4,613 research outputs found
Heat flow and heat generation in greenstone belts
Heat flow has been measured in Precambrian shields in both greenstone belts and crystalline terrains. Values are generally low, reflecting the great age and tectonic stability of the shields; they range typically between 30 and 50 mW/sq m, although extreme values of 18 and 79 mW/sq m have been reported. For large areas of the Earth's surface that are assumed to have been subjected to a common thermotectonic event, plots of heat flow against heat generation appear to be linear, although there may be considerable scatter in the data. The relationship is expressed as: Q = Q sub o + D A sub o in which Q is the observed heat flow, A sub o is the measured heat generation at the surface, Q sub o is the reduced heat flow from the lower crust and mantle, and D, which has the dimension of length, represents a scale depth for the distribution of radiogenic elements. Most authors have not used data from greenstone belts in attempting to define the relationship within shields, considering them unrepresentative and preferring to use data from relatively homogeneous crystalline rocks. A discussion follows
Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks
Recent observations of greatly amplified magnetic fields () around supernova shocks are consistent with the predictions of the
non-linear theory of particle acceleration (NLT), if the field is generated
upstream of the shock by cosmic ray induced streaming instability. The high
acceleration efficiencies and large shock modifications predicted by NLT need
however to be mitigated to confront observations, and this is usually assumed
to be accomplished by some form of turbulent heating. We show here that
magnetic fields with the strength inferred from observations have an important
dynamical role on the shock, and imply a shock modification substantially
reduced with respect to the naive unmagnetized case. The effect appears as soon
as the pressure in the turbulent magnetic field becomes comparable with the
pressure of the thermal gas. The relative importance of this unavoidable effect
and of the poorly known turbulent heating is assessed. More specifically we
conclude that even in the cases in which turbulent heating may be of some
importance, the dynamical reaction of the field cannot be neglected, as instead
is usually done in most current calculations.Comment: 4 pages, 1 figure, accepted for publication in ApJ Letter
Data report: late Miocene to early Pliocene coccolithophore and foraminiferal preservation at Site U1338 from scanning electron microscopy
The late Miocene to early Pliocene carbonate-rich sediments recovered at Integrated Ocean Drilling Program (IODP) Site U1338 during the Expedition 320/321 Pacific Equatorial Age Transect (PEAT) program contain abundant calcareous nanno- and microfossils. Geochemical proxies from benthic and planktonic foraminiferal and coccolithophore calcite could be very useful at this location; however, good preservation of the calcite is crucial for the proxies to be robust. Here, we evaluate the preservation of specific benthic and planktonic foraminifer species and coccolithophores in fine fraction sediment at Site U1338 using backscattered electron (topography mode) scanning electron microscopy (BSE-TOPO SEM). Both investigated foraminiferal species, Cibicidoides mundulus and Globigerinoides sacculifer, have undergone some alteration. The C. mundulus show minor evidence for dissolution, and only some specimens show evidence of overgrowth. The Gs. sacculifer show definite signs of alteration and exhibit variable preservation, ranging from fair to poor; some specimens show minor overgrowth and internal recrystallization but retain original features such as pores, spine pits, and internal testwall growth structure, whereas in other specimens the recrystallization and overgrowth disguise many of the original features. Secondary electron and BSE-TOPO SEM images show that coccolith calcite preservation is moderate or moderate to poor. Slight to moderate etching has removed central heterococcolith features, and a small amount of secondary overgrowth is also visible. Energy dispersive spectroscopy analyses indicate that the main sedimentary components of the fine fraction sediment are biogenic CaCO3 and SiO2, with some marine barite. Based on the investigations in this data report, geochemical analyses on benthic foraminifers are unlikely to be affected by preservation, although geochemical analyses on the planktonic foraminifers should be treated cautiously because of the fair to poor and highly variable preservation
Predicting Market Direction from Direct Speech by Business Leaders
Direct quotations from business leaders can communicate to the wider public the latent state of their organization as well as the beliefs of the organization\u27s leaders. Candid quotes from business leaders can have dramatic effects upon the share price of their organization. For example, Gerald Ratner in 1991 stated that his company\u27s products were crap and consequently his company (Ratners) lost in excess of 500 million pounds in market value. Information in quotes from business leaders can be used to make an estimation of the organization\u27s immediate future financial prospects and therefore can form part of a trading strategy. This paper describes a contextual classification strategy to label direct quotes from business leaders contained in news stories. The quotes are labelled as either: 1. positive, 2. negative or 3. neutral. A trading strategy aggregates the quote classifications to issue a buy, sell or hold instruction. The quote based trading strategy is evaluated against trading strategies which are based upon whole news story classification on the NASDAQ market index. The evaluation shows a clear advantage for the quote classification strategy over the competing strategies
Pan-African alkali granites and syenites of Kerala as imprints of taphrogenic magmatism in the South Indian shield
Granite and syenite plutons with alkaline affinities ranging in age from 550 to 750 Ma sporadically puncture the Precambrian granulites of the Kerala region. All the bodies are small (20 to 60 sq km), E-W to NW-SE elongated elliptical intrusives with sharp contacts and lie on or close to major late Proterozoic lineaments. Geochemical plots of A-F-M and An-Ab-Or relations show an apparent alkali enrichment trend on the former, but the plutons define relatively distinct fields on the latter. Most of the plutons are adamellitic to granitic by chemistry. The variations of SiO2 with log sub 10 K2O/MgO (1) brings out the distinct alkaline nature of the plutons. Some of the granites are extremely potassic, like the Peralimala pluton, which shows up to 11.8 percent K2O. On a SiO2-Al2O3-Na2O+K2O (mol percent) plot, the plutons vary from peraluminous to peralkaline, but none are nepheline normative. Low MgO, low to moderate CaO and high Fe2O3/FeO values are other common characteristics. Among trace elements, depletion of Ba, Sr and Rb with high K/Ba and K/Rb values are typical. Overall, the plutons show a trend of decreasing K/Rb ratio with increasing K content. Individual plutons show more clearly defined trends similar to those from granitic masses characterized by plagioclase fractionation
Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries
In the present paper we discuss the modifications introduced into the
first-order Fermi shock acceleration process due to a finite extent of
diffusive regions near the shock or due to boundary conditions leading to an
increased particle escape upstream and/or downstream the shock. In the
considered simple example of the planar shock wave we idealize the escape
phenomenon by imposing a particle escape boundary at some distance from the
shock. Presence of such a boundary (or boundaries) leads to coupled steepening
of the accelerated particle spectrum and decreasing of the acceleration time
scale. It allows for a semi-quantitative evaluation and, in some specific
cases, also for modelling of the observed steep particle spectra as a result of
the first-order Fermi shock acceleration. We also note that the particles close
to the upper energy cut-off are younger than the estimate based on the
respective acceleration time scale. In Appendix A we present a new
time-dependent solution for infinite diffusive regions near the shock allowing
for different constant diffusion coefficients upstream and downstream the
shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted
Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks
Plasma processes close to SNR shocks result in the amplification of magnetic
fields and in the acceleration of electrons, injecting them into the diffusive
acceleration mechanism. The acceleration of electrons and the B field
amplification by the collision of two plasma clouds, each consisting of
electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel
guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of
25 keV are considered. A quasi-planar shock forms at the front of the dense
plasma cloud. It is mediated by a circularly left-hand polarized
electromagnetic wave with an electric field component along the guiding
magnetic field. Its propagation direction is close to that of the guiding field
and orthogonal to the collision boundary. It has a low frequency and a
wavelength that equals several times the ion inertial length, which would be
indicative of a dispersive Alfven wave close to the ion cyclotron resonance
frequency of the left-handed mode (ion whistler), provided that the frequency
is appropriate. However, it moves with the super-alfvenic plasma collision
speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such
as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the
magnetic amplitude of this wave to values well in excess of those of the
quasi-parallel guiding field and of the filamentation modes results in a
quasi-perpendicular shock. We present evidence for the instability of this mode
to a four wave interaction. The waves developing upstream of the dense cloud
give rise to electron acceleration ahead of the collision boundary. Energy
equipartition between the ions and the electrons is established at the shock
and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy
The local Galactic magnetic field in the direction of Geminga
The Milagro hot spot A, close to the Galactic anticenter direction, has been
tentatively attributed to cosmic rays from a local reservoir (at a distance
~100 pc), freely streaming along diverging and smooth magnetic field lines.
This is at variance with the geometry of the ~kpc scale Galactic magnetic
field, which is known to be aligned with the spiral arms. We investigate the
information available on the geometry of the magnetic field on the scales (~100
pc) of relevance here. The magnetic field immediately upstream of the
heliosphere has been investigated by previous authors by modeling the
interaction of this field with the solar wind. At larger distances, we use the
dispersion measure and the rotation measure of nearby pulsars (especially
towards the third Galactic quadrant). Additional information about the local
field towards the North Polar Spur is taken from previous studies of the
diffuse radio emission and the polarization of starlight. The asymmetry of the
heliosphere with respect to the incoming interstellar medium implies a magnetic
field almost orthogonal to the local spiral arm, in the general direction of
hot spot A, but more to the south. This is in good agreement with the nearby
pulsar data on the one side, and the North Polar Spur data on the other. The
local magnetic field on scales of ~100 parsecs around the Sun seems to be
oriented so as to provide a direct connection between the Solar system and a
possible site of the Geminga supernova; the residual angular difference and the
shape and orientation of the Milagro hot spot can be attributed to the field
trailing in the wake of the heliosphere.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
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