629 research outputs found
Twisted flux tube emergence from the convection zone to the corona
3D numerical simulations of a horizontal magnetic flux tube emergence with
different twist are carried out in a computational domain spanning the upper
layers of the convection zone to the lower corona. We use the Oslo Staggered
Code to solve the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along the magnetic field lines. The emergence
of the magnetic flux tube input at the bottom boundary into a weakly magnetized
atmosphere is presented. The photospheric and chromospheric response is
described with magnetograms, synthetic images and velocity field distributions.
The emergence of a magnetic flux tube into such an atmosphere results in varied
atmospheric responses. In the photosphere the granular size increases when the
flux tube approaches from below. In the convective overshoot region some 200km
above the photosphere adiabatic expansion produces cooling, darker regions with
the structure of granulation cells. We also find collapsed granulation in the
boundaries of the rising flux tube. Once the flux tube has crossed the
photosphere, bright points related with concentrated magnetic field, vorticity,
high vertical velocities and heating by compressed material are found at
heights up to 500km above the photosphere. At greater heights in the magnetized
chromosphere, the rising flux tube produces a cool, magnetized bubble that
tends to expel the usual chromospheric oscillations. In addition the rising
flux tube dramatically increases the chromospheric scale height, pushing the
transition region and corona aside such that the chromosphere extends up to 6Mm
above the photosphere. The emergence of magnetic flux tubes through the
photosphere to the lower corona is a relatively slow process, taking of order 1
hour.Comment: 53 pages,79 figures, Submitted to Ap
The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone
We present three-dimensional numerical simulations of the rise and
fragmentation of twisted, initially horizontal magnetic flux tubes which evolve
into emerging Omega-loops. The flux tubes rise buoyantly through an
adiabatically stratified plasma that represents the solar convection zone. The
MHD equations are solved in the anelastic approximation, and the results are
compared with studies of flux tube fragmentation in two dimensions. We find
that if the initial amount of field line twist is below a critical value, the
degree of fragmentation at the apex of a rising Omega-loop depends on its
three-dimensional geometry: the greater the apex curvature of a given
Omega-loop, the lesser the degree of fragmentation of the loop as it approaches
the photosphere. Thus, the amount of initial twist necessary for the loop to
retain its cohesion can be reduced substantially from the two-dimensional
limit. The simulations also suggest that as a fragmented flux tube emerges
through a relatively quiet portion of the solar disk, extended crescent-shaped
magnetic features of opposite polarity should form and steadily recede from one
another. These features eventually coalesce after the fragmented portion of the
Omega-loop emerges through the photosphere.Comment: 17 pages, 17 figures, uses AAS LaTeX macros v5.0. ApJ, in pres
Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7
Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive
tool for the study of subtle charge, orbital, and spin superlattice orders
driven by the valence electrons, which therefore escape detection in
conventional x-ray diffraction (XRD). Although the power of REXS has been
demonstrated by numerous studies of complex oxides performed in the soft x-ray
regime, the cross section and photon wavelength of the material-specific
elemental absorption edges ultimately set the limit to the smallest
superlattice amplitude and periodicity one can probe. Here we show -- with
simulations and REXS on Mn-substituted SrRuO -- that these
limitations can be overcome by performing resonant scattering experiments at
the absorption edge of a suitably-chosen, dilute impurity. This establishes
that -- in analogy with impurity-based methods used in electron-spin-resonance,
nuclear-magnetic resonance, and M\"ossbauer spectroscopy -- randomly
distributed impurities can serve as a non-invasive, but now momentum-dependent
probe, greatly extending the applicability of resonant x-ray scattering
techniques
High Resolution Observations using Adaptive Optics: Achievements and Future Needs
Over the last few years, several interesting observations were obtained with
the help of solar Adaptive Optics (AO). In this paper, few observations made
using the solar AO are enlightened and briefly discussed. A list of
disadvantages with the current AO system are presented. With telescopes larger
than 1.5m are expected during the next decade, there is a need to develop the
existing AO technologies for large aperture telescopes. Some aspects of this
development are highlighted. Finally, the recent AO developments in India are
also presented
Cerebrospinal fluid neurogranin in an inducible mouse model of neurodegeneration: A translatable marker of synaptic degeneration
Synapse impairment is thought to be an early event in Alzheimer's disease (AD); dysfunction and loss of synapses are linked to cognitive symptoms that precede neuronal loss and neurodegeneration. Neurogranin (Ng) is a somatodendritic protein that has been shown to be reduced in brain tissue but increased in the cerebrospinal fluid (CSF) of AD patients compared to age-matched controls. High levels of CSF Ng have been shown to reflect a more rapid AD progression. To gauge the translational value of Ng as a biomarker, we developed a new, highly sensitive, digital enzyme-linked immunosorbent assay (ELISA) on the Simoa platform to measure Ng in both mouse and human CSF. We investigated and confirmed that Ng levels are increased in the CSF of patients with AD compared to controls. In addition, we explored how Ng is altered in the brain and CSF of transgenic mice that display progressive neuronal loss and synaptic degeneration following the induction of p25 overexpression. In this model, we found that Ng levels increased in CSF when neurodegeneration was induced, peaking after 2âŻweeks, while they decreased in brain. Our data suggest that CSF Ng is a biomarker of synaptic degeneration with translational value
Application of realistic effective interactions to the structure of the Zr isotopes
We calculate the low-lying spectra of the zirconium isotopes Z=40 with
neutron numbers from N=52 to N=60 using the 1p1/20g9/2 proton and
2s1d0g7/20h11/2 neutron sub-shells to define the model space. Effective
proton-proton, neutron--neutron and proton-neutron interactions have been
derived using 88Sr as closed core and employing perturbative many-body
techniques. The starting point is the nucleon-nucleon potential derived from
modern meson exchange models. The comprehensive shell-model calculation
performed in this work provides a qualitative reproduction of essential
properties such as the sub-shell closures in 96Zr and 98Zr.Comment: To appear in Phys Rev C, june 2000, 8 figs, Revtex latex styl
Interpretation of positive troponin results among patients with and without myocardial infarction.
Measuring cardiac troponins is integral to diagnosing acute myocardial infarction (AMI); however, troponins may be elevated without AMI, and the use of multiple different assays confounds comparisons. We considered characteristics and serial troponin values in emergency department chest pain patients with and without AMI to interpret troponin excursions. We compared serial troponin in 124 AMI and non-AMI patients from the observational Performance of Triage Cardiac Markers in the Clinical Setting (PEARL) study who presented with chest pain and had at least one troponin value exceeding the 99th percentile of normal. Because 8 assays were used during data collection, we employed a method of scaling the troponin value to the corresponding assay\u27s 99th percentile upper reference limit to standardize the results. In 81 AMI patients, 96% had elevated troponin at the first test following initial elevation, compared to 73% of the 43 non-AMI patients
Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest
Precision astrometry at microarcsecond accuracy has application to a wide
range of astrophysical problems. This paper is a study of the science questions
that can be addressed using an instrument that delivers parallaxes at about 4
microarcsec on targets as faint as V = 20, differential accuracy of 0.6
microarcsec on bright targets, and with flexible scheduling. The science topics
are drawn primarily from the Team Key Projects, selected in 2000, for the Space
Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities
of this mission to illustrate the importance of the next level of astrometric
precision in modern astrophysics. SIM PlanetQuest is currently in the detailed
design phase, having completed all of the enabling technologies needed for the
flight instrument in 2005. It will be the first space-based long baseline
Michelson interferometer designed for precision astrometry. SIM will contribute
strongly to many astronomical fields including stellar and galactic
astrophysics, planetary systems around nearby stars, and the study of quasar
and AGN nuclei. SIM will search for planets with masses as small as an Earth
orbiting in the `habitable zone' around the nearest stars using differential
astrometry, and could discover many dozen if Earth-like planets are common. It
will be the most capable instrument for detecting planets around young stars,
thereby providing insights into how planetary systems are born and how they
evolve with time. SIM will observe significant numbers of very high- and
low-mass stars, providing stellar masses to 1%, the accuracy needed to
challenge physical models. Using precision proper motion measurements, SIM will
probe the galactic mass distribution and the formation and evolution of the
Galactic halo. (abridged)Comment: 54 pages, 28 figures, uses emulateapj. Submitted to PAS
A Parametrization of Cosmic Ray Shower Profiles Based on Shower Width
Cosmic ray (CR) air showers, detected via the air fluorescence technique, are
reconstructed in part using functions that parameterize the longitudinal
profile of each shower. The profile parameterization yields the position of
shower maximum, X_max, which is sensitive to the incident CR particle type:
e.g. p, C/N/O, Fe or photons. The integral of the profile is directly related
to the shower energy. The Pierre Auger fluorescence reconstruction uses the
Gaisser-Hillas 4-parameter form. The HiRes group has used both the
Gaisser-Hillas form and a 3-parameter Gaussian in Age form. Historically
analytic shower theory suggested yet other forms; the best known is a
3-parameter form popularized by Greisen. Our work now uses the shower full
width half-maximum, "fwhm", and shower asymmetry parameter, "f", to unify the
parameterization of all three profile functions. Furthermore shower profiles
expressed in terms of the new parameters: (fwhm, f) have correlations greatly
reduced over Gaisser-Hillas parameters (X_0, lambda). This allows shower
profile reconstructions to add constraints (if needed) on the mostly
uncorrelated parameters (fwhm, f).Comment: 12 pages, 6 figures, to be published in J. Phys G: Nucl. & Part. Phy
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