797 research outputs found
The structure of current layers and degree of field line braiding in coronal loops
One proposed resolution to the long-standing problem of solar coronal heating
involves the buildup of magnetic energy in the corona due to turbulent motions
at the photosphere that braid the coronal field, and the subsequent release of
this energy via magnetic reconnection. In this paper the ideal relaxation of
braided magnetic fields modelling solar coronal loops is followed. A sequence
of loops with increasing braid complexity is considered, with the aim of
understanding how this complexity influences the development of small scales in
the magnetic field, and thus the energy available for heating. It is
demonstrated that the ideally accessible force-free equilibrium for these
braided fields contains current layers of finite thickness. It is further shown
that for any such braided field, if a force-free equilibrium exists then it
should contain current layers whose thickness is determined by length scales in
the field line mapping. The thickness and intensity of the current layers
follow scaling laws, and this allows us to extrapolate beyond the numerically
accessible parameter regime and to place an upper bound on the braid complexity
possible at coronal plasma parameters. At this threshold level the braided loop
contains -- of available free magnetic energy, more
than sufficient for a large nanoflare.Comment: To appear in ApJ. 20 pages, 10 figure
A study of marine luminescence signatures, part 1
Fluorescent excitation and emission spectral data on chlorophyll and Gelbstoff in natural sea waters from the Atlantic, Gulf, and Pacific coasts show that algae particulates are totally absorbing over much of the near ultraviolet and visible spectra and act approximately as quantum counters; plant pigments absorb energy and transfer a large portion to chlorophyll where some fraction is emitted as chlorophyll fluorescence. Gelbstoff data do not exhibit quantum counter action because of their low concentration. It is concluded that luminescence data of natural sea waters are useful in monitoring algal and Gelbstoff as well as pollutant concentrations
On Statistical Aspects of Qjets
The process by which jet algorithms construct jets and subjets is inherently
ambiguous and equally well motivated algorithms often return very different
answers. The Qjets procedure was introduced by the authors to account for this
ambiguity by considering many reconstructions of a jet at once, allowing one to
assign a weight to each interpretation of the jet. Employing these weighted
interpretations leads to an improvement in the statistical stability of many
measurements. Here we explore in detail the statistical properties of these
sets of weighted measurements and demonstrate how they can be used to improve
the reach of jet-based studies.Comment: 29 pages, 6 figures. References added, minor modification of the
text. This version to appear in JHE
Qjets: A Non-Deterministic Approach to Tree-Based Jet Substructure
Jet substructure is typically studied using clustering algorithms, such as
kT, which arrange the jets' constituents into trees. Instead of considering a
single tree per jet, we propose that multiple trees should be considered,
weighted by an appropriate metric. Then each jet in each event produces a
distribution for an observable, rather than a single value. Advantages of this
approach include: 1) observables have significantly increased statistical
stability; and, 2) new observables, such as the variance of the distribution,
provide new handles for signal and background discrimination. For example, we
find that employing a set of trees substantially reduces the observed
fluctuations in the pruned mass distribution, enhancing the likelihood of new
particle discovery for a given integrated luminosity. Furthermore, the
resulting pruned mass distributions for (background) QCD jets are found to be
substantially wider than that for (signal) jets with intrinsic mass scales,
e.g. jets containing a W decay. A cut on this width yields a substantial
enhancement in significance relative to a cut on the standard pruned jet mass
alone. In particular the luminosity needed for a given significance requirement
decreases by a factor of two relative to standard pruning.Comment: Minor changes to match journal versio
Compendium of marine luminescence signatures, part 2 (appendix C)
Chlorophyll and Gelbstoff excitation and emission spectra of sea water samples are assembled according to geographic sites from the Atlantic and Gulf coasts, the west coast, and a location north of Hawaii. Data were taken by fluorescent spectrophotometer and include also laboratory algal cultures for comparison with the sea water samples
Magnetic field line braiding in the solar atmosphere
AbstractUsing a magnetic carpet as model for the near surface solar magnetic field we study its effects on the propagation of energy injectected by photospheric footpoint motions. Such a magnetic carpet structure is topologically highly non-trivial and with its magnetic nulls exhibits qualitatively different behavior than simpler magnetic fields. We show that the presence of magnetic fields connecting back to the photosphere inhibits the propagation of energy into higher layers of the solar atmosphere, like the solar corona. By applying certain types of footpoint motions the magnetic field topology is is greatly reduced through magnetic field reconnection which facilitates the propagation of energy and disturbances from the photosphere.</jats:p
Topological constraints in the reconnection of vortex braids
We study the relaxation of a topologically nontrivial vortex braid with zero net helicity in a barotropic fluid. The aim is to investigate the extent to which the topology of the vorticity field—characterized by braided vorticity field lines—determines the dynamics, particularly the asymptotic behavior under vortex reconnection in evolution at high Reynolds numbers (25 000). Analogous to the evolution of braided magnetic fields in plasma, we find that the relaxation of our vortex braid leads to a simplification of the topology into large-scale regions of opposite swirl, consistent with an inverse cascade of the helicity. The change of topology is facilitated by a cascade of vortex reconnection events. During this process, the existence of regions of positive and negative kinetic helicities imposes a lower bound for the kinetic energy. For the enstrophy, we derive analytically a lower bound given by the presence of unsigned kinetic helicity, which we confirm in our numerical experiments
Braided magnetic fields:equilibria, relaxation and heating
We examine the dynamics of magnetic flux tubes containing non-trivial field
line braiding (or linkage), using mathematical and computational modelling, in
the context of testable predictions for the laboratory and their significance
for solar coronal heating. We investigate the existence of braided force-free
equilibria, and demonstrate that for a field anchored at perfectly-conducting
plates, these equilibria exist and contain current sheets whose thickness
scales inversely with the braid complexity - as measured for example by the
topological entropy. By contrast, for a periodic domain braided exact
equilibria typically do not exist, while approximate equilibria contain thin
current sheets. In the presence of resistivity, reconnection is triggered at
the current sheets and a turbulent relaxation ensues. We finish by discussing
the properties of the turbulent relaxation and the existence of constraints
that may mean that the final state is not the linear force-free field predicted
by Taylor's hypothesis.Comment: To appear in Plasma Physics and Controlled Fusio
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