18,122 research outputs found
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
The Estimation of Conditional Densities
We discuss a number of issues in the smoothed nonparametric estimation of kernel conditional probability density functions for stationary processes. The kernel conditional density estimate is a ratio of joint and marginal density estimates. We point out the different implications of leading choices of bandwidths in numerator and denominator for the ability of the estimate to integrate to one and to have finite moments. Again bearing in mind different bandwidth possibilities, we discuss asymptotic theory for the estimate: asymptotic bias and variance are calculated under various conditions, an extended discussion of bandwidth choice is included, and a central limit theorem is given.Conditional density estimation, serial dependence, bandwidth choice.
Coherent versus non-coherent decode-and-forward relaying aided cooperative space-time shift keying
Motivated by the recent concept of Space-Time Shift Keying (STSK), we propose a novel cooperative STSK family, which is capable of achieving a flexible rate-diversity tradeoff, in the context of cooperative space-time transmissions. More specifically, we first propose a Coherent cooperative STSK (CSTSK) scheme, where each Relay Node (RN) activates Decode-and-Forward (DF) transmissions, depending on the success or failure of Cyclic Redundancy Checking (CRC). We invoke a bitto- STSK mapping rule, where according to the input bits, one of the Q pre-assigned dispersion vectors is activated to implicitly convey log2(Q) bits, which are transmitted in combination with the classic log2(L)-bit modulated symbol. Additionally, we introduce a beneficial dispersion vector design, which enables us to dispense with symbol-level Inter-Relay Synchronization (IRS). Furthermore, the Destination Node (DN) is capable of jointly detecting the signals received from the source-destination and relay-destination links, using a low-complexity single-stream-based Maximum Likelihood (ML) detector, which is an explicit benefit of our Inter-Element Interference (IEI)-free system model. More importantly, as a benefit of its design flexibility, our cooperative CSTSK arrangement enables us to adapt the number of the RNs, the transmission rate as well as the achievable diversity order. Moreover, we also propose a Differentially-encoded cooperative STSK (DSTSK) arrangement, which dispenses with CSI estimation at any of the nodes, while retaining the fundamental benefits of the cooperative CSTSK scheme
A Ternary Lattice Boltzmann Model for Amphiphilic Fluids
A lattice Boltzmann model for amphiphilic fluid dynamics is presented. It is
a ternary model, in that it conserves mass separately for each chemical species
present (water, oil, amphiphile), and it maintains an orientational degree of
freedom for the amphiphilic species. Moreover, it models fluid interactions at
the microscopic level by introducing self-consistent forces between the
particles, rather than by positing a Landau free energy functional. This
combination of characteristics fills an important need in the hierarchy of
models currently available for amphiphilic fluid dynamics, enabling efficient
computer simulation and furnishing new theoretical insight. Several
computational results obtained from this model are presented and compared to
existing lattice-gas model results. In particular, it is noted that lamellar
structures, which are precluded by the Peierls instability in two-dimensional
systems with kinetic fluctuations, are not observed in lattice-gas models, but
are easily found in the corresponding lattice Boltzmann models. This points out
a striking difference in the phenomenology accessible to each type of model.Comment: 12 pages, 6 figures included with graphic
First-principles method of propagation of tightly bound excitons: exciton band structure of LiF and verification with inelastic x-ray scattering
We propose a simple first-principles method to describe propagation of
tightly bound excitons. By viewing the exciton as a composite object (an
effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic
kernel to encapsulate the exciton propagation and decay for all binding energy.
Applied to prototypical LiF, our approach produces three exciton bands, which
we verified quantitatively via inelastic x-ray scattering. The proposed
real-space picture is computationally inexpensive and thus enables study of the
full exciton dynamics, even in the presence of surfaces and impurity
scattering. It also provides intuitive understanding to facilitate practical
exciton engineering in semiconductors, strongly correlated oxides, and their
nanostructures.Comment: 5 pages, 4 figures. Accepted by PR
Constraints on the average magnetic field strength of relic radio sources 0917+75 and 1401-33 from XMM-Newton observations
We observed two relic radio sources, 0917+75 and 1401-33, with the XMM-Newton
X-ray observatory. We did not detect any X-ray emission, thermal or
non-thermal, in excess of the local background level from either target. This
imposes new upper limits on the X-ray flux due to inverse Compton scattering of
photons from the cosmic microwave background by relativistic electrons in the
relic sources, and new lower limits on the magnetic field strength from the
relative strength of the radio and X-ray emission. The combination of radio and
X-ray observations provides a measure of the magnetic field independent of
equipartition or minimum energy assumptions. Due to increasing sensitivity of
radio observations, the known population of cluster relics has been growing;
however, studies of non-thermal X-ray emission from relics remain scarce. Our
study adds to the small sample of relics studied in X-rays. In both relics, our
field strength lower limits are slightly larger than estimates of the
equipartition magnetic field.Comment: 11 pages, 5 figures. Accepted by MNRA
Direct observation of ultrafast thermal and non-thermal lattice deformation of polycrystalline Aluminum film
The dynamics of thermal and non-thermal lattice deformation of nanometer
thick polycrystalline aluminum film has been studied by means of femtosecond
(fs) time-resolved electron diffraction. We utilized two different pump
wavelengths: 800 nm, the fundamental of Ti: sapphire laser and 1250 nm
generated by a home-made optical parametric amplifier(OPA). Our data show that,
although coherent phonons were generated under both conditions, the diffraction
intensity decayed with the characteristic time of 0.9+/-0.3 ps and 1.7+/-0.3 ps
under 800 nm and 1250 nm excitation, respectively. Because the 800 nm laser
excitation corresponds to the strong interband transition of aluminum due to
the 1.55 eV parallel band structure, our experimental data indicate the
presence of non-thermal lattice deformation under 800 nm excitation, which
occurs on a time-scale that is shorter than the thermal processes dominated by
electron-phonon coupling under 1250 nm excitation
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