440 research outputs found
Diffusion of energetic particles in turbulent MHD plasmas
In this paper we investigate the transport of energetic particles in
turbulent plasmas. A numerical approach is used to simulate the effect of the
background plasma on the motion of energetic protons. The background plasma is
in a dynamically turbulent state found from numerical MHD simulations, where we
use parameters typical for the heliosphere. The implications for the transport
parameters (i.e. pitch-angle diffusion coefficients and mean free path) are
calculated and deviations from the quasi-linear theory are discussed.Comment: Accepted for publication in Ap
Solar energetic particle access to distant longitudes through turbulent field-line meandering
Context. Current solar energetic particle (SEP) propagation models describe the effects of interplanetary plasma turbulence on SEPs as diffusion, using a Fokker-Planck (FP) equation. However, FP models cannot explain the observed fast access of SEPs across the average magnetic field to regions that are widely separated in longitude within the heliosphere without using unrealistically strong cross-field diffusion.
Aims. We study whether the recently suggested early non-diffusive phase of SEP propagation can explain the wide SEP events with realistic particle transport parameters.
Methods. We used a novel model that accounts for the SEP propagation along field lines that meander as a result of plasma turbulence. Such a non-diffusive propagation mode has been shown to dominate the SEP cross-field propagation early in the SEP event history. We compare the new model to the traditional approach, and to SEP observations.
Results. Using the new model, we reproduce the observed longitudinal extent of SEP peak fluxes that are characterised by a Gaussian profile with σ = 30 − 50◦ , while current diffusion theory can only explain extents of 11◦ with realistic diffusion coefficients. Our model also reproduces the timing of SEP arrival at distant longitudes, which cannot be explained using the diffusion model.
Conclusions. The early onset of SEPs over a wide range of longitudes can be understood as a result of the effects of magnetic fieldline random walk in the interplanetary medium and requires an SEP transport model that properly describes the non-diffusive early phase of SEP cross-field propagation
The band structure of BeTe - a combined experimental and theoretical study
Using angle-resolved synchrotron-radiation photoemission spectroscopy we have
determined the dispersion of the valence bands of BeTe(100) along ,
i.e. the [100] direction. The measurements are analyzed with the aid of a
first-principles calculation of the BeTe bulk band structure as well as of the
photoemission peaks as given by the momentum conserving bulk transitions.
Taking the calculated unoccupied bands as final states of the photoemission
process, we obtain an excellent agreement between experimental and calculated
spectra and a clear interpretation of almost all measured bands. In contrast,
the free electron approximation for the final states fails to describe the BeTe
bulk band structure along properly.Comment: 21 pages plus 4 figure
Energetic particle transport across the mean magnetic field: before diffusion
Current particle transport models describe the propagation of charged particles across the mean field direction in turbulent plasmas as diffusion. However, recent studies suggest that at short time- scales, such as soon after solar energetic particle (SEP) injection, particles remain on turbulently meandering field lines, which results in non-diffusive initial propagation across the mean magnetic field. In this work, we use a new technique to investigate how the particles are displaced from their original field lines, and quantify the parameters of the transition from field-aligned particle propagation along meandering field lines to particle diffusion across the mean magnetic field. We show that the initial decoupling of the particles from the field lines is slow, and particles remain within a Larmor radius from their initial meandering field lines for tens to hundreds of Larmor periods, for 0.1-10 MeV protons in turbulence conditions typical of the solar wind at 1 AU. Subsequently, particles decouple from their initial field lines and after hundreds to thousands of Larmor periods reach time-asymptotic diffusive behaviour consistent with particle diffusion across the mean field caused by the meandering of the field lines. We show that the typical duration of the pre-diffusive phase, hours to tens of hours for 10 MeV protons in 1 AU solar wind turbulence conditions, is significant for SEP propagation to 1 AU and must be taken into account when modelling SEP propagation in the interplanetary space
Solar interacting protons versus interplanetary protons in the core plus halo model of diffusive shock acceleration and stochastic re-acceleration
With the first observations of solar γ-rays from the decay of pions, the relationship of protons producing ground level enhancements (GLEs) on the Earth to those of similar energies producing the γ-rays on the Sun has been debated. These two populations may be either independent and simply coincident in large flares, or they may be, in fact, the same population stemming from a single accelerating agent and jointly distributed at the Sun and also in space. Assuming the latter, we model a scenario in which particles are accelerated near the Sun in a shock wave with a fraction transported back to the solar surface to radiate, while the remainder is detected at Earth in the form of a GLE. Interplanetary ions versus ions interacting at the Sun are studied for a spherical shock wave propagating in a radial magnetic field through a highly turbulent radial ray (the acceleration core) and surrounding weakly turbulent sector in which the accelerated particles can propagate toward or away from the Sun. The model presented here accounts for both the first-order Fermi acceleration at the shock front and the second-order, stochastic re-acceleration by the turbulence enhanced behind the shock. We find that the re-acceleration is important in generating the γ-radiation and we also find that up to 10% of the particle population can find its way to the Sun as compared to particles escaping to the interplanetary space
Developing fencing policies in dryland ecosystems
The daily energy requirements of animals are determined by a combination of physical and physiological factors, but food availability may challenge the capacity to meet nutritional needs. Western gorillas (Gorilla gorilla) are an interesting model for investigating this topic because they are folivore-frugivores that adjust their diet and activities to seasonal variation in fruit availability. Observations of one habituated group of western gorillas in Bai-Hokou, Central African Republic (December 2004-December 2005) were used to examine seasonal variation in diet quality and nutritional intake. We tested if during the high fruit season the food consumed by western gorillas was higher in quality (higher in energy, sugar, fat but lower in fibre and antifeedants) than during the low fruit season. Food consumed during the high fruit season was higher in digestible energy, but not any other macronutrients. Second, we investigated whether the gorillas increased their daily intake of carbohydrates, metabolizable energy (KCal/g OM), or other nutrients during the high fruit season. Intake of dry matter, fibers, fat, protein and the majority of minerals and phenols decreased with increased frugivory and there was some indication of seasonal variation in intake of energy (KCal/g OM), tannins, protein/fiber ratio, and iron. Intake of non-structural carbohydrates and sugars was not influenced by fruit availability. Gorillas are probably able to extract large quantities of energy via fermentation since they rely on proteinaceous leaves during the low fruit season. Macronutrients and micronutrients, but not digestible energy, may be limited for them during times of low fruit availability because they are hind-gut fermenters. We discuss the advantages of seasonal frugivores having large dietary breath and flexibility, significant characteristics to consider in the conservation strategies of endangered species
From Sun to Interplanetary Space: What is the Pathlength of Solar Energetic Particles?
Solar energetic particles (SEPs), accelerated during solar eruptions, propagate in turbulent solar wind before being
observed with in situ instruments. In order to interpret their origin through comparison with remote sensing
observations of the solar eruption, we thus must deconvolve the transport effects due to the turbulent magnetic
fields from the SEP observations. Recent research suggests that the SEP propagation is guided by the turbulent
meandering of the magnetic fieldlines across the mean magnetic field. However, the lengthening of the distance the
SEPs travel, due to the fieldline meandering, has so far not been included in SEP event analysis. This omission can
cause significant errors in estimation of the release times of SEPs at the Sun. We investigate the distance traveled
by the SEPs by considering them to propagate along fieldlines that meander around closed magnetic islands that
are inherent in turbulent plasma. We introduce a fieldline random walk model which takes into account the
physical scales associated to the magnetic islands. Our method remedies the problem of the diffusion equation
resulting in unrealistically short pathlengths, and the fractal dependence of the pathlength of random walk on the
length of the random-walk step. We find that the pathlength from the Sun to 1au can be below the nominal Parker
spiral length for SEP events taking place at solar longitudes 45E to 60W, whereas the western and behind-the-limb
particles can experience pathlengths longer than 2au due to fieldline meandering
Interchange Slip-Running Reconnection and Sweeping SEP Beams
We present a new model to explain how particles (solar energetic particles;
SEPs), accelerated at a reconnection site that is not magnetically connected to
the Earth, could eventually propagate along the well-connected open flux tube.
Our model is based on the results of a low-beta resistive magnetohydrodynamics
simulation of a three-dimensional line-tied and initially current-free bipole,
that is embedded in a non-uniform open potential field. The topology of this
configuration is that of an asymmetric coronal null-point, with a closed fan
surface and an open outer spine. When driven by slow photospheric shearing
motions, field lines, initially fully anchored below the fan dome, reconnect at
the null point, and jump to the open magnetic domain. This is the standard
interchange mode as sketched and calculated in 2D. The key result in 3D is
that, reconnected open field lines located in the vicinity of the outer spine,
keep reconnecting continuously, across an open quasi-separatrix layer, as
previously identified for non-open-null-point reconnection. The apparent
slipping motion of these field lines leads to form an extended narrow magnetic
flux tube at high altitude. Because of the slip-running reconnection, we
conjecture that if energetic particles would be traveling through, or be
accelerated inside, the diffusion region, they would be successively injected
along continuously reconnecting field lines that are connected farther and
farther from the spine. At the scale of the full Sun, owing to the super-radial
expansion of field lines below 3 solar radii, such energetic particles could
easily be injected in field lines slipping over significant distances, and
could eventually reach the distant flux tube that is well-connected to the
Earth
A dinuclear ruthenium(II) phototherapeutic that targets duplex and quadruplex DNA
With the aim of developing a sensitizer for photodynamic therapy, a previously reported luminescent dinuclear complex that functions as a DNA probe in live cells was modified to produce a new iso-structural derivative containing RuII(TAP)2 fragments (TAP = 1,4,5,8-tetraazaphenanthrene). The structure of the new complex has been confirmed by a variety of techniques including single crystal X-ray analysis. Unlike its parent, the new complex displays Ru → L-based 3MLCT emission in both MeCN and water. Results from electrochemical studies and emission quenching experiments involving guanosine monophosphate are consistent with an excited state located on a TAP moiety. This hypothesis is further supported by detailed DFT calculations, which take into account solvent effects on excited state dynamics. Cell-free steady-state and time-resolved optical studies on the interaction of the new complex with duplex and quadruplex DNA show that the complex binds with high affinity to both structures and indicate that its photoexcited state is also quenched by DNA, a process that is accompanied by the generation of the guanine radical cation sites as photo-oxidization products. Like the parent complex, this new compound is taken up by live cells where it primarily localizes within the nucleus and displays low cytotoxicity in the absence of light. However, in complete contrast to [{RuII(phen)2}2(tpphz)]4+, the new complex is therapeutically activated by light to become highly phototoxic toward malignant human melanoma cell lines showing that it is a promising lead for the treatment of this recalcitrant cancer
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