hypothesis the sharp far–infrared turnovers in the spectra of several radio–quiet galactic nuclei [1–3] are attributed to the modifications of synchrotron emission arising from the presence of a thermal background plasma. We calculate the synchrotron emission from a power-law distribution of relativistic electrons N(γ) = Noγ–s in a large–scale random magnetic field of strength B embedded in a thermal plasma of density ne. Two major modifications of the classical vacuum theory of synchrotron emission are established [4]:

A) synchrotron sources can be optically thick only in a small frequency range around the Razin -Tsytovich frequency, whereas at smaller and higher frequencies the sources are optically thin;

B) at frequencies above VR the synchrotron intensity in a plasma behaves exactly the same way as in the vacuum case, whereas at frequencies below VR the intensity is exponentially reduced,

I show that a common thermal instability of conventional plasma discharges does not occur if plasma turbulence impedes current flow. Normally, thermal discharges drain current into a single hotter filament, because classical Coulomb resistance falls as a filament heats. Anomalous resistance reverses this, stabilizing the circuit. Only small levels of turbulence are needed.

Significant progress has been made in comparing numerical simulations with radio images of astrophysical jets. One is no longer forced to compare density slices through an axisymmetric jet simulation with observed Stokes I images. With the advent of MHD codes and modern supercomputers, models can now be constrained by all four Stokes parameters. In this paper, recent efforts to simulate synchrotron emission images of extragalactic radio jets are reviewed.

Relativistic jets are formed by plasma injected into the rapidly rotating magnetosphere of accretion disks. The accretion disk generates a dipolar or a quadrupolar magnetosphere which also immerses the black hole. We present self-consistent solutions for the structure of this magnetosphere based on the Grad-Schlüter-Shafranov equation for the poloidal magnetic flux. The resulting supermagnetosonic jets consist of a nested family of magnetic surfaces filled with normal plasma flowing along the rotational axis of the central object.

By using two-dimensional magnetohydrodynamic (MHD) code, the following non-steady MHD jets and outflows are studied in relation to jets ejected from central regions of galaxies: (1) a jet driven by gas pressure gradient, collimated by vertical magnetic fields (as a model of bipolar flows ejected from a hot bubble created by starbursts); (2) a magnetic twist jet which is accelerated and collimated by J × B force in relaxing magnetic twists generated by the interaction of poloidal fields with a rotating disk (as a model of jets from active galactic nuclei); (3) a magnetic-loop-outflow resulting from the Parker instability, which may account for the initial acceleration of the magnetic twist jet and nonthermal emissions near nuclear accretion disks.

There is a long-standing mystery about the formation of the jets and lobes from AGN's. In the present paper, a “gravo-magnetodynamic” picture is proposed for the production of the jets and lobes from AGN's: The primordial large scale magnetic field, which is squeezed by the gravitational contraction of the material to the central object, serves in driving out the spinning jets in bipolar directions, and at the same time, serves as a drain extracting angular momentum of the disk material, enhancing the accretion and therefore enhancing the liberation of the gravitational energy at the center.

Radio gaps, i.e. regions of strongly reduced radio (and also optical) intensity between the galactic nucleus (AGN) and the radio jets in active radio galaxies, are a rather prominent feature of many of these sources. With the aim to explain this phenomenon we consider the evolution of the relativistic electron spectrum in such a source with the relevant plasma kinetic equations. We consider a possible acceleration process for relativistic electrons and positrons near the accreting central object which is based on the continuous interplay of stochastic Fermi acceleration processes and radiation losses of energetic particles. The resulting distribution function heats the background medium efficiently through the oblique Langmuir synchrotron instability once the threshold plasma energy density for the modulation instability is exceeded resulting in the outflow of material. By the concerted action of synchrotron losses and adiabatic focussing in the ordered, spatially decaying magnetic field Bo the isotropic relativistic electron distribution function rapidly develops into two oppositely directed beam distributions, explaining the formation of compact, superluminal radio jets near the AGN and the radio gap phenomenon. The stability of the electron/positron beam in the magnetized background plasma is studied by treating it as a cold beam. The linear growth rates for parallel propagating longitudinal and transversal waves are calculated. Whereas the former give rise to heating of the background plasma, the low-frequency transversal (Alfvén-) waves scatter the beam to an isotropic distribution function which then reveals itself again by synchrotron radiation. The mean free path for this isotropization process is calculated and compared with the length of the radio gaps. A discussion of the results is given in Achatz et al. (1989).

The jets in strong radio galaxies (SRG) terminate in hot spots (HS), which are considered as the region where the jet plasma is shocked by interaction with the intergalactic medium. Little is known about the physical parameters in the jets, in particular the velocities. The jump conditions, which are a consequence of conservation of four-momentum, magnetic flux and particle number, provide a means to connect the physical parameters in the jet with those in the HS. The synchrotron radiation which is observed from the HS has a power law spectrum. If the radiating electrons are accelerated by first order Fermi processes, the spectral index in the test particle approximation is dependent on the jump conditions only. These two ingredients, the jump conditions for relativistic MHD shocks, and first order Fermi acceleration, allow to obtain estimates of the jet velocity. It is concluded that the jets in SRG have mildly relativistic velocities of ≃ 0.3c and fast magnetosonic Mach numbers of ≃ 3. Furthermore, the fact that the jets in weaker radio galaxies show no HS is attributed to submagnetosonic velocities. If the shock heats mainly protons, then the postshock plasma heats up to temperatures of a few 1011 K. How the protons can be heated and the electrons be accelerated, is subject to the subsequent contribution (Lesch, Appl and Camenzind, these proceedings). In addition, strong magnetic fields reduce the shock strength considerably. Therefore, within the framework of magnetically collimated jets, the VLBI knots are unlikely to be shocks. Details of this work are found in Appl and Camenzind (1988).

In this progress-report I wish to describe the work my scientific collaborators and I have done over the last six years.