319 research outputs found
Quasi-geostrophic modes in the Earth's fluid core with an outer stably stratified layer
Seismic waves sensitive to the outermost part of the Earth's liquid core seem
to be affected by a stably stratified layer at the core-mantle boundary. Such a
layer could have an observable signature in both long-term and short-term
variations of the magnetic field of the Earth, which are used to probe the flow
at the top of the core. Indeed, with the recent SWARM mission, it seems
reasonable to be able to identify waves propagating in the core with period of
several months, which may play an important role in the large-scale dynamics.
In this paper, we characterize the influence of a stratified layer at the top
of the core on deep quasi-geostrophic (Rossby) waves. We compute numerically
the quasi-geostrophic eigenmodes of a rapidly rotating spherical shell, with a
stably stratified layer near the outer boundary. Two simple models of
stratification are taken into account, which are scaled with commonly adopted
values of the Brunt-V{\"a}is{\"a}l{\"a} frequency in the Earth's core. In the
absence of magnetic field, we find that both azimuthal wavelength and frequency
of the eigenmodes control their penetration into the stratified layer: the
higher the phase speed, the higher the permeability of the stratified layer to
the wave motion. We also show that the theory developed by Takehiro and Lister
[2001] for thermal convection extends to the whole family of Rossby waves in
the core. Adding a magnetic field, the penetrative behaviour of the
quasi-geostrophic modes (the so-called fast branch) is insensitive to the
imposed magnetic field and only weakly sensitive to the precise shape of the
stratification. Based on these results, the large-scale and high frequency
modes (1 to 2 month periods) may be detectable in the geomagnetic data measured
at the Earth's surface, especially in the equatorial area where the modes can
be trapped
Inertia-gravity waves in geophysical vortices
Pancake-like vortices are often generated by turbulence in geophysical flows.
Here, we study the inertia-gravity oscillations that can exist within such
geophysical vortices, due to the combined action of rotation and gravity. We
consider a fluid enclosed within a triaxial ellipsoid, which is stratified in
density with a constant Brunt-V\"ais\"al\"a frequency (using the Boussinesq
approximation) and uniformly rotating along a (possibly) tilted axis with
respect to gravity. The wave problem is then governed by a mixed
hyperbolic-elliptic equation for the velocity. As in the rotating
non-stratified case considered by Vantieghem (2014, Proc. R. Soc. A, 470,
20140093, doi:10.1098/rspa.2014.0093), we find that the spectrum is pure point
in ellipsoids (i.e. only consists of eigenvalues) with smooth polynomial
eigenvectors. Then, we characterise the spectrum using numerical computations
(obtained with a bespoke Galerkin method) and asymptotic spectral theory.
Finally, the results are discussed in light of natural applications (e.g. for
Mediterranean eddies or Jupiter's vortices).Comment: 22 pages, 9 figures and 1 table. Accepted 5 February 2024 for
publication in Proc. R. Soc.
On gravito-inertial surface waves
In geophysical environments, wave motions that are shaped by the action of
gravity and global rotation bear the name of gravito-inertial waves. We present
a geometrical description of gravito-inertial surface waves, which are
low-frequency waves existing in the presence of a solid boundary. We consider
an idealized fluid model for an incompressible fluid enclosed in a smooth
compact three-dimensional domain, subject to a constant rotation vector. The
fluid is also stratified in density under a constant Brunt-V{\"a}is{\"a}l{\"a}
frequency. The spectral problem is formulated in terms of the pressure, which
satisfies a Poincar\'e equation within the domain, and a Kelvin equation on the
boundary. The Poincar\'e equation is elliptic when the wave frequency is small
enough, such that we can use the Dirichlet-to-Neumann operator to reduce the
Kelvin equation to a pseudo-differential equation on the boundary. We find that
the wave energy is concentrated on the boundary for large covectors, and can
exhibit surface wave attractors for generic domains. In an ellipsoid, we show
that these waves are square-integrable and reduce to spherical harmonics on the
boundary
The spectrum of the Poincar{\'e} operator in an ellipsoid
We reprove the fact, due to Backus, that the Poincar{\'e} operator in
ellipsoids admits a pure point spectrum with polynomial eigenfunctions.We then
show that the eigenvalues of the Poincar{\'e} operator restricted to polynomial
vector fields of fixed degree admitsa limit repartition given by a probability
measure that we construct explicitely. For that, we use Fourier integral
operators and ideas comingfrom Alan Weinstein and the first author in the
seventies. The starting observation is that the orthogonal polynomials in
ellipsoids satisfy a PDE
Optimal finite measurements and Gauss quadratures
We exhibit measurements for optimal state estimation which have a finite
number of outcomes. This is achieved by a connection between finite optimal
measurements and Gauss quadratures. The example we consider to illustrate this
connection is that of state estimation on qubits, all in a same pure state.
Extensions to state estimation of mixed states are also discussed.Comment: 7 pages, 2 figures, paragraph added at the end of the article
(extension of the problem to general states
HETEROGENEITY OF ZOOXANTHELLAE DENSITY IN THE CORAL ACROPORA GLOBICEPS AROUND MOOREA (FRENCH POLYNESIA)
Understanding the natural heterogeneity of coral zooxanthellae density appears as very important to understand variations in coral survival due to bleaching (the loss of these micro-algal symbionts). Heterogeneity of coral bleaching at different observation scales (within a colony, among neighbouring colonies of the same species or on a wider scale) remains largely misunderstood. The present work explores intracolonial, spatial and temporal variations of zooxanthellae density in the coral Acropora globiceps Dana 1846, over a period of three months on the forereef of Moorea, French Polynesia.
In our study, intracolonial zooxanthellae densities did not vary significantly. However, zooxanthellae densities differed slightly between inner and outer branches but this trend was not significant at 6, 12 and 18 meters depth. On a wider scale, zooxanthellae densities also vary spatially : a positive correlation was observed between depth and symbiont density (density increases when light intensity decreases, so according to depth). Moreover, the location of colonies exposed to different hydrodynamical conditions was not a parameter controling the concentration of zooxanthellae.
Finally, the temporal variation of zooxanthellae densities did not show significant variations even if it decreased slightly over the study period.
The results of this study highlight the importance of accounting for variations within and among colonies to determine zooxanthellae densities and to assess the evolution of zooxanthellae populations
High temperature structural and magnetic properties of cobalt nanowires
We present in this paper the structural and magnetic properties of high
aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in
situ X ray diffraction (XRD) and SQUID characterizations. We show that the
anisotropic shapes, the structural and texture properties are preserved up to
500 K. The coercivity can be modelled by u0Hc=2(Kmc+Kshape)/Ms with Kmc the
magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant
and Ms the saturation magnetization. Hc decreases linearly when the temperature
is increased due to the loss of the Co magnetocrystalline anisotropy
contribution. At 500K, 50% of the room temperature coercivity is preserved
corresponding to the shape anisotropy contribution only. We show that the
coercivity drop is reversible in the range 300 - 500 K in good agreement with
the absence of particle alteration. Above 525 K, the magnetic properties are
irreversibly altered either by sintering or by oxidation.Comment: 8 pages, 7 figures, submitted to Journal of Solid State Chemistr
Preliminary results of lifetime measurements in neutron-rich 53Ti
To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data
Microfossils from the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa
The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a pylome interpreted as a sophisticated circular excystment structure, and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicate that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy
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