Nonlinear field line resonances. Effect of Hall term on plasma compression: 1D Hall-MHD modeling

A model is presented that describes the excitation of small-scale density perturbations and electromagnetic fields by standing shear Alfvén waves in a Cartesian geometry. The model includes the effects of plasma betas and the Hall term effects. The characteristics of magnetospheric density cavities and the formation of the significant peak in density are discussed in the paper.     

Magnetotelluric response of a two-dimensional sloping contact by the finite element method

Summary The finite element method, with triangular elements, is used to study the effect of a two-dimensional sloping contact on the surface electromagnetic fields. It is found in the case ofH-polarization and small slopes that the electric field and the apparent resistivity near the contact, on the conductive side, are higher than their asymptotic values. In the case ofE-polarization the apparent resistivity and phase values on the conductive side fall off less rapidly to their asymptotic values with decreasing slope resulting in higher apparent resistivity and phase values on the conductive side, than those expected for a vertical contact. The peak in the amplitude and phase of the normalized vertical magnetic field shifts from the resistive side for a vertical contact to the conductive side for a sloping contact. Far from the sloping contact, on the conductive side, higher values are observed for the normalized vertical magnetic field than in the case of a vertical contact.     

Transport of juvenile gem clams (Gemma gemma) in headland wake

Accumulation of bivalve recruits in the bottom convergence at the center of coastal eddies has been suggested as a possible mechanism resulting in locally abundant adult populations. We investigated transport of juvenile gem clams (Gemma gemma) in a headland wake to determine whether they accumulated, and where. Velocity measurements during three flood tides showed that a wake consistently formed, but that flow speeds were too slow to transport juvenile clams to the eddy center. Instead, the clams were deposited just inside the wake perimeter, where shear velocities decreased to levels below critical erosion velocities of the clams. This result demonstrated that accumulation in a coastal flow separation can occur even in the absence of a well-defined eddy or a strong bottom convergence. Juvenile gem clams were carried, probably as bedload, to regions in the wake dominated by sediments with similar grain sizes, rather than similar fall velocities, suggesting that bedload transport was particularly dependent on particle diameter in this flow regime. Adult gem clam populations tended to be locally abundant in regions receiving transported juveniles, but clam transport on any specific flood tide was not sufficient to fully predict the adult distributions.     

Ce-anomalous lavas from Rabaul caldera,Papua New Guinea

REE abundances for a suite of Rabaul caldera lavas ranging from basalt to dacite are reported. Three of the lavas contain negative Ce anomalies. These ‘anomalous’ lavas are older and have a geochemistry different from the younger lavas which form the main Rabaul caldera. The consistent geochemical differences between the two groups of lavas suggest that the Ce anomaly is inherited from the source material from which the magmas were derived and is not due to differences in crystallization or fractionation paths. Ocean floor lavas containing zeolite assemblages have been shown to possess a Ce anomaly and it is concluded that those lavas having a Ce anomaly were derived from lithosphere which had been depleted in Ce during weathering and hydrothermal processes.     

Delineation of lithostructural domains in northern Orissa (India) from Landsat Thematic Mapper imagery

Image-based reconnaissance geological mapping at 1:100,000 scale using Landsat TM data has delineated a college of Precambrian lithostructural domains within a 50,000-km² region which encompasses the northern portion of the Archaean to Proterozoic granulite-grade Eastern Ghats Tectonic Province and the adjacent Archaean-Mesoproterozoic Singhbhum Craton. The domains identified in the present study display distinctive internal structures on satellite imagery. Most are bounded by clearly recognisable major shear zones and faults on imagery; displacement directions may frequently be ascertained through local reorientation of planar structures adjacent to large strike-slip shear zones and through juxtaposition of domains with different structuring.

The macroscopic/megascopic structural overview provided by the Landsat interpretation, supported by preliminary field investigation, suggests that the N-S shortening (E-W fold/thrust packages with associated NE and NW strike-slip faulting), is the dominant structural style in the northernmost part of the Eastern Ghats Tectonic Province, followed by regional dextral transpression, expressed in the form of major strike-slip faults. The largest of these structures (Kerajang Fault), which may be traced for over 250 km, has demonstrable Paleozoic-Mesozoic dextral motion associated with coal basin formation. A precursor Kerajang shear zone with dextral movement in excess of 100 km of indeterminate age may also have been instrumental in juxtaposing the Singhbhum Nucleus into its present position to the north of the Eastern Ghats Tectonic Province.     

Geochemical comparison of woodlawn and mount painter acid volcanics,Southeastern Australia

Major‐ and trace‐element chemistry (including rare‐earth elements), total‐rock Rb‐Sr and U‐Pb and zircon U‐Pb data are used in an attempt to distinguish between two essentially coeval, felsic volcanic suites: the predominantly submarine Woodlawn suite which is associated with massive Cu‐Pb‐Zn sulphide mineralization and the terrestrial Mt Painter suite, with minor vein‐type mineralization. The Woodlawn samples are the unmineralized equivalents of the volcanics in the immediate ore environment.

Alteration perturbs some of the major‐ and trace‐element chemistry, particularly Ca and alkalis, thereby precluding their usefulness. REE patterns exhibit a significant light to heavy rare‐earth enrichment with an average La/Yb of 12 in the Mt Painter volcanics compared with 5.6 in the Woodlawn volcanics. Both suites have a marked negative Eu anomaly, with that of the Woodlawn samples more pronounced (‐45.5) than in the Mt Painter volcanics (‐29.2). A hydrothermally‐altered sample from Woodlawn has apparently lost about 50% of its light rare‐earth elements.

Initial ⁸⁷Sr/⁸⁶Sr ratios at about 0.711 are the same for rocks from both suites and differences in initial lead‐isotopic ratios appear negligible.

Zircons from both suites are a mixture of clear euhedral crystals and rounded discrete crystals or rounded cores overgrown by clear zircon. The U‐Pb data substantiate the morphological features in that the zircon suites both contain older inherited Pb but the Mt Painter zircons contain a greater proportion.

Cs concentrations and Cs/Rb and Ti/Zr ratios can be used to distinguish between the Woodlawn suite and the Mt Painter suite.     

Rare earth element geochemistry of Late Cenozoic alkaline lavas of the McMurdo Volcanic Group, Antarctica

Rare earth element (REE) concentrations were determined in 16 Ross Island and northern Victoria Land alkaline lava samples which were representative of four lava lineages of the McMurdo Volcanic Group, Antarctica. A kaersutite and two feldspar mineral separates were also analysed.

Two of the lava lineages, a basanite to nepheline benmoreite and a basanite to phonolite, have similar chondrite-normalized REE fractionation patterns, with a continuous enrichment of light and heavy REE and depletion of middle REE. The patterns result from the fractionation of olivine, clinopyroxene, spinels, feldspar, kaersutite and apatite. Kaersutite is an important fractionated phase responsible for the middle REE depletion.

Another of the lava lineages is mildly potassic with trachyandesite to peralkaline K-trachyte lavas which have partly overlapping REE fractionation patterns. There is a depletion in REE from tristanite to K-trachyte. Fractionation of olivine, clinopyroxene, feldspar and apatite probably control the REE chemistry of the lineage, greater degrees of apatite fractionation deplete the K-trachyte in REE relative to the tristanite. Feldspar fractionation in the genesis of the peralkaline K-trachyte is shown by a large negative Eu anomaly (Eu/Eu* = 0.10).

A nepheline hawaiite to anorthoclase phonolite lava lineage from the Erebus Centre shows enrichment of REE, although minor overlapping in the middle REE does occur. Anorthoclase phonolite has a positive Eu anomaly (Eu/Eu* = 1.31), indicating possible accumulation of anorthoclase. The lineage resulted from fractionation of olivine, clinopyroxene, magnetite and apatite.     

Interaction between nulls and emission in the pulsar B0834+06

We present a detailed study of the single pulses of the bright radio pulsar B0834+06, and offer evidence that the dominant periodic modulation in this pulsar's emission governs the occurrence of nulls. The nulls of B0834+06 constitute approximately 9 per cent of the total pulses and we demonstrate that they do not occur at random in the pulse sequence. On the contrary, they are found to occur preferentially close to the minimum of the pulsar's emission cycle, whose period jitters around a central value of P ₃≈ 2.17 rotation periods. It is likely that the intrinsic duration of the nulls averages about 0.2 times the pulsar rotation period. Surprisingly, the clearly distinct population of nulls and partial nulls of B0834+06 exhibit a two-peak profile slightly broader than that of the normal emission. This is in contrast to the profile of extremely weak normal pulses, which is narrower than the overall profile. A flow/counterflow model for the pulsar's two components can reproduce the essential observed features of the emission in its dominant mode, with nulls occurring at the point where the minima of the two systems are aligned. This suggests that the observed nulling rate is determined by the chance positioning of our sightline with respect to the system. If the flow is interpreted as part of a circulating carousel, a fit yields a best estimate of 14 'sparks'.     

A gap in the North American Central Plains conductivity anomaly

The palaeotectonic models of the North American continent have been deeply influenced by the concept of a North American Central Plains conductivity anomaly (NACPCA) which was postulated as a result of large area, wide-spaced, array studies, using the Magnetic Deep Sounding (MDS) method.

A recent closely spaced magnetotelluric (MT) survey across this proposed anomaly near the Canadian-U.S. boundary by Jones and Savage (1986) failed to detect the NACPCA at the location suggested by the MDS. However, this survey did verify a conductivity anomaly discovered by Rankin and Kao (1979) and a new anomaly 75 km east of NACPCA. Our present study verifies this latter anomaly and quantitative results are presented. The striking isotropy in apparent resistivity does not prevent the interpretation of anomalous resistivity distributions. This is discussed in the text.

The failure of the MDS array studies to detect our anomaly is readily explainable by the wide station spacing; however, the failure of the MT survey to detect the NACPCA leads us to the conclusion that it may not be a continuous feature.