7,564 research outputs found
Does a Non-Magnetic Solar Chromosphere Exist?
Enhanced chromospheric emission which corresponds to an outwardly increasing
semiempirical temperature structure can be produced by wave motion without any
increase in the mean gas temperture. Hence, the sun may not have a classical
chromosphere in magnetic field free internetwork regions. Other significant
differences between the properties of dynamic and static atmospheres should be
considered when analyzing chromospheric observations.Comment: 4 pages, uuencoded compressed postscript file including three
figures. APJL accepte
On the Formation of Active Regions
Magneto-convection can produce an active region without an initial coherent
flux tube. A simulation was performed where uniform, untwisted, horizontal
magnetic field of 1 kG strenght was advected into the bottom of a computational
domain 48 Mm wide by 20 Mm deep. The up and down convective motions produce a
hierarchy of magnetic loops with a wide range of scales, with smaller loops
riding "piggy back" in a serpentine fashion on larger loops. When a large loop
approaches the surface it produces an small active region with a compact
leading spot and more diffuse following spots
The Effects of Weak Spatiotemporal Noise on a Bistable One-Dimensional System
We treat analytically a model that captures several features of the
phenomenon of spatially inhomogeneous reversal of an order parameter. The model
is a classical Ginzburg-Landau field theory restricted to a bounded
one-dimensional spatial domain, perturbed by weak spatiotemporal noise having a
flat power spectrum in time and space. Our analysis extends the Kramers theory
of noise-induced transitions to the case when the system acted on by the noise
has nonzero spatial extent, and the noise itself is spatially dependent. By
extending the Langer-Coleman theory of the noise-induced decay of a metastable
state, we determine the dependence of the activation barrier and the Kramers
reversal rate prefactor on the size of the spatial domain. As this is increased
from zero and passes through a certain critical value, a transition between
activation regimes occurs, at which the rate prefactor diverges. Beyond the
transition, reversal preferentially takes place in a spatially inhomogeneous
rather than in a homogeneous way. Transitions of this sort were not discovered
by Langer or Coleman, since they treated only the infinite-volume limit. Our
analysis uses higher transcendental functions to handle the case of finite
volume. Similar transitions between activation regimes should occur in other
models of metastable systems with nonzero spatial extent, perturbed by weak
noise, as the size of the spatial domain is varied.Comment: 16 page
Extremal Black Holes in Dynamical Chern-Simons Gravity
Rapidly rotating black hole solutions in theories beyond general relativity
play a key role in experimental gravity, as they allow us to compute
observables in extreme spacetimes that deviate from the predictions of general
relativity. Such solutions are often difficult to find in
beyond-general-relativity theories due to the inclusion of additional fields
that couple to the metric non-linearly and non-minimally. In this paper, we
consider rotating black hole solutions in one such theory, dynamical
Chern-Simons gravity, where the Einstein-Hilbert action is modified by the
introduction of a dynamical scalar field that couples to the metric through the
Pontryagin density. We treat dynamical Chern-Simons gravity as an effective
field theory and work in the decoupling limit, where corrections are treated as
small perturbations from general relativity. We perturb about the
maximally-rotating Kerr solution, the so-called extremal limit, and develop
mathematical insight into the analysis techniques needed to construct solutions
for generic spin. First we find closed-form, analytic expressions for the
extremal scalar field, and then determine the trace of the metric perturbation,
giving both in terms of Legendre decompositions. Retaining only the first three
and four modes in the Legendre representation of the scalar field and the
trace, respectively, suffices to ensure a fidelity of over 99% relative to full
numerical solutions. The leading-order mode in the Legendre expansion of the
trace of the metric perturbation contains a logarithmic divergence at the
extremal Kerr horizon, which is likely to be unimportant as it occurs inside
the perturbed dynamical Chern-Simons horizon. The techniques employed here
should enable the construction of analytic, closed-form expressions for the
scalar field and metric perturbations on a background with arbitrary rotation.Comment: 25+9 pages (single column), 10 figures, 1 table; matches published
versio
Immersed Boundary Smooth Extension: A high-order method for solving PDE on arbitrary smooth domains using Fourier spectral methods
The Immersed Boundary method is a simple, efficient, and robust numerical
scheme for solving PDE in general domains, yet it only achieves first-order
spatial accuracy near embedded boundaries. In this paper, we introduce a new
high-order numerical method which we call the Immersed Boundary Smooth
Extension (IBSE) method. The IBSE method achieves high-order accuracy by
smoothly extending the unknown solution of the PDE from a given smooth domain
to a larger computational domain, enabling the use of simple Cartesian-grid
discretizations (e.g. Fourier spectral methods). The method preserves much of
the flexibility and robustness of the original IB method. In particular, it
requires minimal geometric information to describe the boundary and relies only
on convolution with regularized delta-functions to communicate information
between the computational grid and the boundary. We present a fast algorithm
for solving elliptic equations, which forms the basis for simple, high-order
implicit-time methods for parabolic PDE and implicit-explicit methods for
related nonlinear PDE. We apply the IBSE method to solve the Poisson, heat,
Burgers', and Fitzhugh-Nagumo equations, and demonstrate fourth-order pointwise
convergence for Dirichlet problems and third-order pointwise convergence for
Neumann problems
Neoglacial cooling culminates in rapid sea ice oscillations in eastern Fram Strait
EGU2011-407
The spatial and temporal distribution of sea ice in the subpolar North Atlantic is mainly controlled by the advection of warm Atlantic Water via the Norwegian and West Spitsbergen Current in eastern Fram Strait. Simultaneously, polar water and sea ice from the Arctic Ocean is transported southward by the East Greenland Current. Hence, variations in the strength of this oceanic circulation regime may either stimulate or reduce the sea ice extent.
Based on organic geochemical studies of a high-resolution sediment core from eastern Fram Strait we provide new evidence for the highly variable character of the sea ice conditions in this area. The combination of the sea ice proxy IP25 (Belt et al., 2007) with phytoplankton derived biomarkers (e.g. brassicasterol, dinosterol; Volkman 2006) enables a reliable reconstruction of sea surface and sea ice conditions, respectively (Müller et al., 2009; 2010). By means of these biomarkers, we trace gradually increasing sea ice occurrences from the Mid to the Late Holocene – consistent with the neoglacial cooling trend. Throughout the past ca. 3,000 years (BP) we observe a significant short-term variability in the biomarker records, which points to rapid advances and retreats of the sea ice cover at the continental margin of West Spitsbergen. The co-occurrence of IP25 and phytoplankton markers, however, suggests that the primary productivity benefits from these sea ice surges. As such, higher amounts of open-water phytoplankton biomarkers together with peak abundances of IP25 indicate recurring periods of enhanced ice-edge phytoplankton blooms at the core site. To what extent a seesawing of temperate Atlantic Water may account for these sea ice fluctuations requires further investigation. Concurrent variations in Siberian river discharge (Stein et al., 2004) or Norwegian glacier extents (Nesje et al., 2001), however, strengthen that these fluctuations may be assigned to variations in the North Atlantic/Arctic Oscillation (NAO/AO) and (hence) a weakened/accelerated Atlantic Water input and Arctic sea ice export
- …