3,187 research outputs found
How large can the electron to proton mass ratio be in Particle-In-Cell simulations of unstable systems?
Particle-in-cell (PIC) simulations are widely used as a tool to investigate
instabilities that develop between a collisionless plasma and beams of charged
particles. However, even on contemporary supercomputers, it is not always
possible to resolve the ion dynamics in more than one spatial dimension with
such simulations. The ion mass is thus reduced below 1836 electron masses,
which can affect the plasma dynamics during the initial exponential growth
phase of the instability and during the subsequent nonlinear saturation. The
goal of this article is to assess how far the electron to ion mass ratio can be
increased, without changing qualitatively the physics. It is first demonstrated
that there can be no exact similarity law, which balances a change of the mass
ratio with that of another plasma parameter, leaving the physics unchanged.
Restricting then the analysis to the linear phase, a criterion allowing to
define a maximum ratio is explicated in terms of the hierarchy of the linear
unstable modes. The criterion is applied to the case of a relativistic electron
beam crossing an unmagnetized electron-ion plasma.Comment: To appear in Physics of Plasma
Characterization of the initial filamentation of a relativistic electron beam passing through a plasma
The linear instability that induces a relativistic electron beam passing
through a return plasma current to filament transversely is often related to
some filamentation mode with wave vector normal to the beam or confused with
Weibel modes. We show that these modes may not be relevant in this matter and
identify the most unstable mode on the two-stream/filamentation branch as the
main trigger for filamentation. This sets both the characteristic transverse
and longitudinal filamentation scales in the non-resistive initial stage.Comment: 4 page, 3 figures, to appear in PR
Slotted Rotatable Target Assembley and Systematic Error Analysis for a Search for Long Range Spin Dependent Interactions from Exotic Vector Boson Exchange Using Neutron Spin Rotation
We discuss the design and construction of a novel target array of nonmagnetic test masses used in a neutron polarimetry measurement made in search for new possible exotic spin dependent neutron–atominteractions of Nature at sub-mm length scales. This target was designed to accept and efficiently transmit a transversely polarized slow neutron beam through a series of long open parallel slots bounded by flat rectangular plates. These openings possessed equal atom density gradients normal to the slots from the flat test masses with dimensions optimized to achieve maximum sensitivity to an exotic spin-dependent interaction from vector boson exchanges with ranges in the mm - μm regime. The parallel slots were oriented differently in four quadrants that can be rotated about the neutron beam axis in discrete 90°increments using a Geneva drive. The spin rotation signals from the 4 quadrants were measured using a segmented neutron ion chamber to suppress possible systematic errors from stray magnetic fields in the target region. We discuss the per-neutron sensitivity of the target to the exotic interaction, the design constraints, the potential sources of systematic errors which could be present in this design, and our estimate of the achievable sensitivity using this method
Recent Clean Air Act Developments—2006
Air is the ultimate public good. No one owns it but everyone uses it. Protecting it is clearly a great challenge. Since its inception in the mid-1960s, the federal Clean Air Act (CAA) has been a crucial component in managing this resource, but it has also been a source of strife and discontent. The law was forward-looking for its time, pressing polluters to radically change their ways, while still accounting for the fact that we would never be a zero-pollution society. It has also changed with the times, as problems that could not have been imagined in 1970, such as atmospheric ozone depletion and acid rain, have since been addressed, while other issues have been revisited as scientific understanding of atmospheric processes, such as those governing the formation of smog and the distribution of airborne toxics, has improved
Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock
The prompt emissions of gamma-ray bursts are seeded by radiating
ultrarelativistic electrons. Internal shocks propagating through a jet launched
by a stellar implosion, are expected to amplify the magnetic field & accelerate
electrons. We explore the effects of density asymmetry & a quasi-parallel
magnetic field on the collision of plasma clouds. A 2D relativistic PIC
simulation models the collision of two plasma clouds, in the presence of a
quasi-parallel magnetic field. The cloud density ratio is 10. The densities of
ions & electrons & the temperature of 131 keV are equal in each cloud. The mass
ratio is 250. The peak Lorentz factor of the electrons is determined, along
with the orientation & strength of the magnetic field at the cloud collision
boundary. The magnetic field component orthogonal to the initial plasma flow
direction is amplified to values that exceed those expected from shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification, caused by a current sheet which
develops in response to the differing deflection of the incoming upstream
electrons & ions. The electron deflection implies a charge separation of the
upstream electrons & ions; the resulting electric field drags the electrons
through the magnetic field, whereupon they acquire a relativistic mass
comparable to the ions. We demonstrate how a magnetic field structure
resembling the cross section of a flux tube grows in the current sheet of the
shock transition layer. Plasma filamentation develops, as well as signatures of
orthogonal magnetic field striping. Localized magnetic bubbles form. Energy
equipartition between the ion, electron & magnetic energy is obtained at the
shock transition layer. The electronic radiation can provide a seed photon
population that can be energized by secondary processes (e.g. inverse Compton).Comment: 12 pages, 15 Figures, accepted to A&
Seasonal patterns of carbon dioxide and water fluxes in three representative tundra ecosystems in northern Alaska
© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 3, no 1 (2012): art4, doi:10.1890/ES11-00202.1.Understanding the carbon dioxide and water fluxes in the Arctic is essential for accurate assessment and prediction of the responses of these ecosystems to climate change. In the Arctic, there have been relatively few studies of net CO2, water, and energy exchange using micrometeorological methods due to the difficulty of performing these measurements in cold, remote regions. When these measurements are performed, they are usually collected only during the short summer growing season. We established eddy covariance flux towers in three representative Alaska tundra ecosystems (heath tundra, tussock tundra, and wet sedge tundra), and have collected CO2, water, and energy flux data continuously for over three years (September 2007–May 2011). In all ecosystems, peak CO2 uptake occurred during July, with accumulations of 51–95 g C/m2 during June–August. The timing of the switch from CO2 source to sink in the spring appears to be regulated by the number of growing degree days early in the season, indicating that warmer springs may promote increased net CO2 uptake. However, this increased uptake in the spring may be lost through warmer temperatures in the late growing season that promote respiration, if this respiration is not impeded by large amounts of precipitation or cooler temperatures. Net CO2 accumulation during the growing season was generally lost through respiration during the snow covered months of September–May, turning the ecosystems into net sources of CO2 over measurement period. The water balance from June to August at the three ecosystems was variable, with the most variability observed in the heath tundra, and the least in the tussock tundra. These findings underline the importance of collecting data over the full annual cycle and across multiple types of tundra ecosystems in order to come to a more complete understanding of CO2 and water fluxes in the Arctic.This research was funded by the National Science
Foundation Office of Polar Programs (OPP 0632264),
with a grant during the International Polar Year,
‘Collaborative Research on Carbon, Water, and Energy
Balance of the Arctic Landscape at Flagship Observatories
and in a PanArctic Network’. Trac
Long-term experimental warming and nutrient additions increase productivity in tall deciduous shrub tundra
© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 5 (2014): art72, doi:10.1890/ES13-00281.1.Warming Arctic temperatures can drive changes in vegetation structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic biome is currently increasing in deciduous shrub cover and this increase is expected to continue with climate warming. However, little is known how current deciduous shrub communities will respond to future climate induced warming and nutrient increase. We examined the plant and ecosystem response to a long-term (18 years) nutrient addition and warming experiment in an Alaskan arctic tall deciduous shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems. In addition, we used a meta-analysis approach to compare the treatment effect size for aboveground biomass among seven long-term studies conducted across multiple plant community types within the Arctic. We found that biomass, productivity, and aboveground N pools increased with nutrient additions and warming, while species diversity decreased. Both nutrient additions and warming caused the dominant functional group, deciduous shrubs, to increase biomass and proportional C and N allocation to aboveground stems but decreased allocation to belowground stems. For all response variables except soil C and N pools, effects of nutrients plus warming were largest. Soil C and N pools were highly variable and we could not detect any response to the treatments. The biomass response to warming and fertilization in tall deciduous shrub tundra was greater than moist acidic and moist non-acidic tundra and more similar to the biomass response of wet sedge tundra. Our data suggest that in a warmer and more nutrient-rich Arctic, tall deciduous shrub tundra will have greater total deciduous shrub biomass and a higher proportion of woody tissue that has a longer residence time, with a lower proportion of C and N allocated to belowground stems.This research was supported by NSF
grants DEB-0516041, DEB-0516509 and the Arctic
LTER (DEB-0423385)
PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front
We study with particle-in-cell (PIC) simulations the stability of fast
magnetosonic shocks. They expand across a collisionless plasma and an
orthogonal magnetic field that is aligned with one of the directions resolved
by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed
when it enters a layer with a reduced density of mobile ions, which decreases
the shock speed by up to 15\% in 1D simulations. In the 2D simulations, the
density of mobile ions in the layer varies sinusoidally perpendicularly to the
shock normal. We resolve one sine period. This variation only leads to small
changes in the shock speed evidencing a restoring force that opposes a shock
deformation. As the shock propagates through the layer, the ion density becomes
increasingly spatially modulated along the shock front and the magnetic field
bulges out where the mobile ion density is lowest. The perturbed shock
eventually reaches a steady state. Once it leaves the layer, the perturbations
of the ion density and magnetic field oscillate along its front at a frequency
close to the lower-hybrid frequency; the shock is mediated by a standing wave
composed of obliquely propagating lower-hybrid waves. We perform three 2D
simulations with different box lengths along the shock front. The shock front
oscillations are aperiodically damped in the smallest box with the fastest
variation of the ion density, strongly damped in the intermediate one, and
weakly damped in the largest box. The shock front oscillations perturb the
magnetic field in a spatial interval that extends by several electron skin
depths upstream and downstream of the shock front and could give rise to
Whistler waves that propagate along the shock's magnetic field overshoot.
Similar waves were observed in hybrid and PIC simulations and by the MMS
satellite mission.Comment: 25 pages, 12 figures, accepted for publication in Physica Script
Instabilities for a relativistic electron beam interacting with a laser irradiated plasma
The effects of a radiation field (RF) on the unstable modes developed in
relativistic electron beam--plasma interaction are investigated assuming that
, where is the frequency of the RF and
is the plasma frequency. These unstable modes are parametrically
coupled to each other due to the RF and are a mix between two--stream and
parametric instabilities. The dispersion equations are derived by the
linearization of the kinetic equations for a beam--plasma system as well as the
Maxwell equations. In order to highlight the effect of the radiation field we
present a comparison of our analytical and numerical results obtained for
nonzero RF with those for vanishing RF. Assuming that the drift velocity
of the beam is parallel to the wave vector of the
excitations two particular transversal and parallel configurations of the
polarization vector of the RF with respect to are
considered in detail. It is shown that in both geometries resonant and
nonresonant couplings between different modes are possible. The largest growth
rates are expected at the transversal configuration when is
perpendicular to . In this case it is demonstrated that in general
the spectrum of the unstable modes in -- plane is split into two
distinct domains with long and short wavelengths, where the unstable modes are
mainly sensitive to the beam or the RF parameters, respectively. In parallel
configuration, , and at short wavelengths
the growth rates of the unstable modes are sensitive to both beam and RF
parameters remaining insensitive to the RF at long wavelengths.Comment: 23 pages, 5 figure
Moving forward in circles: challenges and opportunities in modelling population cycles
Population cycling is a widespread phenomenon, observed across a multitude of taxa in both laboratory and natural conditions. Historically, the theory associated with population cycles was tightly linked to pairwise consumer–resource interactions and studied via deterministic models, but current empirical and theoretical research reveals a much richer basis for ecological cycles. Stochasticity and seasonality can modulate or create cyclic behaviour in non-intuitive ways, the high-dimensionality in ecological systems can profoundly influence cycling, and so can demographic structure and eco-evolutionary dynamics. An inclusive theory for population cycles, ranging from ecosystem-level to demographic modelling, grounded in observational or experimental data, is therefore necessary to better understand observed cyclical patterns. In turn, by gaining better insight into the drivers of population cycles, we can begin to understand the causes of cycle gain and loss, how biodiversity interacts with population cycling, and how to effectively manage wildly fluctuating populations, all of which are growing domains of ecological research
- …