2,459 research outputs found
Planetary gyre, time-dependent eddies, torsional waves, and equatorial jets at the Earth's core surface
We report a calculation of time-dependent quasi-geostrophic core flows for
1940-2010. Inverting recursively for an ensemble of solutions, we evaluate the
main source of uncertainties, namely the model errors arising from interactions
between unresolved core surface motions and magnetic fields. Temporal
correlations of these uncertainties are accounted for. The covariance matrix
for the flow coefficients is also obtained recursively from the dispersion of
an ensemble of solutions. Maps of the flow at the core surface show, upon a
planetary-scale gyre, time-dependent large-scale eddies at mid-latitudes and
vigorous azimuthal jets in the equatorial belt. The stationary part of the flow
predominates on all the spatial scales that we can resolve. We retrieve
torsional waves that explain the length-of-day changes at 4 to 9.5 years
periods. These waves may be triggered by the nonlinear interaction between the
magnetic field and sub-decadal non-zonal motions within the fluid outer core.
Both the zonal and the more energetic non-zonal interannual motions were
particularly intense close to the equator (below 10 degrees latitude) between
1995 and 2010. We revise down the amplitude of the decade fluctuations of the
planetary scale circulation and find that electromagnetic core-mantle coupling
is not the main mechanism for angular momentum exchanges on decadal time scales
if mantle conductance is 3 10 8 S or lower
Stochastic modelling of regional archaeomagnetic series
SUMMARY We report a new method to infer continuous time series of the
declination, inclination and intensity of the magnetic field from
archeomagnetic data. Adopting a Bayesian perspective, we need to specify a
priori knowledge about the time evolution of the magnetic field. It consists in
a time correlation function that we choose to be compatible with present
knowledge about the geomagnetic time spectra. The results are presented as
distributions of possible values for the declination, inclination or intensity.
We find that the methodology can be adapted to account for the age
uncertainties of archeological artefacts and we use Markov Chain Monte Carlo to
explore the possible dates of observations. We apply the method to intensity
datasets from Mari, Syria and to intensity and directional datasets from Paris,
France. Our reconstructions display more rapid variations than previous studies
and we find that the possible values of geomagnetic field elements are not
necessarily normally distributed. Another output of the model is better age
estimates of archeological artefacts
The structure of radiative shock waves. III. The model grid for partially ionized hydrogen gas
The grid of the models of radiative shock waves propagating through partially
ionized hydrogen gas with temperature 3000K <= T_1 <= 8000K and density
10^{-12} gm/cm^3 <= \rho_1 <= 10^{-9}gm/cm^3 is computed for shock velocities
20 km/s <= U_1 <= 90 km/s. The fraction of the total energy of the shock wave
irreversibly lost due to radiation flux ranges from 0.3 to 0.8 for 20 km/s <=
U_1 <= 70 km/s. The postshock gas is compressed mostly due to radiative cooling
in the hydrogen recombination zone and final compression ratios are within 1
<\rho_N/\rho_1 \lesssim 10^2, depending mostly on the shock velocity U_1. The
preshock gas temperature affects the shock wave structure due to the
equilibrium ionization of the unperturbed hydrogen gas, since the rates of
postshock relaxation processes are very sensitive to the number density of
hydrogen ions ahead the discontinuous jump. Both the increase of the preshock
gas temperature and the decrease of the preshock gas density lead to lower
postshock compression ratios. The width of the shock wave decreases with
increasing upstream velocity while the postshock gas is still partially ionized
and increases as soon as the hydrogen is fully ionized. All shock wave models
exhibit stronger upstream radiation flux emerging from the preshock outer
boundary in comparison with downstream radiation flux emerging in the opposite
direction from the postshock outer boundary. The difference between these
fluxes depends on the shock velocity and ranges from 1% to 16% for 20 km/s <=
U_1 <= 60 km/s. The monochromatic radiation flux transported in hydrogen lines
significantly exceeds the flux of the background continuum and all shock wave
models demonstrate the hydrogen lines in emission.Comment: 11 pages, 11 figures, LaTeX, to appear in A
Envelope tomography of long-period variable stars: I. The Schwarzschild mechanism and the Balmer emission lines
This paper is the first one in a series devoted to the study of the dynamics
of the atmospheres of long-period variable stars. Results from a two-month-long
monitoring of the Mira variables RT Cyg and X Oph around maximum light with the
ELODIE spectrograph at the Haute-Provence Observatory are presented. The
monitoring covers phases 0.80 to 1.16 for RT Cyg and phases 0.83 to 1.04 for X
Oph. The cross-correlation profile of the spectrum of RT Cyg with a K0 III mask
confirms that the absorption lines of RT Cyg in the optical domain appear
double around maximum light. No line doubling was found in the optical spectrum
of X Oph around maximum light, indicating that this feature is not common to
all long-period variables. This paper also presents the application to RT Cyg
of a new tomographic technique deriving the velocity field across the
atmosphere by cross-correlating the optical spectrum with numerical masks
constructed from synthetic spectra and probing layers of increasing depths.
This technique reveals that both the temporal evolution of the line doubling,
and its variation with depth in the atmosphere of RT Cyg, are consistent with
the ``Schwarzschild scenario''. This scenario relates the temporal evolution of
the red and blue peaks of the double absorption lines to the progression of a
shock wave in the atmosphere. The temporal evolution of the Balmer Halpha, H
beta, Hgamma and Hdelta emission lines around maximum light is also presented
for RT Cyg and X Oph. The velocity variations of Halpha and of the absorption
lines are discussed in the framework of two competing models for the formation
of Balmer emission lines in long-period variable stars.Comment: 11 pages, 8 figures, Latex, accepted for publication in Astronomy and
Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/Html/ps.htm
Search for surface magnetic fields in Mira stars. First detection in chi Cyg
In order to complete the knowledge of the magnetic field and of its influence
during the transition from Asymptotic Giant Branch to Planetary Nebulae stages,
we have undertaken a search for magnetic fields at the surface of Mira stars.
We used spectropolarimetric observations, collected with the Narval instrument
at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman
signature in the visible part of the spectrum. We present the first
spectropolarimetric observations of the S-type Mira star chi Cyg, performed
around its maximum light. We have detected a polarimetric signal in the Stokes
V spectra and we have established its Zeeman origin. We claim that it is likely
to be related to a weak magnetic field present at the photospheric level and in
the lower part of the stellar atmosphere. We have estimated the strength of its
longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the
variation of the magnetic field strength across the circumstellar envelope of
chi Cyg. This is the first detection of a weak magnetic field at the stellar
surface of a Mira star and we discuss its origin in the framework of shock
waves periodically propagating throughout the atmosphere of these radially
pulsating stars. At the date of our observations of chi Cyg, the shock wave
reaches its maximum intensity, and it is likely that the shock amplifies a weak
stellar magnetic field during its passage through the atmosphere. Without such
an amplification by the shock, the magnetic field strength would have been too
low to be detected. For the first time, we also report strong Stokes Q and U
signatures (linear polarization) centered onto the zero velocity (i.e., at the
shock front position). They seem to indicate that the radial direction would be
favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21
November 2013
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