2,138 research outputs found
The appearance, motion, and disappearance of three-dimensional magnetic null points
N.A.M. acknowledges support from NASA grants NNX11AB61G, NNX12AB25G, and NNX15AF43G; NASA contract NNM07AB07C; and NSF SHINE grants AGS-1156076 and AGS-1358342 to SAO. C.E.P. acknowledges support from the St Andrews 2013 STFC Consolidated grant.While theoretical models and simulations of magnetic reconnection often assume symmetry such that the magnetic null point when present is co-located with a flow stagnation point, the introduction of asymmetry typically leads to non-ideal flows across the null point. To understand this behavior, we present exact expressions for the motion of three-dimensional linear null points. The most general expression shows that linear null points move in the direction along which the magnetic field and its time derivative are antiparallel. Null point motion in resistive magnetohydrodynamics results from advection by the bulk plasma flow and resistive diffusion of the magnetic field, which allows non-ideal flows across topological boundaries. Null point motion is described intrinsically by parameters evaluated locally; however, global dynamics help set the local conditions at the null point. During a bifurcation of a degenerate null point into a null-null pair or the reverse, the instantaneous velocity of separation or convergence of the null-null pair will typically be infinite along the null space of the Jacobian matrix of the magnetic field, but with finite components in the directions orthogonal to the null space. Not all bifurcating null-null pairs are connected by a separator. Furthermore, except under special circumstances, there will not exist a straight line separator connecting a bifurcating null-null pair. The motion of separators cannot be described using solely local parameters because the identification of a particular field line as a separator may change as a result of non-ideal behavior elsewhere along the field line.Publisher PDFPeer reviewe
Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory
We examine whether the Mott transition of a half-filled, two-orbital Hubbard
model with unequal bandwidths occurs simultaneously for both bands or whether
it is a two-stage process in which the orbital with narrower bandwith localizes
first (giving rise to an intermediate `orbital-selective' Mott phase). This
question is addressed using both dynamical mean-field theory, and a
representation of fermion operators in terms of slave quantum spins, followed
by a mean-field approximation (similar in spirit to a Gutzwiller
approximation). In the latter approach, the Mott transition is found to be
orbital-selective for all values of the Coulomb exchange (Hund) coupling J when
the bandwidth ratio is small, and only beyond a critical value of J when the
bandwidth ratio is larger. Dynamical mean-field theory partially confirms these
findings, but the intermediate phase at J=0 is found to differ from a
conventional Mott insulator, with spectral weight extending down to arbitrary
low energy. Finally, the orbital-selective Mott phase is found, at
zero-temperature, to be unstable with respect to an inter-orbital
hybridization, and replaced by a state with a large effective mass (and a low
quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been
extende
Neutrino Fluxes from Active Galaxies: a Model-Independent Analysis
There are tantalizing hints that jets, powered by supermassive black holes at
the center of active galaxies, are true cosmic proton accelerators. They
produce photons of TeV energy, possible higher, and may be the enigmatic source
of the highest energy cosmic rays. Photoproduction of neutral pions by
accelerated protons on UV light is the source of the highest energy photons, in
which most of the bolometric luminosity of the galaxy may be emitted. The case
that proton beams power active galaxies is, however, far from conclusive.
Neutrinos from the decay of charged pions represent an uncontrovertible
signature for the proton induced cascades. We show that their flux can be
estimated by model-independent methods, based on dimensional analysis and
textbook particle physics. Our calculations also demonstrate why different
models for the proton blazar yield very similar results for the neutrino flux,
consistent with the ones obtained here.Comment: Latex 2.09 with epsf.sty. 12 pages, 2 postscript figures. Compressed
postscript version of paper with figures also available soon at
http://phenom.physics.wisc.edu/pub/preprints/1997/madph-97-982.ps.Z or at
ftp://phenom.physics.wisc.edu/pub/preprints/1997/madph-97-982.ps.
The Origin of Galactic Cosmic Rays
Motivated by recent measurements of the major components of the cosmic
radiation around 10 TeV/nucleon and above, we discuss the phenomenology of a
model in which there are two distinct kinds of cosmic ray accelerators in the
galaxy. Comparison of the spectra of hydrogen and helium up to 100 TeV per
nucleon suggests that these two elements do not have the same spectrum of
magnetic rigidity over this entire region and that these two dominant elements
therefore receive contributions from different sources.Comment: To be published in Physical Review D, 13 pages, with 3 figures,
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Cluster magnetic fields from large-scale-structure and galaxy-cluster shocks
The origin of the micro-Gauss magnetic fields in galaxy clusters is one of
the outstanding problem of modern cosmology. We have performed
three-dimensional particle-in-cell simulations of the nonrelativistic Weibel
instability in an electron-proton plasma, in conditions typical of cosmological
shocks. These simulations indicate that cluster fields could have been produced
by shocks propagating through the intergalactic medium during the formation of
large-scale structure or by shocks within the cluster. The strengths of the
shock-generated fields range from tens of nano-Gauss in the intercluster medium
to a few micro-Gauss inside galaxy clusters.Comment: 4 pages, 2 color figure
Renormalized spin coefficients in the accumulated orbital phase for unequal mass black hole binaries
We analyze galactic black hole mergers and their emitted gravitational waves.
Such mergers have typically unequal masses with mass ratio of the order 1/10.
The emitted gravitational waves carry the inprint of spins and mass quadrupoles
of the binary components. Among these contributions, we consider here the
quasi-precessional evolution of the spins. A method of taking into account
these third post-Newtonian (3PN) effects by renormalizing (redefining) the 1.5
PN and 2PN accurate spin contributions to the accumulated orbital phase is
developed.Comment: 10 pages, to appear in Class. Quantum Grav. GWDAW13 Proceedings
Special Issue, v2: no typos conjectur
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
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