Mapping of lithological units, structural units and surface drainage using Skylab data

There are no author-identified significant results in this report

Mapping of lithologic and structural units using multispectral imagery

The author has identified the following significant results. ERTS-1 MSS imagery covering the Afar-Triangle/Ethiopia and adjacent regions (Ethiopian Plateau, Somali Plateau, and parts of Yemen and Saudi Arabi) was applied to the mapping of lithologic and structural units of the test area at a scale 1:1,000,000. Results of the geological evaluation of the ERTS-1 imagery of the Afar have proven the usefullness of this type of satellite data for regional geological mapping. Evaluation of the ERTS images also resulted in new aspects of the structural setting and tectonic development of the Afar-Triangle, where three large rift systems, the oceanic rifts of the Red Sea and Gulf of Aden and the continental East African rift system, seem to meet each other. Surface structures mapped by ERTS do not indicate that the oceanic rift of the Gulf of Aden (Sheba Ridge) continues into the area of continental crust west of the Gulf of Tadjura. ERTS data show that the Wonji fault belt of the African rift system does not enter or cut through the central Afar. The Aysha-Horst is not a Horst but an autochthonous spur of the Somali Plateau

Transmission Line Analogy for Relativistic Poynting-Flux Jets

Radio emission, polarization, and Faraday rotation maps of the radio jet of the galaxy 3C 303 have shown that one knot of this jet carries a {\it galactic}-scale electric current and that it is magnetically dominated. We develop the theory of magnetically dominated or Poynting-flux jets by making an analogy of a Poynting jet with a transmission line or waveguide carrying a net current and having a potential drop across it (from the jet's axis to its radius) and a definite impedance which we derive. Time-dependent but not necessarily small perturbations of a Poynting-flux jet are described by the "telegrapher's equations." These predict the propagation speed of disturbances and the effective wave impedance for forward and backward propagating wave components. A localized disturbance of a Poynting jet gives rise to localized dissipation in the jet which may explain the enhanced synchrotron radiation in the knots of the 3C 303 jet, and also in the apparently stationary knot HST-1 in the jet near the nucleus of the nearby galaxy M87. For a relativistic Poynting jet on parsec scales, the reflected voltage wave from an inductive termination or load can lead to a backward propagating wave which breaks down the magnetic insulation of the jet giving $|{\bf E}| /|{\bf B}|\geq 1$. At the threshold for breakdown, $|{\bf E}|/|{\bf B}|=1$, positive and negative particles are directly accelerated in the ${\bf E \times B}$ direction which is approximately along the jet axis. Acceleration can occur up to Lorentz factors $\sim 10^7$. This particle acceleration mechanism is distinct from that in shock waves and that in magnetic field reconnection.Comment: 8 pages, 6 figure

A multi-frequency study of the peculiar interacting system Arp 206

Arp 206 is a nearby, relatively large, and bright interacting system comprising unequal members: NGC 3432 and UGC 5983. A third anonymous galaxy, Arp 206c, is visible in the field. The CCD images show a well-developed bridge between NGC 3432 and UGC 5983. On the other hand, the complex H I tails are not visible in the optical. In the total H I map, the bridge is lost in a general envelope encompassing both galaxies. The bridge also appears to have some radio emission. On the Total H I map the system is rather edge-on, far more than it would appear in optical wavelengths. UGC 5983 falls exactly in line with NGC 3432. The velocity of the centers of mass of NGC 3432 and UGC 5983 are 530 km s(exp -1) and 630 km s(exp -1), respectively. In view of the considerable damage sustained by NGC 3432 and the apparent low mass of UGC 5983, it appears that the passage must have been at near parabolic speed, with a small pericentric distance and a very low inclination with rspect to the disk of NGC 3432. The apparent distribution of H I along the z axis of the galaxy could be accounted for by projection effects. The tidal appendage found at higher velocities, which rises at a P.A. approx. equal to 25 degrees west of the main body of the galaxy is probably the tail, the part of the tidal damage away from the perturbing companion. The bridge may be rising north-east from the galaxy and then continue under to the south of the galaxy. The relative sizes of the appendages would indicate that the pericenter was crossed recently. Any further inferences about the collision parameters will need to await the results of detailed computational modelling of the interaction. The authors also summarize the observational characteristics of NGC 3432, UGC 5983, and Arp 206c

The Cosmic Large-Scale Structure in X-rays (CLASSIX) Cluster Survey I: Probing galaxy cluster magnetic fields with line of sight rotation measures

To search for a signature of an intracluster magnetic field, we compare measurements of Faraday rotation of polarised extragalactic radio sources in the line of sight of galaxy clusters with those outside. We correlated a catalogue of 1383 rotation measures (RM) of extragalactic polarised radio sources with X-ray luminous galaxy clusters from the CLASSIX survey (combining REFLEX II and NORAS II). We compared the RM in the line of sight of clusters within their projected radii of r_500 with those outside and found a significant excess of the dispersion of the RM in the cluster regions. Since the observed RM is the result of Faraday rotation in several presumably uncorrelated magnetised cells of the intracluster medium, the observations correspond to quantities averaged over several magnetic field directions and strengths. Therefore the interesting quantity is the standard deviation of the RM for an ensemble of clusters. We found a standard deviation of the RM inside r_500 of about 120 +- 21 rad m^-2. This compares to about 56 +- 8 rad m^-2 outside. We show that the most X-ray luminous and thus most massive clusters contribute most to the observed excess RM. Modelling the electron density distribution in the intracluster medium with a self-similar model, we found that the dispersion of the RM increases with the column density, and we deduce a magnetic field value of about 2 - 6 (l/10kpc)^-1/2 microG assuming a constant magnetic field strength, where l is the size of the coherently magnetised intracluster medium cells. This magnetic field energy density amounts to a few percent of the average thermal energy density in clusters. When we assumed the magnetic energy density to be a constant fraction of the thermal energy density, we deduced a slightly lower value for this fraction of 3 - 10 (l/10kpc)^-1/2 per mille.Comment: 7 pages, 6 figures, in press, Astronomy and Astrophysics, 201

Problems of the rotating-torsion-balance limit on the photon mass

We discuss the problems (and the promise) of the ingenious method introduced by Lakes, and recently improved on by Luo, to detect a possible small photon mass $\mu$ by measuring the ambient magnetic vector potential from large scale magnetic fields. We also point out how an improved ``indirect'' limit can be obtained using modern measurements of astrophysical magnetic fields and plasmas and that a good ``direct'' limit exists using properties of the solar wind.Comment: 4 pages, revised title and content

Strong magnetic fields and large rotation measures in protogalaxies by supernova seeding

We present a model for the seeding and evolution of magnetic fields in protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy provide magnetic seed fields, which are subsequently amplified by compression, shear flows and random motions. We implement the model into the MHD version of the cosmological N-body / SPH simulation code GADGET and we couple the magnetic seeding directly to the underlying multi-phase description of star formation. We perform simulations of Milky Way-like galactic halo formation using a standard LCDM cosmology and analyse the strength and distribution of the subsequent evolving magnetic field. A dipole-shape divergence-free magnetic field is injected at a rate of 10^{-9}G / Gyr within starforming regions, given typical dimensions and magnetic field strengths in canonical SN remnants. Subsequently, the magnetic field strength increases exponentially on timescales of a few ten million years. At redshift z=0, the entire galactic halo is magnetized and the field amplitude is of the order of a few $\mu$G in the center of the halo, and 10^{-9} G at the virial radius. Additionally, we analyse the intrinsic rotation measure (RM) of the forming galactic halo over redshift. The mean halo intrinsic RM peaks between redshifts z=4 and z=2 and reaches absolute values around 1000 rad m^{-2}. While the halo virializes towards redshift z=0, the intrinsic RM values decline to a mean value below 10 rad m^{-2}. At high redshifts, the distribution of individual starforming, and thus magnetized regions is widespread. In our model for the evolution of galactic magnetic fields, the seed magnetic field amplitude and distribution is no longer a free parameter, but determined self-consistently by the star formation process occuring during the formation of cosmic structures.Comment: 13 pages, 14 figures, accepted to MNRAS after moderate revisio

Remarkable symmetries in the Milky Way disk's magnetic field

Using a new, expanded compilation of extragalactic source Faraday rotation measures (RM) we investigate the broad underlying magnetic structure of the Galactic disk at latitudes $|b|$ $\lesssim 15^{\circ}$ over all longitudes $l$, where our total number of RM's in this low-latitude range of the Galactic sky is comparable to those in the combined Canadian Galactic Plane Survey(CGPS) at $|b| < 4^{\circ}$ and the Southern Galactic Plane (SGPS) $|b| < 1.5^{\circ}$ survey. We report newly revealed, remarkably coherent patterns of RM at $|b|$ $\lesssim 15^{\circ}$ from $l \sim 270^{\circ}$ to $\sim 90^\circ$ and RM($l$) features of unprecedented clarity that replicate in $l$ with opposite sign on opposite sides of the Galactic center. They confirm a highly patterned bisymmetric field structure toward the inner disc, an axisymmetic pattern toward the outer disc, and a very close coupling between the CGPS/SGPS RM's at $|b| \lesssim 3^{\circ}$ ("mid-plane") and our new RM's up to $|b| \sim 15^{\circ}$ ("near-plane"). Our analysis also shows the approximate $z$-height -- the vertical height of the coherent component of the disc field above the Galactic disc's mid-plane -- to be $\sim 1.5$kpc out to $\sim 6$ kpc from the Sun. This identifies the approximate height of the transition layer to the halo field structure. We find no RM sign change across the plane within $|b| \sim 15^{\circ}$ in any longitude range. The prevailing {\it disc} field pattern, and its striking degree of large scale ordering confirm that our side of the Milky Way has a very organized underlying magnetic structure, for which the inward spiral pitch angle is $5.5^{\circ}\, \pm 1^{\circ}$ at all $|b|$ up to $\sim 12^{\circ}$ in the inner semicircle of Galactic longitudes. It decreases to $\sim 0^{\circ}$ toward the anticentre.Comment: 7 pages, 5 figures, Version 3. Accepted 2011 for publication in Publications of the Astronomical Society of Australia(PASA