2,122 research outputs found
The spatial damping of magnetohydrodynamic waves in a flowing partially ionised prominence plasma
Solar prominences are partially ionised plasmas displaying flows and
oscillations. These oscillations show time and spatial damping and, commonly,
have been explained in terms of magnetohydrodynamic (MHD) waves. We study the
spatial damping of linear non-adiabatic MHD waves in a flowing partially
ionised plasma, having prominence-like physical properties. We consider single
fluid equations for a partially ionised hydrogen plasma including in the energy
equation optically thin radiation, thermal conduction by electrons and
neutrals, and heating. Keeping the frequency real and fixed, we have solved the
obtained dispersion relations for the complex wavenumber, k, and have analysed
the behaviour of the damping length, wavelength and the ratio of the damping
length to the wavelength, versus period, for Alfven, fast, slow and thermal
waves.Comment: 28 pages, 9 figure
The statistical significance of the N-S asymmetry of solar activity revisited
The main aim of this study is to point out the difficulties found when trying
to assess the statistical significance of the North-South asymmetry (hereafter
SSNSA) of the most usually considered time series of solar activity. First of
all, we distinguish between solar activity time series composed by integer or
non-integer and dimensionless data, or composed by non-integer and dimensional
data. For each of these cases, we discuss the most suitable statistical tests
which can be applied and highlight the difficulties to obtain valid information
about the statistical significance of solar activity time series. Our results
suggest that, apart from the need to apply the suitable statistical tests,
other effects such as the data binning, the considered units and the need, in
some tests, to consider groups of data, affect substantially the determination
of the statistical significance of the asymmetry. Our main conclusion is that
the assessment of the statistical significance of the N-S asymmetry of solar
activity is a difficult matter and that an absolute answer cannot be given,
since many different effects influence the results given by the statistical
tests. In summary, the quantitative results about the statistical significance
of the N-S asymmetry of solar activity provided by different authors, as well
as the studies about its behaviour, must be considered with care because they
depend from the chosen values of different parameters or from the considered
units.Comment: Astronomy and Astrophysics Latex, 9 pages, 4 figure
Relativistic bound states in Yukawa model
The bound state solutions of two fermions interacting by a scalar exchange
are obtained in the framework of the explicitly covariant light-front dynamics.
The stability with respect to cutoff of the J= and J=
states is studied. The solutions for J= are found to be stable for
coupling constants below the critical value
and unstable above it. The asymptotic behavior of the
wave functions is found to follow a law. The coefficient
and the critical coupling constant are calculated from an
eigenvalue equation. The binding energies for the J= solutions
diverge logarithmically with the cutoff for any value of the coupling constant.
For a wide range of cutoff, the states with different angular momentum
projections are weakly split.Comment: 22 pages, 13 figures, .tar.gz fil
A new vibrational level of the H molecular ion
A new state of the H molecular ion with binding energy of
1.09 a.u. below the first dissociation limit is predicted, using
highly accurate numerical nonrelativistic quantum calculations. It is the first
L=0 excited state, antisymmetric with respect to the exchange of the two
protons. It manifests itself as a huge p-H scattering length of
Bohr radii.Comment: 6 pages + 3 figure
Long-term variation in the Sun's activity caused by magnetic Rossby waves in the tachocline
Long-term records of sunspot number and concentrations of cosmogenic
radionuclides (10Be and 14C) on the Earth reveal the variation of the Sun's
magnetic activity over hundreds and thousands of years. We identify several
clear periods in sunspot, 10Be, and 14C data as 1000, 500, 350, 200 and 100
years. We found that the periods of the first five spherical harmonics of the
slow magnetic Rossby mode in the presence of a steady toroidal magnetic field
of 1200-1300 G in the lower tachocline are in perfect agreement with the time
scales of observed variations. The steady toroidal magnetic field can be
generated in the lower tachocline either due to the steady dynamo magnetic
field for low magnetic diffusivity or due to the action of the latitudinal
differential rotation on the weak poloidal primordial magnetic field, which
penetrates from the radiative interior. The slow magnetic Rossby waves lead to
variations of the steady toroidal magnetic field in the lower tachocline, which
modulate the dynamo magnetic field and consequently the solar cycle strength.
This result constitutes a key point for long-term prediction of the cycle
strength. According to our model, the next deep minimum in solar activity is
expected during the first half of this century.Comment: 4 pages, 4 figures, accepted in ApJ
Effects of the dexmedetomidine sedation on intraocular pressure and on the central cornea thickness in the dog
Objetivo: Determinar el efecto de la sedación con dexmedetomidina
sobre la presión intraocular (PIO) y el espesor central de la córnea (ECC)
en perros
Material y métodos: Estudio realizado en 10 perros a los que se midió la
PIO y ECC basales y tras administración tópica de tropicamida 1%.
Seguidamente sedamos con dexmedetomidina 5μg/kg IV y valoramos PIO
y ECC a los 5,10, 15 y 20 minutos post-sedación. Los valores medios se
compararon mediante la prueba t de Student para muestras pareadas.
Resultados: Los valores medios basales de PIO fueron media ± D.E.
10,95 ± 1,70 mmHg; y 571 ± 21,42 μm para el ECC. No existe asociación
significativa entre PIO y ECC (r= -0,2399). La midriasis no varió
significativamente los valores de PIO (P= 0,3665) pero sí el ECC
(P=5,6109x10-6). La sedación con dexmedetomidina no varía
significativamente los valores de PIO ni ECC (P>0,05).
Conclusiones: La midriasis provocada por tropicamida 1% disminuye
significativamente el ECC pero no la PIO. La sedación con
dexmedetomidina 5 mg/kg IV no varía significativamente los valores
basales de PIO ni del ECC.Objective: to determine the effects of the dexmedetomidine sedation on
intraocular pressure (IOP) and on the central corneal thickness (CCT).
Material and methods: this study has been performed over 10 dogs
treated in the Veterinary Clinical Hospital of Córdoba University. The IOP and the CCT were measured before and after administration of one drop
of 1% tropicamide. Thereafter, they were sedated with dexmedetomidine
5 μg/kg IV, and IOP and CCT were evaluated at 5, 10, 15 and 20 minutes
after sedation. A t-Student test was performed with paired samples of
mean values in order to compare both groups.
Results: Basal values of IOP were 10.95 ± 1.70 mmHg, whereas CCT
mean values were 571 ± 21.42μm. There were no statistically significant
association between IOP and CCT (Pearson correlation r= -0.2399).
Mydriasis did not significantly change the values of IOP (P= 0.3665), but
did the CCT ones (P= 5.6109x10-6). No statistically significant differences
were found between the IOP nor the CCT values before and after sedation
with dexmedetomidine (P>0.05).
Conclusions: tropicamide-induced mydriasis does not affect IOP value,
but it causes a significant decrease of the CCT value. Sedation with 5
g/kg IV dexmedetomidine has not statistically significant effect on IOP
or CCT
Antiproton-Hydrogen annihilation at sub-kelvin temperatures
The main properties of the interaction of ultra low-energy antiprotons ( a.u.) with atomic hydrogen are established. They include the
elastic and inelastic cross sections and Protonium (Pn) formation spectrum. The
inverse Auger process () is taken into account in the
framework of an unitary coupled-channels model. The annihilation cross-section
is found to be several times smaller than the predictions made by the black
sphere absorption models. A family of nearthreshold metastable
states is predicited. The dependence of Protonium formation probability on the
position of such nearthreshold S-matrix singularities is analysed. An
estimation for the annihilation cross section is obtained.Comment: latex.tar.gz file, 22 pages, 9 figure
The importance of few-nucleon physics at low energy
This manuscript originated from the discussion at the workshop on the "Future
of Few-body Low Energy Experimental Physics" (FFLEEP), which was held at the
University of Trento on December 4-7, 2002 and has been written in its present
form on March 19, 2003. It illustrates a selection of theoretical advancements
in the nuclear few-body problem, including two- and many-nucleon interactions,
the three-nucleon bound and scattering system, the four-body problem, the
A-body (A4) problem, and fields of related interest, such as reactions of
astrophysical interest and few-neutron systems. Particular attention is called
to the contradictory situation one experiences in this field: while theory is
currently advancing and has the potential to inspire new experiments, the
experimental activity is nevertheless rapidly phasing out. If such a trend will
continue, advancements in this area will become critically difficult.Comment: 29 pages, 21 figures. Manuscript originated from the discussion at
the workshop on the "Future of Few-body Low Energy Experimental Physics"
(FFLEEP), University of Trento, December 4-7, 2002, written in its present
form on March 19, 2003, circulated mainly among the participants to the
FFLEEP workshop. Since the authors have been repeatedly solicited to make the
manuscript accessible to a larger audience potentially interested in its
scientific content, they have decided to post it on this archiv
Synthesis for Polynomial Lasso Programs
We present a method for the synthesis of polynomial lasso programs. These
programs consist of a program stem, a set of transitions, and an exit
condition, all in the form of algebraic assertions (conjunctions of polynomial
equalities). Central to this approach is the discovery of non-linear
(algebraic) loop invariants. We extend Sankaranarayanan, Sipma, and Manna's
template-based approach and prove a completeness criterion. We perform program
synthesis by generating a constraint whose solution is a synthesized program
together with a loop invariant that proves the program's correctness. This
constraint is non-linear and is passed to an SMT solver. Moreover, we can
enforce the termination of the synthesized program with the support of test
cases.Comment: Paper at VMCAI'14, including appendi
Limits on the low energy antinucleon-nucleus annihilations from the Heisenberg principle
We show that the quantum uncertainty principle puts some limits on the
effectiveness of the antinucleon-nucleus annihilation at very low energies.
This is caused by the fact that the realization a very effective short-distance
reaction process implies information on the relative distance of the reacting
particles. Some quantitative predictions are possible on this ground, including
the approximate A-independence of antinucleon-nucleus annihilation rates.Comment: 10 pages, no figure
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