|
Diciembre de 2006
Lost tsunami
Authors: Maria Teresa Pareschi, Enzo Boschi et al
Link: Click here
Abstract
Numerical simulations support the occurrence of a catastrophic
tsunami impacting all of the eastern Mediterranean in early
Holocene. The tsunami was triggered by a debris avalanche from
Mt. Etna (Sicily, Italy) which entered the Ionian Sea in the
order of minutes. Simulations show that the
|
resulting tsunami waves were able to destabilize
soft marine sediments across the Ionian Sea floor. This generated
the well-known, sporadically located, "homogenite"
deposits of the Ionian Sea, and the widespread megaturbidite
deposits of the Ionian and Sirte Abyssal Plains. It is possible
that, ~8 ka B.P., the Neolithic village of Atlit-Yam (Israel)
was abandoned because of impact by the same Etna tsunami. Two
other Pleistocenic megaturbidite deposits of the Ionian Sea
can be explained by previous sector collapses from the Etna
area.
|
|
Diciembre de 2006
Solar Mass Ejection Imager (SMEI) observations of coronal
mass ejections (CMEs) in the heliosphere
Authors: D. F. Webb, D. R. Mizuno et al
Link: Click here
Abstract
The Solar Mass Ejection Imager (SMEI) on the Coriolis spacecraft
has been obtaining white light
|
images
of nearly the full sky every 102 minutes for three years. We present
statistical results of analysis of the SMEI observations of coronal
mass ejections (CMEs) traveling through the inner heliosphere;
139 CMEs were observed during the first 1.5 years of operations.
At least 30 of these CMEs were observed by SMEI to propagate out
to 1 AU and beyond and were associated with major geomagnetic
storms at Earth. Most of these were observed as frontside halo
events by the SOHO LASCO coronagraphs. |
|
Diciembre de 2006
Tarapacá intermediate-depth earthquake (Mw 7.7, 2005,
northern Chile): A slab-pull event with horizontal fault plane
constrained from seismologic and geodetic observations
Authors: S. Peyrat, J. Campos et al
Link: Click here
Abstract
A large (Mw 7.7) intermediate-depth earthquake occurred on 13
June 2005 in the Tarapacá region of the northern Chile
seismic gap. Source parameters are inferred from teleseismic
broadbands, strong motions, GPS and InSAR data. Relocated hypocenter
is found at ~98 km depth within the subducting slab. The 21-days
aftershock distribution, constrained by a
|
postseismic
temporary array, indicates a sub-horizontal fault plane lying
between the planes of the double seismic zone and an upper bound
of the rupture area of 60 km × 30 km. Teleseismic inversion
shows a slab-pull down dip extension mechanism on a nearly horizontal
plane. Total seismic and geodetic moments are ~5.5 × 1020
N.m, with an averaged slip of 6.5 m from geodesy. The earthquake
rupture is peculiar in that the effective velocity is slow, 3.5
Km.s-1 for a high stress-drop, 21-30 MPa. We propose that rupture
was due to the reactivation by hydraulic embrittlement of a inherited
major lithospheric fault within the subducting plate. The stress-drop
suggests that the region of the slab between planes of the double
seismic zone can sustain high stresses. |
|
Diciembre de 2006
Uranium in the Earth's lower mantle
Authors: Laurent Gautron, Steeve Greaux et al
Link: Click here
Abstract
The distribution of the radiogenic heat sources strongly influences
the geodynamics and thermal behaviour of the Earth. About 11
TW is produced by the radioactive decay of uranium (25% of the
|
total
heat flux at Earth surface), and 55% of this energy comes from
the lower mantle. Here we report the first experimental evidence
that aluminous CaSiO3 perovskite is the major, or even the only,
host of uranium in the Earth lower mantle, since such a phase
is able to incorporate up to 35 wt% UO2 (or 4 at% of U). The aluminous
Ca-perovskite could be the main U-bearing constituent of a dense
and radiogenic reservoir proposed in a recent model and located
in the bottom half of the lower mantle. |
|
Noviembre de 2006
Effect of pressure on the viscosity of Fe-S and Fe-C liquids
up to 16 GPa
Authors: H. Terasaki, A. Suzuki et al
Link: Click here
Abstract
The viscosity of liquid Fe78S22 was measured up to 16 GPa and
1723 K using a high-speed CCD camera system (125 frames/s maximum).
In order to prevent a chemical reaction between the
|
sample
and a Re viscosity marker sphere, the Re spheres were coated with
alumina to a thickness of 1-2 µm. The measured viscosity
coefficients were 3.2-8.5 mPa-s and the activation volume was
estimated to be 1.46 cm3/mol. The viscosity of liquid Fe86C14
was also measured up to 4.5 GPa and 1843 K. The viscosity coefficients
were 3.8-6.3 mPa-s. There was no large difference in viscosity
coefficient and activation volume between the Fe-S and Fe-C eutectic
liquids in the range of measurement. Viscosities of Fe-S and Fe-C
eutectic liquids are likely to remain low in the planetary interior.
|
|
Diciembre de 2006
Interaction between forearc and oceanic plate at the south-central
Chilean margin as seen in local seismic data
Authors: C. Haberland, A. Rietbrock et al
Link: Click here
Abstract
We installed a dense, amphibious, temporary seismological network
to study the seismicity and structure of the seismogenic zone
in southern Chile between 37° and 39°S, the nucleation
area of the great 1960 Chile earthquake. 213 local earthquakes
with 14.754 onset times were used for a simultaneous inversion
for the 1-D velocity
|
model
and precise earthquake locations. Relocated artificial shots suggest
an accuracy of the earthquake hypocenter of about 1 km (horizontally)
and 500 m (vertically). Crustal events along trench-parallel and
transverse, deep-reaching faults reflect the interseismic transpressional
deformation of the forearc crust due to the subduction of the
Nazca plate. The transverse faults seems to accomplish differential
lateral stresses between subduction zone segments. Many events
situated in an internally structured, planar seismicity patch
at 20 to 40 km depth near the coast indicate a stress concentration
at the plate's interface at 38°S which might in part be induced
by the fragmented forearc structure. |
|
Noviembre de 2006
A model of the ULF magnetic and electric field generated
from a dust devil
Authors: W. M. Farrell, J. R. Marshall et al
Link: Click here
Abstract
It has been demonstrated that terrestrial dust devils emit ULF
magnetic radiation. On Mars, dust devils may also generate such
magnetic emissions, which might be used as a hazard alert for
manned missions. Specifically, grains in dust devils become
charged via contact electrification, and it has been proposed
that the cyclonic motion of these charged grains in the vortex
wind fields
|
accounts
for the magnetic emission. To test this hypothesis in general
and the possible Mars application, a computer simulation of the
contact electrification/wind blowing phenomena was created, with
the charge distribution and resulting magnetic fields monitored
as a function of time. The results indicate that indeed a fluctuating
charge distribution in a vortex wind can account for the ULF magnetic
fields measured from a dust devil. The contact electrification
process is a function of composition, and we demonstrate that
the various compositions will give rise to different magnetic
field responses from the dust devil. We also demonstrate that
this system of swirling charged grains develops vertical currents
and associated electric fields, as suggested in preceding works.
|
|
Noviembre de 2006
Shear wave splitting beneath the central Tien Shan and tectonic
implications
Authors: Aibing Li and Chizheng Chen
Link: Click here
Abstract
Shear-wave splitting analyses were performed at 30 seismic stations
in the central Tien Shan and its vicinity. Fast orientation
at most of the stations on the Tien Shan range is ENE-WSW, parallel
to the strike of the Tien Shan. It largely reflects a
|
coherent deformation of the lithosphere in response
to the NS shortening. Anisotropy with NNE-SSW fast axes is observed
within adjacent basins, around the Issyk Kul and near the northwestern
Tarim, where the lithosphere is presumably strong, less deformed.
The NNE-SSW orientation, which coincides with the surface velocity
of the central Tien Shan, can be best explained by shear between
the lithosphere and asthenosphere on a regional scale rather
than small-scale convection as proposed in previous studies.
The dextral Talasso-Fergana fault seems to have little effect
on anisotropy at its nearby stations, suggesting this fault
is probably confined to the shallow part of the lithosphere.
|
|
Noviembre de 2006
Analysis of gravity and topography in the GLIMPSE study region:
Isostatic compensation and uplift of the Sojourn and Hotu Matua
Ridge systems
Authors: Nicholas Harmon, Donald W. Forsyth et al
Link: Click here
Abstract
The Gravity Lineations Intraplate Melting Petrologic and Seismic
Expedition (GLIMPSE) Experiment investigated the formation of
a series of non-hot spot, intraplate volcanic ridges in the
South Pacific and their relationship to cross-grain gravity
lineaments detected by satellite altimetry. Using shipboard
gravity measurements and a simple model of surface loading of
a thin elastic plate, we estimate effective elastic thicknesses
ranging from ~2 km beneath the Sojourn Ridge to a maximum of
10 km beneath the Southern Cross Seamount. These elastic thicknesses
are lower than predicted for the 3-9 Ma seafloor on which the
volcanoes lie, perhaps due to reheating and thinning of the
plate during emplacement. Anomalously low apparent densities
estimated for the Matua and Southern Cross seamounts of 2050
and 2250 kg m-3, respectively, probably are
|
artifacts
caused by the assumption of only surface loading, ignoring the
presence of subsurface loading in the form of underplated crust
and/or low-density mantle. Using satellite free-air gravity and
shipboard bathymetry, we calculate the age-detrended, residual
mantle Bouguer anomaly (rMBA). The rMBA corrects the free-air
anomaly for the direct effects of topography, including the thickening
of the crust beneath the seamounts and volcanic ridges due to
surface loading of the volcanic edifices. There are broad, negative
rMBA anomalies along the Sojourn and Brown ridges and the Hotu
Matua seamount chain that extend nearly to the East Pacific Rise.
These negative rMBA anomalies connect to negative free-air anomalies
in the western part of the study area that have been recognized
previously as the beginnings of the cross-grain gravity lineaments.
Subtracting the topographic effects of surface loading by the
ridges and seamounts from the observed topography reveals that
the ridges are built on broad bands of anomalously elevated seafloor.
This swell topography and the negative rMBA anomalies contradict
the predictions of lithospheric cracking models for the origin
of gravity lineaments and associated volcanic ridges, favoring
models with a dynamic mantle component such as small-scale convection
or channelized asthenospheric return flow. |
Noviembre
de 2006
Configurational anisotropy in single-domain
and pseudosingle-domain grains of magnetite
Authors: Wyn Williams, Adrian R. Muxworthy et al
Link: Click here
Abstract
In classical domain theory, single-domain (SD) grains change
their magnetization by coherent rotation, where the energy barrier
to domain reversal is provided by the magnetocrystalline anisotropy
or by shape anisotropy for elongated grains. However, numerical
micromagnetic models have shown that domain structure in SD
grains is rarely perfectly uniform. For example, magnetite has
significant "flowering" of its magnetization even
in grains that approach the room temperature superparamagnetic
(SP) size of ~30 nm. The flowering deforms slightly to accommodate
the grain shape and thereby produces anisotropy independent
of magnetocrystalline effects but dependent on magnetization
direction within the grain. This can be similar in magnitude
to that of magnetocrystalline anisotropy, even for
|
equidimensional
grains (where distance from the centroid to the grain faces is
equal). The interaction of the domain structure and grain geometry
is termed configurational anisotropy and has been studied mainly
in relation to man-made isotropic magnetic media but received
little attention in rock magnetism. In this paper we examine configurational
anisotropy in SD to pseudo-single-domain (PSD) grains of magnetite
using a three-dimensional finite element/boundary integral (FEBI)
micromagnetic model. Equidimensional grains of magnetite of three
different shapes are considered: a cube, an octahedron, and a
regular tetrahedron, and in each case the effects magnetocrystalline
anisotropy were removed in order to isolate the configurational
anisotropy. The numerical models predict that very large coercivities
are possible even for SD equidimensional grains. For tetrahedral
grains coercivities of ~120 mT were obtained, which otherwise
would require a grain elongation of ~1:1.75. Depending on the
orientation of the principle crystalline axis to the grain shape,
the configurational anisotropy may increase or decrease the overall
energy barrier to domain reversal. |
Noviembre
de 2006
Anomaly of heat content in the northern
Atlantic in the last 7 years: Is the ocean warming or cooling?
Authors: V. O. Ivchenko, N. C. Wells et al
Link: Click here
Abstract
Whether the North Atlantic Ocean is warming or cooling is an
important question both in physical oceanography and climate
change. The Argo profiling buoys provide an accurate and stable
instrument for determining the tendencies in heat content from
the surface to 2000 m from 1999 to 2005. To calculate temperature
and heat content anomalies two reference climatologies are used.
|
These are the well known WOA2001 climatology (Stephens et al.,
2002), and a new WOCE Global Hydrographic climatology (Gouretski
and Koltermann, 2004). The former climatology is used for our
main results, and the latter is used for evaluating the sensitivity
of our results to the climatology. Our scheme allows us to estimate
the anomaly of heat content (AHC) in the North Atlantic and its
smaller sub-domains (i.e. 10° boxes) for the period 1999-2005.
We have found a dipole structure in the time averaged AHC: negative
values are concentrated in the southern and middle latitudes of
the North Atlantic whilst positive values are found north of 50°N.
The upper 1500 m of the North Atlantic is warming throughout the
period 1999 to 2005. |
Noviembre
de 2006
Modeling mass transfer processes using random walk particle
tracking
Authors: P. Salamon, D. Fernàndez-Garcia et al
Link: Click here
Abstract
The complexity of mass transfer processes often complicates
solute transport simulations. We present a new approach for
the implementation of the multirate mass transfer model into
random walk particle tracking. This novel method allows for
a spatially heterogeneous distribution of mass transfer coefficients
as well as hydrodynamic parameters in three dimensions, and
it is well suited for avoiding numerical dispersion and solving
computationally demanding transport simulations. For this purpose
the normalized zeroth spatial moments of the multirate transport
equations are derived and used as phase transition probabilities.
Performing a simple Bernoulli trial on the appropriate phase
transition
|
probabilities
the particle distribution between the mobile domain and any immobile
domain can be determined. The approach is compared satisfactorily
to analytical and semianalytical solutions for one-dimensional,
advective-dispersive transport with different types of mass transfer.
Aspects of the numerical implementation of this approach are presented,
and it is demonstrated that two restrictive criteria for the time
step size have to be considered. By adjusting the time step size
for each grid cell on the basis of the cell specific velocity
field and mass transfer rate a correct simulation of solute transport
is assured, while at the same time computational efficiency is
preserved. Finally, an example is presented evaluating the effect
of a heterogeneous intraparticle pore diffusion in a synthetic
aquifer. The results demonstrate that for this specific case the
heterogeneous distribution of mass transfer rates does not have
a significant influence on mean solute transport behavior but
that at low concentration ranges, differences between the different
mass transfer models become visible. |
Noviembre
de 2006
U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon
geochronology of granitoid rocks in eastern Zambia: Terrane subdivision
of the Mesoproterozoic Southern Irumide Belt
Authors: S. P. Johnson, B. De Waele et al
Link: Click here
Abstract
The Southern Irumide Belt (SIB) is a structurally and metamorphically
complex region of mainly Mesoproterozoic igneous rocks in southern
and eastern Zambia, northern Mozambique and northern Malawi
that was strongly overprinted in the Neoproterozoic to Cambrian
Damara-Lufilian-Zambezi (DLZ) orogeny. Because of the scarcity
of geological data from this region, little is known about the
timing of tectonomagmatic events; however, this belt has traditionally
been considered to be a southerly continuation of the adjacent
Irumide Belt (IB). Here we provide 27 new U-Pb sensitive high-resolution
ion microprobe (SHRIMP) zircon ages that constrain the Paleoproterozoic
to Cambrian
|
tectonomagmatic history of this belt and which,
for the first time, allow for direct comparison with the adjoining
IB. The SIB is floored by a predominantly late Paleoproterozoic
basement, which was intruded by voluminous continental margin
arc-related magmas between 1.09 and 1.04 Ga and accompanied
by high-temperature/low-pressure metamorphism. In contrast,
the IB is floored by a late Paleoproterozoic basement that is
generally older than 2.0 Ga, contains significant mid-Mesoproterozoic
plutonic rocks that are not present within the SIB, and underwent
moderate-pressure/moderate-temperature compressional metamorphism
and S-type granitoid magmatism at circa 1.02 Ga. These data
indicate that the crust underlying the SIB is not a continuation
of that underlying the IB but represents an allocthonous continental
margin arc terrane juxtaposed against the Congo-Tanzania-Bangweulu
Craton during the late Mesoproterozoic Irumide orogeny. Reworking
and shearing of the SIB occurred during the DLZ orogen, resulting
in the present-day architecture as a series of stacked terranes
which have been exploited by voluminous posttectonic granitoid
batholiths.
|
Noviembre
de 2006
Magnetic memory and coupling between spin-canted and defect
magnetism in hematite
Authors: Özden Özdemir, David J. Dunlop et al
Link: Click here
Abstract
Saturation isothermal remanent magnetization (SIRM) has been
studied on submicron hematite powders with grain sizes between
0.12 and 0.52 µm and on 2 to 5 mm natural hematite single
crystals before and after zero-field cycling through the Morin
transition (T M). SIRM cooling and warming curves for single-domain
(SD) crystals are similar to those of multidomain (MD) hematites.
Both have similar remanence losses at T M (97-98% of the original
SIRM), a similar defect moment below T M (2-3% of initial SIRM),
and similar memory (30-40% of initial SIRM). Regardless of grain
size, higher SIRM memory ratios are associated with higher defect
moments below T M. In SD and MD hematites alike, room temperature
magnetic memory seems to be an amplification of residual weak
ferromagnetism
|
that
persists even at very low temperatures, much below T M. Applying
a strong field to initially demagnetized SD and MD hematites at
20 K produced a substantial SIRM, which spontaneously increased
by a factor 10-28 upon crossing the transition at T M. These observations
imply that some spins do not participate in the general rotation
from the ferromagnetic c plane to the antiferromagnetic c axis
below T M. The defect moment of these spins serves to restore
preferred directions of spins and ferromagnetic domains during
zero-field warming through the Morin transition and is thus responsible
for the memory phenomenon. The existence of a weak ferromagnetic
moment in antiferromagnetic hematite below T M is also indicated
by colloid patterns observed by Gallon. We propose that the mechanism
of memory is clusters of spins pinned magnetoelastically by lattice
defects. These spins rotate only partially out of the basal plane
during cooling through T M. Some basal plane anisotropy, also
magnetoelastic in origin, must remain below T M in order to explain
the existence of low-temperature SIRM and also to guide spin nuclei
into preferred orientations above T M on rewarming through the
transition in zero field. |
Noviembre
de 2006
Gas age-ice age differences and the chronology of the Vostok
ice core, 0-100 ka
Authors: M. L. Bender, G. Floch et al
Link: Click here
Abstract
Gas is trapped in polar ice at depths of ~50-120 m and is therefore
significantly younger than the ice in which it is embedded.
The age difference is not well constrained for slowly accumulating
ice on the East Antarctic Plateau, introducing a significant
uncertainty into chronologies of the oldest deep ice cores (Vostok,
Dome Fuji, and Dome C). We recorrelate the gas records of Vostok
and Greenland Ice Sheet Project 2 (GISP2) cores in part on the
basis of new CH4 data and use these records to construct six
Vostok chronologies that use different assumptions to
|
calculate
gas age-ice age differences. We then evaluate these chronologies
by comparing times of climate events at Vostok with correlative
events in the well-dated Byrd ice core (West Antarctica). From
this evaluation we identify two leading chronologies for the Vostok
core that are based on recent models of firn temperature, firn
densification, and thinning of upstream ice. One chronology involves
calculating gas age-ice age differences from these models. The
second, new, approach involves calculating ice depths in the core
that are contemporaneous with depths in the same ice core whose
gas ages are well constrained. This latter approach circumvents
problems associated with highly uncertain accumulation rates in
the Vostok core. The uncertainty in Vostok chronologies derived
by correlating into the GISP2 gas record remains about ±1
kyr, and high-precision correlations continue to be difficult.
|
Noviembre
de 2006
Airborne gravity over Lake Vostok and adjacent highlands of
East Antarctica
Authors: John W. Holt, Thomas G. Richter et al
Link: Click here
Abstract
Lake Vostok and a 1200 km transect were the targets of aerogeophysical
surveys in East Antarctica during the austral summer of 2000/2001.
The measurement of gravity anomalies for geologic studies was
the primary goal. A total of 24,459 line-km of data were acquired.
Favorable weather, aircraft navigation, and instrument performance
contributed to excellent data quality. Multiple carrier-phase
GPS solutions to determine aircraft-induced accelerations were
available for each flight. Raw
|
gravity
and GPS position solutions were initially filtered to compensate
for hardware filtering within the gravity meter. Filtering of
remaining high-frequency noise was accomplished with a spatial,
moving average smoother. Due to upward continuation effects imposed
by the ice cover, the theoretically estimated minimum resolvable
gravity feature size for the Lake Vostok survey is 8 km, consistent
with an analysis of power spectra comparing the gravity signal
to noise calculated from geographically repeated lines. Comparison
of gravity results with subice topography indicates that the gravity
data are sensitive to real features including the existence of
major crustal structures. Repeated lines and crossovers were analyzed
to estimate uncertainties for the Lake Vostok data set, with both
of these repeatability measures indicating relative accuracy in
the 2 mGal range for the unleveled data and 1 mGal after leveling.
|
Noviembre
de 2006
Low-temperature magnetic properties of magnetite using first-order
reversal curve analysis: Implications for the pseudo-single-domain
state
Author: A. V. Smirnov
Link: Click here
Abstract
Low-temperature magnetic hysteresis properties of nearly stoichiometric
magnetite were studied using the first-order reversal curve
(FORC) technique. FORC distributions were measured at 20 K after
cooling in zero magnetic field or cooling in the presence of
a strong (1.5 T) magnetic field. Five polycrystalline samples
ranged in mean grain size and shape, representing mostly single-domain
(SD), pseudo-single-domain (PSD), and multidomain (MD) magnetic
hysteresis behavior. The FORC distributions measured from PSD
|
magnetite
after zero field cooling (ZFC) manifest an unusual behavior, characterized
by two distinct maxima located almost symmetrically with respect
to the zero interaction field (Hu = 0) axis on a FORC diagram.
In contrast, the low-temperature FORC distributions for SD and
MD samples have a single maximum located at the Hu = 0 axis. The
appearance of two maxima on low-temperature FORC distributions
is interpreted as a manifestation of a perminvar-like behavior
due to stabilization of the magnetic domain structure by irregularities
of crystalline lattice induced by twinning in monoclinic magnetite.
The experimental data suggest that the effect of monoclinic twinning
on remagnetization is only pronounced within a certain grain size
range. The double-maxima ZFC FORC distribution appears to be a
distinctive property of PSD hysteresis behavior, which cannot
be reproduced by mixing SD and MD magnetites. |
Noviembre
de 2006
Day plots of mixtures of superparamagnetic, single-domain,
pseudosingle-domain, and multidomain magnetites
Authors: David J. Dunlop, Brian Carter-Stiglitz et al
Link: Click here
Abstract
We test how well a few hysteresis parameters (saturation remanence
M rs , coercive force H c and remanent coercivity H cr ) serve
to determine the proportions of end-members in binary mixtures.
Our end-members are six magnetites whose grain sizes are within
the superparamagnetic (SP), stable single-domain (SD, three
samples), pseudo-SD (PSD), and multidomain (MD) ranges (Carter-Stiglitz
et al., 2001). The three SD magnetites have contrasting origins
and properties: (1) bacterial magnetite crystals of a single
size and coercivity, arranged in chains; (2) natural volcanic
magnetites with a narrow distribution of coercivities; and (3)
synthetic magnetites precipitated in glass, with a broader coercivity
distribution. Our parameter mixing theory assumes linear magnetization
curves of the end-members between zero field and the largest
coercive force H c (that of the SD phase, if present). Similarly
remanent hysteresis
|
curves
should be linear up to the maximum remanent coercive force H cr
. Three of our mixtures (SP plus bacterial SD, PSD plus bacterial
SD, MD plus volcanic SD) had acceptable agreement between predicted
and measured dependences of H c , H cr and the curve of M rs /M
s versus. H cr /H c (Day plot) on end-member concentrations. A
nonlinear approximation to remanent hysteresis curves gave a reasonable
fit to MD plus glass SD results. In this case, H cr /H c for the
most MD-rich mixture is larger than H cr /H c of either end-member.
Such behavior is characteristic of bimodal mixtures in which H
cr is largely determined by the hard (SD) phase and H c by the
soft (MD) phase. The only mixture that could not be modeled by
linear or nonlinear parameter theory was MD plus bacterial SD.
The bacterial SD hysteresis loop descends almost vertically at
-H c because of the extremely narrow range of particle sizes and
coercivities. In general, linear and nonlinear parameter mixing
models are adequate if only an approximate fit to real data is
needed. An inversion method using complete magnetization curves
as end-member basis functions is preferable as an unmixing technique.
However, comparison of measured data to type curves, for example,
on a Day plot, gives a quick indication of what end-member phases
might be involved in the mix and provides additional insight before
beginning an inversion. |
Noviembre
de 2006
Semiempirically derived heating function of the corona heliosphere
during the Whole Sun Month
Authors: M. Guhathakurta, E. C. Sittler Jr. et al
Link: Click here
Abstract
In spite of many recent developments in observations and models
of the solar corona, the exact form of the heating function
of the solar wind plasma is unknown. Here we attempt to make
some progress on that problem by applying a previously developed
simplified magnetohydrodynamic model to a unique synthesis of
remote sensing coronal observations from SOHO and Spartan 201,
and in-situ data
|
from Ulysses spacecraft during the quiescent
phase of solar cycle 23 (1995-1997) which also included the
Whole Sun Month (August/September 1996) study period. Our analysis
provides for the first time an empirically derived estimate
of the heat flux and temperature profile of the corona and interplanetary
medium over the solar polar regions as a function of latitude
and radial distance for this data set. The latitudinal and radial
variation is extended to include the equatorial regions but
with less fidelity given the significant structure associated
with the streamer belt. Our results suggest that both thermal
and nonthermal heating terms contribute to the temperature and
heat flux in the low corona, particularly above the poles, with
the non-thermal component possibly providing as much as half
of the total heat input.
|
Noviembre
de 2006
Introduction to special section on "Results of the National
Science Foundation Geospace Environment Modeling Inner Magnetosphere/Storms
Assessment Challenge"
Author: Michael W. Liemohn
Link: Click here
Abstract
The National Science Foundation sponsors the Geospace Environment
Modeling (GEM) program, devoted to understanding the scientific
underpinnings for the creation of a Geospace General Circulation
Model. The GEM program is organized into finite-length campaigns,
one of which is the Inner Magnetosphere/Storms (IM/S) Campaign,
which is ending in 2006. The final activity of the IM/S Campaign
was the IM/S
|
Assessment
Challenge (IMSAC), in which several events were selected for intense,
community-wide examination. The primary purposes of the IMSAC
were (1) to quantify the ability of the current set of inner magnetospheric
models to predict the fields and plasma of this region during
storms and (2) to codify the consensus understanding of inner
magnetospheric physics. This paper introduces the reader to the
IMSAC and presents a general answer to each of the IMSAC goals.
In addition, highlights of the scientific advancements during
the IM/S Campaign are presented for the plasmasphere, ring current,
and radiation belts, along with a synthesis view of the inner
magnetosphere. The biggest need for inner magnetospheric research
in the near future is the continued progression toward a coupled,
interconnected understanding of this region, especially the nonlinear
feedback mechanisms between the plasma populations, the electric
and magnetic fields, and plasma waves. |
Noviembre
de 2006
Closed loop GPR data inversion for soil hydraulic and electric
property determination
Authors: S. Lambot , E. C. Slob et al
Link: Click here
Abstract
We propose an integrated electromagnetic-hydrodynamic inverse
modeling approach for identifying field-scale unsaturated soil
hydraulic properties and electric profiles from off-ground
|
time-lapse
ground-penetrating radar data. Hydrodynamic modeling based on
the one-dimensional Richards' equation with homogeneous soil hydraulic
parameters is used to physically constrain the full-wave radar
electromagnetic inverse problem in a closed loop. Significant
effects can be observed of water dynamics on the time-lapse radar
data and numerical studies demonstrate the uniqueness of the inverse
solution. The results suggest that the method is promising for
non-invasive characterization of the shallow subsurface and monitoring
of its dynamics. |
Noviembre
de 2006
Crustal earthquake bursts in California and Japan: Their patterns
and relation to volcanoes
Authors:John E. Vidale , Katie L. Boyle et al
Link: Click here
Abstract
We analyze 153 bursts of earthquakes in southern California
and Japan. The burst patterns are similar in southern California
and Japan; they fill a spectrum between "swarm-like"
sequences without obvious mainshocks and mainshocks with Omori-law-abiding
aftershocks. In agreement with our previous work, the "swarm-like"
sequences in Japan have more events, are more voluminous, and
tend to expand with time, when
|
compared
to "mainshock-aftershock" type sequences. In both regions,
we find that the sequences starting with their largest events
tend to be much shorter in duration. Bursts within 50 km of volcanoes
are similar in character to those elsewhere except they tend to
have longer duration. We hypothesize that swarminess is a proxy
for fluid pressure redistribution and/or aseismic slip driving
the seismicity bursts, and conversely, the mainshock-aftershock-style
sequences have end-member behavior that results solely from a
cascade of elastic failures. The complexity of the spatial seismicity
distribution does not correlate with the style of swarm observed,
indicating that fluid conditions and composition are likely more
influential than geometry in determining the patterns we observe.
|
Noviembre
de 2006
Variation of spherical harmonic power as a function of harmonic
order for Earth's core and crustal magnetic field and for Mars'
crustal field
Author: Christopher G. A. Harrison
Link: Click here
Abstract
The variation of power of the spherical harmonics of Earth's
magnetic field as a function of degree is well known. At Earth's
surface the power decreases as a function of degree until the
crustal component dominates. At the core surface, the spectrum
is flat apart from the dipole term, and each degree contributes
about equally to the power. However, as a function of order
the power tends to decrease for high harmonic orders. These
represent sources at low geographic latitudes, and so a field
in which high-order power is low represents one in which low-latitude
sources are weaker than high-latitude sources. Thus the explanation
for the reduction in power for
|
high-order
harmonics in Earth's core field is that in the core, low-latitude
sources of the nondipole field are less intense than high-latitude
sources. Earth's crustal field is represented by harmonic degrees
greater than 15 and also shows the power falling off as a function
of order, implying that there is an increase in the intensity
of magnetic anomalies from equator to poles. This can be explained
if the magnetizations have been produced by a dipole field aligned
along the spin axis because such a field is twice as strong at
the poles as at the equator. For the Martian field there is no
central dipole source, but there are strong crustal sources that
were created when Mars had a planetary dynamo and presumably an
axial dipolar field. Martian harmonics show the same pattern of
variation as a function of order as those on Earth. One explanation
for this is that these crustal sources were magnetized with a
dipolar field aligned along the present spin axis, implying that
there has not been much Martian polar wander since the crust was
magnetized. |
Noviembre
de 2006
Cost-effective allocation of watershed management practices
using a genetic algorithm
Authors: Mazdak Arabi , Rao S. Govindaraju et al
Link: Click here
Abstract
Implementation of conservation programs are perceived as being
crucial for restoring and protecting waters and watersheds from
nonpoint source pollution. Success of these programs depends
to a great extent on planning tools that can assist the watershed
management process. Herein a novel optimization methodology
is presented for deriving watershed-scale sediment and nutrient
control plans that incorporate multiple, and often conflicting,
objectives. The
|
method
combines the use of a watershed model (SWAT), representation of
best management practices, an economic component, and a genetic
algorithm-based spatial search procedure. For two small watersheds
in Indiana located in the midwestern portion of the United States,
selection and placement of best management practices by optimization
was found to be nearly 3 times more cost-effective than targeting
strategies for the same level of protection specified in terms
of maximum monthly sediment, phosphorus, and nitrogen loads. Conversely,
for the same cost, the optimization plan reduced the maximum monthly
loads by a factor of 2 when compared to the targeting plan. The
optimization methodology developed in this paper can facilitate
attaining water quality goals at significantly lower costs than
commonly used cost share and targeting strategies. |
Noviembre
de 2006
El Niño-Southern Oscillation determines the salinity
of the freshwater lens under a coral atoll in the Pacific Ocean
Authors: M. van der Velde , M. Javaux et al
Link: Click here
Abstract
The freshwater resources of coral atolls occur mainly as lenses
floating on salt water underneath the islands. The size and
shape of these lenses are determined by hydrogeologic characteristics,
the rainfall recharge rate and its temporal variation, plus
extractions (Underwood et al., 1992; Jones and Banner, 2003;
Jocson et al., 2002). In the South Pacific, rainfall exhibits
|
seasonal
as well as interannual variability related to the El Niño-Southern
Oscillation (ENSO) (Ropelewski and Halpert, 1987). We used electric
conductivity measurements from pumped wells on Tongatapu to show
a moderate ENSO control on the temporal fluctuation of the pumped
freshwater salinity. The salinity dynamics depended on low or
increased rainfall recharge during respectively dry El Niño
periods or wet La Niña events. ENSO events cause a large
variation around the mean salinity and determine the relative
salinity over the time-scale of several years, while a smaller
variation is introduced by seasonal rainfall. The Southern Oscillation
Index (SOI) (Troup, 1965; Stone et al., 1996) was used to predict
freshwater salinity with a lag time of 10 months. |