18 research outputs found
Long-term variations of turbulent transport coefficients in a solar-like convective dynamo simulation
The Sun, aside from its eleven year sunspot cycle is additionally subject to
long term variation in its activity. In this work we analyse a solar-like
convective dynamo simulation, containing approximately 60 magnetic cycles,
exhibiting equatorward propagation of the magnetic field, multiple frequencies,
and irregular variability, including a missed cycle and complex parity
transitions between dipolar and quadrupolar modes. We compute the turbulent
transport coefficients, describing the effects of the turbulent velocity field
on the mean magnetic field, using the test-field method. The test-field
analysis provides a plausible explanation of the missing cycle in terms of the
reduction of in advance of the reduced surface activity,
and enhanced downward turbulent pumping during the event to confine some of the
magnetic field at the bottom of the convection zone, where local maximum of
magnetic energy is observed during the event. At the same time, however, a
quenching of the turbulent magnetic diffusivities is observed, albeit
differently distributed in depth compared to the other transport coefficients.
Therefore, dedicated mean-field modelling is required for verification.Comment: 11 pages, 12 figures, accepted by AN for 14th Potsdam Thinksho
Separating the scales in a compressible interstellar medium
We apply Gaussian smoothing to obtain mean density, velocity, magnetic and
energy density fields in simulations of the interstellar medium based on
three-dimensional magnetohydrodynamic equations in a shearing box
in size. Unlike alternative averaging procedures,
such as horizontal averaging, Gaussian smoothing retains the three-dimensional
structure of the mean fields. Although Gaussian smoothing does not obey the
Reynolds rules of averaging, physically meaningful central statistical moments
are defined as suggested by Germano (1992). We discuss methods to identify an
optimal smoothing scale and the effects of this choice on the results.
From spectral analysis of the magnetic, density and velocity fields, we find a
suitable smoothing length for all three fields, of . We discuss the properties of third-order statistical moments in
fluctuations of kinetic energy density in compressible flows and suggest their
physical interpretation. The mean magnetic field, amplified by a mean-field
dynamo, significantly alters the distribution of kinetic energy in space and
between scales, reducing the magnitude of kinetic energy at intermediate
scales. This intermediate-scale kinetic energy is a useful diagnostic of the
importance of SN-driven outflows
The small-scale dynamo in a multiphase supernova-driven medium
Magnetic fields grow quickly, even at early cosmological times, suggesting
the action of a small-scale dynamo (SSD) in the interstellar medium (ISM) of
galaxies. Many studies have focused on idealized, isotropic, homogeneous,
turbulent driving of the SSD. Here we analyze more realistic simulations of
supernova-driven turbulence to understand how it drives an SSD. We find that
SSD growth rates are intermittently variable as a result of the evolving
multiphase ISM structure. Rapid growth in the magnetic field typically occurs
in hot gas, with the highest overall growth rates occurring when the fractional
volume of hot gas is large. SSD growth rates correlate most strongly with
vorticity, which also correlates well with gas temperature. Rotational energy
exceeds irrotational energy in all phases, but particularly in the hot phase
while SSD growth is most rapid. Supernova (SN) rate does not significantly
affect the ISM average kinetic energy density. Rather, higher temperatures
associated with high SN rates tend to increase SSD growth rates. SSD saturates
with total magnetic energy density around 5% of equipartition to kinetic energy
density, increasing slightly with magnetic Prandtl number. While magnetic
energy density in the hot gas can exceed that of the other phases when SSD
grows most rapidly, it saturates below 5% of equipartition with kinetic energy
in the hot gas, while in the cold gas it attains 100%. Fast, intermittent
growth of the magnetic field appears to be a characteristic behavior of
SN-driven, multiphase turbulence.Comment: 27 pages, 12 figures, 4 table
Galaxies in box: A simulated view of the interstellar medium
We review progress in the development of physically realistic three
dimensional simulated models of the galaxy.We consider the scales from star
forming molecular clouds to the full spiral disc. Models are computed using
hydrodynamic (HD) or magnetohydrodynamic (MHD) equations and may include cosmic
ray or tracer particles. The range of dynamical scales between the full galaxy
structure and the turbulent scales of supernova (SN) explosions and even cloud
collapse to form stars, make it impossible with current computing tools and
resources to resolve all of these in one model. We therefore consider a
hierarchy of models and how they can be related to enhance our understanding of
the complete galaxy.Comment: Chapter in Large Scale Magnetic Fields in the Univers
Effects of Anacetrapib in Patients with Atherosclerotic Vascular Disease
BACKGROUND:
Patients with atherosclerotic vascular disease remain at high risk for cardiovascular events despite effective statin-based treatment of low-density lipoprotein (LDL) cholesterol levels. The inhibition of cholesteryl ester transfer protein (CETP) by anacetrapib reduces LDL cholesterol levels and increases high-density lipoprotein (HDL) cholesterol levels. However, trials of other CETP inhibitors have shown neutral or adverse effects on cardiovascular outcomes.
METHODS:
We conducted a randomized, double-blind, placebo-controlled trial involving 30,449 adults with atherosclerotic vascular disease who were receiving intensive atorvastatin therapy and who had a mean LDL cholesterol level of 61 mg per deciliter (1.58 mmol per liter), a mean non-HDL cholesterol level of 92 mg per deciliter (2.38 mmol per liter), and a mean HDL cholesterol level of 40 mg per deciliter (1.03 mmol per liter). The patients were assigned to receive either 100 mg of anacetrapib once daily (15,225 patients) or matching placebo (15,224 patients). The primary outcome was the first major coronary event, a composite of coronary death, myocardial infarction, or coronary revascularization.
RESULTS:
During the median follow-up period of 4.1 years, the primary outcome occurred in significantly fewer patients in the anacetrapib group than in the placebo group (1640 of 15,225 patients [10.8%] vs. 1803 of 15,224 patients [11.8%]; rate ratio, 0.91; 95% confidence interval, 0.85 to 0.97; P=0.004). The relative difference in risk was similar across multiple prespecified subgroups. At the trial midpoint, the mean level of HDL cholesterol was higher by 43 mg per deciliter (1.12 mmol per liter) in the anacetrapib group than in the placebo group (a relative difference of 104%), and the mean level of non-HDL cholesterol was lower by 17 mg per deciliter (0.44 mmol per liter), a relative difference of -18%. There were no significant between-group differences in the risk of death, cancer, or other serious adverse events.
CONCLUSIONS:
Among patients with atherosclerotic vascular disease who were receiving intensive statin therapy, the use of anacetrapib resulted in a lower incidence of major coronary events than the use of placebo. (Funded by Merck and others; Current Controlled Trials number, ISRCTN48678192 ; ClinicalTrials.gov number, NCT01252953 ; and EudraCT number, 2010-023467-18 .)
Supernova induced processing of interstellar dust: impact of ISM gas density and gas turbulence
| openaire: EC/H2020/818665/EU//UniSDynQuantifying the efficiency of dust destruction in the interstellar medium (ISM) due to supernovae (SNe) is crucial for the understanding of galactic dust evolution. We present 3D hydrodynamic simulations of an SN blast wave propagating through the ISM. The interaction between the forward shock of the remnant and the surrounding ISM leads to destruction of ISM dust by the shock heated gas. We consider the dust processing due to ion sputtering, accretion of atoms/molecules and grain-grain collisions. Using 2D slices from the simulation timeseries, we apply post-processing calculations using the Paperboats code. We find that efficiency of dust destruction depends strongly on the rate of grain shattering due to grain-grain collisions. The effective dust destruction is similar to previous theoretical estimates when grain-grain collisions are omitted, but with grain shattering included, the net destruction efficiency is roughly one order of magnitude higher. This result indicates that the dust destruction rate in the ISM may have been severely underestimated in previous work, which only exacerbates the dust-budget crises seen in galaxies at high redshifts.Peer reviewe
Supernova dust destruction in the magnetized turbulent ISM
Abstract Dust in the interstellar medium (ISM) is critical to the absorption and intensity of emission profiles used widely in astronomical observations, and necessary for star and planet formation. Supernovae (SNe) both produce and destroy ISM dust. In particular the destruction rate is difficult to assess. Theory and prior simulations of dust processing by SNe in a uniform ISM predict quite high rates of dust destruction, potentially higher than the supernova dust production rate in some cases. Here we show simulations of supernova-induced dust processing with realistic ISM dynamics including magnetic field effects and demonstrate how ISM inhomogeneity and magnetic fields inhibit dust destruction. Compared to the non-magnetic homogeneous case, the dust mass destroyed within 1 Myr per SNe is reduced by more than a factor of two, which can have a great impact on the ISM dust budget
Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers
Funding Information: We acknowledge fruitful discussions with A. Brandenburg, I. Rogachevskii, A. Schekochihin and J. Schober during the Nordita programme on ‘Magnetic field evolution in low density or strongly stratified plasmas’. Computing resources from CSC during the Mahti pilot project and from Max Planck Computing and Data Facility (MPCDF) are gratefully acknowledged. This project, including all authors, has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Project UniSDyn, grant agreement number 818665). This work was done in collaboration with the COFFIES DRIVE Science Center. | openaire: EC/H2020/818665/EU//UniSDynMagnetic fields on small scales are ubiquitous in the Universe. Although they can often be observed in detail, their generation mechanisms are not fully understood. One possibility is the so-called small-scale dynamo (SSD). Prevailing numerical evidence, however, appears to indicate that an SSD is unlikely to exist at very low magnetic Prandtl numbers (PrM) such as those that are present in the Sun and other cool stars. Here we have performed high-resolution simulations of isothermal forced turbulence using the lowest PrM values achieved so far. Contrary to earlier findings, the SSD not only turns out to be possible for PrM down to 0.0031 but also becomes increasingly easier to excite for PrM below about 0.05. We relate this behaviour to the known hydrodynamic phenomenon referred to as the bottleneck effect. Extrapolating our results to solar values of PrM indicates that an SSD would be possible under such conditions.Peer reviewe
Steady states of the Parker instability: the effects of rotation
We model the Parker instability in vertically stratified isothermal gas using
non-ideal MHD three-dimensional simulations. Rotation, especially differential,
more strongly and diversely affects the nonlinear state than the linear stage
(where we confirm the most important conclusions of analytical models), and
stronger than any linear analyses predict. Steady state magnetic fields are
stronger and cosmic ray energy density higher than in comparable nonrotating
systems. Transient gas outflows induced by the nonlinear instability persist
longer, of order 2 Gyr, with rotation. Stratification combined with
(differential) rotation drives helical flows, leading to mean-field dynamo.
Consequently, the nonlinear state becomes oscillatory (while both the linear
instability and the dynamo are non-oscillatory). The horizontal magnetic field
near the midplane reverses its direction propagating to higher altitudes as the
reversed field spreads buoyantly. The spatial pattern of the large-scale
magnetic field may explain the alternating magnetic field directions in the
halo of the edge-on galaxy NGC 4631. Our model is unique in producing a
large-scale magnetic structure similar to such observation. Furthermore, our
simulations show that the mean kinetic helicity of the magnetically driven
flows has the sign opposite to that in the conventional non-magnetic flows.
This has profound consequences for the nature of the dynamo action and
large-scale magnetic field structure in the coronae of spiral galaxies which
remain to be systematically explored and understood. We show that the energy
density of cosmic rays and magnetic field strength are not correlated at scales
of order a kiloparsec