Decay Length Scale Research Articles

FORMATION OF SOLAR FILAMENTS BY STEADY AND NONSTEADY CHROMOSPHERIC HEATING

It has been established that cold plasma condensations can form in a magnetic loop subject to localized heating of the footpoints. In this paper, we use grid-adaptive numerical simulations of the radiative hydrodynamic equations to parametrically investigate the filament formation process in a pre-shaped loop with both steady and finite-time chromospheric heating. Compared to previous works, we consider low-lying loops with shallow dips, and use a more realistic description for the radiative losses. We demonstrate for the first time that the onset of thermal instability satisfies the linear instability criterion. The onset time of the condensation is roughly \sim 2 hr or more after the localized heating at the footpoint is effective, and the growth rate of the thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the amplitude and the decay length scale characterizing this localized chromospheric heating. We show how single or multiple condensation segments may form in the coronal portion. In the asymmetric heating case, when two segments form, they approach and coalesce, and the coalesced condensation later drains down into the chromosphere. With a steady heating, this process repeats with a periodicity of several hours. While our parametric survey confirms and augments earlier findings, we also point out that steady heating is not necessary to sustain the condensation. Once the condensation is formed, it can keep growing also when the localized heating ceases. Finally, we show that the condensation can survive continuous buffeting by perturbations resulting from the photospheric p-mode waves.

OBSERVED DAMPING OF THE SLOW MAGNETOACOUSTIC MODE

Spectroscopic and stereoscopic imaging observations of slow magnetoacoustic wave propagation within a coronal loop are investigated to determine the decay length scale of the slow magnetoacoustic mode in three dimensions and the density profile within the loop system. The slow wave is found to have an e-folding decay length scale of $20,000^{+4000}_{-3000}$km with a uniform density profile along the loop base. These observations place quantitive constraints on the modelling of wave propagation within coronal loops. Theoretical forward modelling suggests that magnetic field line divergence is the dominant damping factor and thermal conduction is insufficient, given the observed parameters of the coronal loop temperature, density and wave mode period.

Irreversible Rearrangements, Correlated Domains, and Local Structure in Aging Glasses

Bidisperse colloidal suspensions of temperature-sensitive microgel spheres were quenched from liquid to glass states by a rapid temperature drop, and then the glass was permitted to age. Irreversible rearrangements, events that dramatically change a particle's local environment, were observed to be closely related to dynamic heterogeneity. The rate of these irreversible events decreased during aging and the the number of particles required to move as part of these irreversible rearrangements increased. Thus, the slowing dynamics of aging were governed by growing, correlated domains of particles. Additionally, short-range order developed, and a spatial decay length scale associated with orientational order was found to grow during aging.

Transport of Photogenerated Charge Carriers in Polymer Semiconductors

The electrical transport of photogenerated charge carriers in disordered polymer semiconductors is reviewed. We emphasize that the mobility parameter in these disordered semiconducting systems is not a well-defined quantity. We highlight the utility of scanning probe photocurrent technique on variety of polymers in an asymmetric-electrode patterned configuration. The multiple length and time scales present in carrier transport processes are indicated by the large observed decay length scales in these systems.

Large-amplitude jumps and non-Gaussian dynamics in highly concentrated hard sphere fluids

Our microscopic stochastic nonlinear Langevin equation theory of activated dynamics has been employed to study the real-space van Hove function of dense hard sphere fluids and suspensions. At very short times, the van Hove function is a narrow Gaussian. At sufficiently high volume fractions, such that the entropic barrier to relaxation is greater than the thermal energy, its functional form evolves with time to include a rapidly decaying component at small displacements and a long-range exponential tail. The "jump" or decay length scale associated with the tail increases with time (or particle root-mean-square displacement) at fixed volume fraction, and with volume fraction at the mean alpha relaxation time. The jump length at the alpha relaxation time is predicted to be proportional to a measure of the decoupling of self-diffusion and structural relaxation. At long times corresponding to mean displacements of order a particle diameter, the volume fraction dependence of the decay length disappears. A good superposition of the exponential tail feature based on the jump length as a scaling variable is predicted at high volume fractions. Overall, the theoretical results are in good accord with recent simulations and experiments. The basic aspects of the theory are also compared with a classic jump model and a dynamically facilitated continuous time random-walk model. Decoupling of the time scales of different parts of the relaxation process predicted by the theory is qualitatively similar to facilitated dynamics models based on the concept of persistence and exchange times if the elementary event is assumed to be associated with transport on a length scale significantly smaller than the particle size.

High biomass, low export regimes in the Southern Ocean

This paper investigates ballasting and remineralization controls of carbon sedimentation in the Twilight Zone (100–1000 m) of the Southern Ocean. Size-fractionated (<1 μm, 1–51 μm, >51 μm) suspended particulate matter was collected by large-volume in-situ filtration from the upper 1000 m in the Subantarctic (55°S, 172°W) and Antarctic (66°S, 172°W) zones of the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX) in January–February 2002. Particles were analyzed for major chemical constituents (POC, P, biogenic Si, CaCO3), and digital and SEM image analyses of particles were used to aid in the interpretation of the chemical profiles. Twilight Zone waters at 66°S in the Antarctic had a steeper decrease in POC with depth than at 55°S in the Subantarctic, with lower POC concentrations in all size fractions at 66°S than at 55°S, despite up to an-order-of magnitude higher POC in surface waters at 66°S. The decay length scale of >51-μm POC was significantly shorter in the upper Twilight Zone at 66°S (δe=26 m) compared to 55°S (δe=81 m). Particles in the carbonate-producing 55°S did not have higher excess densities than particles from the diatom-dominated 66°S, indicating that there was no direct ballast effect that accounted for deeper POC penetration at 55°S. An indirect ballast effect due to differences in particle packaging and porosities cannot be ruled out, however, as aggregate porosities were high (∼97%) and variable. Image analyses point to the importance of particle loss rates from zooplankton grazing and remineralization as determining factors for the difference in Twilight Zone POC concentrations at 55°S and 66°S, with stronger and more focused shallow remineralization at 66°S. At 66°S, an abundance of large (several mm long) fecal pellets from the surface to 150 m, and almost total removal of large aggregates by 200 m, reflected the actions of a single or few zooplankton species capable of grazing diatoms in the euphotic zone, coupled with a more diverse particle-feeding zooplankton community immediately below. Surface waters with high biomass levels and high proportion of biomass in the large-size fraction were associated with low particle loading at depth, with all indications implying conditions of low export. The 66°S region exhibits this “high biomass, low export” (HBLE) condition, with very high >51-μm POC concentrations at the surface (∼2.1 μM POC), but low concentrations below 200 m (<0.07 μM POC). The 66°S region remained HBLE after iron fertilization. Iron addition at 55°S caused a 10 fold increase in >51-μm biomass concentrations in the euphotic zone, bringing surface POC concentrations to levels found at 66°S (∼3.8 μM), and a concurrent decrease in POC concentrations below 200 m. The 55°S region, which began with moderate levels of biomass and stronger particle export, transitioned to being HBLE after iron fertilization. We propose that iron addition to already HBLE waters will not cause mass sedimentation events. The stability of an iron-induced HBLE condition is unknown. Better understanding of biological pump processes in non-HBLE Subantarctic waters is needed.

Observation of sub‐inertial internal tides in McMurdo Sound, Antarctica

McMurdo Sound, Antarctica, at 78°S is poleward of the critical latitudes for both diurnal and semi‐diurnal internal tides, which are, respectively, 30°S and 75°S. Yet fieldwork during April 2003 and November 2004 observed energy at both frequencies. High temporal resolution density profiles in April 2003 and mooring observations at 240 m depth in November 2004 show a diurnal internal tide with displacements of up to 12 m. The horizontal decay length scales for diurnal evanescent internal tides indicate that they were locally generated. The November 2004 mooring observations also show a semi‐diurnal tide with displacements up to 12 m, although some of the energy in these waves could be baroclinic inertial motion, which has a similar frequency at this latitude. The semi‐diurnal internal waves have a larger decay length scale than the diurnal internal waves, so are not necessarily locally generated.

The role of membrane-like stresses in determining the stability and sensitivity of the Antarctic ice sheets: back pressure and grounding line motion

Membrane stresses act along thin bodies which are relatively well lubricated on both surfaces. They operate in ice sheets because the bottom is either sliding, or is much less viscous than the top owing to stress and heat softening of the basal ice. Ice streams flow over very well lubricated beds, and are restrained at their sides. The ideal of the perfectly slippery bed is considered in this paper, and the propagation of mechanical effects along an ice stream considered by applying spatially varying horizontal body forces. Propagation distances depend sensitively on the rheological index, and can be very large for ice-type rheologies.A new analytical solution for ice-shelf profiles and grounded tractionless stream profiles is presented, which show blow up of the profile in a finite distance upstream at locations where the flux is non-zero. This is a feature of an earlier analytical solution for a floating shelf.The length scale of decay of membrane stresses from the grounding line is investigated through scale analysis. In ice sheets, such effects decay over distances of several tens of kilometres, creating a vertical boundary layer between sheet flow and shelf flow, where membrane stresses adjust. Bounded, physically reasonable steady surface profiles only exist conditionally in this boundary layer. Where bounded steady profiles exist, adjacent profile equilibria for the whole ice sheet corresponding to different grounded areas occur (neutral equilibrium). If no solution in the boundary layer can exist, the ice-sheet profile must change.The conditions for existence can be written in terms of whether the basal rate factor (sliding or internal deformation) is too large to permit a steady solution. The critical value depends extremely sensitively on ice velocity and the back stress applied at the grounding line. High ice velocity and high stress both favour the existence of solutions and stability. Changes in these parameters can cause the steady solution existence criterion to be traversed, and the ice-sheet dynamics to change.A finite difference model which represents both neutral equilibrium and the dynamical transition is presented, and preliminary investigations into its numerical sensitivity are carried out. Evidence for the existence of a long wavelength instability is presented through the solution of a numerical eigenproblem, which will hamper predictability.

Particle‐in‐Cell Simulations of a Nonlinear Transverse Electromagnetic Wave in a Pulsar Wind Termination Shock

A 2.5-dimensional particle-in-cell code is used to investigate the propagation of a large-amplitude, superluminal, nearly transverse electromagnetic (TEM) wave in a relativistically streaming electron-positron plasma with and without a shock. In the freestreaming, unshocked case, the analytic TEM dispersion relation is verified, and the streaming is shown to stabilize the wave against parametric instabilities. In the confined, shocked case, the wave induces strong, coherent particle oscillations, heats the plasma, and modifies the shock density profile via ponderomotive effects. The wave decays over {approx}> 10{sup 2} skin depths; the decay length scale depends primarily on the ratio between the wave frequency and the effective plasma frequency, and on the wave amplitude. The results are applied to the termination shock of the Crab pulsar wind, where the decay length-scale ({approx}> 0.05''?) might be comparable to the thickness of filamentary, variable substructure observed in the optical and X-ray wisps and knots.

Wavelength and decay length of density overshoot structure in supercritical, collisionless bow shocks

Above some critical Mach number, quasiperpendicular collisionless shocks are known to exhibit “overshoot” and “undershoot” structure thought to be associated with the motion of ions trapped at the shock front. Using spacecraft potential data from the Cluster spacecraft, the overshoot/undershoot density structure at 56 crossings of the quasiperpendicular bow shock is studied. The envelope of the absolute value of the density, in most cases, decays exponentially and these envelopes are fitted to a decaying function from which we calculate the decay length scale. The overshoot/undershoot wavelength is also estimated using the zero crossings of the density profile and a good correlation between the average wavelength and the convected ion gyroradius is found: the wavelength is approximately two to three times the ion gyroradius. There is no evidence of a strong correlation between the wavelength and the ion inertial length. Similar results are found for the decay length, which also seems to be ordered by the convected ion gyroradius.

A STUDY OF LIQUID METAL ATOMIZATION USING CLOSE-COUPLED NOZZLES, PART 2: ATOMIZATION BEHAVIOR

In this article we examine the gas dynamic and atomization behavior of a convergent and a converging-diverging (c-d) close-coupled nozzle used to fabricate fine metal powders by gas atomization. Part 1 of this article characterized and compared the gas dynamic behavior of the two nozzles over a range of nozzle stagnation pressures. Part 2 focuses on the liquid metal atomization behavior of these nozzles. A high-speed Schlieren technique is used to visualize liquid metal breakup behavior simultaneously with the supersonic gas flow pattern in which it occurs. At all conditions examined, atomization is characterized by a coarse but "prompt" primary breakup stage followed by a more energetic secondary breakup stage that takes place over an extended distance from the nozzle tip. The reduction in particle size observed with increasing stagnation pressure is believed to be due in large part to the increase in the supersonic length of the gas jets that extend away from the nozzle tip because of enhanced secondary breakup. As expected, the c-d nozzle did not produce finer powder than the convergent nozzle at stagnation pressures where they produced similarly long gas-only supersonic jets. Overall the present experiments suggest that particle size refinement is influenced by the ratio of the velocity decay length scale and the secondary breakup length scale, a quantity that is physically related to the gas-to-liquid metal mass flux ratio. Because such length scales have not been previously considered, new opportunities may exist to significantly improve particle size control and energy efficiency in this costly powder manufacturing process.

Organic matter remineralisation and biogenic silica dissolution in a deep fjord in British Columbia, Canada: a regression analysis of upper ocean sediment-trap fluxes

Remineralisation of sinking particulate matter can be inferred from sediment-trap studies where fluxes decrease with depth. However, during a 4-year experiment of monthly sediment-trap deployments at two stations in Jervis Inlet, British Columbia, Canada, fluxes often increased with depth, a common observation in coastal and some oceanic settings. A regression analysis that simultaneously estimates the composition of ‘additional’ fluxes to deep sediment traps and inverse decay length (IDL) scales of the primary (operationally termed ‘anticipated’) flux is applied to the data from Jervis Inlet. IDL scales for organic carbon (OC), nitrogen (N) and biogenic silica (BSi) decreased with depth in a manner well described by the power function, and are used to generated curves of flux versus depth in the absence of additional fluxes. The curve for OC flux is comparable to previously published models, but in Jervis Inlet remineralisation was somewhat higher in the upper water column, and lower in deep waters, than implied from open-ocean curves. The first upper-ocean estimates for dissolution rates of sinking BSi are presented; conversion to time-dependent rate constants shows that BSi dissolution in Jervis Inlet was similar to results from other studies where dissolution was rapid. A fitting of the flux curves to the reactive continuum model demonstrates that settling organic matter in Jervis Inlet had 3–4 reactive components, while BSi had 6–7 reactive components and over-all was less susceptible to decay. Additional fluxes to deep sediment traps in Jervis Inlet appear to have resulted partly from resuspended sediment increasingly winnowed with distance from topographic boundaries. Resuspended fluxes were particularly abundant during deepwater renewals, and another source supplied additional fluxes to mid-depth and deep traps on a continual basis. Included in the analysis is an error term, which is interpreted as the degree to which anticipated fluxes were over- or under-represented in deep sediment traps. Although for the deep depth intervals in Jervis Inlet errors of regression analyses were negligible, for the more shallow intervals they were not. An enigma of the results from Jervis Inlet is that it appears OC, N and Al were generally over-collected at mid-depth sediment traps, while BSi was under-collected.

Competition of superconductivity and antiferromagnetism in ad-wave vortex lattice

The d-wave vortex lattice state is studied within the framework of Bogoliubov-de Gennes (BdG) mean field theory. We allow antiferromagnetic (AFM) order to develop self-consistently along with d-wave singlet superconducting (dSC) order in response to an external magnetic field that generates vortices. The resulting AFM order has strong peaks at the vortex centers, and changes sign, creating domain walls along lines where $\nabla \times j_s \approx 0$. The length scale for decay of this AFM order is found to be much larger than the bare d-wave coherence length, $\xi$. Coexistence of dSC and AFM order in this system is shown to induce $\pi$-triplet superconducting order. Competition between different orders is found to suppress the local density of states at the vortex center and comparison to recent experimental findings is discussed.

Radiating Instability of Nonzonal Oceanic Shear Flows with Application to the Kuroshio Extension Current

This paper proposes a mechanism to explain the fairly energetic internal temperature fluctuations observed in the regions far away from the axis ofthe Kuroshio Extension Current (KEC). Based upon the typical observed time and length scales, flow speeds and orientations, phase propagation, and length scale of decay in the crossflow direction, the radiating instabilities of nonzonal shear flows are explored for the cases of different flow orientations, vertical shears, and Rossby radii of deformation. The results show that, as the orientation of an eastward flow becomes more northward, the number of radiating unstable modes within the typical range of the observed wavelengths becomes larger, the length scale of decay becomes larger, and the surface layer of the far field in the two-layer system tends to become more intensified than the bottom layer. A similar effect is found in the cases of a stronger vertical shear and a larger Rossby radius of deformation. Therefore, it is concluded that the radiating instabilities of the KEC in the western North Pacific can be a source of internal temperature fluctuations observed in the regions away from the KEC, especially in the region near the western boundary, where the flow orientation may be nonzonal, the vertical shear is strong, and the Rossby radius of deformation is large.

The turbulent bottom boundary layer and its influence on local dynamics over the continental shelf

We report on measurements of the structure of the bottom boundary layer on the continental shelf off Vancouver Island. A time series of vertical profiles obtained with the microstructure profiler FLY II revealed large temporal variations in the dissipation rate and in the density structure. The near-bottom current structure was simultaneously measured at fixed heights with conventional current meters. The data reveal the association between the predominantly diurnal tide and the variations in the structure of the bottom boundary layer. A clear distinction appears between the turbulent bottom boundary layer (8–40 m deep in a total water depth of 138 m) and the well-mixed layer (20–40 m deep). The two layers vary independently, with horizontal advection dominating the fluctuations in the thickness of the well-mixed layer while local dissipation is more closely related to the thickness of the turbulent layer. Variations in the density structure of the bottom layer are related to the strength and direction of the vertical shear and to the regional distribution of isopycnals. Current veering is commonly concentrated above the well-mixed layer. The evolution of the characteristics of the bottom layers is followed through a tidal cycle and related to local dissipation and other variables. Microstructure measurements from six locations over the southern portion of the Vancouver Island continental shelf are used to estimate the influence of turbulent energy dissipation on regional-scale flows. That fraction of the dissipation taking place in the bottom boundary layer is attributed to barotropic tidal flows, while that occurring above it is associated with nearly geostrophic baroclinic flows. Results give a lower limit of ∼ 1070 km for the length scale of shelf wave decay, in good agreement with current models; the estimates of tidal friction based on our dissipation measurements are however much lower than required by astronomical observations. An estimate of 230 h is obtained for the spin-down time of the local Tully eddy, rather longer than the decay time of 68 h obtained from observations.