9,871 research outputs found
Delayed Babcock-Leighton dynamos in the diffusion-dominated regime
Context. Solar dynamo models of Babcock-Leighton type typically assume the
rise of magnetic flux tubes to be instantaneous. Solutions with
high-magnetic-diffusivity have too short periods and a wrong migration of their
active belts. Only the low-diffusivity regime with advective meridional flows
is usually considered. Aims. In the present paper we discuss these assumptions
and applied a time delay in the source term of the azimuthally averaged
induction equation. This delay is set to be the rise time of magnetic flux
tubes which supposedly form at the tachocline. We study the effect of the
delay, which adds to the spacial non-locality a non-linear temporal one, in the
advective but particularly in the diffusive regime. Methods. Fournier et al.
(2017) obtained the rise time according to stellar parameters such as rotation,
and the magnetic field strength at the bottom of the convection zone. These
results allowed us to constrain the delay in the mean-field model used in a
parameter study. Results. We identify an unknown family of solutions. These
solutions self-quench, and exhibit longer periods than their non-delayed
counterparts. Additionally, we demonstrate that the non-linear delay is
responsible for the recover of the equatorward migration of the active belts at
high turbulent diffusivities. Conclusions. By introducing a non-linear temporal
non-locality (the delay) in a Babcock-Leighton dynamo model, we could obtain
solutions quantitatively comparable to the solar butterfly diagram in the
diffusion-dominated regime.Comment: 11 pages, 10 Figure
Field-dependent diamagnetic transition in magnetic superconductor
The magnetic penetration depth of single crystal
was measured down to 0.4 K in dc fields up
to 7 kOe. For insulating , Sm spins order at the
N\'{e}el temperature, K, independent of the applied field.
Superconducting ( K) shows a
sharp increase in diamagnetic screening below which varied from
4.0 K () to 0.5 K ( 7 kOe) for a field along the c-axis. If the
field was aligned parallel to the conducting planes, remained
unchanged. The unusual field dependence of indicates a spin freezing
transition that dramatically increases the superfluid density.Comment: 4 pages, RevTex
Density Power Spectrum of Compressible Hydrodynamic Turbulent Flows
Turbulent flows are ubiquitous in astrophysical environments, and
understanding density structures and their statistics in turbulent media is of
great importance in astrophysics. In this paper, we study the density power
spectra, , of transonic and supersonic turbulent flows through one
and three-dimensional simulations of driven, isothermal hydrodynamic turbulence
with root-mean-square Mach number in the range of 1 \la M_{\rm rms} \la 10.
From one-dimensional experiments we find that the slope of the density power
spectra becomes gradually shallower as the rms Mach number increases. It is
because the density distribution transforms from the profile with {\it
discontinuities} having for to
the profile with {\it peaks} having for . We also find that the same trend is carried to three-dimension; that is,
the density power spectrum flattens as the Mach number increases. But the
density power spectrum of the flow with has the Kolmogorov
slope. The flattening is the consequence of the dominant density structures of
{\it filaments} and {\it sheets}. Observations have claimed different slopes of
density power spectra for electron density and cold H I gas in the interstellar
medium. We argue that while the Kolmogorov spectrum for electron density
reflects the {\it transonic} turbulence of in the warm
ionized medium, the shallower spectrum of cold H I gas reflects the {\it
supersonic} turbulence of a few in the cold neutral medium.Comment: To appear in ApJ Lett. Pdf file with full resolution figures can be
downloaded from http://canopus.cnu.ac.kr/ryu/kimryu.pd
Geothermal studies - Yellowstone National Park /test site 11/, Wyoming
Summary report of diamond drilling in thermal areas of Yellowstone National Park, and method for determining heat flow in thermal area
Measurements of the absolute value of the penetration depth in high- superconductors using a tunnel diode resonator
A method is presented to measure the absolute value of the London penetration
depth, , from the frequency shift of a resonator. The technique
involves coating a high- superconductor (HTSC) with film of low - Tc
material of known thickness and penetration depth. The method is applied to
measure London penetration depth in YBa2Cu3O{7-\delta} (YBCO)
Bi2Sr2CaCu2O{8+\delta} (BSCCO) and Pr{1.85}Ce{0.15}CuO{4-\delta}\lambda (0)\lambda \approx 2790$ \AA, reported for the first
time.Comment: RevTex 4 (beta 4). 4 pages, 4 EPS figures. Submitted to Appl. Phys.
Let
Long-range Ni/Mn structural order in epitaxial double perovskite La2NiMnO6 thin films
We report and compare the structural, magnetic, and optical properties of
ordered La2NiMnO6 thin films and its disordered LaNi0.5Mn0.5O3 counterpart. An
x-ray diffraction study reveals that the B-site Ni/Mn ordering induces
additional XRD reflections as the crystal symmetry is transformed from a
pseudocubic perovskite unit cell in the disordered phase to a monoclinic form
with larger lattice parameters for the ordered phase. Polarized Raman
spectroscopy studies reveal that the ordered samples are characterized by
additional phonon excitations that are absent in the disordered phase. The
appearance of these additional phonon excitations is interpreted as the
clearest signature of Brillouin zone folding as a result of the long-range
Ni/Mn ordering in La2NiMnO6. Both ordered and disordered materials display a
single ferromagnetic-to-paramagnetic transition. The ordered films display also
a saturation magnetization close to 4.8 mB/f.u. and a transition temperature
(FM-TC) around 270 K, while the disordered ones have only a 3.7 mB/f.u.
saturation magnetization and a FM-TC around 138 K. The differences in their
magnetic behaviours are understood based on the distinct local electronic
configurations of their Ni/Mn cations.Comment: 15 pages, 5 fig
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