39 research outputs found
The helicity constraint in turbulent dynamos with shear
The evolution of magnetic fields is studied using simulations of forced
helical turbulence with strong imposed shear. After some initial exponential
growth, the magnetic field develops a large scale travelling wave pattern. The
resulting field structure possesses magnetic helicity, which is conserved in a
periodic box by the ideal MHD equations and can hence only change on a
resistive time scale. This constrains strongly the growth time of the large
scale magnetic field, but less strongly the length of the cycle period.
Comparing with the case without shear, the time scale for large scale field
amplification is shortened by a factor Q, which depends on the relative
importance of shear and helical turbulence, and which controls also the ratio
of toroidal to poloidal field. The results of the simulations can be reproduced
qualitatively and quantitatively with a mean-field alpha-Omega dynamo model
with alpha-effect and the turbulent magnetic diffusivity coefficients that are
less strongly quenched than in the corresponding alpha^2-dynamo.Comment: 8 pages, 11 figs, submitted to MN (revised, newly added Sect 6
The Sun's Preferred Longitudes and the Coupling of Magnetic Dynamo Modes
Observations show that solar activity is distributed non-axisymmetrically,
concentrating at "preferred longitudes". This indicates the important role of
non-axisymmetric magnetic fields in the origin of solar activity. We
investigate the generation of the non-axisymmetric fields and their coupling
with axisymmetric solar magnetic field. Our kinematic generation (dynamo) model
operating in a sphere includes solar differential rotation, which approximates
the differential rotation obtained by inversion of helioseismic data, modelled
distributions of the turbulent resistivity, non-axisymmetric mean helicity, and
meridional circulation in the convection zone. We find that (1) the
non-axisymmetric modes are localised near the base of the convection zone,
where the formation of active regions starts, and at latitudes around
; (2) the coupling of non-axisymmetric and axisymmetric modes
causes the non-axisymmetric mode to follow the solar cycle; the phase relations
between the modes are found. (3) The rate of rotation of the first
non-axisymmetric mode is close to that determined in the interplanetary space.Comment: 22 pages, 18 figures. Accepted for publication in the Astrophysical
Journa
PP-wave and Non-supersymmetric Gauge Theory
We extend the pp-wave correspondence to a non supersymmetric example. The
model is the type 0B string theory on the pp-wave R-R background. We explicitly
solve the model and give the spectrum of physical states. The field theory
counterpart is given by a sector of the large N SU(N) x SU(N) CFT living on a
stack of N electric and N magnetic D3-branes. The relevant effective coupling
constant is g_{eff}=g_sN/J^2. The string theory has a tachyon in the spectrum,
whose light-cone energy can be exactly computed as a function of g_{eff}. We
argue that the perturbative analysis in g_{eff} in the dual gauge theory is
reliable, with corrections of non perturbative type. We find a precise
state/operator map, showing that the first perturbative corrections to the
anomalous dimensions of the operators have the behavior expected from the
string analysis.Comment: 19 pages. Revised versio
The Gluonic Field of a Heavy Quark in Conformal Field Theories at Strong Coupling
We determine the gluonic field configuration sourced by a heavy quark
undergoing arbitrary motion in N=4 super-Yang-Mills at strong coupling and
large number of colors. More specifically, we compute the expectation value of
the operator tr[F^2+...] in the presence of such a quark, by means of the
AdS/CFT correspondence. Our results for this observable show that signals
propagate without temporal broadening, just as was found for the expectation
value of the energy density in recent work by Hatta et al. We attempt to shed
some additional light on the origin of this feature, and propose a different
interpretation for its physical significance. As an application of our general
results, we examine when the quark undergoes oscillatory motion,
uniform circular motion, and uniform acceleration. Via the AdS/CFT
correspondence, all of our results are pertinent to any conformal field theory
in 3+1 dimensions with a dual gravity formulation.Comment: 1+38 pages, 16 eps figures; v2: completed affiliation; v3: corrected
typo, version to appear in JHE
Methane and water spectroscopic database for TROPOMI/Sentinel-5 Precursor in the 2.3 ÎŒm region
The ESA project âSEOM-Improved Atmospheric Spectroscopy Databases (IAS)â will improve the spectroscopic database for retrieval of the data products CO, CH4, O3 and SO2 column amounts measured by the TROPOMI instrument (TROPOspheric Monitoring Instrument) aboard the Sentinel-5 Precursor. The project was launched in February 2014 with 3 years duration extended to 4 years recently. The spectroscopy of CO, CH4 and O3 in the 2.3 ?m region is covered first while UV measurements of SO2 and UV/FIR/IR measurements of ozone will be carried out later.
Measurements were mainly taken with a high resolution Fourier Transform spectrometer combined with a coolable
multi reflection cell. Cavity ring down measurements served for validation. The analysis has been completed. A
clear improvement can be seen when using the new data for CH4, H2O and CO retrieval from ground-based high
resolution solar occultation measurements obtained with instrumentation in the TCCON and NDACC network
Negative Refractive Index in Hydrodynamical Systems
We discuss the presence of exotic electromagnetic phenomena in systems with
finite charge density which are described by hydrodynamics. We show that such
systems generically have negative refractive index for low frequency
electromagnetic waves, i.e. the energy flux and the phase velocity of the wave
propagate in opposite directions. We comment on possible phenomenological
applications, focusing on the Quark Gluon Plasma.Comment: 16 pages, 2 figure
Early-Time Energy Loss in a Strongly-Coupled SYM Plasma
We carry out an analytic study of the early-time motion of a quark in a
strongly-coupled maximally-supersymmetric Yang-Mills plasma, using the AdS/CFT
correspondence. Our approach extracts the first thermal effects as a small
perturbation of the known quark dynamics in vacuum, using a double expansion
that is valid for early times and for (moderately) ultrarelativistic quark
velocities. The quark is found to lose energy at a rate that differs
significantly from the previously derived stationary/late-time result: it
scales like T^4 instead of T^2, and is associated with a friction coefficient
that is not independent of the quark momentum. Under conditions representative
of the quark-gluon plasma as obtained at RHIC, the early energy loss rate is a
few times smaller than its late-time counterpart. Our analysis additionally
leads to thermally-corrected expressions for the intrinsic energy and momentum
of the quark, in which the previously discovered limiting velocity of the quark
is found to appear naturally.Comment: 39 pages, no figures. v2: Minor corrections and clarifications.
References added. Version to be published in JHE