4,512 research outputs found
Collisionless reconnection: The sub-microscale mechanism of magnetic field line interaction
Magnetic field lines are quantum objects carrying one quantum
of magnetic flux and have finite radius . Here
we argue that they possess a very specific dynamical interaction. Parallel
field lines reject each other. When confined to a certain area they form
two-dimensional lattices of hexagonal structure. We estimate the filling factor
of such an area. Antiparallel field lines, on the other hand, attract each
other. We identify the physical mechanism as being due to the action of the
gauge potential field which we determine quantum mechanically for two parallel
and two antiparallel field lines. The distortion of the quantum electrodynamic
vacuum causes a cloud of virtual pairs. We calculate the virtual pair
production rate from quantum electrodynamics and estimate the virtual pair
cloud density, pair current and Lorentz force density acting on the field lines
via the pair cloud. These properties of field line dynamics become important in
collisionless reconnection, consistently explaining why and how reconnection
can spontaneously set on in the field-free centre of a current sheet below the
electron-inertial scale.Comment: 13 journal pages, 6 figures, submitted to Ann. Geophy
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
Motion in a Bose condensate: IX. Crow instability of antiparallel vortex pairs
The Gross-Pitaevskii (GP) equation admits a two-dimensional solitary wave
solution representing two mutually self-propelled, anti-parallel straight line
vortices. The complete sequence of such solitary wave solutions has been
computed by Jones and Roberts (J. Phys. A, 15, 2599, 1982). These solutions are
unstable with respect to three-dimensional perturbations (the Crow
instability). The most unstable mode has a wavelength along the direction of
the vortices of the same order as their separation. The growth rate associated
with this mode is evaluated here, and it is found to increase very rapidly with
decreasing separation. It is shown, through numerical integrations of the GP
equation that, as the perturbations grow to finite amplitude, the lines
reconnect to produce a sequence of almost circular vortex rings.Comment: Submitted to J. Phys. A: Math. Gen.; Corrected reference
Recommended from our members
Mitotic antipairing of homologous and sex chromosomes via spatial restriction of two haploid sets.
Pairing homologous chromosomes is required for recombination. However, in nonmeiotic stages it can lead to detrimental consequences, such as allelic misregulation and genome instability, and is rare in human somatic cells. How mitotic recombination is prevented-and how genetic stability is maintained across daughter cells-is a fundamental, unanswered question. Here, we report that both human and mouse cells impede homologous chromosome pairing by keeping two haploid chromosome sets apart throughout mitosis. Four-dimensional analysis of chromosomes during cell division revealed that a haploid chromosome set resides on either side of a meridional plane, crossing two centrosomes. Simultaneous tracking of chromosome oscillation and the spindle axis, using fluorescent CENP-A and centrin1, respectively, demonstrates collective genome behavior/segregation of two haploid sets throughout mitosis. Using 3D chromosome imaging of a translocation mouse with a supernumerary chromosome, we found that this maternally derived chromosome is positioned by parental origin. These data, taken together, support the identity of haploid sets by parental origin. This haploid set-based antipairing motif is shared by multiple cell types, doubles in tetraploid cells, and is lost in a carcinoma cell line. The data support a mechanism of nuclear polarity that sequesters two haploid sets along a subcellular axis. This topological segregation of haploid sets revisits an old model/paradigm and provides implications for maintaining mitotic fidelity
Interactions of vortices with rarefaction solitary waves in a Bose-Einstein condensate and their role in the decay of superfluid turbulence
There are several ways to create the vorticity-free solitary waves --
rarefaction pulses -- in condensates: by the process of strongly nonequilibrium
condensate formation in a weakly interacting Bose gas, by creating local
depletion of the condensate density by a laser beam, and by moving a small
object with supercritical velocities. Perturbations created by such waves
colliding with vortices are studied in the context of the Gross-Pitaevskii
model. We find that the effect of the interactions consists of two competing
mechanisms: the creation of vortex line as rarefaction waves acquire
circulation in a vicinity of a vortex core and the loss of the vortex line to
sound due to Kelvin waves that are generated on vortex lines by rarefaction
pulses. When a vortex ring collides with a rarefaction wave, the ring either
stabilises to a smaller ring after emitting sound through Kelvin wave radiation
or the entire energy of the vortex ring is lost to sound if the radius of the
ring is of the order of the healing length. We show that during the time
evolution of a tangle of vortices, the interactions with rarefaction pulses
provide an important dissipation mechanism enhancing the decay of superfluid
turbulence.Comment: Revised paper accepted by Phys. Rev.
Reentrant superconductivity in superconductor/ferromagnetic-alloy bilayers
We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state
establishing due to the proximity effect in superconducting Nb/Cu41Ni59
bilayers. Using a special wedge-type deposition technique, series of 20-35
samples could be fabricated by magnetron sputtering during one run. The layer
thickness of only a few nanometers, the composition of the alloy, and the
quality of interfaces were controlled by Rutherford backscattering
spectrometry, high resolution transmission electron microscopy, and Auger
spectroscopy. The magnetic properties of the ferromagnetic alloy layer were
characterized with superconducting quantum interference device (SQUID)
magnetometry. These studies yield precise information about the thickness, and
demonstrate the homogeneity of the alloy composition and magnetic properties
along the sample series. The dependencies of the critical temperature on the Nb
and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant
thickness dF of the magnetic alloy layer and dS of the superconducting layer,
respectively. All types of non-monotonic behaviors of Tc versus dF predicted by
the theory could be realized experimentally: from reentrant superconducting
behavior with a broad extinction region to a slight suppression of
superconductivity with a shallow minimum. Even a double extinction of
superconductivity was observed, giving evidence for the multiple reentrant
behavior predicted by theory. All critical temperature curves were fitted with
suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F
spin-switch core structure were calculated using these parameters. Finally,
superconducting spin-switch fabrication issues are discussed in detail in view
of the achieved results.Comment: 34 pages, 9 figure
- …