5,088 research outputs found
Consequences of spontaneous reconnection at a two-dimensional non-force-free current layer
Magnetic neutral points, where the magnitude of the magnetic field vanishes
locally, are potential locations for energy conversion in the solar corona. The
fact that the magnetic field is identically zero at these points suggests that
for the study of current sheet formation and of any subsequent resistive
dissipation phase, a finite beta plasma should be considered, rather than
neglecting the plasma pressure as has often been the case in the past. The
rapid dissipation of a finite current layer in non-force-free equilibrium is
investigated numerically, after the sudden onset of an anomalous resistivity.
The aim of this study is to determine how the energy is redistributed during
the initial diffusion phase, and what is the nature of the outward transmission
of information and energy. The resistivity rapidly diffuses the current at the
null point. The presence of a plasma pressure allows the vast majority of the
free energy to be transferred into internal energy. Most of the converted
energy is used in direct heating of the surrounding plasma, and only about 3%
is converted into kinetic energy, causing a perturbation in the magnetic field
and the plasma which propagates away from the null at the local fast
magnetoacoustic speed. The propagating pulses show a complex structure due to
the highly non-uniform initial state. It is shown that this perturbation
carries no net current as it propagates away from the null. The fact that,
under the assumptions taken in this paper, most of the magnetic energy released
in the reconnection converts internal energy of the plasma, may be highly
important for the chromospheric and coronal heating problem
Shock-Wave Heating Model for Chondrule Formation: Prevention of Isotopic Fractionation
Chondrules are considered to have much information on dust particles and
processes in the solar nebula. It is naturally expected that protoplanetary
disks observed in present star forming regions have similar dust particles and
processes, so study of chondrule formation may provide us great information on
the formation of the planetary systems.
Evaporation during chondrule melting may have resulted in depletion of
volatile elements in chondrules. However, no evidence for a large degree of
heavy-isotope enrichment has been reported in chondrules. In order to meet this
observed constraint, the rapid heating rate at temperatures below the silicate
solidus is required to suppress the isotopic fractionation.
We have developed a new shock-wave heating model taking into account the
radiative transfer of the dust thermal continuum emission and the line emission
of gas molecules and calculated the thermal history of chondrules. We have
found that optically-thin shock waves for the thermal continuum emission from
dust particles can meet the rapid heating constraint, because the dust thermal
emission does not keep the dust particles high temperature for a long time in
the pre-shock region and dust particles are abruptly heated by the gas drag
heating in the post-shock region. We have also derived the upper limit of
optical depth of the pre-shock region using the radiative diffusion
approximation, above which the rapid heating constraint is not satisfied. It is
about 1 - 10.Comment: 58 pages, including 5 tables and 15 figures, accepted for publication
in The Astrophysical Journa
Identification of a novel retroviral gene unique to human immunodeficiency virus type 2 and simian immunodeficiency virus SIVMAC
Human and simian immunodeficiency-associated retroviruses are extraordinarily complex, containing at least five genes, tat, art, sor, R, and 3' orf, in addition to the structural genes gag, pol, and env. Recently, nucleotide sequence analysis of human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus SIVMAC revealed the existence of still another open reading frame, termed X, which is highly conserved between these two viruses but absent from HIV-1. In this report, we demonstrate for the first time that the X open reading frame represents a functional retroviral gene in both HIV-2 and SIVMAC and that it encodes a virion-associated protein of 14 and 12 kilodaltons, respectively. We also describe the production of recombinant TrpE/X fusion proteins in Escherichia coli and show that sera from some HIV-2-infected individuals specifically recognize these proteins
Ambipolar diffusion : self-similar solutions and MHD code testing. Cylindrical symmetry
Funding: This research has been supported by the Spanish Ministry of Science, Innovation and Universities through projects AYA2014-55078-P and PGC2018-095832-B-I00. The authors are also grateful to the European Research Council for support through the Synergy Grant number 810218 (ERC-2018-SyG). DNS acknowledges support by the Research Council of Norway through its Centres of Excellence scheme, project number 262622, and through grants of computing time from the Programme for Supercomputing. AWH gratefully acknowledges the financial support of STFC through the Consolidated grant, ST/S000402/1, to the University of St Andrews.Ambipolar diffusion is a process occurring in partially ionised astrophysical systems that imparts a complicated mathematical and physical nature to Ohm's law. The numerical codes that solve the magnetohydrodynamic (MHD) equations have to be able to deal with the singularities that are naturally created in the system by the ambipolar diffusion term. The global aim is to calculate a set of theoretical self-similar solutions to the nonlinear diffusion equation with cylindrical symmetry that can be used as tests for MHD codes which include the ambipolar diffusion term. First, following the general methods developed in the applied mathematics literature, we obtained the theoretical solutions as eigenfunctions of a nonlinear ordinary differential equation. Phase-plane techniques were used to integrate through the singularities at the locations of the nulls, which correspond to infinitely sharp current sheets. In the second half of the paper, we consider the use of these solutions as tests for MHD codes. To that end, we used the Bifrost code, thereby testing the capabilities of these solutions as tests as well as (inversely) the accuracy of Bifrost's recently developed ambipolar diffusion module. The obtained solutions are shown to constitute a demanding, but nonetheless viable, test for MHD codes that incorporate ambipolar diffusion. The Bifrost code is able to reproduce the theoretical solutions with sufficient accuracy up to very advanced diffusive times. Using the code, we also explored the asymptotic properties of our theoretical solutions in time when initially perturbed with either small or finite perturbations. The functions obtained in this paper are relevant as physical solutions and also as tests for general MHD codes. They provide a more stringent and general test than the simple Zeldovich-Kompaneets-Barenblatt-Pattle solution.PostprintPeer reviewe
Profits, Commercial Food Supplier Involvement, and School Vending Machine Snack Food Availability: Implications for Implementing the New Competitive Foods Rule
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/107388/1/josh12165.pd
Physical Activity Breaks and Facilities in US Secondary Schools
BACKGROUND Research on physical activity breaks and facilities (indoor and outdoor) in secondary schools is relatively limited. METHODS School administrators and students in nationally representative samples of 8th (middle school) and 10th/12th grade (high school) students were surveyed annually from 2008‐2009 to 2011‐2012. School administrators reported information about physical activity breaks and facilities. Students self‐reported height, weight, and physical activity. RESULTS The prevalence of physical activity breaks and indoor and outdoor facilities (dichotomized by median split) differed significantly by region of the country, school size, student race/ethnicity, and school socioeconomic status (SES). Breaks were associated with lower odds of overweight (adjusted odds ratio [AOR] = 0.91, 95% confidence interval [CI]: 0.83‐1.00) and obesity (AOR = 0.86, 95% CI: 0.75‐0.99) among middle school students. Among low‐SES middle school students and schools, higher indoor facilities were associated with lower rates of overweight and obesity. Among high school students, higher indoor and outdoor facilities were associated with 19‐42% higher odds of moderate‐to‐vigorous physical activity. CONCLUSIONS Physical activity breaks and school facilities may help to address high rates of overweight/obesity and low physical activity levels among secondary students, especially lower‐SES students. Students in all schools should have equal access to these resources.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108694/1/josh12206.pd
Optimization of inhomogeneous electron correlation factors in periodic solids
A method is presented for the optimization of one-body and inhomogeneous
two-body terms in correlated electronic wave functions of Jastrow-Slater type.
The most general form of inhomogeneous correlation term which is compatible
with crystal symmetry is used and the energy is minimized with respect to all
parameters using a rapidly convergent iterative approach, based on Monte Carlo
sampling of the energy and fitting energy fluctuations. The energy minimization
is performed exactly within statistical sampling error for the energy
derivatives and the resulting one- and two-body terms of the wave function are
found to be well-determined. The largest calculations performed require the
optimization of over 3000 parameters. The inhomogeneous two-electron
correlation terms are calculated for diamond and rhombohedral graphite. The
optimal terms in diamond are found to be approximately homogeneous and
isotropic over all ranges of electron separation, but exhibit some
inhomogeneity at short- and intermediate-range, whereas those in graphite are
found to be homogeneous at short-range, but inhomogeneous and anisotropic at
intermediate- and long-range electron separation.Comment: 23 pages, 15 figures, 1 table, REVTeX4, submitted to PR
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