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PubMed2004

Near-field optical patterning and structuring based on local-field enhancement at the extremity of a metal tip.

We present a particular approach and the associated results allowing the nanostructuration of a thin photosensitive polymer film. This approach based on a scanning near-field optical microscopy configuration uses the field-enhancement (FE) effect, a so-called lightning-rod effect appearing at the extremity of a metallic tip when illuminated with an incident light polarized along the tip axis. The local enhancement of the electromagnetic field straight below the tip's apex is observed directly through a photoisomerization reaction, inducing the growth of a topographical nanodot characterized in situ by atomic-force microscopy using the same probe. From a survey of the literature, we first review the different experimental approaches offered to nanostructure materials by near-field optical techniques. We describe more particularly the FE effect approach. An overview of the theoretical approach of this effect is then given before presenting some experimental results so as theoretical results using the finite-element method. These results show the influence on the nanostructuration of the polymer of a few experimental parameters such as the polarization state, the illumination mode and the tip's geometry. Finally, the potentiality of this technique for some applications in the field of lithography and high-density data storage is shown via the fabrication of nano-patterns.

Journal Article↗
PubMed1999

Interesting times on Krakatau: stand dynamics in the 1990s.

The Krakatau Islands, Indonesia, have provided an opportunity for ecologists to track primary succession from the 'clean slate' of 1883, through forest closure in the 1920s, to the contemporary period, in which successional changes take the form of alterations in composition and stature of forest stands rather than gross changes in ecosystem type. This paper reports on permanent forest plots established on the islands in 1989, and fully surveyed again in both 1992 and 1997. Since 1989, the plots have been subject to natural disturbance phenomena in the form of varying combinations of, for example, deposition of volcanic ejecta, landslides, lightning strikes, storm damage and drought. These effects have been concentrated between 1992 and 1997, during which the volcano Anak Krakatau has deposited ash on the islands of Sertung and Panjang, but not on Rakata. Data on stand responses are presented for growth rates (dbh (diameter at breast height, 1.3 m) increment), stem recruitment and mortality, biomass changes partitioned into mortality, ingrowth and growth of established trees), and compositional shifts. The discussion focuses on evaluation of questions and successional models framed earlier in the programme. One general finding is that the stand dominants as of 1989 have tended to decline in number within the plots, generally through low levels of recruitment failing to balance rates of mortality. The effects of disturbance to the plots appear to be evident in terms of mortality and recruitment, dbh increment, and changes in biomass. The patterns of change in the eight plots are quite varied, such that relatively few generalizations are possible. The difficulties of establishing meaningful baseline rates for tree growth and stand biomass are discussed.

Biomass↗
PubMed1999

Electron kinetic effects in atmosphere breakdown by an intense electromagnetic pulse.

A physical model is proposed for description of electron kinetics driven by a powerful electromagnetic pulse in the Earth's atmosphere. The model is based on a numerical solution to the Boltzmann kinetic equation for two groups of electrons. Slow electrons (with energies below a few keV) are described in a two-term approximation assuming a weak anisotropy of the electron distribution function. Fast electrons (with energies above a few keV) are described by a modified macroparticle method, taking into account the electron acceleration in the electric field, energy losses in the continuous deceleration approximation, and the multiple pitch angle scattering. The model is applied to a problem of the electric discharge in a nitrogen, which is preionized by an external gamma-ray source. It is shown that the runaway electrons have an important effect on the energy distribution of free electrons, and on the avalanche ionization rate. This mechanism might explain the observation of multiple lightning discharges observed in the Ivy-Mike thermonuclear test in the early 1950's.

Journal Article↗
PubMed2001

Long plasma channels generated by femtosecond laser pulses.

Generation of a long plasma channel by femtosecond laser pulses is investigated. The results show that the balance between the nonlinear self-focusing of the laser beam and plasma defocusing forms a long plasma channel, which guides the laser beam to propagate a long distance in air. This phenomenon can be used to trigger lightning.

Journal Article↗
PubMed2002

Synchronization and coarsening (without self-organized criticality) in a forest-fire model.

We study the long-time dynamics of a forest-fire model with deterministic tree growth and instantaneous burning of entire forests by stochastic lightning strikes. Asymptotically the system organizes into a coarsening self-similar mosaic of synchronized patches within which trees regrow and burn simultaneously. We show that the average patch length grows linearly with time as t--> infinity. The number density of patches of length L, N(L,t), scales as -2N(L/ ), and within a mean-field rate equation description we find that this scaling function decays as N(x) approximately e(-1/x) for x-->0, and as e(-x) for x--> infinity. In one dimension, we develop an event-driven cluster algorithm to study the asymptotic behavior of large systems. Our numerical results are consistent with mean-field predictions for patch coarsening.

Journal Article↗
PubMed2002

Symmetry transforms for ideal magnetohydrodynamics equilibria.

A method for constructing ideal magnetohydrodynamics (MHD) equilibria is introduced. The method consists of the application of symmetry transforms to any known MHD equilibrium [ O. I. Bogoyavlenskij, Phys. Rev. E. 62, 8616, (2000)]. The transforms break the geometrical symmetries of the field-aligned solutions and produce continuous families of the nonsymmetric MHD equilibria. The method of symmetry transforms also allows to obtain MHD equilibria with current sheets and exact solutions with noncollinear vector fields B and V. A model of the nonsymmetric astrophysical jets outside of their accretion disks is developed. The total magnetic and kinetic energy of the jet is finite in any layer c(1)<z<c(2). The jet possesses current sheets and is highly collimated in view of a rapid decrease of the magnetic field B in the transverse direction. The method gives also the MHD equilibria that model ball lightning with dynamics of plasma inside the fireball.

Journal Article↗
PubMed2003

Metal nanoplasmas as bright sources of hard X-ray pulses.

We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and "lightning rod" effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities approximately 10(14)-10(15) W cm(-2). Our experiments show a 13-fold enhancement in hard x-ray yield (10-200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions.

Journal Article↗
PubMed2003

Resonant behavior of dielectric objects (electrostatic resonances).

Resonant behavior of dielectric objects occurs at certain frequencies for which the object permittivity is negative and the free-space wavelength is large in comparison with the object dimensions. Unique physical features of these resonances are studied and a novel technique for the calculation of resonance values of permittivity, and hence resonance frequencies, is proposed. Scale invariance of resonance frequencies, unusually strong orthogonality properties of resonance modes, and a two-dimensional phenomenon of "twin" spectra are reported. The paper concludes with brief discussions of optical controllability of these resonances in semiconductor nanoparticles and a plausible, electrostatic resonance based, mechanism for nucleation and formation of ball lightning.

Biosensing Techniques↗
PubMed2006

Fireball ejection from a molten hot spot to air by localized microwaves.

A phenomenon of fireball ejection from hot spots in solid materials (silicon, germanium, glass, ceramics, basalt, etc.) to the atmosphere is presented. The hot spot is created in the substrate material by the microwave-drill mechanism [Jerby, Science 298, 587 (2002)10.1126/science.1077062]. The vaporized drop evolved from the hot spot is blown up, and forms a stable fireball buoyant in the air. The experimental observations of fireball ejection from silicate hot spots are referred to the Abrahamson-Dinniss theory [Nature (London) 403, 519 (2000)10.1038/35000525] suggesting a mechanism for ball-lightning initiation in nature. The fireballs observed in our experiments tend to absorb the available microwave power entirely, similarly to the plasmon resonance effect in submicron wavelengths [Nie and Emory, Science 275, 1102 (1997)10.1126/science.275.5303.1102].

Journal Article↗
PubMed2003

Finite-element time-domain algorithms for modeling linear Debye and Lorentz dielectric dispersions at low frequencies.

We present what we believe to be the first algorithms that use a simple scalar-potential formulation to model linear Debye and Lorentz dielectric dispersions at low frequencies in the context of finite-element time-domain (FETD) numerical solutions of electric potential. The new algorithms, which permit treatment of multiple-pole dielectric relaxations, are based on the auxiliary differential equation method and are unconditionally stable. We validate the algorithms by comparison with the results of a previously reported method based on the Fourier transform. The new algorithms should be useful in calculating the transient response of biological materials subject to impulsive excitation. Potential applications include FETD modeling of electromyography, functional electrical stimulation, defibrillation, and effects of lightning and impulsive electric shock.

Algorithms↗
PubMed1984

Plantar ulcers in hereditary sensory neuropathy. A plea for conservative treatment.

Hereditary sensory neuropathy is a rare syndrome characterized by the occurrence, in childhood or early adult life, of perforating ulcers of the feet, lightning pains, and loss of cutaneous sensation and tendon reflexes in the lower extremities. Three patients with hereditary sensory neuropathy were family members. Trophic ulcers may be caused by diseases other than diabetes, syphilis, and leprosy.

Adult↗
PubMed2006

Accidental deaths caused by electricity in Sweden, 1975-2000.

This study analyzes accidental fatalities caused by electricity--at work and during leisure time--to evaluate risk factors, the role of alcohol, and to identify possible preventive strategies. In Sweden, data on fatalities by electrocution from 1975 through 2000 were collected from the National Cause-of-Death Register. Additional cases were found in the archives of The Swedish National Electrical Safety Board. Suicides and deaths by lightning were excluded. Two hundred and eighty-five deaths were found, including occupational (n=132), leisure time (n=151), and unknown (n=2). Most deaths were caused by aerial power lines, and the most common place for an electrical injury was a railway area or residential property. Postmortem blood from 20% (n=47) of the tested cases was found positive for alcohol, and these persons were killed mainly during leisure time. During the study period, the overall incidence of electricity-related fatalities has decreased, in spite of increased use of electricity. This indicates that safety improvements have been successful.

Accidents↗
PubMed1999

Electrical injuries in Canadian burn care. Identification of unsolved problems.

Over the past decade, the Firefighters' Burn Treatment Unit of the University of Alberta Hospital in Edmonton, Alberta, Canada, has treated 1399 inpatients suffering from thermal injury. Regional burn care is provided in a 10-bed intensive care unit with 18 plastic surgery reconstructive beds for a large referral region of central and northern Alberta, portions of the Northwest Territories, and neighboring provinces of British Columbia and Saskatchewan. Of the total burn inpatients during this period, 74 electrical injuries were treated (5.3% of all admissions): 71 were males (95.9%) and 3 females (4.1%). The mean age of all patients was 33.9 +/- 12.6 years (range 1-67). Compared to our general population of thermally injured patients, those with electrical injuries had smaller injuries [9.9 +/- 12.9% TBSA (range 1-65) versus 15.1 +/- 10.1], shorter length of hospitalization [18.6 +/- 7.3 days (range 1-80) versus 26.2 +/- 0.8], and substantially lower mortality once reaching the hospital (0% versus 4%). Electrical injuries were classified as flash in 30 cases, contact in 42 cases, and lightning in 2 cases; 74.3% of injuries occurred during work-related activities. A total of 118 operative procedures were performed during the acute admission (1.6 procedures per patient), including 19 amputations: 12 in the upper and 7 in the lower extremity. The mean time of amputation was 9.3 +/- 5.3 days after admission. In contact injuries of the upper extremity, 14 patients suffered amputations or neurologic injury that required reconstruction with free tissue transfers and nerve grafts. Long-term functional outcome of these patients using sensory testing, the Jebsen-Taylor hand function test, and wound coverage has revealed that these patients have substantial persistent sensory impairment of their upper extremities postinjury despite reconstruction, although many remain active and functional with acceptable wound coverage. Based on our analysis of electrical injury as it presents to one typical Canadian burn unit, our patients suffer limb loss on a delayed basis, which leads to substantial morbidity. Reconstruction of the upper extremity with microsurgical techniques after profound electrical injury has provided acceptable coverage, but in many instances is associated with poor or marginal sensory recovery limiting reemployment options for patients with upper extremity electrical burns. Further understanding of the cellular biology of delayed tissue loss after electric injury would offer the potential for reduction in amputation rate and improvement in functional outcome and overall morbidity.

Accidents, Occupational↗
PubMed2003

White-light filaments for atmospheric analysis.

Most long-path remote spectroscopic studies of the atmosphere rely on ambient light or narrow-band lasers. High-power femtosecond laser pulses have been found to propagate in the atmosphere as dynamically self-guided filaments that emit in a continuum from the ultraviolet to the infrared. This white light exhibits a directional behavior with enhanced backward scattering and was detected from an altitude of more than 20 kilometers. This light source opens the way to white-light and nonlinear light detection and ranging applications for atmospheric trace-gas remote sensing or remote identification of aerosols. Air ionization inside the filaments also opens promising perspectives for laser-induced condensation and lightning control. The mobile femtosecond-terawatt laser system, Teramobile, has been constructed to study these applications.

Journal Article↗
PubMed2004

Radio and plasma wave observations at Saturn from Cassini's approach and first orbit.

We report data from the Cassini radio and plasma wave instrument during the approach and first orbit at Saturn. During the approach, radio emissions from Saturn showed that the radio rotation period is now 10 hours 45 minutes 45 +/- 36 seconds, about 6 minutes longer than measured by Voyager in 1980 to 1981. In addition, many intense impulsive radio signals were detected from Saturn lightning during the approach and first orbit. Some of these have been linked to storm systems observed by the Cassini imaging instrument. Within the magnetosphere, whistler-mode auroral hiss emissions were observed near the rings, suggesting that a strong electrodynamic interaction is occurring in or near the rings.

Journal Article↗
PubMed2005

Cassini Imaging Science: initial results on Saturn's atmosphere.

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of approximately 375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.

Atmosphere↗
PubMed1998

Large-scale nitrogen oxide plumes in the tropopause region and implications for ozone

Continuous measurements of nitrogen oxide and ozone were performed from a commercial airliner during 1 year at cruising altitudes below and above the tropopause. The upper tropospheric nitrogen oxides distribution was found to be strongly influenced by large-scale plumes extending about 100 to 1300 kilometers along the flight track. The plumes were frequently observed downwind of thunderstorms and frontal systems, which most probably caused upward transport of polluted air from the continental boundary layer or nitrogen oxide production in lightning strokes, or both. Particularly in summer, average ozone concentrations in the plumes were enhanced compared to the tropospheric background levels.

Journal Article↗
PubMed1996

Thunderstorm associated asthma: a detailed analysis of environmental factors.

OBJECTIVES: To seek associations between meteorological factors, concentrations of air pollutants or pollen, and an asthma epidemic which occurred in London on 24 and 25 June 1994 after a thunderstorm. DESIGN: Retrospective study of patients' accident and emergency department records, with bivariate and multivariate analysis of environmental factors and data collection for the two months surrounding the epidemic. SETTING: The accident and emergency department of St Mary's Hospital in west central London. SUBJECTS: 148 patients presenting with asthma between 1 June and 31 July 1994, of whom 40 presented in the 24 hours after the storm. RESULTS: The asthma epidemic was significantly associated with a drop in air temperature six hours previously and a high grass pollen concentration nine hours previously. Non-epidemic asthma was significantly associated with lightning strikes, increase in humidity or sulphur dioxide concentration, a drop in temperature or high rainfall the previous day, and a decrease in maximum air pressure or changes in grass pollen counts over the previous two days. CONCLUSIONS: New episodes of asthma during the epidemic on 24 and 25 June 1994 were associated with a fall in air temperature and a rise in grass pollen concentration. Non-epidemic asthma was significantly associated with a greater number of environmental changes. This may indicate that the patients with thunderstorm associated asthma were a separate population, sensitive to different environmental stimuli.

Air Pollutants↗