Extension of the solar limb at sub-millimeter and millimeter wavelengths

Solar limb scanning at 5 wavelengths from 0.35 to 2 mm on the JCMT has revealed significant limb extension which increases rapidly with wavelength, in agreement with other measurements. This appears to be related to the increasing opacity of overlapping spicules which appear to become optically thick at about 1mm.     

X-ray luminosities of galaxies in groups

We have derived the X-ray luminosities of a sample of galaxies in groups, making careful allowance for contaminating intragroup emission. The L _X: L _B and L _X: L _FIR relations of spiral galaxies in groups appear to be indistinguishable from those in other environments, however the elliptical galaxies fall into two distinct classes. The first class is central-dominant group galaxies, which are very X-ray luminous and may be the focus of group cooling flows. All other early-type galaxies in groups belong to the second class, which populates an almost constant band of L _X/ L _B over the range 9.8< log  L _B<11.3 . The X-ray emission from these galaxies can be explained by a superposition of discrete galactic X-ray sources together with a contribution from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In the region where the optical luminosity of the non-central group galaxies overlaps with the dominant galaxies, the dominant galaxies are over an order of magnitude more luminous in X-rays.
We also compared these group galaxies with a sample of isolated early-type galaxies, and used previously published work to derive L _X: L _B relations as a function of environment. The non-dominant group galaxies have mean L _X/ L _B ratios very similar to those of isolated galaxies, and we see no significant correlation between L _X/ L _B and environment. We suggest that previous findings of a steep L _X: L _B relation for early-type galaxies result largely from the inclusion of group-dominant galaxies in samples.     

1.3–0.9 Ga Oaxaquia (Mexico): Remnant of an arc/backarc on the northern margin of Amazonia

Rocks with ages of ca. 1 Ga occur in central and southern Mexico as inliers surrounded by ubiquitous Mesozoic and Cenozoic rocks. They appear to share a common history consisting of: (i) ca. 1300–1200 Ma arc magmatism and deposition of sediments including evaporites; (ii) ca.1160–1100 Ma intrusion of syenite, granite and anorthosite, the later part of which is synchronous with migmatization; (iii) intrusion of a ca. 1035–1010 Ma anorthosite–gabbro–charnockite–granite (AMCG) suite; (iv) a 1000–980 Ma granulite facies tectonothermal event with a stretching axis parallel to the long axis of Oaxaquia; (v) gradual exhumation at 750 and/or 545 Ma; and (vi) 517 Ma intrusion of an isolated calcalkaline granitoid pluton. The common Precambrian geological record of these outcrops suggests that they belonged to a single terrane (Oaxaquia) and formed a juvenile arc/backarc bordering a continent that underwent collision with, and overthrusting of, the Avalonian arc at 1000–980 Ma. This buried Oaxaquia to 25–30 km and was followed by further supra-subduction zone magmatism at ca. 917 Ma. These Precambrian rocks are unconformably overlain by uppermost Cambrian and Silurian platform rocks containing Gondwanan fauna and ca. 1 detrital zircons of Oaxacan provenance. The neighbouring Mixteca terrane includes lower Paleozoic, rift-passive margin sedimentary rocks that also contain 900–750 Ma detrital zircons probably derived from the Goiás arc in eastern Amazonia. The arc-backarc tectonic setting inferred for the 1300–900 Ma rocks also suggests that Oaxaquia lay on an active periphery of Amazonia until ca. 900 Ma, well after the amalgamation of Rodinia. This precludes a location for Oaxaquia off southern and western Amazonia that are inferred to have been juxtaposed against eastern Laurentia; contiguity with eastern Amazonia is also unlikely given the absence of the 900–750 Ma convergent tectonics in the Goiás arc. This leaves northern Amazonia as the most likely position, a location that requires the least relative displacement between Oaxaquia and Amazonia. The inferred 750 and 545 Ma exhumation episodes of Oaxaquia correspond to two proposed breakup stages of Rodinia.     

Exploring an Ediacaran ‘nursery’: growth,ecology and evolution in a rangeomorph palaeocommunity

The recent discovery of a fossilized assemblage of juvenile Ediacaran rangeomorph macro‐organisms, fern‐like impressions from the Drook Formation of Newfoundland, suggests that the morphological diversity present within the earliest known communities of the Ediacaran biota was relatively high. The fledgling population of these Proterozoic organisms was smothered by volcanic material from a nearby eruption, preserving the standing community on the seafloor. As many as six different genera are identified, present less than three million years after the end of the Gaskiers glacial event. This hints at a hidden evolutionary history for these organisms that is yet to be discovered. Co‐occurring filamentous impressions, and other fossils interpreted to represent the remains of decayed and ‘effaced’ organisms, suggest that the bedding plane exhibits unusual ecological completeness, with the assemblage as a whole documenting an early example of secondary community succession. Discoveries such as this reveal the potential for high‐quality preservation of soft‐bodied Ediacaran organisms in siliciclastic successions, and provide valuable insights into rangeomorph development and palaeoecology.     

A Non-Linear Force-Free Field Model for the Evolving Magnetic Structure of Solar Filaments

In this paper the effect of a small magnetic element approaching the main body of a solar filament is considered through non-linear force-free field modeling. The filament is represented by a series of magnetic dips. Once the dips are calculated, a simple hydrostatic atmosphere model is applied to determine which structures have sufficient column mass depth to be visible in Hα. Two orientations of the bipole are considered, either parallel or anti-parallel to the overlying arcade. The magnetic polarity that lies closest to the filament is then advected towards the filament. Initially for both the dominant and minority polarity advected elements, right/left bearing barbs are produced for dextral/sinsitral filaments. The production of barbs due to dominant polarity elements is a new feature. In later stages the filament breaks into two dipped sections and takes a highly irregular, non-symmetrical form with multiple pillars. The two sections are connected by field lines with double dips even though the twist of the field is less than one turn. Reconnection is not found to play a key role in the break up of the filament. The non-linear force-free fields produce very different results to extrapolated linear-force free fields. For the cases considered here the linear force-free field does not produce the break up of the filament nor the production of barbs as a result of dominant polarity elements.     

Where Do Solar Filaments Form?: Consequences for Theoretical Models

This paper examines the locations where large, stable solar filaments form relative to magnetic bipoles lying underneath them. The study extends the earlier work of F. Tang to include two additional classification categories for stable filaments and to consider their population during four distinct phases of the solar cycle. With this new classification scheme, results show that over 92% of filaments form in flux distributions that are nonbipolar in nature where the filament lies either fully (79%) or partially (13%) above a polarity inversion line (PIL) external to any single bipole. Filaments that form within a single bipole (traditionally called Type A) are not as common as previously thought. These results are a significant departure from those of F. Tang. Consistency with the earlier work is shown when our data are regrouped to conform to the two-category classification scheme for filaments adopted by F. Tang. We also demonstrate that only filaments that form along the external PIL lying between two bipoles (62% of the full sample, traditionally called Type B) show any form of solar cycle dependence, where their number significantly increases with magnetic activity over the solar cycle. Finally, current observations and theoretical models for the formation of filaments are discussed in the context of the present results. We conclude that key elements in the formation of the majority of filaments considered within this study must be the convergence of magnetic flux resulting in either flux cancellation or coronal reconnection.     

Direct shear testing of polished slickensided surfaces

A series of ring shear and direct shear tests were performed to measure the drained residual strength of three clay soils. For each of the soils, slickensided direct shear specimens were prepared by wire-cutting intact specimens, and polishing the resulting shear plane on a variety of surfaces to align the clay particles in the direction of shear. Drained direct shear tests were then conducted on each of the polished specimens. The resulting shear strengths were compared with the residual strengths measured in the ring shear device to evaluate the effectiveness of the different polishing techniques for creating slickensided surfaces. Test results indicated that the measurement of residual strengths along preformed slickensided surfaces is extremely sensitive to both the soil type and the slickenside preparation technique that is used. Consequently, this approach does not appear to be a viable alternative to conventional repeated direct shear or ring shear tests to measure residual shear strengths.