32,944 research outputs found
Ion dynamics and the magnetorotational instability in weakly-ionized discs
The magnetorotational instability (MRI) of a weakly ionized, differentially
rotating, magnetized plasma disk is investigated in the multi-fluid framework.
The disk is threaded by a uniform vertical magnetic field and charge is carried
by electrons and ions only. The inclusion of ion dynamics causes significant
modification to the conductivity tensor in a weakly ionized disk. The parallel,
Pedersen and Hall component of conductivity tensor become time dependent
quantities resulting in the AC and DC part of the conductivity. The
conductivity may change sign leading to the significant modification of the
parameter window in which MRI may operate.
The effect of ambipolar and Hall diffusion on the linear growth of the MRI is
examined in the presence of time dependent conductivity tensor. We find that
the growth rate in ambipolar regime can become somewhat larger than the
rotational frequency, especially when the departure from ideal MHD is
significant. Further, the instability operates on large scale lengths. This has
important implication for the angular momentum transport in the disk.Comment: 13 pages, 12 figure
Hall instability of solar flux tubes
The magnetic network which consists of vertical flux tubes located in
intergranular lanes is dominated by Hall drift in the photosphere-lower
chromosphere region (). In the internetwork regions, Hall drift
dominates above in the photosphere and below in the
chromosphere. Although Hall drift does not cause any dissipation in the ambient
plasma, it can destabilise the flux tubes and magnetic elements in the presence
of azimuthal shear flow. The physical mechanism of this instability is quite
simple: the shear flow twists the radial magnetic field and generates azimuthal
field; torsional oscillations of the azimuthal field in turn generates the
radial field completing feedback loop. The maximum growth rate of Hall
instability is proportional to the absolute value of the shear gradient and is
dependent on the ambient diffusivity. The diffusivity also determines the most
unstable wavelength which is smaller for weaker fields.
We apply the result of local stability analysis to the network and
internetwork magnetic elements and show that the maximum growth rate for
kilogauss field occurs around and decreases with increasing altitude.
However, for a field, the maximum growth rate remains almost constant
in the entire photosphere-lower chromosphere except in a small region of lower
photosphere. For shear flow gradient , the Hall growth time
is 10 minute near the footpoint. Therefore, network fields are likely to be
unstable in the photosphere, whereas internetwork fields could be unstable in
the entire photosphere-chromosphere. Thus the Hall instability can play an
important role in generating low frequency turbulence which can heat the
chromosphere.Comment: 8 page, 4 figure
The Effects of Malpractice Pressure and Liability Reforms on Physicians’ Perceptions of Medical Care
Considerable evidence suggests that the medical malpractice liability system neither provides compensation to patients who suffer negligent medical injury nor seeks to penalize physicians whose negligence causes patient injury. The relationship between liability reforms, malpractice pressure and physician perceptions of medical care is examined
Flavor Changing Neutral Currents, an Extended Scalar Sector, and the Higgs Production Rate at the LHC
We study extensions of the standard model with additional colored scalar
fields which can couple directly to quarks. Natural suppression of flavor
changing neutral currents implies minimal flavor violation, and fixes the
scalars to transform as (8,2)_1/2 under the SU(3) X SU(2) X U(1) gauge
symmetry. We explore the phenomenology of the standard model with one
additional (8,2)_1/2 scalar, and discuss how this extension can modify flavor
physics and the Higgs boson production rate at the LHC. Custodial SU(2)
symmetry can be implemented for the octet scalars since they transform as a
real color representation. Additional weak scale degrees of freedom needed for
gauge unification are discussed.Comment: Minor change
Loop-Induced Stochastic Bias at Small Wavevectors
Primordial non-Gaussianities enhanced at small wavevectors can induce a power
spectrum of the galaxy overdensity that differs greatly from that of the matter
overdensity at large length scales. In previous work, it was shown that
"squeezed" three-point and "collapsed" four-point functions of the curvature
perturbation can generate these non-Gaussianities and give rise to
so-called scale-dependent and stochastic bias in the galaxy overdensity power
spectrum. We explore a third way to generate non-Gaussianities enhanced at
small wavevectors: the infrared behavior of quantum loop contributions to the
four-point correlations of . We show that these loop effects lead to
stochastic bias, which can be observable in the context of quasi-single field
inflation.Comment: 10 pages, 4 figure
Continued monitoring of acute kidney injury survivors might not be necessary in those regaining an estimated glomerular filtration rate > 60 mL/min at 1 year
Background. Severe acute kidney injury (AKI) among hospitalized patients often necessitates initiation of short-term dialysis. Little is known about the long-term outcome of those who recover to normal renal function. The aim of this study was to determine the long-term renal outcome of patients experiencing AKI requiring dialysis secondary to hypoperfusion injury and/or sepsis who recovered to apparently normal renal function.
Methods. All adult patients with AKI requiring dialysis in our centre between 1 January 1980 and 31 December 2010 were identified. We included patients who had estimated glomerular filtration rate (eGFR) >60 mL/min/1.73 m2 12 months or later after the episode of AKI. Patients were followed up until 3 March 2015. The primary outcome was time to chronic kidney disease (CKD) (defined as eGFR persistently <60 mL/min/1.73 m2) from first dialysis for AKI.
Results. Among 2922 patients with a single episode of dialysis-requiring AKI, 396 patients met the study inclusion criteria. The mean age was 49.8 (standard deviation 16.5) years and median follow-up was 7.9 [interquartile range (IQR) 4.8–12.7] years. Thirty-five (8.8%) of the patients ultimately developed CKD after a median of 5.3 (IQR 2.8–8.0) years from first dialysis for AKI giving an incidence rate of 1 per 100 person-years. Increasing age, diabetes and vascular disease were associated with higher risk of progression to CKD [adjusted hazard ratios (95% confidence interval): 1.06 (1.03, 1.09), 3.05 (1.41, 6.57) and 3.56 (1.80, 7.03), respectively].
Conclusions. Recovery from AKI necessitating in-hospital dialysis was associated with a very low risk of progression to CKD. Most of the patients who progressed to CKD had concurrent medical conditions meriting monitoring of renal function. Therefore, it seems unlikely that regular follow-up of renal function is beneficial in patients who recover to eGFR >60 mL/min/1.73 m2 by 12 months after an episode of AKI
Renal replacement modality and stroke risk in end-stage renal disease—a national registry study
Background:
The risk of stroke in end-stage renal disease (ESRD) on renal replacement therapy (RRT) is up to 10-fold greater than the general population. However, whether this increased risk differs by RRT modality is unclear.
Methods:
We used data contained in the Scottish Renal Registry and the Scottish Stroke Care Audit to identify stroke in all adult patients who commenced RRT for ESRD from 2005 to 2013. Incidence rate was calculated and regression analyses were performed to identify variables associated with stroke. We explored the effect of RRT modality at initiation and cumulative dialysis exposure by time-dependent regression analysis, using transplant recipients as the reference group.
Results:
A total of 4957 patients commenced RRT for ESRD. Median age was 64.5 years, 41.5% were female and 277 patients suffered a stroke (incidence rate was 18.6/1000 patient-years). Patients who had stroke were older, had higher blood pressure and were more likely to be female and have diabetes. On multivariable regression older age, female sex, diabetes and higher serum phosphate were associated with risk of stroke. RRT modality at initiation was not. On time-dependent analysis, haemodialysis (HD) exposure was independently associated with increased risk of stroke.
Conclusions:
In patients with ESRD who initiate RRT, HD use independently increases risk of stroke compared with transplantation. Use of peritoneal dialysis did not increase risk on adjusted analysis
- …