974 research outputs found
Origin of the hemispheric asymmetry of solar activity
The frequency spectrum of the hemispheric asymmetry of solar activity shows
enhanced power for the period ranges around 8.5 years and between 30 and 50
years. This can be understood as the sum and beat periods of the superposition
of two dynamo modes: a dipolar mode with a (magnetic) period of about 22 years
and aquadrupolar mode with a period between 13 and 15 years. An updated
Babcock-Leighton-type dynamo model with weak driving as indicated by stellar
observations shows an excited dipole mode and a damped quadrupole mode in the
correct range of periods. Random excitation of the quadrupole by stochastic
fluctuations of the source term for the poloidal field leads to a time
evolution of activity and asymmetry that is consistent with the observational
results.Comment: Astronomy & Astrophysics, accepte
Surface flux transport modeling for solar cycles 15--21: effects of cycle-dependent tilt angles of sunspot groups
We model the surface magnetic field and open flux of the Sun from 1913 to
1986 using a surface flux transport model, which includes the observed
cycle-to-cycle variation of sunspot group tilts. The model reproduces the
empirically derived time evolution of the solar open magnetic flux, and the
reversal times of the polar fields. We find that both the polar field and the
axial dipole moment resulting from this model around cycle minimum correlate
with the strength of the following cycle.Comment: Accepted for publication by Ap
Penumbral structure and outflows in simulated sunspots
Sunspots are concentrations of magnetic field on the visible solar surface
that strongly affect the convective energy transport in their interior and
surroundings. The filamentary outer regions (penumbrae) of sunspots show
systematic radial outward flows along channels of nearly horizontal magnetic
field. These flows were discovered 100 years ago and are present in all fully
developed sunspots. Using a comprehensive numerical simulation of a sunspot
pair, we show that penumbral structures with such outflows form when the
average magnetic field inclination to the vertical exceeds about 45 degrees.
The systematic outflows are a component of the convective flows that provide
the upward energy transport and result from anisotropy introduced by the
presence of the inclined magnetic field.Comment: 19 pages, 8 figures, main Science article + supporting online
material combined into one fil
Shock-wave therapy of gastric outlet syndrome caused by a gallstone
A patient with gastric outlet syndrome (Bouveret's syndrome) caused by a large gallstone impacted in the duodenal bulb was successfully treated by extracorporeal shock-wave lithotripsy. Thus, open abdominal surgery could be avoided. For disintegration of the stone, three consecutive lithotripsy procedures were necessary. Thereafter, stone fragments could be extracted endoscopically. Extracorporeal shock-wave lithotripsy could become a non-surgical alternative in patients with obstruction of the duodenum caused by a gallstone
Mesogranular structure in a hydrodynamical simulation
We analyse mesogranular flow patterns in a three-dimensional hydrodynamical
simulation of solar surface convection in order to determine its
characteristics. We calculate divergence maps from horizontal velocities
obtained with the Local Correlation Tracking (LCT) method. Mesogranules are
identified as patches of positive velocity divergence. We track the
mesogranules to obtain their size and lifetime distributions. We vary the
analysis parameters to verify if the pattern has characteristic scales. The
characteristics of the resulting flow patterns depend on the averaging time and
length used in the analysis. We conclude that the mesogranular patterns do not
exhibit intrinsic length and time scales
Observing and modeling the poloidal and toroidal fields of the solar dynamo
Context. The solar dynamo consists of a process that converts poloidal field
to toroidal field followed by a process which creates new poloidal field from
the toroidal field.
Aims. Our aim is to observe the poloidal and toroidal fields relevant to the
global solar dynamo and see if their evolution is captured by a
Babcock-Leighton dynamo.
Methods. We use synoptic maps of the surface radial field from the KPNSO/VT
and SOLIS observatories to construct the poloidal field as a function of time
and latitude, and Wilcox Solar Observatory and SOHO/MDI full disk images to
infer the longitudinally averaged surface azimuthal field. We show that the
latter is consistent with an estimate of that due to flux emergence and
therefore closely related to the subsurface toroidal field.
Results. We present maps of the poloidal and toroidal magnetic field of the
global solar dynamo. The longitude-averaged azimuthal field observed at the
surface results from flux emergence. At high latitudes this component follows
the radial component of the polar fields with a short time lag (1-3 years). The
lag increases at lower latitudes. The observed evolution of the poloidal and
toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo
model.Comment: A&
Limits to solar cycle predictability: Cross-equatorial flux plumes
Within the Babcock-Leighton framework for the solar dynamo, the strength of a
cycle is expected to depend on the strength of the dipole moment or net
hemispheric flux during the preceding minimum, which depends on how much flux
was present in each hemisphere at the start of the previous cycle and how much
net magnetic flux was transported across the equator during the cycle. Some of
this transport is associated with the random walk of magnetic flux tubes
subject to granular and supergranular buffeting, some of it is due to the
advection caused by systematic cross-equatorial flows such as those associated
with the inflows into active regions, and some crosses the equator during the
emergence process.
We aim to determine how much of the cross-equatorial transport is due to
small-scale disorganized motions (treated as diffusion) compared with other
processes such as emergence flux across the equator. We measure the
cross-equatorial flux transport using Kitt Peak synoptic magnetograms,
estimating both the total and diffusive fluxes. Occasionally a large sunspot
group, with a large tilt angle emerges crossing the equator, with flux from the
two polarities in opposite hemispheres. The largest of these events carry a
substantial amount of flux across the equator (compared to the magnetic flux
near the poles). We call such events cross-equatorial flux plumes. There are
very few such large events during a cycle, which introduces an uncertainty into
the determination of the amount of magnetic flux transported across the equator
in any particular cycle. As the amount of flux which crosses the equator
determines the amount of net flux in each hemisphere, it follows that the
cross-equatorial plumes introduce an uncertainty in the prediction of the net
flux in each hemisphere. This leads to an uncertainty in predictions of the
strength of the following cycle.Comment: A&A, accepte
An analytic interface dynamo over a shear layer of finite depth
Parker's analytic Cartesian interface dynamo is generalized to the case of a
shear layer of finite thickness and low resistivity ("tachocline"), bounded by
a perfect conductor ("radiative zone") on the one side, and by a highly
diffusive medium ("convective zone") supporting an -effect on the other
side. In the limit of high diffusivity contrast between the shear layer and the
diffusive medium, thought to be relevant for the Sun, a pair of exact
dispersion relations for the growth rate and frequency of dynamo modes is
analytically derived. Graphic solution of the dispersion relations displays a
somewhat unexpected, non-monotonic behaviour, the mathematical origin of which
is elucidated. The dependence of the results on the parameter values (dynamo
number and shear layer thickness) is investigated. The implications of this
result for the solar dynamo problem are discussed.Comment: 11 pages, 4 figures Geophys. Astrophys. Fluid Dyn., in pres
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