Fast magnetoacoustic wave trains in magnetic funnels of the solar corona

Context: Fast magneto-acoustic waves are highly dispersive in waveguides, so they can generate quasi-periodic wave trains if a localised, impulsive driver is applied. Such wave trains have been observed in the solar corona and may be of use as a seismological tool since they depend upon the plasma structuring perpendicular to the direction of propagation. Aims. We extend existing models of magnetoacoustic waveguides to consider the effects of an expanding magnetic field. The funnel geometry employed includes a field-aligned density structure. Methods: We performed 2D numerical simulations of impulsively generated fast magneto-acoustic perturbations. The effects of the density contrast ratio, density stratification, and spectral profile of the driver upon the excited wave trains were investigated. Results: The density structure acts as a dispersive waveguide for fast magneto-acoustic waves and generates a quasi-periodic wave train similar to previous models. The funnel geometry leads to generating additional wave trains that propagate outside the density structure. These newly discovered wave trains are formed by the leakage of transverse perturbations, but they propagate upwards owing to the refraction caused by the magnetic funnel. Conclusions: The results of our funnel model may be applicable to wave trains observed propagating in the solar corona. They demonstrate similar properties to those found in our simulations

Does the Sun Shrink with Increasing Magnetic Activity?

We have analyzed the full set of SOHO/MDI f- and p-mode oscillation frequencies from 1996 to date in a search for evidence of solar radius evolution during the rising phase of the current activity cycle. Like Antia et al. (2000), we find that a significant fraction of the f-mode frequency changes scale with frequency; and that if these are interpreted in terms of a radius change, it implies a shrinking sun. Our inferred rate of shrinkage is about 1.5 km/y, which is somewhat smaller than found by Antia et al. We argue that this rate does not refer to the surface, but rather to a layer extending roughly from 4 to 8 Mm beneath the visible surface. The rate of shrinking may be accounted for by an increasing radial component of the rms random magnetic field at a rate that depends on its radial distribution. If it were uniform, the required field would be ~7 kG. However, if it were inwardly increasing, then a 1 kG field at 8 Mm would suffice. To assess contribution to the solar radius change arising above 4Mm, we analyzed the p-mode data. The evolution of the p-mode frequencies may be explained by a magnetic^M field growing with activity. The implications of the near-surface magnetic field changes depend on the anisotropy of the random magnetic field. If the field change is predominantly radial, then we infer an additional shrinking at a rate between 1.1-1.3 km/y at the photosphere. If on the other hand the increase is isotropic, we find a competing expansion at a rate of 2.3 km/y. In any case, variations in the sun's radius in the activity cycle are at the level of 10^{-5} or less, hence have a negligible contribution to the irradiance variations.Comment: 10 pages (ApJ preprint style), 4 figures; accepted for publication in Ap

Leadership Traits, Tools, and Practices: Decision Making in a Crisis

Crises often present complex, uncertain, and unstable situations where routine decision making is not enough. Crises are typically unpredictable yet leaders must prepare to make decisions using a variety of traits, tools and practices. While all leadership is dependent on many factors and subject to many variables, in a crisis, those variables are magnified. Effective decision making during a crisis is a key trait of crisis leaders and is developed over time and with practice. Using the classic Delphi Technique, the researcher obtained qualitative data from experts in crisis management concerning (a) the difference between non-crisis and crisis decision making, (b) the traits and tools of a crisis leader, and (c) evidence of effective crisis leadership practices. This research method was selected because of its flexibility, its use of experts, and the varied locations of those experts. Literature reviewed for this study considered traditional leadership as well as crisis leadership. Crisis leaders use traditional decision making strategies, tools and practices as well as those adapted to a crisis environment. This study seeks to capture some of that data and disseminate it to the community of practice as well as the research community. The goal of any research is to improve the field of practice, add to the body of knowledge, and increase awareness of an idea, concept, or theory. Recognizing the complexity of crisis environments, the researcher suggests recommendations that may assist the crisis management community to improve decision making and to share traits, tools, and practices of effective crisis leaders

Transverse oscillations of two coronal loops

We study transverse fast magnetohydrodynamic waves in a system of two coronal loops modeled as smoothed, dense plasma cylinders in a uniform magnetic field. The collective oscillatory properties of the system due to the interaction between the individual loops are investigated from two points of view. Firstly, the frequency and spatial structure of the normal modes are studied. The system supports four trapped normal modes in which the loops move rigidly in the transverse direction. The direction of the motions is either parallel or perpendicular to the plane containing the axes of the loops. Two of these modes correspond to oscillations of the loops in phase, while in the other two they move in antiphase. Thus, these solutions are the generalization of the kink mode of a single cylinder to the double cylinder case. Secondly, we analyze the time-dependent problem of the excitation of the pair of tubes. We find that depending on the shape and location of the initial disturbance, different normal modes can be excited. The frequencies of normal modes are accurately recovered from the numerical simulations. In some cases, because of the simultaneous excitation of several eigenmodes, the system shows beating and the phase lag between the loops is $\pi/2$.Comment: Accepted for publication in The Astrophysical Journa

High frequency and high wavenumber solar oscillations

We determine the frequencies of solar oscillations covering a wide range of degree (100< l <4000) and frequency (1.5 <\nu<10 mHz) using the ring diagram technique applied to power spectra obtained from MDI (Michelson Doppler Imager) data. The f-mode ridge extends up to degree of approximately 3000, where the line width becomes very large, implying a damping time which is comparable to the time period. The frequencies of high degree f-modes are significantly different from those given by the simple dispersion relation \omega^2=gk. The f-mode peaks in power spectra are distinctly asymmetric and use of asymmetric profile increases the fitted frequency bringing them closer to the frequencies computed for a solar model.Comment: Revised version. 1.2 mHz features identified as artifacts of data analysis. Accepted for publication in Ap

The resonant damping of fast magnetohydrodynamic oscillations in a system of two coronal slabs

Observations of transversal coronal loop oscillations very often show the excitation and damping of oscillations in groups of coronal loops rather than in individual and isolated structures. We present results on the oscillatory properties (periods, damping rates, and spatial distribution of perturbations) for resonantly damped oscillations in a system of two inhomogeneous coronal slabs and compare them to the properties found in single slab loop models. A system of two identical coronal loops is modeled, in Cartesian geometry, as being composed by two density enhancements. The linear magnetohydrodynamic (MHD) wave equations for oblique propagation of waves are solved and the damping of the different solutions, due to the transversal inhomogeneity of the density profile, is computed. The physics of the obtained results is analyzed by an examination of the perturbed physical variables. We find that, due to the interaction between the loops, the normal modes of oscillation present in a single slab split into symmetric and antisymmetric oscillations when a system of two identical slabs is considered. The frequencies of these solutions may differ from the single slab results when the distance between the loops is of the order of a few slab widths. Oblique propagation of waves weakens this interaction, since solutions become more confined to the edges of the slabs. The damping is strong for surface-like oscillations, while sausage body-like solutions are unaffected. For some solutions, and small slab separations, the damping in a system of two loops differs substantially from the damping of a single loop.Comment: 25 pages, 9 figure

An Instrument to Assess Subjective Task Value Beliefs Regarding the Decision to Pursue Postgraduate Training

Objectives. To develop and validate an instrument to assess subjective ratings of the perceived value of various postgraduate training paths followed using expectancy-value as a theoretical framework; and to explore differences in value beliefs across type of postgraduate training pursued and type of pharmacy training completed prior to postgraduate training. Methods. A survey instrument was developed to sample 4 theoretical domains of subjective task value: intrinsic value, attainment value, utility value, and perceived cost. Retrospective self-report methodology was employed to examine respondents’ (N=1,148) subjective task value beliefs specific to their highest level of postgraduate training completed. Exploratory and confirmatory factor analytic techniques were used to evaluate and validate value belief constructs. Results. Intrinsic, attainment, utility, cost, and financial value constructs resulted from exploratory factor analysis. Cross-validation resulted in a 26-item instrument that demonstrated good model fit. Differences in value beliefs were noted across type of postgraduate training pursued and pharmacy training characteristics. Conclusions. The Postgraduate Training Value Instrument demonstrated evidence of reliability and construct validity. The survey instrument can be used to assess value beliefs regarding multiple postgraduate training options in pharmacy and potentially inform targeted recruiting of individuals to those paths best matching their own value beliefs

Stability of an MHD shear flow with a piecewise linear velocity profile

In this paper we present the results of the stability analysis of a simple shear flow of an incompressible fluid with a piecewise linear velocity profile in the presence of a magnetic field. In the flow, a finite transitional magnetic-free layer with a linear velocity profile is sandwiched by two semi-infinite regions. One of these regions is magnetic-free and the flow velocity in the region is constant. The other region is magnetic and the fluid in it is quiescent. The magnetic field is constant and parallel to the flow in the transitional layer. The fluid density is constant both in the magnetic as well as the magnetic-free regions, while it has a jump-type discontinuity at the boundary between the transitional layer and the magnetic region. The effect of gravity is included in the model, and it is assumed that the lighter fluid is overlaying the heavier one, thus no Rayleigh-Taylor instability is present. The dispersion equation governing the normal-mode stability of the flow is derived and its properties are analysed. We study stability of two cases: (i) magnetic-free flow in the presence of gravity, and (ii) magnetic flow without gravity. In the first case, the flow stability is controlled by the Rayleigh number, R. In the second case, the control parameter is the inverse squared Alfvénic Mach number, H . Stability of a particular monochromatic perturbation also depends on its dimensionless wavenumber α. We combine the analytical and numerical approaches to obtain the neutral stability curves in the (α,R)-plane in the case of the magnetic-free flow, and in the (α,H)-plane in the case of the magnetic flow. The dependence of the instability increment on R in the first case, and on H in the second case is treated. We apply the results of the analysis to the stability of a strongly subsonic portion of the heliopause. Our main conclusion is as follows: The inclusion of a transitional layer near the heliopause into the model increases by an order of magnitude the strength of the interstellar magnetic field required to stabilize this portion of the heliopause in comparison with the corresponding stabilizing strength of the magnetic field required when modelling the heliopause as a tangential discontinuity