167 research outputs found
Dynamics of isolated magnetic bright points derived from Hinode/SOT G-band observations
Small-scale magnetic fields in the solar photosphere can be identified in
high-resolution magnetograms or in the G-band as magnetic bright points (MBPs).
Rapid motions of these fields can cause magneto-hydrodynamical waves and can
also lead to nanoflares by magnetic field braiding and twisting. The MBP
velocity distribution is a crucial parameter for estimating the amplitudes of
those waves and the amount of energy they can contribute to coronal heating.
The velocity and lifetime distributions of MBPs are derived from solar G-band
images of a quiet sun region acquired by the Hinode/SOT instrument with
different temporal and spatial sampling rates. We developed an automatic
segmentation, identification and tracking algorithm to analyse G-Band image
sequences to obtain the lifetime and velocity distributions of MBPs. The
influence of temporal/spatial sampling rates on these distributions is studied
and used to correct the obtained lifetimes and velocity distributions for these
digitalisation effects. After the correction of algorithm effects, we obtained
a mean MBP lifetime of (2.50 +- 0.05) min and mean MBP velocities, depending on
smoothing processes, in the range of (1 - 2) km/s. Corrected for temporal
sampling effects, we obtained for the effective velocity distribution a
Rayleigh function with a coefficient of (1.62 +- 0.05) km/s. The x- and y-
components of the velocity distributions are Gaussians. The lifetime
distribution can be fitted by an exponential function.Comment: Astronomy and Astrophysics (in press
Asymmetric polarity reversals, bimodal field distribution, and coherence resonance in a spherically symmetric mean-field dynamo model
Using a mean-field dynamo model with a spherically symmetric helical
turbulence parameter alpha which is dynamically quenched and disturbed by
additional noise, the basic features of geomagnetic polarity reversals are
shown to be generic consequences of the dynamo action in the vicinity of
exceptional points of the spectrum. This simple paradigmatic model yields long
periods of constant polarity which are interrupted by self-accelerating field
decays leading to asymmetric polarity reversals. It shows the recently
discovered bimodal field distribution, and it gives a natural explanation of
the correlation between polarity persistence time and field strength. In
addition, we find typical features of coherence resonance in the dependence of
the persistence time on the noise.Comment: 5 pages, 7 figure
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Determination of Mesurol (Methiocarb) in Oregon Wines
The following is a report on a limited survey we conducted testing for the presence of Mesurol (Methiocarb) in Oregon wines. This study was undertaken as a result of recent publicity (resulting from testing in British Columbia) concerning the finding of Methiocarb residues in some wines, including Oregon wines; and in response to a WAB request to conduct the study and to review the "status" of Mesurol (1)
Three-dimensional non-LTE radiative transfer effects in Fe I lines I. Flux sheet and flux tube geometries
In network and active region plages, the magnetic field is concentrated into
structures often described as flux tubes (FTs) and sheets (FSs). 3-D radiative
transfer (RT) is important for energy transport in these concentrations. It is
also expected to be important for diagnostic purposes but has rarely been
applied for that purpose. Using true 3-D, non-LTE (NLTE) RT in FT/FS models, we
compute Fe line profiles commonly used to diagnose the Sun's magnetic field by
comparing the results with those obtained from LTE/1-D (1.5-D) NLTE
calculations. Employing a multilevel iron atom, we study the influence of basic
parameters such as Wilson depression, wall thickness, radius/width, thermal
stratification or magnetic field strength on all Stokes parameters in the
thin-tube approximation. The use of different levels of approximations of RT
may lead to considerable differences in profile shapes, intensity contrasts,
equivalent widths, and the determination of magnetic field strengths. In
particular, LTE, which often provides a good approach in planar 1-D
atmospheres, is a poor approximation in our flux sheet model for some of the
most important diagnostic Fe I lines (524.7nm, 525.0nm, 630.1nm, and 630.2nm).
The observed effects depend on parameters such as the height of line formation,
field strength, and internal temperature stratification. Differences between
the profile shapes may lead to errors in the determination of magnetic fields
on the order of 10 to 20%, while errors in the determined temperature can reach
300-400K. The empirical FT models NET and PLA turn out to minimize the effects
of 3D RT, so that results obtained with these models by applying LTE may also
remain valid for 3-D NLTE calculations. Finally, horizontal RT is found to only
insignificantly smear out structures such as the optically thick walls of FTs
and FSs, allowing features as narrow as 10km to remain visible.Comment: 20 pages, 21 figures, accepted for publication to "Astronomy and
Astrophysics
Photospheric logarithmic velocity spirals as MHD wave generation mechanisms
High-resolution observations of the solar photosphere have identified a wide variety of spiralling motions in the solar plasma. These spirals vary in properties, but are observed to be abundant at the solar surface. In this work, these spirals are studied for their potential as magnetohydrodynamic (MHD) wave generation mechanisms. The inter-granular lanes, where these spirals are commonly observed, are also regions where the magnetic field strength is higher than average. This combination of magnetic field and spiralling plasma is a recipe for the generation of Alfvén waves and other MHD waves. This work employs numerical simulations of a self-similar magnetic flux tube embedded in a realistic, gravitationally stratified, solar atmosphere to study the effects of a single magnetic flux tube perturbed by a logarithmic velocity spiral driver. The expansion factor of the logarithmic spiral driver is varied and multiple simulations are run for a range of values of the expansion factor centred around observational constraints. The simulations are analysed using ‘flux surfaces’ constructed from the magnetic field lines so that the vectors perpendicular, parallel and azimuthal to the local magnetic field vector can be calculated. The results of this analysis show that the Alfvén wave is the dominant wave for lower values of the expansion factor, whereas for the higher values the parallel component is dominant. This transition occurs within the range of the observational constraints, meaning that spiral drivers, as observed in the solar photosphere, have the potential to generate a variety of MHD wave modes
Improving Oral Hygiene Skills by Computer-Based Training: A Randomized Controlled Comparison of the Modified Bass and the Fones Techniques
Background: Gingivitis and other plaque-associated diseases have a high prevalence in western communities even though the majority of adults report daily oral hygiene. This indicates a lack of oral hygiene skills. Currently, there is no clear evidence as to which brushing technique would bring about the best oral hygiene skills. While the modified Bass technique is often recommended by dentists and in textbooks, the Fones technique is often recommended in patient brochures. Still, standardized comparisons of the effectiveness of teaching these techniques are lacking.
Methodology/Principal Findings: In a final sample of n=56 students, this multidisciplinary, randomized, examiner-blinded, controlled study compared the effects of parallel and standardized interactive computer presentations teaching either the Fones or the modified Bass technique. A control group was taught the basics of tooth brushing alone. Oral hygiene skills (remaining plaque after thorough oral hygiene) and gingivitis were assessed at baseline and 6, 12, and 28 weeks after the intervention. We found a significant group×time interaction for gingivitis (F(4/102)=3.267; p=0.016; e=0.957; ?2=0.114) and a significant main effect of group for oral hygiene skills (F(2/51)=7.088; p=0.002; ?2=0.218). Fones was superior to Bass; Bass did not differ from the control group. Group differences were most prominent after 6 and 12 weeks.
Conclusions/Significance: The present trial indicates an advantage of teaching the Fones as compared to the modified Bass technique with respect to oral hygiene skills and gingivitis. Future studies are needed to analyze whether the disadvantage of teaching the Bass technique observed here is restricted to the teaching method employed.
Trial Registration: German Clinical Trials Register http://www.drks.de/DRKS0000348
Stokes Diagnostis of 2D MHD-simulated Solar Magnetogranulation
We study the properties of solar magnetic fields on scales less than the
spatial resolution of solar telescopes. A synthetic infrared
spectropolarimetric diagnostics based on a 2D MHD simulation of
magnetoconvection is used for this. We analyze two time sequences of snapshots
that likely represent two regions of the network fields with their immediate
surrounding on the solar surface with the unsigned magnetic flux density of 300
and 140 G. In the first region we find from probability density functions of
the magnetic field strength that the most probable field strength at logtau_5=0
is equal to 250 G. Weak fields (B < 500 G) occupy about 70% of the surface,
while stronger fields (B 1000 G) occupy only 9.7% of the surface. The magnetic
flux is -28 G and its imbalance is -0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3 %, 0.3 %, -40 G, and -0.10.
We estimate the distribution of line-of-sight velocities on the surface of log
tau_5=-1. The mean velocity is equal to 0.4 km/s in the first simulated region.
The averaged velocity in the granules is -1.2 km/s and in the intergranules is
2.5 km/s. In the second region, the corresponding values of the mean velocities
are equal to 0, -1.8, 1.5 km/s. In addition we analyze the asymmetry of
synthetic Stokes-V profiles of the Fe I 1564.8 nm line. The mean values of the
amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude
asymmetry is increased to 10% while the area asymmetry is only slightly varied.Comment: 24 pages, 12 figure
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Horizontal Magnetic Fields in the Solar Photosphere
The results of 2D MHD simulations of solar magnetogranulation are used to
analyze the horizontal magnetic fields and the response of the synthesized
Stokes profiles of the FeI 1564.85 nm line to the magnetic fields. Selected
1.5-h series of the 2D MHD models reproduces a region of the network fields
with their immediate surrounding on the solar surface with the unsigned
magnetic flux density of 192 G. According to the magnetic field distribution
obtained, the most probable absolute strength of the horizontal magnetic field
at an optical depth of tau_5 = 1 (tau_5 denotes tau at lambda = 500 nm) is 50
G, while the mean value is 244 G. On average, the horizontal magnetic fields
are stronger than the vertical fields to heights of about 400 km in the
photosphere due to their higher density and the larger area they occupy. The
maximum factor by which the horizontal fields are greater is 1.5. Strong
horizontal magnetic flux tubes emerge at the surface as spots with field
strengths of more than 500 G. These are smaller than granules in size, and have
lifetimes of 3.6 min. They form in the photosphere due to the expulsion of
magnetic fields by convective flows coming from deep subphotospheric layers.
The data obtained qualitatively agree with observations with the Hinode space
observatory.Comment: 15 pages, 7 figures, published by Astronomicheskii Zhurnal (in
Russian) and Astronomy Reports (in English
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