3,982 research outputs found
Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
The last solar minimum activity period, and the consequent minimum modulation
conditions for cosmic rays, was unusual. The highest levels of galactic protons
were recorded at Earth in late 2009 in contrast to expectations. Proton spectra
observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the
Resurs-DK1 satellite are presented together with the solutions of a
comprehensive numerical model for the solar modulation of cosmic rays. The
model is used to determine what mechanisms were mainly responsible for the
modulation of protons during this period, and why the observed spectrum for
2009 was the highest ever recorded. From mid-2006 until December 2009 we find
that the spectra became significantly softer because increasingly more low
energy protons had reached Earth. To simulate this effect, the rigidity
dependence of the diffusion coefficients had to decrease significantly below ~3
GeV. The modulation minimum period of 2009 can thus be described as relatively
more "diffusion dominated" than previous solar minima. However, we illustrate
that drifts still had played a significant role but that the observable
modulation effects were not as well correlated with the waviness of the
heliospheric current sheet as before. Protons still experienced global gradient
and curvature drifts as the heliospheric magnetic field had decreased
significantly until the end of 2009, in contrast to the moderate decreases
observed during previous minimum periods. We conclude that all modulation
processes contributed to the observed increases in the proton spectra for this
period, exhibiting an intriguing interplay of these major mechanisms
Variational approach to gravitational theories with two independent connections
A new variational approach for general relativity and modified theories of
gravity is presented. In addition to the metric tensor, two independent affine
connections enter the action as dynamical variables. In the matter action the
dependence upon one of the connections is left completely unspecified. When the
variation is applied to the Einstein-Hilbert action the Einstein field
equations are recovered. However when applied to and Scalar-Tensor
theories, it yields gravitational field equations which differ from their
equivalents obtained with a metric or Palatini variation and reduce to the
former ones only when no connections appear in the matter action.Comment: 11 pages, no figure
Spinning test particles and clock effect in Kerr spacetime
We study the motion of spinning test particles in Kerr spacetime using the
Mathisson-Papapetrou equations; we impose different supplementary conditions
among the well known Corinaldesi-Papapetrou, Pirani and Tulczyjew's and analyze
their physical implications in order to decide which is the most natural to
use. We find that if the particle's center of mass world line, namely the one
chosen for the multipole reduction, is a spatially circular orbit (sustained by
the tidal forces due to the spin) then the generalized momentum of the test
particle is also tangent to a spatially circular orbit intersecting the center
of mass line at a point. There exists one such orbit for each point of the
center of mass line where they intersect; although fictitious, these orbits are
essential to define the properties of the spinning particle along its physical
motion. In the small spin limit, the particle's orbit is almost a geodesic and
the difference of its angular velocity with respect to the geodesic value can
be of arbitrary sign, corresponding to the spin-up and spin-down possible
alignment along the z-axis. We also find that the choice of the supplementary
conditions leads to clock effects of substantially different magnitude. In
fact, for co-rotating and counter-rotating particles having the same spin
magnitude and orientation, the gravitomagnetic clock effect induced by the
background metric can be magnified or inhibited and even suppressed by the
contribution of the individual particle's spin. Quite surprisingly this
contribution can be itself made vanishing leading to a clock effect
undistiguishable from that of non spinning particles. The results of our
analysis can be observationally tested.Comment: IOP macros, eps figures n. 12, to appear on Classical and Quantum
Gravity, 200
Spin precession in the Schwarzschild spacetime: circular orbits
We study the behavior of nonzero rest mass spinning test particles moving
along circular orbits in the Schwarzschild spacetime in the case in which the
components of the spin tensor are allowed to vary along the orbit, generalizing
some previous work.Comment: To appear on Classical and Quantum Gravity, 200
Deformed Special Relativity as an effective theory of measurements on quantum gravitational backgrounds
In this article we elaborate on a recently proposed interpretation of DSR as
an effective measurement theory in the presence of non-negligible (albeit
small) quantum gravitational fluctuations. We provide several heuristic
arguments to explain how such a new theory can emerge and discuss the possible
observational consequences of this framework.Comment: 11 pages, no figure
Reconstruction of gravity: Rip cosmology, finite-time future singularities and thermodynamics
We demonstrate that there appear finite-time future singularities in
gravity with being the torsion scalar. We reconstruct a model of
gravity with realizing the finite-time future singularities. In addition, it is
explicitly shown that a power-low type correction term ()
such as a term can remove the finite-time future singularities in
gravity. Moreover, we study models with realizing inflation in the early
universe, the CDM model, Little Rip cosmology and Pseudo-Rip
cosmology. It is demonstrated that the disintegration of bound structures for
Little Rip and Pseudo-Rip cosmologies occurs in the same way as in gravity with
corresponding dark energy fluid. We also discuss that the time-dependent matter
instability in the star collapse can occur in gravity. Furthermore, we
explore thermodynamics in gravity and illustrate that the second law of
thermodynamics can be satisfied around the finite-time future singularities for
the universe with the temperature inside the horizon being the same as that of
the apparent horizon.Comment: 38 pages, no figure, version accepted for publication in Physical
Review
A Born-Infeld-like f(R) gravity
Several features of an theory in which there is a maximum value for
the curvature are analyzed. The theory admits the vaccuum solutions of GR, and
also the radiation evolution for the scale factor of the standard cosmological
model. Working in the Jordan frame, a complete analysis of the phase space is
performed, and its results supported with examples obtainted by numerical
integration. In particular, we showed that theory has nonsingular cosmological
solutions which after the bounce enter a phase of de Sitter expansion and
subsequently relax to a GR-like radiation-dominated evolution.Comment: Latex file, 14 pages, 7 figures (jpg format), including more detailed
discussions than previous version, accepted for publication in Physical
Review
Gyroscopic Precession and Inertial Forces in Axially Symmetric Stationary Spacetimes
We study the phenomenon of gyroscopic precession and the analogues of
inertial forces within the framework of general relativity. Covariant
connections between the two are established for circular orbits in stationary
spacetimes with axial symmetry. Specializing to static spacetimes, we prove
that gyroscopic precession and centrifugal force both reverse at the photon
orbits. Simultaneous non-reversal of these in the case of stationary spacetimes
is discussed. Further insight is gained in the case of static spacetime by
considering the phenomena in a spacetime conformal to the original one.
Gravi-electric and gravi-magnetic fields are studied and their relation to
inertial forces is established.Comment: 21 pages, latex, no figures, http://202.41.67.76/~nayak/gpifass.te
Финансово-экономический механизм функционирования кредитных союзов в Украине
Цель статьи - исследование финансово-экономического механизма функционирования кредитных союзов как альтернативного финансирования населения, что позволяет максимально приблизить финансовые услуги к потребителям, создать надлежащие условия для долгосрочного кредитования с применением современной рыночной инфраструктуры и финансовых инструментов
The uterine junctional zone: A 3-dimensional ultrasound study of patients with endometriosis
The uterine junctional zone (JZ) alterations are correlated with adenomyosis. An accurate evaluation of the JZ may be obtained by 3-dimensional transvaginal sonography (TVS). The aim of the present prospective study was to assess the value of detectable alterations by 3-dimensional TVS of the JZ in patients with pelvic endometriosis (diagnosed by laparoscopy and histologic condition) and to compare these findings with those of women without pelvic endometriosis
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