8,245 research outputs found

### Dynamic Power Spectral Analysis of Solar Measurements from Photospheric, Chromospheric, and Coronal Sources

An important aspect in the power spectral analysis of solar variability is the quasistationary and quasiperiodic nature of solar periodicities. In other words, the frequency, phase, and amplitude of solar periodicities vary on time scales ranging from active region lifetimes to solar cycle time scales. Here, researchers employ a dynamic, or running, power spectral density analysis to determine many periodicities and their time-varying nature in the projected area of active sunspot groups (S sub act). The Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor (SMM/ACRIM) total solar irradiance (S), the Nimbus-7 MgII center-to-wing ratio (R (MgII sub c/w)), the Ottawa 10.7 cm flux (F sub 10.7), and the GOES background x ray flux (X sub b) for the maximum, descending, and minimum portions of solar cycle 21 (i.e., 1980 to 1986) are used. The technique dramatically illustrates several previously unrecognized periodicities. For example, a relatively stable period at about 51 days has been found in those indices which are related to emerging magnetic fields. The majority of solar periodicities, particularly around 27, 150 and 300 days, are quasiperiodic because they vary in amplitude and frequency throughout the solar cycle. Finally, it is shown that there are clear differences between the power spectral densities of solar measurements from photospheric, chromospheric, and coronal sources

### Energy Loss from Reconnection with a Vortex Mesh

Experiments in superfluid 4He show that at low temperatures, energy
dissipation from moving vortices is many orders of magnitude larger than
expected from mutual friction. Here we investigate other mechanisms for energy
loss by a computational study of a vortex that moves through and reconnects
with a mesh of small vortices pinned to the container wall. We find that such
reconnections enhance energy loss from the moving vortex by a factor of up to
100 beyond that with no mesh. The enhancement occurs through two different
mechanisms, both involving the Kelvin oscillations generated along the vortex
by the reconnections. At relatively high temperatures the Kelvin waves increase
the vortex motion, leading to more energy loss through mutual friction. As the
temperature decreases, the vortex oscillations generate additional reconnection
events between the moving vortex and the wall, which decrease the energy of the
moving vortex by transfering portions of its length to the pinned mesh on the
wall.Comment: 9 pages, 10 figure

### Vortex nucleation by collapsing bubbles in Bose-Einstein condensates

The nucleation of vortex rings accompanies the collapse of ultrasound bubbles
in superfluids. Using the Gross-Pitaevskii equation for a uniform condensate we
elucidate the various stages of the collapse of a stationary spherically
symmetric bubble and establish conditions necessary for vortex nucleation. The
minimum radius of the stationary bubble, whose collapse leads to vortex
nucleation, was found to be about 28 healing lengths. The time after which the
nucleation becomes possible is determined as a function of bubble's radius. We
show that vortex nucleation takes place in moving bubbles of even smaller
radius if the motion made them sufficiently oblate.Comment: 4 pages, 5 figure

### Parity violation in quasielastic electron-nucleus scattering within the relativistic impulse approximation

We study parity violation in quasielastic (QE) electron-nucleus scattering
using the relativistic impulse approximation. Different fully relativistic
approaches have been considered to estimate the effects associated with the
final-state interactions. We have computed the parity-violating quasielastic
(PVQE) asymmetry and have analyzed its sensitivity to the different ingredients
that enter in the description of the reaction mechanism: final-state
interactions, nucleon off-shellness effects, current gauge ambiguities.
Particular attention has been paid to the description of the weak neutral
current form factors. The PVQE asymmetry is proven to be an excellent
observable when the goal is to get precise information on the axial-vector
sector of the weak neutral current. Specifically, from measurements of the
asymmetry at backward scattering angles good knowledge of the radiative
corrections entering in the isovector axial-vector sector can be gained.
Finally, scaling properties shown by the interference $\gamma-Z$ nuclear
responses are also analyzed.Comment: 15 pages, 11 figure

### Parity violation and dynamical relativistic effects in $(\vec{e},e'N)$ reactions

It is well known that coincidence quasielastic $(\vec{e},e'N)$ reactions are
not appropriate to analyze effects linked to parity violation due the presence
of the fifth electromagnetic (EM) response $R^{TL'}$. Nevertheless, in this
work we develop a fully relativistic approach to be applied to parity-violating
(PV) quasielastic $(\vec{e},e'N)$ processes. This is of importance as a
preliminary step in the subsequent study of inclusive quasielastic PV
$(\vec{e},e')$ reactions. Moreover, our present analysis allows us to
disentangle effects associated with the off-shell character of nucleons in
nuclei, gauge ambiguities and the role played by the lower components in the
nucleon wave functions, i.e., dynamical relativistic effects. This study can
help in getting clear information on PV effects. Particular attention is paid
to the relativistic plane-wave impulse approximation where the explicit
expressions for the PV single-nucleon responses are shown for the first time.Comment: 39 pages, 9 figure

### Direct measurement of quantum phase gradients in superfluid 4He flow

We report a new kind of experiment in which we generate a known superfluid
velocity in a straight tube and directly determine the phase difference across
the tube's ends using a superfluid matter wave interferometer. By so doing, we
quantitatively verify the relation between the superfluid velocity and the
phase gradient of the condensate macroscopic wave function. Within the
systematic error of the measurement (~10%) we find v_s=(hbar/m_4)*(grad phi)

### Global analysis of parity-violating asymmetry data for elastic electron scattering

We perform a statistical analysis of the full set of parity-violating
asymmetry data for elastic electron scattering including the most recent high
precision measurement from $Q$-weak. Given the basis of the present analysis,
our estimates appear to favor non-zero vector strangeness, specifically,
positive (negative) values for the electric (magnetic) strange form factors. We
also provide an accurate estimate of the axial-vector nucleon form factor at
zero momentum transfer, $G_A^{ep}(0)$. Our study shows $G_A^{ep}(0)$ to be
importantly reduced with respect to the currently accepted value. We also find
our analysis of data to be compatible with the Standard Model values for the
weak charges of the proton and neutron.Comment: 6 pages, 4 figures, 2 tables. Accepted for publication in PR

### Spontaneous squeezing of a vortex in an optical lattice

We study the equilibrium states of a vortex in a Bose-Einstein condensate in
a one-dimensional optical lattice. We find that quantum effects can be
important and that it is even possible for the vortex to be strongly squeezed,
which reflects itself in a different quantum mechanical uncertainty of the
vortex position in two orthogonal directions. The latter is observable by
measuring the atomic density after an expansion of the Bose-Einstein condensate
in the lattice.Comment: 8 pages, 3 figures, more details added, some new citation

### Pseudo-contact angle due to superfluid vortices in $^{4}$He

We have investigated spreading of superfluid $^{4}$He on top of polished
MgF$_2$ and evaporated SiO$_{2}$ substrates. Our results show strongly varying
contact angles of 0 - 15 mrad on the evaporated layers. According to our
theoretical calculations, these contact angles can be explained by a spatially
varying distribution of vortex lines, the unpinning velocity of which is
inversely proportional to the liquid depth.Comment: 10 pages, 4 figure

- â€¦