14 research outputs found
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
The 2011 outburst of the recurrent novaT Pyx. Evidence for a face-on bipolar ejection
We report on near-IR interferometric observations of the outburst of the
recurrent nova T Pyx. We obtained near-IR observations of T Pyx at dates
ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC
recombiner, located at the CHARA array, and with the PIONIER and AMBER
recombiners, located at the VLTI array. These data are supplemented with
near-IR photometry and spectra obtained at Mount Abu, India. Slow expansion
velocities were measured (<300km/s) before t=20d (assuming D=3.5kpc). From
t=28d on, the AMBER and PIONIER continuum visibilities (K and H band,
respectively) are best simulated with a two component model consisting of an
unresolved source plus an extended source whose expansion velocity onto the sky
plane is lower than 700km/s. The expansion of the Brgamma line forming region,
as inferred at t=28d and t=35d is slightly larger, implying velocities in the
range 500-800km/s, still strikingly lower than the velocities of 1300-1600km/s
inferred from the Doppler width of the line. Moreover, a remarkable pattern was
observed in the Brgamma differential phases. A semi-quantitative model using a
bipolar flow with a contrast of 2 between the pole and equator velocities, an
inclination of i=15^{\circ} and a position angle P.A.=110^{\circ} provides a
good match to the AMBER observables (spectra, differential visibilities and
phases). At t=48d, a PIONIER dataset confirms the two component nature of the H
band emission, consisting of an unresolved stellar source and an extended
region whose appearance is circular and symmetric within error bars.These
observations are most simply interpreted within the frame of a bipolar model,
oriented nearly face-on. This finding has profound implications for the
interpretation of past, current and future observations of the expanding
nebula.Comment: Accepted Astronomy and Astrophysics (2011
Entropy Stable Finite Volume Approximations for Ideal Magnetohydrodynamics
This article serves as a summary outlining the mathematical entropy analysis of the ideal magnetohydrodynamic (MHD) equations. We select the ideal MHD equations as they are particularly useful for mathematically modeling a wide variety of magnetized fluids. In order to be self-contained we first motivate the physical properties of a magnetic fluid and how it should behave under the laws of thermodynamics. Next, we introduce a mathematical model built from hyperbolic partial differential equations (PDEs) that translate physical laws into mathematical equations. After an overview of the continuous analysis, we thoroughly describe the derivation of a numerical approximation of the ideal MHD system that remains consistent to the continuous thermodynamic principles. The derivation of the method and the theorems contained within serve as the bulk of the review article. We demonstrate that the derived numerical approximation retains the correct entropic properties of the continuous model and show its applicability to a variety of standard numerical test cases for MHD schemes. We close with our conclusions and a brief discussion on future work in the area of entropy consistent numerical methods and the modeling of plasmas
Neck pain patterns and subgrouping based on weekly SMS-derived trajectories
Background
Neck and low back pain represent dynamic conditions that change over time, often with an initial improvement after the onset of a new episode, followed by flare-ups or variations in intensity. Pain trajectories were previously defined based on longitudinal studies of temporal patterns and pain intensity of individuals with low back pain. In this study, we aimed to 1) investigate if the defined patterns and subgroups for low back pain were applicable to neck pain patients in chiropractic practice, 2) explore the robustness of the defined patterns, and 3) investigate if patients within the various patterns differ concerning characteristics and clinical findings.
Methods
Prospective cohort study including 1208 neck pain patients from chiropractic practice. Patients responded to weekly SMS-questions about pain intensity and frequency over 43 weeks. We categorized individual responses into four main patterns based on number of days with pain and variations in pain intensity, and subdivided each into four subgroups based on pain intensity, resulting in 16 trajectory subgroups. We compared baseline characteristics and clinical findings between patterns and between Persistent fluctuating and Episodic subgroups.
Results
All but two patients could be classified into one of the 16 subgroups, with 94% in the Persistent fluctuating or Episodic patterns. In the largest subgroup, “Mild Persistent fluctuating” (25%), mean (SD) pain intensity was 3.4 (0.6) and mean days with pain 130. Patients grouped as “Moderate Episodic” (24%) reported a mean pain intensity of 2.7 (0.6) and 39 days with pain. Eight of the 16 subgroups each contained less than 1% of the cohort. Patients in the Persistent fluctuating pattern scored higher than the other patterns in terms of reduced function and psychosocial factors.
Conclusions
The same subgroups seem to fit neck and low back pain patients, with pain that typically persists and varies in intensity or is episodic. Patients in a Persistent fluctuating pattern are more bothered by their pain than those in other patterns. The low back pain definitions can be used on patients with neck pain, but with the majority of patients classified into 8 subgroups, there seems to be a redundancy in the original model