523 research outputs found
Neutrino kinetics in a magnetized dense plasma
The relativistic kinetic equations (RKE) for lepton plasma in the presence of
a strong external magnetic field are derived in Vlasov approximation. The new
RKE for the electron spin distribution function includes the weak interaction
with neutrinos originated by the axial vector current () and provided
by the parity nonconservation. In a polarized electron gas Bloch equation
describing the evolution of the magnetization density perturbation is derived
from the electron spin RKE being modified in the presence of neutrino fluxes.
Such modified hydrodynamical equation allows to obtain the new dispersion
equation in a magnetized plasma from which the neutrino driven instability of
spin waves can be found. It is shown that this instability is more efficient
e.g. in a magnetized supernova than the analogous one for Langmuir waves
enhanced in an isotropic plasma.Comment: 20 pages, no figures, added subsection 2.3 about the lepton current
conservation, to be published in Astroparticle Physic
Alfven Wave Generation by means of High Orbital Injection of Barium Cloud in Magnetosphere
An analysis of the Alfven wave generation associated with the barium vapor
release at altitudes ~ 5.2 Earth's radii (ER) in the magnetosphere is
presented. Such injections were executed in G-8 and G-10 experiments of the
Combined Radiation and Radiation Effects Satellite (CRRES) mission. It is shown
that the generation of Alfven waves is possible during the total time of plasma
expansion. The maximum intensity of these waves corresponds to the time of
complete retardation of the diamagnetic cavity created by the expansion of
plasma cloud. The Alfven wave exhibits a form of an impulse with an effective
frequency ~ 0.03-0.05 Hz. Due to the background conditions and wave frequency,
the wave mainly oscillates along the geomagnetic field between the mirror
reflection points situated at ~ 0.7 ER. The wave amplitude is sufficient to the
generation of plasma instabilities and longitudinal electric field, and to an
increase in the longitudinal energy of electrons to ~ 1 keV. These processes
are the most probable for altitudes ~ 1 ER. The auroral kilometric radiation
(AKR) at frequencies ~ 100 kHz is associated with these accelerated electrons.
The acceleration of electrons and AKR can be observed almost continuously
during the first minute and then from time to time with pauses about 35-40 s
till 6-8 min after the release. The betatron acceleration of electrons at the
recovery of the geomagnetic field is also discussed. This mechanism could be
responsible for the acceleration of electrons resulting in the aurorae and
ultra short radio wave storm at frequencies 50-300 MHz observed at the 8-10th
min after the release.Comment: Presented at COSPAR 200
Eigenoscillations of the Differentially Rotating Sun: I. 22-year, 4000-year, and quasi-biennial modes
Retrograde waves with frequencies much lower than the rotation frequency
become trapped in the solar radiative interior. The eigenfunctions of the
compressible, nonadiabatic, Rossby-like modes (-mechanism and
radiative losses taken into account) are obtained by an asymptotic method
assuming a very small latitudinal gradient of rotation, without an arbitrary
choice of other free parameters. An integral dispersion relation for the
complex eigenfrequencies is derived as a solution of the boundary value
problem. The discovered resonant cavity modes (called R-modes) are
fundamentally different from the known r-modes: their frequencies are functions
of the solar interior structure, and the reason for their existence is not
related to geometrical effects. The most unstable R-modes are those with
periods of 1--3 yr, 18--30 yr, and 1500--20000 yrs; these three separate period
ranges are known from solar and geophysical data. The growing times of those
modes which are unstable with respect to the -mechanism are and years, respectively. The amplitudes of the R-modes are
growing towards the center of the Sun. We discuss some prospects to develop the
theory of R-modes as a driver of the dynamics in the convective zone which
could explain, e.g., observed short-term fluctuations of rotation, a control of
the solar magnetic cycle, and abrupt changes of terrestrial climate in the
past.Comment: 17 pages, 6 figures, To appear in Astronomy and Astrophysic
Discrete Imaging Models for Three-Dimensional Optoacoustic Tomography using Radially Symmetric Expansion Functions
Optoacoustic tomography (OAT), also known as photoacoustic tomography, is an
emerging computed biomedical imaging modality that exploits optical contrast
and ultrasonic detection principles. Iterative image reconstruction algorithms
that are based on discrete imaging models are actively being developed for OAT
due to their ability to improve image quality by incorporating accurate models
of the imaging physics, instrument response, and measurement noise. In this
work, we investigate the use of discrete imaging models based on Kaiser-Bessel
window functions for iterative image reconstruction in OAT. A closed-form
expression for the pressure produced by a Kaiser-Bessel function is calculated,
which facilitates accurate computation of the system matrix.
Computer-simulation and experimental studies are employed to demonstrate the
potential advantages of Kaiser-Bessel function-based iterative image
reconstruction in OAT
Photons Plus Ultrasound: Imaging and Sensing
Imaging and sensing based on fusing the compelling features of optical and ultrasonic waves is the fastest growing area of research in biomedical optics. The annual SPIE conference on this topic, co-chaired by both of us, has been doubling in size approximately every three years since 2003 (Fig. 1). As of 2009, this conference has become the largest at SPIE Photonics West. Hybrid modalities such as photoacoustic or optoacoustic tomography can provide deep tissue penetration, high ultrasonic resolution, and speckle-free optical contrast. Applications include in vivo functional and molecular imaging of cancer, neurophysiology, and vascular disease in both animals and humans. Major challenges include development of quantitative imaging, improvement of contrast and resolution, and commercialization of the technology. We look forward to seeing significant preclinical and clinical impact from this emerging technology
Photoacoustic imaging based on MEMS mirror scanning
A microelectromechanical systems (MEMS)-based photoacoustic imaging system is reported for
the first time. In this system, the MEMS-based light scanning subsystem and a ring-shaped
polyvinylidene fluoride (PVDF) transducer are integrated into a miniaturized probe that is
capable of three-dimensional (3D) photoacoustic imaging. It is demonstrated that the imaging
system is able to image small objects embedded in phantom materials and in chicken and to in
vivo visualize blood vessels under the skin of a human hand
Electromagnetic effects of neutrinos in an electron gas
We study the electromagnetic properties of a system that consists of an
electron background and a neutrino gas that may be moving or at rest, as a
whole, relative to the background. The photon self-energy for this system is
characterized by the usual transverse and longitudinal polarization functions,
and two additional ones which are the focus of our calculations, that give rise
to birefringence and anisotropic effects in the photon dispersion relations.
Expressions for them are obtained, which depend on the neutrino number
densities and involve momentum integrals over the electron distribution
functions, and are valid for any value of the photon momentum and general
conditions of the electron gas. Those expressions are evaluated explicitly for
several special cases and approximations which are generally useful in
astrophysical and cosmological settings. Besides studying the photon dispersion
relations, we consider the macroscopic electrodynamic equations for this
system, which involve the standard dielectric and permeability constants plus
two additional ones related to the photon self-energy functions. As an
illustration, the equations are used to discuss the evolution of a magnetic
field perturbation in such a medium. This particular phenomena has also been
considered in a recent work by Semikoz and Sokoloff as a mechanism for the
generation of large-scale magnetic fields in the Early Universe as a
consequence of the neutrino-plasma interactions, and allows us to establish
contact with a specific application in a well defined context, with a broader
scope and from a very different point of view.Comment: Revtex 20 page
- …