10,123 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
Pinsker estimators for local helioseismology
A major goal of helioseismology is the three-dimensional reconstruction of
the three velocity components of convective flows in the solar interior from
sets of wave travel-time measurements. For small amplitude flows, the forward
problem is described in good approximation by a large system of convolution
equations. The input observations are highly noisy random vectors with a known
dense covariance matrix. This leads to a large statistical linear inverse
problem.
Whereas for deterministic linear inverse problems several computationally
efficient minimax optimal regularization methods exist, only one
minimax-optimal linear estimator exists for statistical linear inverse
problems: the Pinsker estimator. However, it is often computationally
inefficient because it requires a singular value decomposition of the forward
operator or it is not applicable because of an unknown noise covariance matrix,
so it is rarely used for real-world problems. These limitations do not apply in
helioseismology. We present a simplified proof of the optimality properties of
the Pinsker estimator and show that it yields significantly better
reconstructions than traditional inversion methods used in helioseismology,
i.e.\ Regularized Least Squares (Tikhonov regularization) and SOLA (approximate
inverse) methods.
Moreover, we discuss the incorporation of the mass conservation constraint in
the Pinsker scheme using staggered grids. With this improvement we can
reconstruct not only horizontal, but also vertical velocity components that are
much smaller in amplitude
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
Discrimination of the light CP-odd scalars between in the NMSSM and in the SLHM
The presence of the light CP-odd scalar boson predicted in the
next-to-minimal supersymmetric model (NMSSM) and the simplest little Higgs
model (SLHM) dramatically changes the phenomenology of the Higgs sector. We
suggest a practical strategy to discriminate the underlying model of the CP-odd
scalar boson produced in the decay of the standard model-like Higgs boson. We
define the decay rate of "the non -tagged jet pair" with which we compute
the ratio of decay rates into lepton and jets. They show much different
behaviors between the NMSSM and the SLHM.Comment: 5 pages, 2 figures (5 figure files
Alien Registration- Fournier, Elizabeth D. (Brunswick, Cumberland County)
https://digitalmaine.com/alien_docs/31474/thumbnail.jp
Universal analytic properties of noise. Introducing the J-Matrix formalism
We propose a new method in the spectral analysis of noisy time-series data
for damped oscillators. From the Jacobi three terms recursive relation for the
denominators of the Pad\'e Approximations built on the well-known Z-transform
of an infinite time-series, we build an Hilbert space operator, a J-Operator,
where each bound state (inside the unit circle in the complex plane) is simply
associated to one damped oscillator while the continuous spectrum of the
J-Operator, which lies on the unit circle itself, is shown to represent the
noise. Signal and noise are thus clearly separated in the complex plane. For a
finite time series of length 2N, the J-operator is replaced by a finite order
J-Matrix J_N, having N eigenvalues which are time reversal covariant. Different
classes of input noise, such as blank (white and uniform), Gaussian and pink,
are discussed in detail, the J-Matrix formalism allowing us to efficiently
calculate hundreds of poles of the Z-transform. Evidence of a universal
behaviour in the final statistical distribution of the associated poles and
zeros of the Z-transform is shown. In particular the poles and zeros tend, when
the length of the time series goes to infinity, to a uniform angular
distribution on the unit circle. Therefore at finite order, the roots of unity
in the complex plane appear to be noise attractors. We show that the
Z-transform presents the exceptional feature of allowing lossless undersampling
and how to make use of this property. A few basic examples are given to suggest
the power of the proposed method.Comment: 14 pages, 8 figure
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