3,420 research outputs found
Instability of a supersonic shock free elliptic jet
This paper presents a comparison of the measured and the computed spatial stability properties of an aspect ratio 2 supersonic shock free elliptic jet. The shock free nature of the elliptic jet provides an ideal test of validity of modeling the large scale coherent structures in the initial mixing region of noncircular supersonic jets with linear hydrodynamic stability theory. Both aerodynamic and acoustic data were measured. The data are used to compute the mean velocity profiles and to provide a description of the spatial composition of pressure waves in the elliptic jet. A hybrid numerical scheme is applied to solve the Rayleigh problem governing the inviscid linear spatial stability of the jet. The measured mean velocity profiles are used to provide a qualitative model for the cross sectional geometry and the smooth velocity profiles used in the stability analysis. Computational results are presented for several modes of instability at two jet cross sections. The acoustic measurements show that a varicose instability is the jet's perferred mode of motion. The stability analysis predicts that the Strouhal number varies linearly as a function of axial distance in the jet's initial mixing region, which is in good qualitative agreement with previous measurements
Observation of kink instability during small B5.0 solar flare on 04 June, 2007
Using multi-wavelength observations of SoHO/MDI, SOT-Hinode/blue-continuum
(4504 \AA), G-band (4305 \AA), Ca II H (3968 \AA) and TRACE 171 \AA, we present
the observational signature of highly twisted magnetic loop in AR 10960 during
the period 04:43 UT-04:52 UT at 4 June, 2007. SOT-Hinode/blue-continuum (4504
\AA) observations show that penumbral filaments of positive polarity sunspot
have counter-clock wise twist, which may be caused by the clock-wise rotation
of the spot umbrae. The coronal loop, whose one footpoint is anchored in this
sunspot, shows strong right-handed twist in chromospheric SOT-Hinode/Ca II H
(3968 \AA) and coronal TRACE 171 \AA\, images. The length and the radius of the
loop are 80 Mm and 4.0 Mm respectively. The distance between
neighboring turns of magnetic field lines (i.e. pitch) is estimated as
10 Mm. The total twist angle, 12 (estimated for the
homogeneous distribution of the twist along the loop), is much larger than the
Kruskal -Shafranov instability criterion. We detected clear double structure of
the loop top during 04:47-04:51 UT on TRACE 171 \AA \ images, which is
consistent with simulated kink instability in curved coronal loops
(T{\"o}r{\"o}k et al. 2004). We suggest, that the kink instability of this
twisted magnetic loop triggered B5.0 class solar flare, which occurred between
04:40 UT and 04:51 UT in this active region.Comment: 24 pages, 5 Figures; The Astrophysical Journa
The role of national culture in the strategic use of and investment in ICT: a comparative study of Japanese and Australian organisations
Japanese ICT investment trailed the developed world for many decades, contributing to low productivity growth. A potential root cause is the influence of Japanese culture over the strategic use of and investment in ICT (SUIICT). Empirical research on cultural aspects of SUIICT in Japan is lacking. This study will examine national culture and its association with SUIICT in organisations in Japan compared to Australia. A quantitative survey of ICT decision makers in Japanese and Australian organisations will be fielded based on questions synthesized from McFarlan, McKenney and Pyburn (The Strategic Grid), and Hofstede and Minkov (Cultural Dimensions Model). A contribution to theory will be the development and testing of an empirical model and practical instrument for determining the association between national culture and SUIICT. A practical contribution will be an increase in understanding of both markets allowing for more effective market segmentation and development of ICT offerings for both countries
A Nanoflare Distribution Generated by Repeated Relaxations Triggered by Kink Instability
Context: It is thought likely that vast numbers of nanoflares are responsible
for the corona having a temperature of millions of degrees. Current
observational technologies lack the resolving power to confirm the nanoflare
hypothesis. An alternative approach is to construct a magnetohydrodynamic
coronal loop model that has the ability to predict nanoflare energy
distributions.
Aims: This paper presents the initial results generated by such a model. It
predicts heating events with a range of sizes, depending on where the
instability threshold for linear kink modes is encountered. The aims are to
calculate the distribution of event energies and to investigate whether kink
instability can be predicted from a single parameter.
Methods: The loop is represented as a straight line-tied cylinder. The
twisting caused by random photospheric motions is captured by two parameters,
representing the ratio of current density to field strength for specific
regions of the loop. Dissipation of the loop's magnetic energy begins during
the nonlinear stage of the instability, which develops as a consequence of
current sheet reconnection. After flaring, the loop evolves to the state of
lowest energy where, in accordance with relaxation theory, the ratio of current
to field is constant throughout the loop and helicity is conserved.
Results: The results suggest that instability cannot be predicted by any
simple twist-derived property reaching a critical value. The model is applied
such that the loop undergoes repeated episodes of instability followed by
energy-releasing relaxation. Hence, an energy distribution of the nanoflares
produced is collated.
Conclusions: The final energy distribution features two nanoflare populations
that follow different power laws. The power law index for the higher energy
population is more than sufficient for coronal heating.Comment: 13 pages, 18 figure
Ab Initio Phase Diagram of Chromium to 2.5 TPa
Chromium possesses remarkable physical properties such as hardness and corrosion resistance. Chromium is also a very important geophysical material as it is assumed that lighter Cr isotopes were dissolved in the Earth's molten core during the planet's formation, which makes Cr one of the main constituents of the Earth's core. Unfortunately, Cr has remained one of the least studied 3d transition metals. In a very recent combined experimental and theoretical study (Anzellini et al., Scientific Reports, 2022), the equation of state and melting curve of chromium were studied to 150 GPa, and it was determined that the ambient body-centered cubic (bcc) phase of crystalline Cr remains stable in the whole pressure range considered. However, the importance of the knowledge of the physical properties of Cr, specifically its phase diagram, necessitates further study of Cr to higher pressure. In this work, using a suite of ab initio quantum molecular dynamics (QMD) simulations based on the Z methodology which combines both direct Z method for the simulation of melting curves and inverse Z method for the calculation of solid-solid phase transition boundaries, we obtain the theoretical phase diagram of Cr to 2.5 TPa. We calculate the melting curves of the two solid phases that are present on its phase diagram, namely, the lower-pressure bcc and the higher-pressure hexagonal close-packed (hcp) ones, and obtain the equation for the bcc-hcp solid-solid phase transition boundary. We also obtain the thermal equations of state of both bcc-Cr and hcp-Cr, which are in excellent agreement with both experimental data and QMD simulations. We argue that 2180 K as the value of the ambient melting point of Cr which is offered by several public web resources ("Wikipedia," "WebElements," "It's Elemental," etc.) is most likely incorrect and should be replaced with 2135 K, found in most experimental studies as well as in the present theoretical work
The Flare-energy Distributions Generated by Kink-unstable Ensembles of Zero-net-current Coronal Loops
It has been proposed that the million degree temperature of the corona is due
to the combined effect of barely-detectable energy releases, so called
nanoflares, that occur throughout the solar atmosphere. Alas, the nanoflare
density and brightness implied by this hypothesis means that conclusive
verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a
magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from
the nature of an ideal kink instability. The set of energy-releasing
instabilities is captured by an instability threshold for linear kink modes.
Each point on the threshold is associated with a unique energy release and so
we can predict a distribution of nanoflare energies. When the linear
instability threshold is crossed, the instability enters a nonlinear phase as
it is driven by current sheet reconnection. As the ensuing flare erupts and
declines, the field transitions to a lower energy state, which is modelled by
relaxation theory, i.e., helicity is conserved and the ratio of current to
field becomes invariant within the loop. We apply the model so that all the
loops within an ensemble achieve instability followed by energy-releasing
relaxation. The result is a nanoflare energy distribution. Furthermore, we
produce different distributions by varying the loop aspect ratio, the nature of
the path to instability taken by each loop and also the level of radial
expansion that may accompany loop relaxation. The heating rate obtained is just
sufficient for coronal heating. In addition, we also show that kink instability
cannot be associated with a critical magnetic twist value for every point along
the instability threshold
Evidence for a singularity in ideal magnetohydrodynamics: implications for fast reconnection
Numerical evidence for a finite-time singularity in ideal 3D
magnetohydrodynamics (MHD) is presented. The simulations start from two
interlocking magnetic flux rings with no initial velocity. The magnetic
curvature force causes the flux rings to shrink until they come into contact.
This produces a current sheet between them. In the ideal compressible
calculations, the evidence for a singularity in a finite time is that the
peak current density behaves like for a range of
sound speeds (or plasma betas). For the incompressible calculations consistency
with the compressible calculations is noted and evidence is presented that
there is convergence to a self-similar state. In the resistive reconnection
calculations the magnetic helicity is nearly conserved and energy is
dissipated.Comment: 4 pages, 4 figure
A novel MT-CO2 variant causing cerebellar ataxia and neuropathy: The role of muscle biopsy in diagnosis and defining pathogenicity
Pathogenic variants in mitochondrial DNA (mtDNA) are associated with significant clinical heterogeneity with neuromuscular involvement commonly reported. Non-syndromic presentations of mtDNA disease continue to pose a diagnostic challenge and with genomic testing still necessitating a muscle biopsy in many cases. Here we describe an adult patient who presented with progressive ataxia, neuropathy and exercise intolerance in whom the application of numerous Mendelian gene panels had failed to make a genetic diagnosis. Muscle biopsy revealed characteristic mitochondrial pathology (cytochrome c oxidase deficient, ragged-red fibers) prompting a thorough investigation of the mitochondrial genome. Two heteroplasmic MT-CO2 gene variants (NC_012920.1: m.7887G>A and m.8250G>A) were identified, necessitating single fiber segregation and familial studies – including the biopsy of the patient's clinically-unaffected mother - to demonstrate pathogenicity of the novel m.7887G>A p.(Gly101Asp) variant and establishing this as the cause of the mitochondrial biochemical defects and clinical presentation. In the era of high throughput whole exome and genome sequencing, muscle biopsy remains a key investigation in the diagnosis of patients with non-syndromic presentations of adult-onset mitochondrial disease and fully defining the pathogenicity of novel mtDNA variants
A Novel Pathogenic Variant in MT-CO2 Causes an Isolated Mitochondrial Complex IV Deficiency and Late-Onset Cerebellar Ataxia
Both nuclear and mitochondrial DNA defects can cause isolated cytochrome c oxidase (COX; complex IV) deficiency, leading to the development of the mitochondrial disease. We report a 52-year-old female patient who presented with a late-onset, progressive cerebellar ataxia, tremor and axonal neuropathy. No family history of neurological disorder was reported. Although her muscle biopsy demonstrated a significant COX deficiency, there was no clinical and electromyographical evidence of myopathy. Electrophysiological studies identified low frequency sinusoidal postural tremor at 3 Hz, corroborating the clinical finding of cerebellar dysfunction. Complete sequencing of the mitochondrial DNA genome in muscle identified a novel MT-CO2 variant, m.8163A>G predicting p.(Tyr193Cys). We present several lines of evidence, in proving the pathogenicity of this heteroplasmic mitochondrial DNA variant, as the cause of her clinical presentation. Our findings serve as an important reminder that full mitochondrial DNA analysis should be included in the diagnostic pipeline for investigating individuals with spinocerebellar ataxi
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