1,487 research outputs found
Absolute velocity measurements in sunspot umbrae
In sunspot umbrae, convection is largely suppressed by the strong magnetic
field. Previous measurements reported on negligible convective flows in umbral
cores. Based on this, numerous studies have taken the umbra as zero reference
to calculate Doppler velocities of the ambient active region. To clarify the
amount of convective motion in the darkest part of umbrae, we directly measured
Doppler velocities with an unprecedented accuracy and precision. We performed
spectroscopic observations of sunspot umbrae with the Laser Absolute Reference
Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency
comb enabled the calibration of the high-resolution spectrograph and absolute
wavelength positions. A thorough spectral calibration, including the
measurement of the reference wavelength, yielded Doppler shifts of the spectral
line Ti i 5713.9 {\AA} with an uncertainty of around 5 m s-1. The measured
Doppler shifts are a composition of umbral convection and magneto-acoustic
waves. For the analysis of convective shifts, we temporally average each
sequence to reduce the superimposed wave signal. Compared to convective
blueshifts of up to -350 m s-1 in the quiet Sun, sunspot umbrae yield a
strongly reduced convective blueshifts around -30 m s-1. {W}e find that the
velocity in a sunspot umbra correlates significantly with the magnetic field
strength, but also with the umbral temperature defining the depth of the
titanium line. The vertical upward motion decreases with increasing field
strength. Extrapolating the linear approximation to zero magnetic field
reproduces the measured quiet Sun blueshift. Simply taking the sunspot umbra as
a zero velocity reference for the calculation of photospheric Dopplergrams can
imply a systematic velocity error.Comment: 10 pages, 7 figures, 2 tables, Appendix with 5 figure
Critical conditions for the wetting of soils
The wettability of soil is of great importance for plants and soil biota and in determining whether flooding and soil erosion will occur. The analysis used in common measurements of soil hydrophobicity makes the assumption that water always enters soils if the average contact angle between the soil and water is 90 degrees or lower; these tests have been used for decades. The authors show theoretically and experimentally that water cannot enter many soils unless the contact angle is considerably lower than this, down to approximately 50 degrees. This difference generates serious errors in determining and modeling soil wetting behavior
Particulate emissions from large North American wildfires estimated using a new top-down method
Particulate matter emissions from wildfires affect climate, weather
and air quality. However, existing global and regional aerosol emission
estimates differ by a factor of up to 4 between different methods. Using
a novel approach, we estimate daily total particulate matter (TPM) emissions
from large wildfires in North American boreal and temperate regions. Moderate
Resolution Imaging Spectroradiometer (MODIS) fire location and aerosol
optical thickness (AOT) data sets are coupled with HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) atmospheric
dispersion simulations, attributing identified smoke plumes to sources.
Unlike previous approaches, the method (i)Â combines information from both
satellite and AERONET (AErosol RObotic NETwork) observations to take into account aerosol water uptake
and plume specific mass extinction efficiency when converting smoke AOT to
TPM, and (ii)Â does not depend on instantaneous emission rates observed during
individual satellite overpasses, which do not sample night-time emissions.
The method also allows multiple independent estimates for the same emission
period from imagery taken on consecutive days.
<br><br>
Repeated fire-emitted AOT estimates for the same emission period over 2 to 3
days of plume evolution show increases in plume optical thickness by
approximately 10âŻ% for boreal events and by 40âŻ% for
temperate emissions. Inferred median water volume fractions for aged
boreal and temperate smoke observations are 0.15 and 0.47 respectively,
indicating that the increased AOT is partly explained by aerosol water
uptake. TPM emission estimates for boreal events, which predominantly
burn during daytime, agree closely with bottom-up Global
Fire Emission Database (GFEDv4) and Global Fire Assimilation System
(GFASv1.0) inventories, but are lower by approximately 30âŻ% compared
to Quick Fire Emission Dataset (QFEDv2) PM<sub>2.â5</sub>,
and are higher by approximately a factor of 2 compared to Fire Energetics and
Emissions Research (FEERv1) TPM estimates. The discrepancies are
larger for temperate fires, which are characterized by lower median
fire radiative power values and more significant night-time combustion. The TPM
estimates for this study for the biome are lower than QFED PM<sub>2.â5</sub> by
35âŻ%, and are larger by factors of 2.4, 3.2 and 4
compared with FEER, GFED and GFAS inventories respectively. A large
underestimation of TPM emission by bottom-up GFED and GFAS indicates
low biases in emission factors or consumed biomass estimates for temperate
fires
Rigorous treatment of electrostatics for spatially varying dielectrics based on energy minimization
A novel energy minimization formulation of electrostatics that allows
computation of the electrostatic energy and forces to any desired accuracy in a
system with arbitrary dielectric properties is presented. An integral equation
for the scalar charge density is derived from an energy functional of the
polarization vector field. This energy functional represents the true energy of
the system even in non-equilibrium states. Arbitrary accuracy is achieved by
solving the integral equation for the charge density via a series expansion in
terms of the equation's kernel, which depends only on the geometry of the
dielectrics. The streamlined formalism operates with volume charge
distributions only, not resorting to introducing surface charges by hand.
Therefore, it can be applied to any spatial variation of the dielectric
susceptibility, which is of particular importance in applications to
biomolecular systems. The simplicity of application of the formalism to real
problems is shown with analytical and numerical examples.Comment: 27 pages, 5 figure
Comb-calibrated solar spectroscopy through a multiplexed single-mode fiber channel
We investigate a new scheme for astronomical spectrograph calibration using
the laser frequency comb at the Solar Vacuum Tower Telescope on Tenerife. Our
concept is based upon a single-mode fiber channel, that simultaneously feeds
the spectrograph with comb light and sunlight. This yields nearly perfect
spatial mode matching between the two sources. In combination with the absolute
calibration provided by the frequency comb, this method enables extremely
robust and accurate spectroscopic measurements. The performance of this scheme
is compared to a sequence of alternating comb and sunlight, and to absorption
lines from Earth's atmosphere. We also show how the method can be used for
radial-velocity detection by measuring the well-explored 5-minute oscillations
averaged over the full solar disk. Our method is currently restricted to solar
spectroscopy, but with further evolving fiber-injection techniques it could
become an option even for faint astronomical targets.Comment: 21 pages, 11 figures. A video abstract for this paper is available on
youtube. For watching the video, please follow
https://www.youtube.com/watch?v=oshdZgrt89I . The video abstract is also
available for streaming and download on the related article website of New
Journal of Physic
First-Hitting Times Under Additive Drift
For the last ten years, almost every theoretical result concerning the
expected run time of a randomized search heuristic used drift theory, making it
the arguably most important tool in this domain. Its success is due to its ease
of use and its powerful result: drift theory allows the user to derive bounds
on the expected first-hitting time of a random process by bounding expected
local changes of the process -- the drift. This is usually far easier than
bounding the expected first-hitting time directly.
Due to the widespread use of drift theory, it is of utmost importance to have
the best drift theorems possible. We improve the fundamental additive,
multiplicative, and variable drift theorems by stating them in a form as
general as possible and providing examples of why the restrictions we keep are
still necessary. Our additive drift theorem for upper bounds only requires the
process to be nonnegative, that is, we remove unnecessary restrictions like a
finite, discrete, or bounded search space. As corollaries, the same is true for
our upper bounds in the case of variable and multiplicative drift
The Relevance of Pyrogenic Carbon for Carbon Budgets From Fires: Insights From the FIREX Experiment
Vegetation fires play an important role in global and regional carbon cycles. Due to climate warming and land use shifts, fire patterns are changing and fire impacts increasing in many of the world's regions. Reducing uncertainties in carbon budgeting calculations from fires is therefore fundamental to advance our current understanding and forecasting capabilities. Here we study 20 chamber burns from the FIREX FireLab experiment, which burnt a representative set of North American wildland fuels, to assess the following: (i) differences in carbon emission estimations between the commonly used âconsumed biomassâ approach and the âburnt carbonâ approach; (ii) pyrogenic carbon (PyC) production rates; and (iii) thermal and chemical recalcitrance of the PyC produced, as proxies of its biogeochemical stability. We find that the âconsumed biomassâ approach leads to overestimation of carbon emissions by 2â27% (most values between 2% and 10%). This accounting error arises largely from not considering PyC production and, even if relatively small, can therefore have important implications for mediumâ and longâterm carbon budgeting. A large fraction (34â100%) of this PyC was contained in the charred fine residue, a postfire material frequently overlooked in fire carbon research. However, the most recalcitrant PyC was in the form of woody charcoal, with estimated halfâlives for most samples exceeding 1,000 years. Combustion efficiency was relatively high in these laboratory burns compared to actual wildland fire conditions, likely leading to lower PyC production rates. We therefore argue that the PyC production values obtained here, and associated overestimation of carbon emissions, should be taken as lowâend estimates for wildland fire conditions
Duplex ultrasound imaging alone is sufficient for midterm endovascular aneurysm repair surveillance: A cost analysis study and prospective comparison with computed tomography scan
ObjectiveEarly in our experience with endovascular aortic aneurysm repair (EVAR) we performed both serial computed tomography scans and duplex ultrasound (DU) imaging in our post-EVAR surveillance regimen. Later we conducted a prospective study with DU imaging as the sole surveillance study and determined cost savings and outcome using this strategy.MethodsFrom September 21, 1998, to May 30, 2008, 250 patients underwent EVAR at our hospital. Before July 1, 2004, EVAR patients underwent CT and DU imaging performed every 6 months during the first year and then annually if no problems were identified (group 1). We compared aneurysm sac size, presence of endoleak, and graft patency between the two scanning modalities. After July 1, 2004, patients underwent surveillance using DU imaging as the sole surveillance study unless a problem was detected (group 2). CT and DU imaging charges for each regimen were compared using our 2008 health system pricing and Medicare reimbursements. All DU examinations were performed in our accredited noninvasive vascular laboratory by experienced technologists. Statistical analysis was performed using Pearson correlation coefficient.ResultsDU and CT scans were equivalent in determining aneurysm sac diameter after EVAR (P < .001). DU and CT were each as likely to falsely suggest an endoleak when none existed and were as likely to miss an endoleak. Using DU imaging alone would have reduced cost of EVAR surveillance by 29% (1595 per patient per year were realized in group 2 by eliminating CT scan surveillance. None of the group 2 patients sustained an adverse event such as rupture, graft migration, or limb occlusion as a result of having DU imaging performed as the sole follow-up modality.ConclusionSurveillance of EVAR patients can be performed accurately, safely, and cost-effectively with DU as the sole imaging study
- âŠ