2,494 research outputs found
Giant current fluctuations in an overheated single electron transistor
Interplay of cotunneling and single-electron tunneling in a thermally
isolated single-electron transistor (SET) leads to peculiar overheating
effects. In particular, there is an interesting crossover interval where the
competition between cotunneling and single-electron tunneling changes to the
dominance of the latter. In this interval, the current exhibits anomalous
sensitivity to the effective electron temperature of the transistor island and
its fluctuations. We present a detailed study of the current and temperature
fluctuations at this interesting point. The methods implemented allow for a
complete characterization of the distribution of the fluctuating quantities,
well beyond the Gaussian approximation. We reveal and explore the parameter
range where, for sufficiently small transistor islands, the current
fluctuations become gigantic. In this regime, the optimal value of the current,
its expectation value, and its standard deviation differ from each other by
parametrically large factors. This situation is unique for transport in
nanostructures and for electron transport in general. The origin of this
spectacular effect is the exponential sensitivity of the current to the
fluctuating effective temperature.Comment: 10 pages, 11 figure
Nonequilibrium characteristics in all-superconducting tunnel structures
We study the nonequilibrium characteristics of superconducting tunnel
structures in the case when one of the superconductors is a small island
confined between large superconductors. The state of this island can be probed
for example via the supercurrent flowing through it. We study both the
far-from-equilibrium limit when the rate of injection for the electrons into
the island exceeds the energy relaxation inside it, and the quasiequilibrium
limit when the electrons equilibrate between themselves. We also address the
crossover between these limits employing the collision integral derived for the
superconducting case. The clearest signatures of the nonequilibrium limit are
the anomalous heating effects seen as a supercurrent suppression at low
voltages, and the hysteresis at voltages close to the gap edge ,
resulting from the peculiar form of the nonequilibrium distribution function.Comment: 8 pages, 10 figure
Scavenging of ultrafine particles by rainfall at a boreal site: observations and model estimations
International audienceValues of the scavenging coefficient were determined from observations of ultrafine particles (with diameters in the range 10?510 nm) during rain events at a boreal forest site in Southern Finland between 1996 and 2001. The estimated range of values of the scavenging coefficient was [7×10?6?4×10?5] s?1, which is generally higher than model calculations based only on below-cloud processes (Brownian diffusion, interception, and typical charge effects). A new model that includes below-cloud scavenging processes, mixing of ultrafine particles from the boundary layer (BL) into cloud, followed by cloud condensation nuclei activation and in-cloud removal by rainfall, is presented. The effective scavenging coefficients estimated from this new model have values comparable with those obtained from observations. Results show that ultrafine particle removal by rain depends on aerosol size, rainfall intensity, mixing processes between BL and cloud elements, in-cloud scavenged fraction, in-cloud collection efficiency, and in-cloud coagulation with cloud droplets. Implications for the treatment of scavenging of BL ultrafine particles in numerical models are discussed
Factors of air ion balance in a coniferous forest according to measurements in Hyytiälä, Finland
A new mathematical model describing air ion balance was developed and tested. It has improved approximations and includes dry deposition of ions onto the forest canopy. The model leads to an explicit algebraic solution of the balance equations. This allows simple calculation of both the ionization rate and the average charge of aerosol particles from measurements of air ions and aerosol particles, with some parameters of the forest. Charged aerosol particles are distinguished from cluster ions by their size, which exceeds 1.6 nm diameter. The relative uncertainty of the ionization rate is about the same or less than the relative uncertainties of the measurements. The model was tested with specific air ion measurements carried out simultaneously at two heights at the Hyytiälä forest station, Finland. Earlier studies have shown a difference in the predictions of the ionization rate in the Hyytiälä forest when calculated in two different ways: based on the measurements of the environmental radioactivity and based on the air ion and aerosol measurements. The new model explains the difference as a consequence of neglecting dry deposition of ions in the earlier models. The ionization rate during the 16 h campaign was 5.6±0.8 cm<sup>−3</sup> s<sup>−1</sup> at the height of 2 m and 3.9±0.2 cm<sup>−3</sup> s<sup>−1</sup> at the height of 14 m, between the tops of the trees. The difference points out the necessity to consider the height variation when the ionization rate is used as a parameter in studies of ion-induced nucleation. Additional results are some estimates of the parameters of air ion balance. The recombination sink of cluster ions on the ions of opposite polarity made up 9–13%, the sink on aerosol particles 65–69%, and the sink on forest canopy 18–26% of the total sink of cluster ions. The average lifetime of cluster ions was about 130 s for positive and about 110 s for negative ions. At the height of 2 m, about 70% of the space charge of air was carried by aerosol particles, and at the height of 14 m, about 84%
Waterfalls as sources of small charged aerosol particles
In this study, we measured the mobility distributions of cluster and intermediate ions with an ion spectrometer near a waterfall. We observed that the concentration of negative 1.5–10 nm ions was one-hundred fold higher than a reference point 100 m away from the waterfall. Also, the concentration of positive intermediate ions was found to be higher than that at the reference point by a factor of ten. This difference was observed only at the smallest sizes; above 10 nm the difference was insignificant
100 years of atmospheric and marine observations at the Finnish Utö Island in the Baltic Sea
The Utö Atmospheric and Marine Research Station introduced in
this paper is located on Utö Island
(59°46.84′ N, 21°22.13′ E) at the outer edge of the Archipelago
Sea, by the Baltic Sea towards the Baltic Proper. Meteorological observations at
the island started in 1881 and vertical profiling of seawater temperature
and salinity in 1900. Since 1980, the number of observations at Utö has
rapidly increased, with a large number of new meteorological, air quality,
aerosol, optical and greenhouse gas parameters, and recently, a
variety of marine observations. In this study, we analyze long-term changes
of atmospheric temperature, cloudiness, sea salinity, temperature and ice
cover. Our main dataset consists of 248 367 atmospheric temperature
observations, 1632Â quality-assured vertical seawater temperature and salinity
profiles and 8565Â ice maps, partly digitized for this project. We also use
North Atlantic Oscillation (NAO), major Baltic inflow (MBI) and Baltic Sea
river runoff data from the literature as reference variables to our data. Our
analysis is based on a statistical method utilizing a dynamic linear model. The
results show an increase in the atmospheric temperature at Utö, but the
increase is significantly smaller than on land areas and has taken place only
since the early 1980s, with a rate of
0.4 °C decade−1 during the last 35 years. We also see an increase in
seawater temperatures, especially on the surface, with an increase of
0.3 °C decade−1 for the last 100 years. In deeper water
layers, the increase is smaller and influenced by vertical mixing, which is modulated
by inflow of saline water from the North Sea and freshwater inflow from
rivers and by wind-driven processes influenced by the local bathymetry. The date
when air temperature in the spring exceeds +5 °C became 5 days
earlier from the period 1951–1980 to the period 1981–2010 and the date when sea
surface water temperature exceeds +4 °C changed to 9 days earlier.
Sea ice cover duration at Utö shows a decrease of approximately 50 %
during the last 35Â years. Based on the combined results, it is possible that
the climate at Utö has changed into a new phase, in which the sea ice
no longer reduces the local temperature increase caused by the global
warming.</p
Characterization of air ions in boreal forest air during BIOFOR III campaign
International audienceThe behavior of the concentration of positive small (or cluster) air ions and naturally charged nanometer aerosol particles (aerosol ions) has been studied on the basis of measurements carried out in a boreal forest at the Hyytiälä SMEAR station, Finland, during the BIOFOR III campaign in spring 1999. Statistical characteristics of the concentrations of cluster ions, two classes of aerosol ions of the sizes of 2.5?8 nm and 8?ca 20 nm and the quantities that determine the balance of small ions in the atmosphere have been given for the nucleation event days and non-event days. The dependence of small ion concentration on the ion loss (sink) due to aerosol particles was investigated applying a model of bipolar diffusion charging of particles by small ions. The small ion concentration and the ion sink were closely correlated (correlation coefficient 87%) when the fog events and the hours of high relative humidity (above 97%), as well as nocturnal calms and weak wind (wind speed-1 had been excluded. In the case of nucleation burst events, variations in the concentration of small positive ions were in accordance with the changes caused by the ion sink due to aerosols; no clear indication of positive ion depletion by ion-induced nucleation was found. The estimated average ionization rate of air at the Hyytiälä station in early spring, when the ground was partly covered with snow, was about 4.8 ion pairs cm-3 s-1. The study of the charging state of nanometer aerosol particles (2.5?8 nm) revealed a strong correlation (correlation coefficient 88%) between the concentrations of particles and their charged fraction (positive air ions) during nucleation bursts. The estimated charged fraction of particles, which varied from 3% to 6% considering various nucleation event days, confirms that these particles are almost quasi-steady state charged. Also the particles and air ions in the size range of 8?ca 20 nm showed a good qualitative consistency; the correlation coefficient was 92%
Comparison of Operating Microscope and Exoscope in a Highly Challenging Experimental Setting
BACKGROUND: The use of a digital three-dimensional (3D) exoscope system in neurosurgery is increasing as an alternative to the operative microscope. The objective of this study was to compare a digital 3D exoscope system with a standard operating microscope as a neurosurgical visualization tool in a highly challenging experimental setting. METHODS: End-to-side bypass procedures, each at a depth of 9 cm, were performed in a simulation setting. The quality of the task and the depth effect, visualization, magnification, illumination, and ergonomics were evaluated. RESULTS: No major differences were noted between the microscope and the 3D exoscope in terms of the quality of the work. Working with the 3D exoscope was more time-consuming than working with the microscope. Changing the depth and focus was faster using the operative microscope. The 3D exoscope enabled higher magnification and offered better ergonomic features. CONCLUSIONS: In a highly challenging experimental setting, comparable procedural quality was found for the microscope and the 3D exoscope. Each visualization tool had advantages and disadvantages. Over time and with technologic advances, the digital 3D exoscope may become the main operative visualization system in microneurosurgery.Peer reviewe
Particle concentration and flux dynamics in the atmospheric boundary layer as the indicator of formation mechanism
We carried out column model simulations to study particle fluxes and deposition and to evaluate different particle formation mechanisms at a boreal forest site in Finland. We show that kinetic nucleation of sulphuric acid cannot be responsible for new particle formation alone as the simulated vertical profile of particle number concentration does not correspond to observations. Instead organic induced nucleation leads to good agreement confirming the relevance of the aerosol formation mechanism including organic compounds emitted by the biosphere. <br><br> The simulation of aerosol concentration within the atmospheric boundary layer during nucleation event days shows a highly dynamical picture, where particle formation is coupled with chemistry and turbulent transport. We have demonstrated the suitability of our turbulent mixing scheme in reproducing the most important characteristics of particle dynamics within the boundary layer. Deposition and particle flux simulations show that deposition affects noticeably only the smallest particles in the lowest part of the atmospheric boundary layer
Cluster Multi-spacecraft Determination of AKR Angular Beaming
Simultaneous observations of AKR emission using the four-spacecraft Cluster
array were used to make the first direct measurements of the angular beaming
patterns of individual bursts. By comparing the spacecraft locations and AKR
burst locations, the angular beaming pattern was found to be narrowly confined
to a plane containing the magnetic field vector at the source and tangent to a
circle of constant latitude. Most rays paths are confined within 15 deg of this
tangent plane, consistent with numerical simulations of AKR k-vector
orientation at maximum growth rate. The emission is also strongly directed
upward in the tangent plane, which we interpret as refraction of the rays as
they leave the auroral cavity. The narrow beaming pattern implies that an
observer located above the polar cap can detect AKR emission only from a small
fraction of the auroral oval at a given location. This has important
consequences for interpreting AKR visibility at a given location. It also helps
re-interpret previously published Cluster VLBI studies of AKR source locations,
which are now seen to be only a subset of all possible source locations. These
observations are inconsistent with either filled or hollow cone beaming models.Comment: 5 pages, 4 figures. Geophys. Res. Letters (accepted
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