131,960 research outputs found
The heating of the thermal plasma with energetic electrons in small solar flares
The energetic electrons deduced from hard X-rays in the thick target model may be responsible for heating of soft X-ray plasma in solar flares. It is shown from OSO-7 studies that if a cutoff of 10 keV is assumed, the total electron is comparable to the thermal plasma energy. However, (1) the soft X-ray emission often appears to begin before the hard X-ray burst, (2) in about one-third of flares there is no detectable hard X-ray emission, and (3) for most events the energy content (assuming constant density) of soft X-ray plasma continues to rise after the end of the hard X-ray burst. To understand these problems we have analyzed the temporal relationship between soft X-rays and hard X-rays for 20 small events observed by ISEE-3 during 1980. One example is shown. The start of soft X-ray and hard X-ray bursts is defined as the time when the counting rates of the 4.8 to 5. keV and 25.8 to 43.2 keV channels, respectively, exceed the background by one standard deviation
Spatiotemporal instability of a confined capillary jet
Recent experimental studies on the instability appearance of capillary jets
have revealed the capabilities of linear spatiotemporal instability analysis to
predict the parametrical map where steady jetting or dripping takes place. In
this work, we present an extensive analytical, numerical and experimental
analysis of confined capillary jets extending previous studies. We propose an
extended, accurate analytic model in the limit of low Reynolds flows, and
introduce a numerical scheme to predict the system response when the liquid
inertia is not negligible. Theoretical predictions show a remarkable accuracy
with results from the extensive experimental exploration provided.Comment: Submitted to the Physical Review E (20-March-2008
Tunneling and delocalization in hydrogen bonded systems: a study in position and momentum space
Novel experimental and computational studies have uncovered the proton
momentum distribution in hydrogen bonded systems. In this work, we utilize
recently developed open path integral Car-Parrinello molecular dynamics
methodology in order to study the momentum distribution in phases of high
pressure ice. Some of these phases exhibit symmetric hydrogen bonds and quantum
tunneling. We find that the symmetric hydrogen bonded phase possesses a
narrowed momentum distribution as compared with a covalently bonded phase, in
agreement with recent experimental findings. The signatures of tunneling that
we observe are a narrowed distribution in the low-to-intermediate momentum
region, with a tail that extends to match the result of the covalently bonded
state. The transition to tunneling behavior shows similarity to features
observed in recent experiments performed on confined water. We corroborate our
ice simulations with a study of a particle in a model one-dimensional double
well potential that mimics some of the effects observed in bulk simulations.
The temperature dependence of the momentum distribution in the one-dimensional
model allows for the differentiation between ground state and mixed state
tunneling effects.Comment: 14 pages, 13 figure
Nonuniversal Effects in the Homogeneous Bose Gas
Effective field theory predicts that the leading nonuniversal effects in the
homogeneous Bose gas arise from the effective range for S-wave scattering and
from an effective three-body contact interaction. We calculate the leading
nonuniversal contributions to the energy density and condensate fraction and
compare the predictions with results from diffusion Monte Carlo calculations by
Giorgini, Boronat, and Casulleras. We give a crude determination of the
strength of the three-body contact interaction for various model potentials.
Accurate determinations could be obtained from diffusion Monte Carlo
calculations of the energy density with higher statistics.Comment: 24 pages, RevTex, 5 ps figures, included with epsf.te
Frequency Locking in Spatially Extended Systems
A variant of the complex Ginzburg-Landau equation is used to investigate the
frequency locking phenomena in spatially extended systems. With appropriate
parameter values, a variety of frequency-locked patterns including flats,
fronts, labyrinths and fronts emerge. We show that in spatially
extended systems, frequency locking can be enhanced or suppressed by diffusive
coupling. Novel patterns such as chaotically bursting domains and target
patterns are also observed during the transition to locking
Galaxy alignment on large and small scales
Galaxies are not randomly distributed across the universe but showing
different kinds of alignment on different scales. On small scales satellite
galaxies have a tendency to distribute along the major axis of the central
galaxy, with dependence on galaxy properties that both red satellites and
centrals have stronger alignment than their blue counterparts. On large scales,
it is found that the major axes of Luminous Red Galaxies (LRGs) have
correlation up to 30Mpc/h. Using hydro-dynamical simulation with star
formation, we investigate the origin of galaxy alignment on different scales.
It is found that most red satellite galaxies stay in the inner region of dark
matter halo inside which the shape of central galaxy is well aligned with the
dark matter distribution. Red centrals have stronger alignment than blue ones
as they live in massive haloes and the central galaxy-halo alignment increases
with halo mass. On large scales, the alignment of LRGs is also from the
galaxy-halo shape correlation, but with some extent of mis-alignment. The
massive haloes have stronger alignment than haloes in filament which connect
massive haloes. This is contrary to the naive expectation that cosmic filament
is the cause of halo alignment.Comment: 4 pages, 3 figures, To appear in the proceedings of the IAU Symposium
308 "The Zeldovich Universe: Genesis and Growth of the Cosmic Web
A Thermal-Nonthermal Inverse Compton Model for Cyg X-1
Using Monte Carlo methods to simulate the inverse Compton scattering of soft
photons, we model the spectrum of the Galactic black hole candidate Cyg X-1,
which shows evidence of a nonthermal tail extending beyond a few hundred keV.
We assume an ad hoc sphere of leptons, whose energy distribution consists of a
Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody
photons. The spectral data is used to constrain the nonthermal plasma fraction
and the power-law index assuming a reasonable Maxwellian temperature and
Thomson depth. A small but non-negligible fraction of nonthermal leptons is
needed to explain the power-law tail.Comment: 5 pages, 2 PostScript figure, uses aipproc.sty, to appear in
Proceedings of Fourth Compton Symposiu
Colloidal Dynamics on Disordered Substrates
Using Langevin simulations we examine driven colloids interacting with
quenched disorder. For weak substrates the colloids form an ordered state and
depin elastically. For increasing substrate strength we find a sharp crossover
to inhomogeneous depinning and a substantial increase in the depinning force,
analogous to the peak effect in superconductors. The velocity versus driving
force curve shows criticality at depinning, with a change in scaling exponent
occuring at the order to disorder crossover. Upon application of a sudden pulse
of driving force, pronounced transients appear in the disordered regime which
are due to the formation of long-lived colloidal flow channels.Comment: 4 pages, 4 postscript figure
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