On the Cause of Supra-Arcade Downflows in Solar Flares

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in August, 2013. This version is the accepted versio

No Radio Afterglow from the Gamma-Ray Burst of February 28, 1997

We present radio observations of the gamma-ray burster GRB 970228 made with the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO) spanning a range of postburst timescales from one to 300 days. A search for a time-variable radio source was conducted covering an area which included a fading X-ray source and an optical transient, both of which are thought to be the long wavelength counterparts to the gamma-ray burst. At the position of the optical transient sensitive limits between 10 uJy and 1 mJy can be placed on the absence of a radio counterpart to GRB 970228 between 1.4 and 240 GHz. We apply a simple formulation of a fireball model which has been used with some success to reproduce the behavior of the optical and X-ray light curves. Using this model we conclude that the radio non-detections are consistent with the peak flux density of the afterglow lying between 20-40 uJy and it requires that the optical flux peaked between 4 and 16 hours after the burst.Comment: ApJ Let (submitted

Open-source development experiences in scientific software: the HANDE quantum Monte Carlo project

The HANDE quantum Monte Carlo project offers accessible stochastic algorithms for general use for scientists in the field of quantum chemistry. HANDE is an ambitious and general high-performance code developed by a geographically-dispersed team with a variety of backgrounds in computational science. In the course of preparing a public, open-source release, we have taken this opportunity to step back and look at what we have done and what we hope to do in the future. We pay particular attention to development processes, the approach taken to train students joining the project, and how a flat hierarchical structure aids communicationComment: 6 pages. Submission to WSSSPE

The modulation of stationary waves, and their response to climate change, by parameterized orographic drag

The parameterization of orographic drag processes in atmospheric models remains uncertain due to a lack of observational and theoretical constraints on their formulation and free parameters. While previous studies have demonstrated that parameterized orographic drag acting near the surface has a significant impact on the atmospheric circulation, this work follows a more systematic approach to investigate its impacts on the large scale circulation and the circulation response to climate change. A set of experiments with a comprehensive atmospheric general circulation model is used to ascertain the range of climatological circulations that may arise from parameter uncertainty. It is found that the Northern Hemisphere (NH) wintertime stationary wave field is strongly damped over the North Pacific (NP) and amplified over the North Atlantic (NA) as a result of increased low-level parameterized orographic drag, both of which are shown to be conducive to higher-latitude westerlies. A comparison with the stationary wave field presented in other studies suggests that the too zonal NA jet and equatorward NP jet biases that are prevalent in climate models may be at least partly due to their representation of orographic drag. The amplitude of the stationary wave response to climate change across the experiments is shown to scale with the magnitude of low-level parameterized orographic drag through its influence on the present-day climatological stationary wave amplitudes over different sectors of the NH, which is consistent with linear stationary wave theory. This work highlights the importance of fidelity in a model's basic state for regional climate change projections

Influence of the quasi-biennial oscillation on the extratropical winter stratosphere in an atmospheric general circulation model and in reanalysis data

The interannual variability of the stratospheric polar vortex during winter in both hemispheres is observed to correlate strongly with the phase of the quasi-biennial oscillation (QBO) in tropical stratospheric winds. It follows that the lack of a spontaneously generated QBO in most atmospheric general circulation models (AGCMs) adversely affects the nature of polar variability in such models. This study examines QBO–vortex coupling in an AGCM in which a QBO is spontaneously induced by resolved and parameterized waves. The QBO–vortex coupling in the AGCM compares favorably to that seen in reanalysis data [from the 40-yr ECMWF Re-Analysis (ERA-40)], provided that careful attention is given to the definition of QBO phase. A phase angle representation of the QBO is employed that is based on the two leading empirical orthogonal functions of equatorial zonal wind vertical profiles. This yields a QBO phase that serves as a proxy for the vertical structure of equatorial winds over the whole depth of the stratosphere and thus provides a means of subsampling the data to select QBO phases with similar vertical profiles of equatorial zonal wind. Using this subsampling, it is found that the QBO phase that induces the strongest polar vortex response in early winter differs from that which induces the strongest late-winter vortex response. This is true in both hemispheres and for both the AGCM and ERA-40. It follows that the strength and timing of QBO influence on the vortex may be affected by the partial seasonal synchronization of QBO phase transitions that occurs both in observations and in the model. This provides a mechanism by which changes in the strength of QBO–vortex correlations may exhibit variability on decadal time scales. In the model, such behavior occurs in the absence of external forcings or interannual variations in sea surface temperatures

Sensitivity of simulated climate to conservation of momentum in gravity wave drag parameterization

The Canadian Middle Atmosphere Model is used to examine the sensitivity of simulated climate to conservation of momentum in gravity wave drag parameterization. Momentum conservation requires that the parameterized gravity wave momentum flux at the top of the model be zero and corresponds to the physical boundary condition of no momentum flux at the top of the atmosphere. Allowing momentum flux to escape the model domain violates momentum conservation. Here the impact of momentum conservation in two sets of model simulations is investigated. In the first set, the simulation of present-day climate for two model-lid height configurations, 0.001 and 10 hPa, which are identical below 10 hPa, is considered. The impact of momentum conservation on the climate with the model lid at 0.001 hPa is minimal, which is expected because of the small amount of gravity wave momentum flux reaching 0.001 hPa. When the lid is lowered to 10 hPa and momentum is conserved, there is only a modest impact on the climate in the Northern Hemisphere; however, the Southern Hemisphere climate is more adversely affected by the deflection of resolved waves near the model lid. When momentum is not conserved in the 10-hPa model the climate is further degraded in both hemispheres, particularly in winter at high latitudes, and the impact of momentum conservation extends all the way to the surface. In the second set of simulations, the impact of momentum conservation and model-lid height on the modeled response to ozone depletion in the Southern Hemisphere is considered, and it is found that the response can display significant sensitivity to both factors. In particular, both the lower-stratospheric polar temperature and surface responses are significantly altered when the lid is lowered, with the effect being most severe when momentum is not conserved. The implications with regard to the current round of Intergovernmental Panel on Climate Change model projections are discussed

Numerical Study of the Detonation Wave Structure in Ethylene-oxygen Mixtures

We examine a transition from a weakly to a highly unstable regime of a cellular detonation in stoichiometric ethylene-oxygen systems with varied dilution. The structure and propagation of cellular detonations is calculated using two-dimensional, time-dependent, reactive Euler fluid-dynamics algorithm. A dynamically adapting mesh is used to resolve reaction zones, shocks, contact surfaces, and vortices in flow. A simplified chemical model with Arrhenius kinetics is used for all mixtures. Effects of dilution are modeled by varying the adiabatic index γ and molecular weight of matter. Results are compared to experimental data obtained in a detonation tube by simultaneous visualization of a chemical species (OH), density gradients in the reaction zone, and to soot foil records. Due to a strong sensitivity of the post-shock temperature to variations of γ, the degree of chemical - fluid dynamics coupling inside the detonation structure varies significantly with dilution. Variations of the equation of state with dilution can account for and could be the main mechanism explaining a wide range of detonation behavior observed in the experiments

Direct observations of reaction zone structure in propagating detonations

We report experimental observations of the reaction zone structure of self-sustaining, cellular detonations propagating near the Chapman-Jouguet state in hydrogen-oxygen-argon/nitrogen mixtures. Two-dimensional cross sections perpendicular to the propagation direction were imaged using the technique of planar laser induced fluorescence (PLIF) and, in some cases, compared to simultaneously acquired schlieren images. Images are obtained which clearly show the nature of the disturbances in an intermediate chemical species (OH) created by the variations in the strength of the leading shock front associated with the transverse wave instability of a propagating detonation. The images are compared to 2-D, unsteady simulations with a reduced model of the chemical reaction processes in the hydrogen-oxygen-argon system. We interpret the experimental and numerical images using simple models of the detonation front structure based on the “weak” version of the flow near the triple point or intersection of three shock waves, two of which make up the shock front and the third corresponding to the wave propagating transversely to the front. Both the unsteady simulations and the triple point calculations are consistent with the creation of keystone-shaped regions of low reactivity behind the incident shock near the end of the oscillation cycle within the “cell.

Health-Related Quality of Life in patients with chronic hepatitis C receiving Sofosbuvir-Based Treatment, with and without Interferon: a prospective observational study in Egypt

Background: The Egyptian government introduced the first directly acting antivirals (DAAs) into Egypt through the government funded National Treatment Program. As yet, there has been no investigation into the effects of these new DAAs therapies on patient reported outcomes (PROs). This study aimed to (1) assess the PROs (health-related quality of life (HRQoL), mental health and perceived social support) of HCV patients receiving DAAs therapy prior, during and at the end of therapy; (2) evaluate PROs of Interferon-free (dual) users versus Interferon-containing (triple) users cross the three different time periods; and (3) identify the predictors of HRQoL of DAAs therapy users cross the three different time periods.  Methods: A prospective observational design was used. Patients with chronic HCV undergoing treatment following the Egyptian National Guidelines at one of the national treatment centers were approached. Data collection occurred in the period from February to October 2015. Data was collected at three time points: (1) baseline (time 0: T0), before initiating therapy); (2) 5/6 weeks after initiation of therapy (time 1 of therapy: T1) and at the end of the therapy (Time 2: T2). Four PROs questionnaires were utilized for data collection: (1) Multidimensional Scale of Perceived Social Support (MSPSS), (2) The Depression Anxiety Stress Scales (DASS-21), (3) the Liver Disease Symptom Index-2.0 (LDSI-2.0) for testing disease specific HRQoL and (4) the Center for Adherence Support Evaluation (CASE) Index, alongside the background data sheet.  Results: Sixty-two patients participated. There was a change in HRQoL, symptom experience and mental health across the three different time periods. HRQoL was impaired more after starting the course of therapy (T1) than at baseline (T0) and end of therapy (T2), z ≥ -2.04, p ≤ .04. Also, symptom experience deteriorated more during the treatment period than at the baseline, Z ≥ -1.97, p ≤ .04. Anxiety and stress were significantly higher during the treatment period than at the end of treatment. Perceived social support was significantly higher during the treatment period than at baseline and end of therapy, Z ≥ -2.27, p ≤.023. During the course of therapy, triple users were more likely to report poorer HRQoL and anxiety than dual users (p ≤ .04). By the end of therapy, the two arms of therapy had no significant differences in any of the PROs.  At baseline, the predictor model significantly (p=.000) explained 37.5% of the variation in the HRQoL prior to therapy. Depression was the main variable that contributed to (41.3%) predicting change in HRQoL prior to therapy. During therapy, the model significantly (p=.000) explained 76% of the variation in the HRQoL-T1. Stress-T1, body mass index (BMI)-T1 and HRQoL-T0 significantly and respectively predicted 44.4%, 46.5% and 31.1% of the variation in HRQoL-T1. At the end of therapy, the model significantly (p=.000) predicted 80.5% of the variation in the HRQoL-T2. HRQoL-T1 and anxiety-T2 significantly predicted 72.3% and 61.6% of the variation in HRQoL-T2.  Conclusions: Baseline HRQoL, depression and BMI should be systematically assessed before starting the antiviral therapy for early detection and the improvement of the impairment before the initiation of therapy. Anxiety should be frequently assessed and followed up through the course of antiviral therapy. The triple group required more nursing and practitioner attention due to increased anxiety levels and impaired HRQoL during the treatment therapy