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 ($\lesssim 1 Mm$). In the internetwork regions, Hall drift dominates above $0.25 Mm$ in the photosphere and below $2.5 Mm$ 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 $0.5 Mm$ and decreases with increasing altitude. However, for a $120 G$ 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 $\sim 0.01 s^{-1}$, 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

A three-dimensional numerical method for modelling weakly ionized plasmas

Astrophysical fluids under the influence of magnetic fields are often subjected to single-fluid or two-fluid approximations. In the case of weakly ionized plasmas however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and non-collisional forces. As a result, in dense molecular clouds and proto-stellar accretion discs for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics. Diffusive processes are known to restrict the stability of conventional numerical schemes which are not implicit in nature. Furthermore, recent work establishes that a large Hall term can impose an additional severe stability limit on standard explicit schemes. Following a previous paper which presented the one-dimensional case, we describe a fully three-dimensional method which relaxes the normal restrictions on explicit schemes for multifluid processes. This is achieved by applying the little known Super TimeStepping technique to the symmetric (ambipolar) component of the evolution operator for the magnetic field in the local plasma rest-frame, and the new Hall Diffusion Scheme to the skew-symmetric (Hall) component.Comment: 13 pages, 9 figures, accepted for publication in MNRA

Handling uncertainties in background shapes: the discrete profiling method

A common problem in data analysis is that the functional form, as well as the parameter values, of the underlying model which should describe a dataset is not known a priori. In these cases some extra uncertainty must be assigned to the extracted parameters of interest due to lack of exact knowledge of the functional form of the model. A method for assigning an appropriate error is presented. The method is based on considering the choice of functional form as a discrete nuisance parameter which is profiled in an analogous way to continuous nuisance parameters. The bias and coverage of this method are shown to be good when applied to a realistic example.Comment: Accepted by J.Ins

Post-Carboniferous and Post-Triassic Structures in Southern New Brunswick

Geology of New Brunswick: field guide to excursions, 1973: Trip A-10; B-

Medical student attitudes towards complementary and alternative medicine (CAM) in medical education: A critical review

© 2016 by De Gruyter. Background: This paper aims to remedy a gap in the knowledge by presenting the first critical review of the literature on major themes relating to medical students perceptions and attitudes towards the exponentially growing field of complementary and alternative medicine (CAM). Methods: After a comprehensive database search of the literature, 21 papers were chosen as suitable for the review. Results: The results from these papers were tabled and discussed. The results indicated that medical students lacked knowledge of CAM and are generally positive towards CAM education (especially in the preclinical years, if it provided evidence of efficacy and post-placement). Medical students thought that CAM should generally be incorporated into the medical curriculum mainly so they can confidently undertake referral to CAM practitioners. Being able to communicate with future patients about their CAM use was a major motivation for medical students to learn about CAM and a factor for medical student support of further incorporation of CAM content in the medical curricula. Educational exposure to CAM in many forms and in many papers was shown to significantly affect medical student attitudes to CAM. This may be reflective of the fact that, outside direct CAM training, there may be limited accessible opportunities for medical students and if integration is to occur, educational exposure is most important. Conclusions: The rise of CAM as a social and clinical phenomenon necessitates consideration of further inclusion of these topics in the medical curriculum, if future physicians are to be able to fully discharge their role as care providers in an increasingly medically pluralistic world. However, the inclusion of CAM needs to be done in an objective and critical manner, which is relevant to the learner

Impaired Glucose Metabolism among Those with and without Diagnosed Diabetes and Mortality: A Cohort Study Using Health Survey for England Data.

The extent that controlled diabetes impacts upon mortality, compared with uncontrolled diabetes, and how pre-diabetes alters mortality risk remain issues requiring clarification

Sustained enjoyment of life and mortality at older ages: analysis of the English Longitudinal Study of Ageing

OBJECTIVE:  To test whether the number of reports of enjoyment of life over a four year period is quantitatively associated with all cause mortality, and with death from cardiovascular disease and from other causes. DESIGN AND SETTING:  Longitudinal observational population study using the English Longitudinal Study of Ageing (ELSA), a nationally representative sample of older men and women living in England. PARTICIPANTS:  9365 men and women aged 50 years or older (mean 63, standard deviation 9.3) at recruitment. MAIN OUTCOME MEASURES:  Time to death, based on mortality between the third phase of data collection (wave 3 in 2006) and March 2013 (up to seven years). RESULTS:  Subjective wellbeing with measures of enjoyment of life were assessed in 2002 (wave 1), 2004 (wave 2), and 2006 (wave 3). 2264 (24%) respondents reported no enjoyment of life on any assessment, with 1833 (20%) reporting high enjoyment on one report of high enjoyment of life, 2063 (22%) on two reports, and 3205 (34%) on all three occasions. 1310 deaths were recorded during follow-up. Mortality was inversely associated with the number of occasions on which participants reported high enjoyment of life. Compared with the no high enjoyment group, the hazard ratio for all cause mortality was 0.83 (95% confidence interval 0.70 to 0.99) for two reports of enjoyment of life, and 0.76 (0.64 to 0.89) for three reports, after adjustment for demographic factors, baseline health, mobility impairment, and depressive symptoms. The same association was observed after deaths occurring within two years of the third enjoyment measure were excluded (0.90 (0.85 to 0.95) for every additional report of enjoyment), and in the complete case analysis (0.90 (0.83 to 0.96)). CONCLUSIONS:  This is an observational study, so causal conclusions cannot be drawn. Nonetheless, the results add a new dimension to understanding the significance of subjective wellbeing for health outcomes by documenting the importance of sustained wellbeing over time