10,839 research outputs found
Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot
The discrete nature of the solar magnetic field as it emerges into the corona
through the photosphere indicates that it exists as isolated flux tubes in the
convection zone, and will remain as discrete flux tubes in the corona until it
collides and reconnects with other coronal fields. Collisions of these flux
tubes will in general be three dimensional, and will often lead to
reconnection, both rearranging the magnetic field topology in fundamental ways,
and releasing magnetic energy. With the goal of better understanding these
dynamics, we carry out a set of numerical experiments exploring fundamental
characteristics of three dimensional magnetic flux tube reconnection. We first
show that reconnecting flux tubes at opposite extremes of twist behave very
differently: in some configurations, low twist tubes slingshot while high twist
tubes tunnel. We then discuss a theory explaining these differences: by
assuming helicity conservation during the reconnection one can show that at
high twist, tunneled tubes reach a lower magnetic energy state than slingshot
tubes, whereas at low twist the opposite holds. We test three predictions made
by this theory. 1) We find that the level of twist at which the transition from
slingshot to tunnel occurs is about two to three times higher than predicted on
the basis of energetics and helicity conservation alone, probably because the
dynamics of the reconnection play a large role as well. 2) We find that the
tunnel occurs at all flux tube collision angles predicted by the theory. 3) We
find that the amount of magnetic energy a slingshot or a tunnel reconnection
releases agrees reasonably well with the theory, though at the high
resistivities we have to use for numerical stability, a significant amount of
magnetic energy is lost to diffusion, independent of reconnection.Comment: 21 pages, 15 figures, submitted to Ap
Axial, induced pseudoscalar, and pion-nucleon form factors in manifestly Lorentz-invariant chiral perturbation theory
We calculate the nucleon form factors G_A and G_P of the isovector
axial-vector current and the pion-nucleon form factor G_piN in manifestly
Lorentz-invariant baryon chiral perturbation theory up to and including order
O(p^4). In addition to the standard treatment including the nucleon and pions,
we also consider the axial-vector meson a_1 as an explicit degree of freedom.
This is achieved by using the reformulated infrared renormalization scheme. We
find that the inclusion of the axial-vector meson effectively results in one
additional low-energy coupling constant that we determine by a fit to the data
for G_A. The inclusion of the axial-vector meson results in an improved
description of the experimental data for G_A, while the contribution to G_P is
small.Comment: 21 pages, 9 figures, REVTeX
X-ray measured metallicities of the intra-cluster medium: a good measure for the metal mass?
Aims. We investigate whether X-ray observations map heavy elements in the
Intra-Cluster Medium (ICM) well and whether the X-ray observations yield good
estimates for the metal mass, with respect to predictions on transport mech-
anisms of heavy elements from galaxies into the ICM. We further test the
accuracy of simulated metallicity maps. Methods. We extract synthetic X-ray
spectra from N-body/hydrodynamic simulations including metal enrichment pro-
cesses, which we then analyse with the same methods as are applied to
observations. By changing the metal distribution in the simulated galaxy
clusters, we investigate the dependence of the overall metallicity as a
function of the metal distribution. In addition we investigate the difference
of X-ray weighted metal maps produced by simulations and metal maps extracted
from artifcial X-ray spectra, which we calculate with SPEX2.0 and analyse with
XSPEC12.0. Results. The overall metallicity depends strongly on the
distribution of metals within the galaxy cluster. The more inhomogeneously the
metals are distributed within the cluster, the less accurate is the metallicity
as a measure for the true metal mass. The true metal mass is generally
underestimated by X-ray observations. The difference between the X-ray weighted
metal maps and the metal maps from synthetic X-ray spectra is on average less
than 7% in the temperature regime above T > 3E7 K, i.e. X-ray weighted metal
maps can be well used for comparison with observed metal maps. Extracting the
metal mass in the central parts (r < 500 kpc) of galaxy clusters with X-ray
observations results in metal mass underestimates up to a factor of three.Comment: 7 pages, 9 figures, accepted for publication in A&
Density of states in graphene with vacancies: midgap power law and frozen multifractality
The density of states (DoS), , of graphene is investigated
numerically and within the self-consistent T-matrix approximation (SCTMA) in
the presence of vacancies within the tight binding model. The focus is on
compensated disorder, where the concentration of vacancies, and
, in both sub-lattices is the same. Formally, this model belongs to
the chiral symmetry class BDI. The prediction of the non-linear sigma-model for
this class is a Gade-type singularity . Our numerical data is compatible with this
result in a preasymptotic regime that gives way, however, at even lower
energies to , . We take this finding as an evidence that similar to the case
of dirty d-wave superconductors, also generic bipartite random hopping models
may exhibit unconventional (strong-coupling) fixed points for certain kinds of
randomly placed scatterers if these are strong enough. Our research suggests
that graphene with (effective) vacancy disorder is a physical representative of
such systems.Comment: References updated onl
Magellan Spectroscopy of the Galaxy Cluster RX J1347.5-1145: Redshift Estimates for the Gravitationally Lensed Arcs
We present imaging and spectroscopic observations of the gravitationally
lensed arcs in the field of RX J1347.5-1145, the most X-ray luminous galaxy
cluster known. Based on the detection of the [OII] 3727 emission line, we
confirm that the redshift of one of the arcs is z = 0.806. Its color and [OII]
line strength are consistent with those of distant, actively star forming
galaxies. In a second arc, we tentatively identify a pair of absorption lines
superposed on a red continuum; the lines are consistent with Ca II H & K at z =
0.785. We detected a faint blue continuum in two additional arcs, but no
spectral line features could be measured. We establish lower limits to their
redshifts based on the absence of [OII] emission, which we argue should be
present and detectable in these objects. Redshifts are also given for a number
of galaxies in the field of the cluster.Comment: To appear in The Astrophysical Journal (September 2002). 6 page
Implications of surface noise for the motional coherence of trapped ions
Electric noise from metallic surfaces is a major obstacle towards quantum
applications with trapped ions due to motional heating of the ions. Here, we
discuss how the same noise source can also lead to pure dephasing of motional
quantum states. The mechanism is particularly relevant at small ion-surface
distances, thus imposing a new constraint on trap miniaturization. By means of
a free induction decay experiment, we measure the dephasing time of the motion
of a single ion trapped 50~m above a Cu-Al surface. From the dephasing
times we extract the integrated noise below the secular frequency of the ion.
We find that none of the most commonly discussed surface noise models for ion
traps describes both, the observed heating as well as the measured dephasing,
satisfactorily. Thus, our measurements provide a benchmark for future models
for the electric noise emitted by metallic surfaces.Comment: (5 pages, 4 figures
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