Unstable coronal loops : numerical simulations with predicted observational signatures

We present numerical studies of the nonlinear, resistive magnetohydrodynamic (MHD) evolution of coronal loops. For these simulations we assume that the loops carry no net current, as might be expected if the loop had evolved due to vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical flux tube with line-tied ends. For a given amount of twist in the magnetic field it is well known that once such a loop exceeds a critical length it becomes unstableto ideal MHD instabilities. The early evolution of these instabilities generates large current concentrations. Firstly we show that these current concentrations are consistent with the formation of a current sheet. Magnetic reconnection can only occur in the vicinity of these current concentrations and we therefore couple the resistivity to the local current density. This has the advantage of avoiding resistive diffusion in regions where it should be negligible. We demonstrate the importance of this procedure by comparison with simulations based on a uniform resistivity. From our numerical experiments we are able to estimate some observational signatures for unstable coronal loops. These signatures include: the timescale of the loop brightening; the temperature increase; the energy released and the predicted observable flow speeds. Finally we discuss to what extent these observational signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure

The two-dimensional magnetohydrodynamic Kelvin-Helmholtz instability: compressibility and large-scale coalescence effects

The Kelvin-Helmholtz (KH) instability occurring in a single shear flow configuration that is embedded in a uniform flow-aligned magnetic field, is revisited by means of high resolution two-dimensional (2D) magnetohydrodynamic (MHD) simulations. First, the calculations extend previous studies of magnetized shear flows to a higher compressibility regime. The nonlinear evolution of an isolated KH billow emerging from the fastest growing linear mode for a convective sonic Mach number $M_{cs}=0.7$ layer is in many respects similar to its less compressible counterpart (Mach $M_{cs}=0.5$). In particular, the disruptive regime where locally amplified, initially weak magnetic fields, control the nonlinear saturation process is found for Alfv\'en Mach numbers 4\simlt M_A \simlt 30. The most notable difference between $M_{cs}=0.7$ versus $M_{cs}=0.5$ layers is that higher density contrasts and fast magnetosonic shocklet structures are observed. Second, the use of adaptive mesh refinement allows to parametrically explore much larger computational domains, including up to 22 wavelengths of the linearly dominant mode. A strong process of large-scale coalescence is found, whatever the magnetic field regime. It proceeds through continuous pairing/merging events between adjacent vortices up to the point where the final large-scale vortical structure reaches the domain dimensions. This pairing/merging process is attributed to the growth of subharmonic modes and is mainly controlled by relative phase differences between them. These grid-adaptive simulations demonstrate that even in very weak magnetic field regimes ($M_A \simeq 30$), the large-scale KH coalescence process can trigger tearing-type reconnection events previously identified in cospatial current-vortex sheets.Comment: Published in Physics of Plasmas, figures absent due to file sizes, full version at http://www.phys.uu.nl/~toth/ (follow Publications

Efficacy of non-invasive intelligent volume assured pressure support (iVAPS) and pressure support ventilation (PSV) in clinical practice

Aims of the study: Noninvasive ventilation (NIV) is a well-established treatment option for hypercapnic respiratory failure; however, the best mode of ventilation remains unknown. The aim of this retrospective study was to compare patients’ adherence to NIV using either pressure support ventilation (PSV) or intelligent volume-assured pressure support (iVAPS). Patients and methods: In this retrospective cohort study, we assessed in- and outpatients suffering from hypercapnic respiratory failure of various aetiologies (chronic obstructive pulmonary disease [COPD], obese COPD [body mass index >30 kg/m2], obesity hypoventilation syndrome and other diseases such as amyotrophic lateral sclerosis or interstitial lung disease) after NIV initiation with PSV or iVAPS. Adherence to treatment was compared between these modes using the Wilcoxon test. Within-group differences were tested using linear regression models. Mortality and emergency hospital readmission rates were modelled using Kaplan-Meier estimates and Cox proportional hazards models. Results: Adherence to treatment was similar in both groups throughout the observation period – after 6 weeks: PSV 363 min/night (interquartile range [IQR] 200–448), iVAPS 369 min/night (IQR 310–468) (p = 0.619); after 1 year: PSV 423 (323–500), iVAPS 429 (298–475) (p = 0.901); at the last follow up: PSV 481 (395–586), iVAPS 426 (391–565) (p = 0.284). NIV reduced PaCO2 significantly compared with baseline at all follow-ups: PSV −1.29/−1.49/−1.49 kPa, iVAPS −1.47/−1.23/−1.24 kPa, p <0.001 each, PSV vs iVAPS: p = 0.250, 0.756 and 0.352, respectively. Median survival time (PSV 5.06 years, iVAPS median not reached; p = 0.800) and time to first readmission (PSV 3.6 years, iVAPS 7.33 years, p = 0.200) did not differ between groups. Obese COPD patients had a longer time to hospital readmission than lean COPD patients (3.8 vs 1.5 years, hazard ratio (HR) 0.39, 95% confidence interval [CI] 0.16–0. 74; p = 0.007). Good adherence (>4 h/night and >80% nightly usage) was associated with a lower mortality rate (HR 0.34, 95% CI 0.15–0.77; p = 0.010). Conclusion: In a real-world setting of a mixed population with hypercapnic respiratory failure, iVAPS and PSV seem to be similarly effective in improving gas exchange and demonstrate excellent adherence to treatment. A longer survival was noted in NIV-adherent patients. Randomised controlled studies are necessary to identify patients who might benefit more from hybrid ventilation modes

Current driven rotating kink mode in a plasma column with a non-line-tied free end

First experimental measurements are presented for the kink instability in a linear plasma column which is insulated from an axial boundary by finite sheath resistivity. Instability threshold below the classical Kruskal-Shafranov threshold, axially asymmetric mode structure and rotation are observed. These are accurately reproduced by a recent kink theory, which includes axial plasma flow and one end of the plasma column that is free to move due to a non-line-tied boundary condition.Comment: 4 pages, 6 figure

Conservation of the Illinois flora: A climate change vulnerability assessment of 73 plant species

It will be important for land managers, ecological researchers and policymakers to understand how predicted climate changes may affect the flora of Illinois. A climate change vulnerability assessment was completed in 2011 for the162 Animal Species in Greatest Need of Conservation using NatureServe’s Climate Change Vulnerability Index (CCVI) tool. Here we selected 73plant species found in Illinois and calculated their relative vulnerabilities to predicted climate changes, also using the NatureServe CCVI tool. We selected species from several groups that we felt would be broadly representative of the Illinois flora. These groups included: rare plants, invasive plants, important prairie species, important woodland/savanna species, important forest species, and plants important to society. We compiled and entered data regarding both the species’ exposures to predicted climate changes and their sensitivities to those changes. Exposures were determined by overlapping species range maps for Illinois with maps of temperature and moisture (AET:PET) predictions for the middle of this century. Species sensitivities were determined by interviewing between 4 and 12 experts for each plant species. Experts answered questions found in the CCVI tool regarding the species’ biologies, ecologies and behaviors. Results for each individual survey were averaged for each species. Results fell into one of five vulnerability categories: Extremely Vulnerable, Highly Vulnerable, Moderately Vulnerable, Not Vulnerable/Presumed Stable, and Not Vulnerable/Increase Likely. Results for these 73species in Illinois fell into all 5 vulnerability categories, with the majority (67%) falling into the Presumed Stable category. The species most vulnerable to predicted climate changes were all of conservation concern; most were federal or state listed species. Native species tended to be more vulnerable than non-natives, and plants important to prairies, savannas and forests were equally vulnerable to predicted changes. The four species were ranked as likely to increase in population size or range extent due to predicted climate changes were: Ailanthus altissima(tree of heaven), Ambrosia artemisiifolia(ragweed), Microstegium vimineum(Japanese stiltgrass) and Toxicodendron radicans(poison ivy). We advocate for 8 important next-steps to ensure adequate conservation of Illinois plants in a future with climate change, based on the findings of this report: 1) Investigate the climate change vulnerabilities of all rare plants in Illinois. Rare plants are the most vulnerable group of species in this report. 2) Prioritize research on plants’ abilities to phenologically track changes in seasonality, population genetics, species interactions, dispersal distances, thermal and hydrological tolerancesand soil preferences/tolerances. 3) Continue to monitor population trends. Increase capacity to monitor species with the most uncertain responses to climate changes, the most vulnerable species, and all rare species. 4) Monitor invasive speciesfor changes in populations and behavior. Assess all invasive species using NatureServe’s CCVI tool or another tool. 5) Increase connectivity between natural areas. Increase acreage of natural areas. 6) Managers, policymakers, researchers and the public shouldwork together to fully consider the role that assisted migration should or should not play in Illinois plant conservation. 7) Compile work done by various agencies and NGOs on the climate change vulnerabilities of Illinois species and ecosystems to detect trends, and to identify appropriate research, management and policy priorities. 8) Use adaptive management approaches to care for natural areas in Illinois in order to best achieve land management goals in an uncertain future.Illinois Natural History SurveyIllinois Department of Natural ResourcesIllinois Department of Transportationunpublishednot peer reviewedOpe

Impact of laughter on air trapping in severe chronic obstructive lung disease

Static and dynamic hyperinflation is an important factor of exertional dyspnea in patients with severe COPD. This proof-of-concept intervention trial sought to study whether laughter can reduce hyperinflation through repetitive expiratory efforts in patients with severe COPD. For small groups of patients with severe COPD (n = 19) and healthy controls (n = 10) Pello the clown performed a humor intervention triggering regular laughter. Plethysmography was done before and up to 24 hours after intervention. Laughing and smiling were quantified with video-analysis. Real-time breathing pattern was assessed with the LifeShirt™, and the psychological impact of the intervention was monitored with self-administered questionnaires. The intervention led to a reduction of TLC in COPD (p = 0.04), but not in controls (p = 0.9). TLC reduction was due to a decline of the residual volume. Four (22 [CI 95% 7 to 46] %) patients were ≥10% responders. The frequency of smiling and TLC at baseline were independent predictors of TLC response. The humor intervention improved cheerfulness, but not seriousness nor bad mood. In conclusion, smiling induced by a humor intervention was able to reduce hyperinflation in patients with severe COPD. A smiling-derived breathing technique might complement pursed-lips breathing in patients with symptomatic obstruction

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 $t_c$ is that the peak current density behaves like $|J|_\infty \sim 1/(t_c-t)$ 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