1,488 research outputs found
Recent Developments of NEMO: Detection of Solar Eruptions Characteristics
The recent developments in space instrumentation for solar observations and
telemetry have caused the necessity of advanced pattern recognition tools for
the different classes of solar events. The Extreme ultraviolet Imaging
Telescope (EIT) of solar corona on-board SOHO spacecraft has uncovered a new
class of eruptive events which are often identified as signatures of Coronal
Mass Ejection (CME) initiations on solar disk. It is evident that a crucial
task is the development of an automatic detection tool of CMEs precursors. The
Novel EIT wave Machine Observing (NEMO) (http://sidc.be/nemo) code is an
operational tool that detects automatically solar eruptions using EIT image
sequences. NEMO applies techniques based on the general statistical properties
of the underlying physical mechanisms of eruptive events on the solar disc. In
this work, the most recent updates of NEMO code - that have resulted to the
increase of the recognition efficiency of solar eruptions linked to CMEs - are
presented. These updates provide calculations of the surface of the dimming
region, implement novel clustering technique for the dimmings and set new
criteria to flag the eruptive dimmings based on their complex characteristics.
The efficiency of NEMO has been increased significantly resulting to the
extraction of dimmings observed near the solar limb and to the detection of
small-scale events as well. As a consequence, the detection efficiency of CMEs
precursors and the forecasts of CMEs have been drastically improved.
Furthermore, the catalogues of solar eruptive events that can be constructed by
NEMO may include larger number of physical parameters associated to the dimming
regions.Comment: 12 Pages, 5 figures, submitted to Solar Physic
Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model
We investigate the magnetic instabilities of the nondegenerate (s-band) and a
degenerate (d-band) Hubbard model in two dimensions using many-body effects due
to the particle-particle diagrams and Hund's rule local correlations. The
density of states and the position of Van Hove singularity change depending on
the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly
reduced in the s-band case, and ferromagnetism survives only if electron
density is small, and the band is almost flat at small momenta due to
next-nearest neighbor hopping. In contrast, for the d-band case the reduction
of the Stoner parameter which follows from particle-particle correlations is
much smaller and ferromagnetism survives to a large extent. Inclusion of local
spin-spin correlations has a limited destabilizing effect on the magnetic
states.Comment: 8 pages, 7 figure
Integrable discretizations of derivative nonlinear Schroedinger equations
We propose integrable discretizations of derivative nonlinear Schroedinger
(DNLS) equations such as the Kaup-Newell equation, the Chen-Lee-Liu equation
and the Gerdjikov-Ivanov equation by constructing Lax pairs. The discrete DNLS
systems admit the reduction of complex conjugation between two dependent
variables and possess bi-Hamiltonian structure. Through transformations of
variables and reductions, we obtain novel integrable discretizations of the
nonlinear Schroedinger (NLS), modified KdV (mKdV), mixed NLS, matrix NLS,
matrix KdV, matrix mKdV, coupled NLS, coupled Hirota, coupled Sasa-Satsuma and
Burgers equations. We also discuss integrable discretizations of the
sine-Gordon equation, the massive Thirring model and their generalizations.Comment: 24 pages, LaTeX2e (IOP style), final versio
Quantum Monte Carlo in the Interaction Representation --- Application to a Spin-Peierls Model
A quantum Monte Carlo algorithm is constructed starting from the standard
perturbation expansion in the interaction representation. The resulting
configuration space is strongly related to that of the Stochastic Series
Expansion (SSE) method, which is based on a direct power series expansion of
exp(-beta*H). Sampling procedures previously developed for the SSE method can
therefore be used also in the interaction representation formulation. The new
method is first tested on the S=1/2 Heisenberg chain. Then, as an application
to a model of great current interest, a Heisenberg chain including phonon
degrees of freedom is studied. Einstein phonons are coupled to the spins via a
linear modulation of the nearest-neighbor exchange. The simulation algorithm is
implemented in the phonon occupation number basis, without Hilbert space
truncations, and is exact. Results are presented for the magnetic properties of
the system in a wide temperature regime, including the T-->0 limit where the
chain undergoes a spin-Peierls transition. Some aspects of the phonon dynamics
are also discussed. The results suggest that the effects of dynamic phonons in
spin-Peierls compounds such as GeCuO3 and NaV2O5 must be included in order to
obtain a correct quantitative description of their magnetic properties, both
above and below the dimerization temperature.Comment: 23 pages, Revtex, 11 PostScript figure
Nuclear Polarization of Molecular Hydrogen Recombined on a Non-metallic Surface
The nuclear polarization of molecules formed by recombination
of nuclear polarized H atoms on the surface of a storage cell initially coated
with a silicon-based polymer has been measured by using the longitudinal
double-spin asymmetry in deep-inelastic positron-proton scattering. The
molecules are found to have a substantial nuclear polarization, which is
evidence that initially polarized atoms retain their nuclear polarization when
absorbed on this type of surfac
Single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target
Single-spin asymmetries for semi-inclusive electroproduction of charged pions
in deep-inelastic scattering of positrons are measured for the first time with
transverse target polarization. The asymmetry depends on the azimuthal angles
of both the pion () and the target spin axis () about the virtual
photon direction and relative to the lepton scattering plane. The extracted
Fourier component \cmpi is a signal of the previously unmeasured quark
transversity distribution, in conjunction with the so-called Collins
fragmentation function, also unknown. The Fourier component \smpi of the
asymmetry arises from a correlation between the transverse polarization of the
target nucleon and the intrinsic transverse momentum of quarks, as represented
by the previously unmeasured Sivers distribution function. Evidence for both
signals is observed, but the Sivers asymmetry may be affected by exclusive
vector meson productio
First Measurement of the Tensor Structure Function of the Deuteron
The \Hermes experiment has investigated the tensor spin structure of the
deuteron using the 27.6 GeV/c positron beam of \Hera. The use of a tensor
polarized deuteron gas target with only a negligible residual vector
polarization enabled the first measurement of the tensor asymmetry \At and
the tensor structure function \bd for average values of the Bj{\o}rken
variable and of the squared four-momentum transfer . The quantities \At and \bd are found to be
non-zero. The rise of \bd for decreasing values of can be interpreted to
originate from the same mechanism that leads to nuclear shadowing in
unpolarized scattering
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