Coronal magnetic fields from faraday rotation observations

In the first part of this communication we briefly summarize the results of the first observation of linear polarization in the microwave emission above a solar active region obtained with the Westerbork Synthesis Radio Telescope, taking advantage of the very narrow bandwidths of a multi-channel spectral line receiver. The intensity of the Stokes parameterU, measured at several points close to the line of zero circular polarization, showed a clear sinusoidal trend as a function of ², in accordance to what is expected from Faraday rotation (Alissandrakis and Chiuderi Drago, 1994). Combining the measured period of the Faraday rotation with the observed deplacement of the depolarization line with respect to the photospheric neutral line, the height above the photosphere of the depolarization point and the value of the electron density and the magnetic field at this point are computed. Although the calculations are done in the very simplified assumptions of a bipolar magnetic field and of a density following hydrostatic equilibrium, they represent the first estimate of the coronal magnetic field in an active region, far from sunspots.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Interpretation of the apparent north-south asymmetry and fluctuations of galactic rotation

A model of the apparent north-south asymmetry of the rotation curve of the Galaxy and of the asymmetry appearing for the rotation of its outskirts has been presented, in terms of interaction redshifts. Fluctuations in the rotation curve for neutral hydrogen are discussed and interpreted as expanding motions in the arms. The expansion of the disk of the Galaxy which has been suggested previously as an explanation for the asymmetry has been shown to meet some serious difficulties. A remarkable north-south symmetry both in the spatial distribution and in the kinematics of neutral hydrogen in the subsolar region follows from the present model.     

Structure of the magnetic field near the galactic plane

A method is introduced for constructing two-color maps for the in-plane component of the magnetic field of our galaxy in (R, l) and (DM, l) coordinates. It is shown that, in agreement with the standard models of the galactic magnetic field, the magnetic field in neighboring spiral arms reverses direction. However, the magnetic field in the spiral arm of Sagittarius differs significantly from the standard magnetic field model, with the major difference being that the magnetic fields in the southern and northern hemispheres are oppositely directed in the spiral arm of Sagittarius. It is proposed that this distribution of the magnetic field can be explained best by assuming that the spiral arm of Sagittarius, or, at least, a magnetic spiral arm in that region, is not symmetric with respect to the galactic plane and lies mainly in the northern hemisphere.     

Extragalactic cosmic rays and the galactic magnetic field

The origin and behavior of cosmic rays in the Galaxy depends crucially upon whether the galactic magnetic field has a closed topology, as does the field of Earth, or whether a major fraction of the lines of force connect into extragalactic space. If the latter, then cosmic rays could be of extragalactic origin, or they could be of galactic origin, detained in the Galaxy by the scattering offered by hydromagnetic waves, etc. If, on the other hand, the field is largely closed, then cosmic rays cannot be of extragalactic origin (at least below 10¹⁶ eV). They must be of galactic origin and escape because their collective pressure inflates the galactic field and they push their way out.This paper examines the structure of a galactic field that opens initially into intergalactic space and, with the inclusion of turbulent diffusion, finds no possibility for maintaining a significant magnetic connection with an extragalactic field. Unless some mechanism can be found, we are forced to the conclusion that the field is closed, that cosmic rays are of galactic origin, and that cosmic rays escape from the Galaxy only by pushing their way out.     

The origin of our galactic magnetic field

A serious difficulty with the standard alpha‐omega theory of the origin of galactic magnetic fields involves the question of flux expulsion. This is intimately related to flux freezing. The alpha‐omega theory is shown in the context of the giant superbubble explosions that have a large impact on the physics of the interstellar medium. It is shown that superbubbles alone can duplicate the processes of the alpha‐omega dynamo and produce exponential growth of the galactic magnetic field. The possibility of the blow‐out of pieces of the magnetic field is discussed and it is shown that they have the potential to solve the flux‐expulsion problem. However, such an explanation must lead to apparent ‘gaps’ in the field in the galactic disc. These gaps are probably unavoidable in any dynamo theory and should have important observable consequences, one of which is an explanation for the escape of cosmic rays from the disc (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Differential rotation of the photospheric magnetic field

The differential rotation of the large-scale photospheric magnetic field has been investigated with an autocorrelation technique using synoptic charts of the photospheric field during the interval 1959–66. Near the equator the rotation period of the field is nearly the same as the rotation rate of long-lived sunspots studied by Newton and Nunn. Away from the equatorial zone the field has a significantly shorter rotation period than the spots. Over the entire range of latitudes investigated the average rotation period of the photospheric magnetic field was about 1 1/4 days less than the average rotation period of the material observed with Doppler shifts by Livingston and by Howard and Harvey. Near the equator the photospheric field results agree with the results obtained from recurrent sunspots, while above 15° the photospheric field rotation rates agree with the rotation rates of the K corona and the filaments.     

Hierarchial galactic dynamo and seed magnetic field problem

A new approach to the galactic seed magnetic field problem is briefly discussed. It is shown that, in early stages of galactic evolution, the hierarchial agglomeration and fragmentation processes can account for the generation of a dynamically important magnetic field. The amplification of this field follows an inverse cascade since a non-zero average value of the field amplified on a smaller scale serves as a seed field on the next (earlier) hierarchial scale. In such a scenario, a problem of how to get things started never occurs as any infinitesimally small battery generated seed field (Lazarian 1992a) can be efficiently amplified passing by through a sufficient number of amplification cascades.     

Maintenance of spiral arms and galactic magnetic field configurations

Of the various proposed mechanisms to maintain spiral arms in spiral galaxies, three have been supported by observations, statistics, or theories (bar, companion, extended solid-body rotation curve). It is shown here that in the presence of a central bar or oval distirtion to maintain spiral arms, the global magnetic field lines also follow the spiral shape of the arms. Excluding then barred galaxies, it is confirmed that in the presence of a companion galaxy to maintain spiral arms, the global magnetic lines in a spiral galaxy will either follow thespiral shape of the arms (when tides are larger), or else will follow thering shape of the orbit of matter crossing spiral arms (when tides are small). In the presence of an extended solid-body rotation curve to maintain spiral arms within the solid-body rotation region, the global magnetic field lines also follow the spiral shape of the arms.The results above do not favour the hypothesis that a weak intergalactic magnetic field could have been amplified enough by gravitational contraction of a protogalaxy to give rise to the observed strength of galactic magnetic fields. On the contrary, leakage of galactic magnetic fields into intergalactic/cosmological space is expected.     

Orientation of the galactic magnetic field in the vicinity of the solar system

It is found from analysis of the position angles of the plane of polarization of about 3000 stars (¦b¦ 5° andP 0.5%) that the angle between the magnetic field and the equatorial plane of the galaxy is approximately 0–5°. The distance within which the local magnetic fields of the galaxy have a greater effect on the position angles of the plane of polarization than the galactic magnetic field is estimated to be about 500 pc. The effect of the galactic magnetic field becomes dominant for distancesr 1000 pc.Translated fromAstrofizika, Vol. 39, No. 4, pp. 553–559, November, 1996.     

The dynamical coupling of cosmic rays and magnetic field in galactic disks

In this paper we demonstrate the importance of cosmic rays for the dynamics of the interstellar medium. We present the first 3D-MHD numerical simulations of the Parker instability triggered by cosmic rays accelerated in supernova remnants. We show that in the presence of galactic rotation a net radial magnetic field is produced as a result of the cosmic ray injection. This process provides a very efficient magnetic field amplification within the general frame of so called fast galactic dynamo proposed by Parker (1992). This revised version was published online in September 2006 with corrections to the Cover Date.     

New pulsar rotation measures and the Galactic magnetic field

We measured a sample of 150 pulsar rotation measures (RMs) using the 20-cm receiver of the Parkes 64-m radio telescope. 46 of the pulsars in our sample have not had their RM values previously published, whereas 104 pulsar RMs have been revised. We used a novel quadratic fitting algorithm to obtain an accurate RM from the calibrated polarization profiles recorded across 256 MHz of receiver bandwidth. The new data are used in conjunction with previously known dispersion measures and the NE2001 electron-density model to study models of the direction and magnitude of the Galactic magnetic field.     

Velocity field in the galactic plane

In this paper we present a study of the velocity components of the local centroids in the galactic plane for a point-axial stellar system model in a non-stationary state and with an equatorial plane of symmetry. The main results obtained are:(1) A displacement and attenuation of the maximum of the velocity-distance curve for different values of the direction. (2) The existence of expansion and contraction zones in the galactic plane.It must be pointed out that, in general, these velocity-distance curves require corrections due to the inclination of the galaxy. In the same way, it would be necessary to integrate the light along the line of vision. In the cylindrical case this latter factor has been studied by Catala (1975). However, these corrections do not greatly affect the structural form of the rotation curve as far as the aspects dealt with in this paper are concerned.     

On the structure of the magnetic field near a black hole in active galactic nuclei

Using the Grad-Shafranov equation, we consider a new analytical model of the black hole magnetosphere based on the assumption that the magnetic field is radial near the horizon and uniform (cylindrical) in the jet region. Within this model, we have managed to show that the angular velocity of particles Ω_F near the rotation axis of the black hole can be smaller than Ω_H/2. This result is consistent with the latest numerical simulations.     

Cosmic rays and galactic rotation curves

A theoretical basis for modifying Newtonian dynamics on a galactic scale can be obtained by postulating that cosmic rays interact with graviton exchanges between distant masses. This assumes that these charged particles move under the influence of local electromagnetic fields rather than the weak gravitational fields of distant matter. It leads to an enhancement of graviton exchanges between distant masses via an additional gravitational force term inversely proportional to distance. At planetary and local interstellar distances this predicts an extremely small additional gravitational force, but it can become significant on a galactic scale. The model is used here to predict rotational velocities in a wide range of galaxies including the Milky Way, Andromeda (M31) and some galaxies in the THINGS study. Results are obtained assuming a galactic cosmic ray density consistent with observations in the solar system. This approach is compared with the dark matter hypothesis and with Modified Newtonian Dynamics (MOND), the two primary postulates used to explain the constant rotational velocities observed in most galaxies.