32,040 research outputs found
Does a Non-Magnetic Solar Chromosphere Exist?
Enhanced chromospheric emission which corresponds to an outwardly increasing
semiempirical temperature structure can be produced by wave motion without any
increase in the mean gas temperture. Hence, the sun may not have a classical
chromosphere in magnetic field free internetwork regions. Other significant
differences between the properties of dynamic and static atmospheres should be
considered when analyzing chromospheric observations.Comment: 4 pages, uuencoded compressed postscript file including three
figures. APJL accepte
On the Formation of Active Regions
Magneto-convection can produce an active region without an initial coherent
flux tube. A simulation was performed where uniform, untwisted, horizontal
magnetic field of 1 kG strenght was advected into the bottom of a computational
domain 48 Mm wide by 20 Mm deep. The up and down convective motions produce a
hierarchy of magnetic loops with a wide range of scales, with smaller loops
riding "piggy back" in a serpentine fashion on larger loops. When a large loop
approaches the surface it produces an small active region with a compact
leading spot and more diffuse following spots
Composite oscillator systems for meeting user needs for time and frequency
Frequency standards are used in most navigation and telecommunications systems to provide a long term memory of either frequency, phase, or time epoch. From a systems point of view, the performance aspects of the frequency standard are weighed against other systems characteristics, such as overall performance, cost, size, and accessibility; a number of examples are very briefly reviewed. The theory of phase lock and frequency lock systems is outlined in sufficient detail that total oscillator system performance can be predicted from measurements on the individual components. As an example, details of the performance of a high spectral purity oscillator phase locked to a long term stable oscillator are given. Results for several systems, including the best system stability that can be obtained from present commercially available 5-MHz sources, are shown
Flat-top TIRF illumination boosts DNA-PAINT imaging and quantification
Super-resolution (SR) techniques have extended the optical resolution down to a few nanometers. However, quantitative treatment of SR data remains challenging due to its complex dependence on a manifold of experimental parameters. Among the different SR variants, DNA-PAINT is relatively straightforward to implement, since it achieves the necessary 'blinking' without the use of rather complex optical or chemical activation schemes. However, it still suffers from image and quantification artifacts caused by inhomogeneous optical excitation. Here we demonstrate that several experimental challenges can be alleviated by introducing a segment-wise analysis approach and ultimately overcome by implementing a flat-top illumination profile for TIRF microscopy using a commercially-available beam-shaping device. The improvements with regards to homogeneous spatial resolution and precise kinetic information over the whole field-of-view were quantitatively assayed using DNA origami and cell samples. Our findings open the door to high-throughput DNA-PAINT studies with thus far unprecedented accuracy for quantitative data interpretation
Line formation in solar granulation: II. The photospheric Fe abundance
The solar photospheric Fe abundance has been determined using realistic ab
initio 3D, time-dependent, hydrodynamical model atmospheres. The study is based
on the excellent agreement between the predicted and observed line profiles
directly rather than equivalent width, since the intrinsic Doppler broadening
from the convective motions and oscillations provide the necessary non-thermal
broadening. Thus, three of the four hotly debated parameters (equivalent
widths, microturbulence and damping enhancement factors) in the center of the
recent solar Fe abundance dispute regarding FeI lines no longer enter the
analysis, leaving the transition probabilities as the main uncertainty. Both
FeI (using the samples of lines of both the Oxford and Kiel studies) and FeII
lines have been investigated, which give consistent results: log FeI = 7.44 +-
0.05 and log FeII = 7.45 +- 0.10. Also the wings of strong FeI lines return
consistent abundances, log FeII = 7.42 +- 0.03, but due to the uncertainties
inherent in analyses of strong lines we give this determination lower weight
than the results from weak and intermediate strong lines. In view of the recent
slight downward revision of the meteoritic Fe abundance log Fe = 7.46 +- 0.01,
the agreement between the meteoritic and photospheric values is very good, thus
appearingly settling the debate over the photospheric Fe abundance from FeI
lines.Comment: Accepted for A&
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