2,251 research outputs found
Sunspot group tilt angle measurements from historical observations
Sunspot positions from various historical sets of solar drawings are analysed
with respect to the tilt angles of bipolar sunspot groups. Data by Scheiner,
Hevelius, Staudacher, Zucconi, Schwabe, and Spoerer deliver a series of average
tilt angles spanning a period of 270 years, additional to previously found
values for 20th-century data obtained by other authors. We find that the
average tilt angles before the Maunder minimum were not significantly different
from the modern values. However, the average tilt angles of a period 50 years
after the Maunder minimum, namely for cycles 0 and 1, were much lower and near
zero. The normal tilt angles before the Maunder minimum suggest that it was not
abnormally low tilt angles which drove the solar cycle into a grand minimum.Comment: accepted by Advances in Space Researc
Three-dimensional stability of the solar tachocline
The three-dimensional, hydrodynamic stability of the solar tachocline is
investigated based on a rotation profile as a function of both latitude and
radius. By varying the amplitude of the latitudinal differential rotation, we
find linear stability limits at various Reynolds numbers by numerical
computations. We repeated the computations with different latitudinal and
radial dependences of the angular velocity. The stability limits are all higher
than those previously found from two-dimensional approximations and higher than
the shear expected in the Sun. It is concluded that any part of the tachocline
which is radiative is hydrodynamically stable against small perturbations.Comment: 6 pages, 8 figures, accepted by Astron. & Astrophy
Characterising shear-induced dynamics in flowing complex fluids using differential dynamic microscopy
Microscopic dynamics reveal the origin of the bulk rheological response in
complex fluids. In model systems particle motion can be tracked, but for
industrially relevant samples this is often impossible. Here we adapt
differential dynamic microscopy (DDM) to study flowing highly-concentrated
samples without particle resolution. By combining an investigation of
oscillatory flow, using a novel "echo-DDM" analysis, and steady shear, through
flow-DDM, we characterise the yielding of a silicone oil emulsion on both the
microscopic and bulk level. Through measuring the rate of shear-induced droplet
rearrangements and the flow velocity, the transition from a solid-like to
liquid-like state is shown to occur in two steps: with droplet mobilisation
marking the limit of linear visco-elasticity, followed by the development of
shear localisation and macroscopic yielding. Using this suite of techniques,
such insight could be developed for a wide variety of challenging complex
fluids.Comment: 11 pages, 8 figure
Particle sizing for flowing colloidal suspensions using flow-differential dynamic microscopy
Particle size is a key variable in understanding the behaviour of the
particulate products that underpin much of our modern lives. Typically obtained
from suspensions at rest, measuring the particle size under flowing conditions
would enable advances for in-line testing during manufacture and
high-throughput testing during development. However, samples are often turbid,
multiply scattering light and preventing the direct use of common sizing
techniques. Differential dynamic microscopy (DDM) is a powerful technique for
analysing video microscopy of such samples, measuring diffusion and hence
particle size without the need to resolve individual particles while free of
substantial user input. However, when applying DDM to a flowing sample,
diffusive dynamics are rapidly dominated by flow effects, preventing particle
sizing. Here, we develop "flow-DDM", a novel analysis scheme that combines
optimised imaging conditions, a drift-velocity correction and modelling of the
impact of flow. Flow-DDM allows a decoupling of flow from diffusive motion that
facilitates successful particle size measurements at flow speeds an order of
magnitude higher than for DDM. We demonstrate the generality of the technique
by applying flow-DDM to two separate microscopy methods and flow geometries.Comment: 9 pages, 8 figure
Superpositions of the Orbital Angular Momentum for Applications in Quantum Experiments
Two different experimental techniques for preparation and analyzing
superpositions of the Gaussian and Laguerre-Gassian modes are presented. This
is done exploiting an interferometric method on the one hand and using computer
generated holograms on the other hand. It is shown that by shifting the
hologram with respect to an incoming Gaussian beam different superpositions of
the Gaussian and the Laguerre-Gaussian beam can be produced. An analytical
expression between the relative phase and the amplitudes of the modes and the
displacement of the hologram is given. The application of such orbital angular
momenta superpositions in quantum experiments such as quantum cryptography is
discussed.Comment: 18 pages, 4 figures. to appear in Journal of Optics
Search for non-helical disc dynamos in simulations
The possibility of non-helical large scale dynamo action is investigated
using three-dimensional simulations of global accretion discs as well as
idealized local simulations without rotation and only shear. Particular
emphasis is placed on a certain correlation between vorticity and azimuthal
velocity gradient which has been predicted to drive large scale dynamo action,
independent of the presence or absence of kinetic helicity. In the global disc
simulations two types of behaviours are found: those which do show this type of
velocity correlation and those which do not. The former ones are typically also
the cases where the resistivity is larger. The latter ones show signs typical
of dynamo action based on the usual helicity effect. In the idealized
simulations without rotation and just shear the above correlation is found to
be particularly strong. In both cases there is, as expected, a systematic flux
of magnetic helicity through the midplane. However, very little helicity flux
leaves the domain through the top and bottom boundaries. The idealized
simulations reveal that much of this systematic flux comes from the rotational
component of the helicity flux and does not contribute to its divergence.Comment: 14 pages, 17 figures, 5 tables, submitted to Astron. & Astrophy
Analyzing a Bose polaron across resonant interactions
Recently, two independent experiments reported the observation of long-lived
polarons in a Bose-Einstein condensate, providing an excellent setting to study
the generic scenario of a mobile impurity interacting with a quantum reservoir.
Here, we expand the experimental analysis by disentangling the effects of trap
inhomogeneities and the many-body continuum in one of these experiments. This
makes it possible to extract the energy of the polaron at a well-defined
density as a function of the interaction strength. Comparisons with quantum
Monte-Carlo as well as diagrammatic calculations show good agreement, and
provide a more detailed picture of the polaron properties at stronger
interactions than previously possible. Moreover, we develop a semi-classical
theory for the motional dynamics and three-body loss of the polarons, which
partly explains a previously unresolved discrepancy between theory and
experimental observations for repulsive interactions. Finally, we utilize
quantum Monte-Carlo calculations to demonstrate that the findings reported in
the two experiments are consistent with each other
Interaction-free measurements by quantum Zeno stabilisation of ultracold atoms
Quantum mechanics predicts that our physical reality is influenced by events
that can potentially happen but factually do not occur. Interaction-free
measurements (IFMs) exploit this counterintuitive influence to detect the
presence of an object without requiring any interaction with it. Here we
propose and realize an IFM concept based on an unstable many-particle system.
In our experiments, we employ an ultracold gas in an unstable spin
configuration which can undergo a rapid decay. The object - realized by a laser
beam - prevents this decay due to the indirect quantum Zeno effect and thus,
its presence can be detected without interacting with a single atom. Contrary
to existing proposals, our IFM does not require single-particle sources and is
only weakly affected by losses and decoherence. We demonstrate confidence
levels of 90%, well beyond previous optical experiments.Comment: manuscript with 5 figures, 3 supplementary figure, 1 supplementary
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