15,148 research outputs found
The structure of lightning flashes HF-UHF: 12 September 1975, Atlanta, Georgia
Simultaneous measurement of sferics at 3, 30, 139, and 295 MHz were made during thunderstorms. Wideband electronics and an analogue tape recorder continuously recorded the radiation from lightning with about 300 kHz of bandwidth. The data were obtained during the passage of a cold front. Flashing rate, burst rate and the structure of individual flashes were recorded. The record of a typical flash begins with a sudden burst of closely spaced pulses whose temporal structure is typical of the stepped leader, and ends in a large pulse suggestive of a first return stroke. The remainder of the flash consists of a sequence of pulses of varying amplitude separated by quiet periods of the order of milliseconds. The shape of these pulses and the temporal structure suggest that the first few large pulses are return strokes. Other discharges begin with widely spaced discrete pulses and resemble the preceding discharge less the leader and return stroke phase. The radiation exhibits a similar structure, at each of the frequencies monitored
Lower thermosphere densities of N2, O and Ar under high latitude winter conditions
Measurements of the neutral thermosphere were conducted in northern Scandinavia during the Energy Budget Campaign. These measurements included determinations of N2, O, and Ar densities using rocket-borne experiments. The results obtained in the experiments are presented, taking into account also details regarding the employed experimental methods, and an evaluation of the significance of the data. It is found that there are striking differences in thermospheric distributions of the neutral constituents under different geomagnetic conditions. Under quiet geomagnetic conditions there was reasonable agreement with the United States Standard Atmosphere. The concentrations of N2 and Ar were about 70 percent of the predicted values, while the O concentration was about 2.5 times greater
Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions
Accurate models of alkali and halide ions in aqueous solution are necessary
for computer simulations of a broad variety of systems. Previous efforts to
develop ion force fields have generally focused on reproducing experimental
measurements of aqueous solution properties such as hydration free energies and
ion-water distribution functions. This dependency limits transferability of the
resulting parameters because of the variety and known limitations of water
models. We present a solvent-independent approach to calibrating ion parameters
based exclusively on crystal lattice properties. Our procedure relies on
minimization of lattice sums to calculate lattice energies and interionic
distances instead of equilibrium ensemble simulations of dense fluids. The gain
in computational efficiency enables simultaneous optimization of all parameters
for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that
enforce consistency with periodic table trends. We demonstrate the method by
presenting lattice-derived parameters for the primitive model and the
Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting
parameters successfully reproduce the lattice properties used to derive them
and are free from the influence of any water model. To assess the
transferability of the Lennard-Jones parameters to aqueous systems, we used
them to estimate hydration free energies and found that the results were in
quantitative agreement with experimentally measured values. These
lattice-derived parameters are applicable in simulations where coupling of ion
parameters to a particular solvent model is undesirable. The simplicity and low
computational demands of the calibration procedure make it suitable for
parametrization of crystallizable ions in a variety of force fields.Comment: 9 pages, 5 table
Terahertz Hall Measurements On Optimally Doped Single Crystal Bi-2212
The infrared Hall angle in optimally doped single crystal was measured from 3.05 to 21.75 meV as a continuous function of
temperature from 25 to 300\,K. In the normal state, the temperature dependence
of the real part of the cotangent of the infrared Hall angle obeys the same
power law as dc measurements. The measured Hall frequency is
significantly larger than the expected value based upon ARPES data analyzed in
terms of the relaxation time approximation. This discrepancy as well as the
temperature dependence of and is well
described by a Fermi liquid theory in which current vertex corrections produced
by electron-magnon scattering are included.Comment: 10 pages, 2 figure
A FUSE survey of high-latitude Galactic molecular hydrogen
Measurements of molecular hydrogen (H_2) column densities are presented for
the first six rotational levels (J=0 to 5) for 73 extragalactic targets
observed with FUSE. All of these have a final signal-to-noise ratio larger than
\snlimit, and are located at galactic latitude |b|>20 deg. The individual
observations were calibrated with the FUSE calibration pipeline CalFUSE version
2.1 or higher, and then carefully aligned in velocity. The final velocity
shifts for all the FUSE segments are listed. H_2 column densities or limits are
determined for the 6 lowest rotational (J) levels for each HI component in the
line of sight, using a curve-of-growth approach at low column densities ~16.5),
and Voigt-profile fitting at higher column densities. Detections include 73
measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight
sightlines yield non-detections for Galactic H_2. The measured column densities
range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found
between log N(H_2) and T_01, the excitation temperature of the H_2, as well as
between log N(H_2) and the level population ratios (log (N(J')/N(J))). The
average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is
calculated; however, because there are many HI clouds in each sightline, the
physics of the transition from HI to H_2 can not be studied. Detections also
include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35
IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm
of the Magellanic Stream), although 19 high-velocity clouds are intersected;
this strongly suggests that dust is rare or absent in these objects. Finally,
there are five detections of H_2 in external galaxies.Comment: Accepted for ApJ Supplement. Note: figs 7 and 8 not included because
astro-ph rejects them as too bi
Electromagnetic Moments of the Baryon Decuplet
We compute the leading contributions to the magnetic dipole and electric
quadrupole moments of the baryon decuplet in chiral perturbation theory. The
measured value for the magnetic moment of the is used to determine
the local counterterm for the magnetic moments. We compare the chiral
perturbation theory predictions for the magnetic moments of the decuplet with
those of the baryon octet and find reasonable agreement with the predictions of
the large-- limit of QCD. The leading contribution to the quadrupole
moment of the and other members of the decuplet comes from one--loop
graphs. The pionic contribution is shown to be proportional to (and so
will not contribute to the quadrupole moment of nuclei), while the
contribution from kaons has both isovector and isoscalar components. The chiral
logarithmic enhancement of both pion and kaon loops has a coefficient that
vanishes in the limit. The third allowed moment, the magnetic octupole,
is shown to be dominated by a local counterterm with corrections arising at two
loops. We briefly mention the strange counterparts of these moments.Comment: Uses harvmac.tex, 15 pages with 3 PostScript figures packed using
uufiles. UCSD/PTH 93-22, QUSTH-93-05, Duke-TH-93-5
Terahertz Kerr and Reflectivity Measurements on the Topological Insulator Bi2Se3
We report the first terahertz Kerr measurements on bulk crystals of the
topological insulator Bi2Se3. At T=10K and fields up to 8T, the real and
imaginary Kerr angle and reflectance measurements utilizing both linearly and
circularly polarized incident radiation were measured at a frequency of
5.24meV. A single fluid free carrier bulk response can not describe the
line-shape. Surface states with a small mass and surprisingly large associated
spectral weight quantitatively fit all data. However, carrier concentration
inhomogeneity has not been ruled out. A method employing a gate is shown to be
promising for separating surface from bulk effects.Comment: 10 pages, 5 figure
Effect of dislocations on properties of heteroepitaxial InP solar cells
The apparently unrelated phenomena of temperature dependency, carrier removal and photoluminescence are shown to be affected by the high dislocation densities present in heteroepitaxial InP solar cells. Using homoepitaxial InP cells as a baseline, it is found that the relatively high dislocation densities present in heteroepitaxial InP/GaAs cells lead to increased volumes of dVoc/dt and carrier removal rate and substantial decreases in photoluminescence spectral intensities. With respect to dVoc/dt, the observed effect is attributed to the tendency of dislocations to reduce Voc. Although the basic cause for the observed increase in carrier removal rate is unclear, it is speculated that the decreased photoluminescence intensity is attributable to defect levels introduced by dislocations in the heteroepitaxial cells
Deep UV Luminosity Functions at the Infall Region of the Coma Cluster
We have used deep GALEX observations at the infall region of the Coma cluster
to measure the faintest UV luminosity functions (LFs) presented for a rich
galaxy cluster thus far. The Coma UV LFs are measured to M_UV = -10.5 in the
GALEX FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach
the dwarf early-type galaxy population in Coma for the first time. The
Schechter faint-end slopes (alpha = -1.39 in both GALEX bands) are shallower
than reported in previous Coma UV LF studies owing to a flatter LF at faint
magnitudes. A Gaussian-plus-Schechter model provides a slightly better
parametrization of the UV LFs resulting in a faint-end slope of ~ -1.15 in both
GALEX bands. The two-component model gives faint-end slopes shallower than -1
(a turnover) for the LFs constructed separately for passive and star forming
galaxies. The UV LFs for star forming galaxies show a turnover at M_UV ~ -14
owing to a deficit of dwarf star forming galaxies in Coma with stellar masses
below M*=10^8 Msun. A similar turnover is identified in recent UV LFs measured
for the Virgo cluster suggesting this may be a common feature of local galaxy
clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did
not identify an excess of passive galaxies as would be expected if the missing
dwarf star forming galaxies were quenched inside the cluster. In fact, the LFs
for both dwarf passive and star forming galaxies show the same turnover at
faint magnitudes. We discuss the possible origin of the missing dwarf star
forming galaxies in Coma and their expected properties based on comparisons to
local field galaxies.Comment: accepted for publication in Ap
-meson in nuclear matter
The -nucleon (N) interactions are deduced from the heavy baryon
chiral perturbation theory up to the next-to-leading-order terms. Combining the
relativistic mean-field theory for nucleon system, we have studied the
in-medium properties of -meson. We find that all the elastic scattering
N interactions come from the next-to-leading-order terms. The N
sigma term is found to be about 280130 MeV. The off-shell terms are also
important to the in-medium properties of -meson. On application of the
latest determination of the N scattering length, the ratio of
-meson effective mass to its vacuum value is near , while
the optical potential is about MeV, at the normal nuclear density.Comment: 8 pages, 3 figures, to appear in PRC, many modification
- …