335 research outputs found
Synthetic IRIS spectra of the solar transition region: Effect of high-energy tails
The solar transition region satisfies the conditions for presence of
non-Maxwellian electron energy distributions with high-energy tails at energies
corresponding to the ionization potentials of many ions emitting in the EUV and
UV portions of the spectrum. We calculate the synthetic Si IV, O IV, and S IV
spectra in the far ultra-violet (FUV) channel of the Interface Region Imaging
Spectrograph (IRIS). Ionization, recombination, and excitation rates are
obtained by integration of the cross-sections or their approximations over the
model electron distributions considering particle propagation from the hotter
corona. The ionization rates are significantly affected by the presence of
high-energy tails. This leads to the peaks of the relative abundance of
individual ions to be broadened with pronounced low-temperature shoulders. As a
result, the contribution functions of individual lines observable by IRIS also
exhibit low-temperature shoulders, or their peaks are shifted to temperatures
an order of magnitude lower than for the Maxwellian distribution. The
integrated emergent spectra can show enhancements of Si IV compared toO IV by
more than a factor of two. The high-energy particles can have significant
impact on the emergent spectra and their presence needs to be considered even
in situations without strong local acceleration
Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique
While the dynamics of many complex systems is dominated by activated events,
there are very few simulation methods that take advantage of this fact. Most of
these procedures are restricted to relatively simple systems or, as with the
activation-relaxation technique (ART), sample the conformation space
efficiently at the cost of a correct thermodynamical description. We present
here an extension of ART, the properly-obeying-probability ART (POP-ART), that
obeys detailed balance and samples correctly the thermodynamic ensemble.
Testing POP-ART on two model systems, a vacancy and an interstitial in
crystalline silicon, we show that this method recovers the proper
thermodynamical weights associated with the various accessible states and is
significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure
Probabilistic Phase Space Trajectory Description for Anomalous Polymer Dynamics
It has been recently shown that the phase space trajectories for the
anomalous dynamics of a tagged monomer of a polymer --- for single polymeric
systems such as phantom Rouse, self-avoiding Rouse, Zimm, reptation, and
translocation through a narrow pore in a membrane; as well as for
many-polymeric system such as polymer melts in the entangled regime --- is
robustly described by the Generalized Langevin Equation (GLE). Here I show that
the probability distribution of phase space trajectories for all these
classical anomalous dynamics for single polymers is that of a fractional
Brownian motion (fBm), while the dynamics for polymer melts between the
entangled regime and the eventual diffusive regime exhibits small, but
systematic deviations from that of a fBm.Comment: 8 pages, two figures, 3 eps figure files, minor changes,
supplementary material included moved to the appendix, references expanded,
to appear in J. Phys.: Condens. Matte
Oxidative Activity of Human Polymorphonuclear Leukocytes Stimulated by the Long-Chain Phosphatidic Acids
Summary It has already been suggested that phosphatidic acids (PAs) play an important role in the regulation of signaling pathways involved in the production of reactive oxygen species (ROS) by human polymorphonuclear leukocytes (PMNs). The present study was performed to elucidate the effects of extracellularly added PAs -1,2-distearoyl-(DSPA) and 1-stearoyl-2-arachidonoyl-sn-glycero-phosphate (SAPA) -on the ROS production and on the elastase release by human PMNs. ROS production was monitored by luminol-amplified chemiluminescence and the elastase activity was measured in the supernatant of the PA-stimulated human PMNs by colorimetric assay. Obtained effects were compared with those of cells stimulated by either a chemotactic tripeptide, phorbol ester or calcium ionophore. Our results show that long-chain PAs at concentrations higher than 3 × 10 -5 mol/l stimulate the ROS production by human PMNs, whereas they were ineffective in promoting the elastase release. The chemiluminescence pattern of the SAPA-stimulated cells exhibited a biphasic curve, whereas cell stimulation with DSPA resulted in a monophasic chemiluminescence curve. Stimulation of the ROS production by PAs in dependence of the fatty acid composition required the activity of protein kinases
Amplitude and Frequency Spectrum of Thermal Fluctuations of A Translocating RNA Molecule
Using a combination of theory and computer simulations, we study the
translocation of an RNA molecule, pulled through a solid-state nanopore by an
optical tweezer, as a method to determine its secondary structure. The
resolution with which the elements of the secondary structure can be determined
is limited by thermal fluctuations. We present a detailed study of these
thermal fluctuations, including the frequency spectrum, and show that these
rule out single-nucleotide resolution under the experimental conditions which
we simulated. Two possible ways to improve this resolution are strong
stretching of the RNA with a back-pulling voltage across the membrane, and
stiffening of the translocated part of the RNA by biochemical means.Comment: Significantly expanded compared to previous version, 13 pages, 4
figures, to appear in J. Phys.: Condens. Matte
Pore-blockade Times for Field-Driven Polymer Translocation
We study pore blockade times for a translocating polymer of length ,
driven by a field across the pore in three dimensions. The polymer performs
Rouse dynamics, i.e., we consider polymer dynamics in the absence of
hydrodynamical interactions. We find that the typical time the pore remains
blocked during a translocation event scales as ,
where is the Flory exponent for the polymer. In line with our
previous work, we show that this scaling behaviour stems from the polymer
dynamics at the immediate vicinity of the pore -- in particular, the memory
effects in the polymer chain tension imbalance across the pore. This result,
along with the numerical results by several other groups, violates the lower
bound suggested earlier in the literature. We discuss why
this lower bound is incorrect and show, based on conservation of energy, that
the correct lower bound for the pore-blockade time for field-driven
translocation is given by , where is the viscosity of
the medium surrounding the polymer.Comment: 14 pages, 6 figures, slightly shorter than the previous version; to
appear in J. Phys.: Cond. Ma
Multifrequency behaviour of the anomalous events of PSR J0922+0638
PSR J0922+0638 (B0919+06) shows unexplained anomalous variations in the
on-pulse phase, where the pulse appears to episodically move to an earlier
longitude for a few tens of rotations before reverting to the usual phase for
approximately several hundred to more than a thousand rotations. These events,
where the pulse moves in phase by up to 5, have been previously
detected in observations from 300 to 2000 MHz. We present simultaneous
observations from the Effelsberg 100-m radio telescope at 1350 MHz and the
Bornim (Potsdam) station of the LOw Frequency ARray at 150 MHz. Our
observations present the first evidence for an absence of the anomalous
phase-shifting behaviour at 150 MHz. Instead, the observed intensity at the
usual pulse-phase typically decreases, often showing a pseudo-nulling feature
corresponding to the times when phase shifts are observed at 1350 MHz. The
presence of weak emission at the usual pulse-phase supports the theory that
these shifts may result from processes similar to the 'profile-absorption'
expected to operate for PSR J0814+7429 (B0809+74). A possible mechanism for
this could be intrinsic variations of the emission within the pulsar's beam
combined with absorption by expanding shells of electrons in the line of sight.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Anomalous zipping dynamics and forced polymer translocation
We investigate by Monte Carlo simulations the zipping and unzipping dynamics
of two polymers connected by one end and subject to an attractive interaction
between complementary monomers. In zipping, the polymers are quenched from a
high temperature equilibrium configuration to a low temperature state, so that
the two strands zip up by closing up a "Y"-fork. In unzipping, the polymers are
brought from a low temperature double stranded configuration to high
temperatures, so that the two strands separate. Simulations show that the
unzipping time, , scales as a function of the polymer length as , while the zipping is characterized by anomalous dynamics with . This exponent is in good agreement with
simulation results and theoretical predictions for the scaling of the
translocation time of a forced polymer passing through a narrow pore. We find
that the exponent is robust against variations of parameters and
temperature, whereas the scaling of as a function of the driving force
shows the existence of two different regimes: the weak forcing () and strong forcing ( independent of ) regimes. The crossover
region is possibly characterized by a non-trivial scaling in , matching the
prediction of recent theories of polymer translocation. Although the
geometrical setup is different, zipping and translocation share thus the same
type of anomalous dynamics. Systems where this dynamics could be experimentally
investigated are DNA (or RNA) hairpins: our results imply an anomalous dynamics
for the hairpins closing times, but not for the opening times.Comment: 15 pages, 9 figure
Solar off-limb line widths: Alfven waves, ion-cyclotron waves, and preferential heating
Alfven waves and ion-cyclotron absorption of high-frequency waves are
frequently brought into models devoted to coronal heating and fast solar-wind
acceleration. Signatures of ion-cyclotron resonance have already been observed
in situ in the solar wind (HELIOS spacecrafts) and, recently, in the upper
corona (UVCS/SOHO remote-sensing results). We propose a method to constrain
both the Alfven wave amplitude and the preferential heating induced by
ion-cyclotron resonance, above a partially developed polar coronal hole
observed with the SUMER/SOHO spectrometer. The instrumental stray light
contribution is first substracted from the spectra. By supposing that the
non-thermal velocity is related to the Alfven wave amplitude, it is constrained
through a density diagnostic and the gradient of the width of the Mg X 625 A
line. The temperatures of several coronal ions, as functions of the distance
above the limb, are then determined by substracting the non-thermal component
to the observed line widths. The effect of stray light explains the apparent
decrease with height in the width of several spectral lines, this decrease
usually starting about 0.1-0.2 Rs above the limb. This result rules out any
direct evidence of damping of the Alfven waves, often suggested by other
authors. We also find that the ions with the smallest charge-to-mass ratios are
the hottest ones at a fixed altitude and that they are subject to a stronger
heating, as compared to the others, between 57" and 102" above the limb. This
constitutes a serious clue to ion-cyclotron preferential heating.Comment: 15 pages, 12 figures, 3 table
- …