1,241 research outputs found
Droplet spreading and pinning on heterogeneous substrates
The contact angle of a fluid droplet on an heterogeneous surface is analysed
using the statistical dynamics of the spreading contact line. The statistical
properties of the final droplet radius and contact angle are obtained through
applications of depinning transitions of contact lines with non-local
elasticity and features of pinning-depinning dynamics. Such properties not only
depend on disorder strength and surface details, but also on the droplet volume
and disorder correlation length. Deviations from Wenzel or Cassie/Baxter
behaviour are particularly apparent in the case of small droplet volumes and
small contact angles.Comment: accepted to Phys. Rev. E, 5 figure
Description of understory development in a tree plantation with a new method of data structuring
During May and June, 1974, relevés were obtained from 30 plantation stands in the Saginaw Forest in southeastern Michigan. The canopy trees in these plantations were planted between 1904 and 1938. The understory has developed naturally over the years. The forest plantations offer opportunities for study of the effects of the canopy on the structure and species composition of the understory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43888/1/11258_2004_Article_BF00228485.pd
Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster
International audienceTemporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ?1V/m, bipolar or unipolar profiles, perpendicular scale sizes of ?10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ?10µA/m2, and upward electron beams with characteristic energies of a few hundred eV to a fewkeV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20s, too short for monitoring the evolution of the structure, but yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipolar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary
The normal field instability under side-wall effects: comparison of experiments and computations
We consider a single spike of ferrofluid, arising in a small cylindrical
container, when a vertically oriented magnetic field is applied. The height of
the spike as well as the surface topography is measured experimentally by two
different technologies and calculated numerically using the finite element
method. As a consequence of the finite size of the container, the numerics
uncovers an imperfect bifurcation to a single spike solution, which is forward.
This is in contrast to the standard transcritical bifurcation to hexagons,
common for rotational symmetric systems with broken up-down symmetry. The
numerical findings are corroborated in the experiments. The small hysteresis
observed is explained in terms of a hysteretic wetting of the side wall.Comment: accepted to New Journal of Physic
Prelude to THEMIS tail conjunction study
A close conjunction of several satellites (LANL, GOES, Polar, Geotail, and Cluster) distributed from the geostationary altitude to about 16 <I>R<sub>E</sub></I> downstream in the tail occurred during substorm activity as indicated by global auroral imaging and ground-based magnetometer data. This constellation of satellites resembles what is planned for the THEMIS (Time History of Events and Macroscopic Interactions during Substorms) mission to resolve the substorm controversy on the location of the substorm expansion onset region. In this article, we show in detail the dipolarization and dynamic changes seen by these satellites associated with two onsets of substorm intensification activity. In particular, we find that dipolarization at ~16 <I>R<sub>E</sub></I> downstream in the tail can occur with dawnward electric field and without plasma flow, just like some near-Earth dipolarization events reported previously. The spreading of substorm disturbances in the tail coupled with complementary ground observations indicates that the observed time sequence on the onsets of substorm disturbances favors initiation in the near-Earth region for this THEMIS-like conjunction
Universality in fully developed turbulence
We extend the numerical simulations of She et al. [Phys.\ Rev.\ Lett.\ 70,
3251 (1993)] of highly turbulent flow with Taylor-Reynolds number
up to , employing a reduced wave
vector set method (introduced earlier) to approximately solve the Navier-Stokes
equation. First, also for these extremely high Reynolds numbers ,
the energy spectra as well as the higher moments -- when scaled by the spectral
intensity at the wave number of peak dissipation -- can be described by
{\it one universal} function of for all . Second, the ISR
scaling exponents of this universal function are in agreement with
the 1941 Kolmogorov theory (the better, the large is), as is the
dependence of . Only around viscous damping leads to
slight energy pileup in the spectra, as in the experimental data (bottleneck
phenomenon).Comment: 14 pages, Latex, 5 figures (on request), 3 tables, submitted to Phys.
Rev.
The role of oxygen ions in the formation of a bifurcated current sheet in the magnetotail
Cluster observations in the near-Earth magnetotail have shown that sometimes
the current sheet is bifurcated, i.e. it is divided in two layers. The
influence of magnetic turbulence on ion motion in this region is investigated
by numerical simulation, taking into account the presence of both protons and
oxygen ions. The magnetotail current sheet is modeled as a magnetic field
reversal with a normal magnetic field component , plus a three-dimensional
spectrum of magnetic fluctuations , which represents the
observed magnetic turbulence. The dawn-dusk electric field E is also
included. A test particle simulation is performed using different values of
, E and injecting two different species of particles, O
ions and protons. O ions can support the formation of a double current
layer both in the absence and for large values of magnetic fluctuations
( and , where B is the constant
magnetic field in the magnetospheric lobes).Comment: 8 pages, 8 figures. J. Geophys. Res., in pres
Kinetic simulations of magnetic reconnection in presence of a background O+ population
Particle-in-Cell simulations of magnetic reconnection with an H+ current
sheet and a mixed background plasma of H+ and O+ ions are completed using
physical mass ratios. Four main results are shown. First, the O+ presence
slightly decreases the reconnection rate and the magnetic reconnection
evolution depends mainly on the lighter H+ ion species in the presented
simulations. Second, the Hall magnetic field is characterized by a two-scale
structure in presence of O+ ions: it reaches sharp peak values in a small area
in proximity of the neutral line, and then decreases slowly over a large
region. Third, the two background species initially separate in the outflow
region because H+ and O+ ions are accelerated by different mechanisms occurring
on different time scales and with different strengths. Fourth, the effect of a
guide field on the O+ dynamics is studied: the O+ presence does not change the
reconnected flux and all the characteristic features of guide field magnetic
reconnection are still present. Moreover, the guide field introduces an O+
circulation pattern between separatrices that enhances high O+ density areas
and depletes low O+ density regions in proximity of the reconnection fronts.
The importance and the validity of these results are finally discussed
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