78 research outputs found
On Predicting the Solar Cycle using Mean-Field Models
We discuss the difficulties of predicting the solar cycle using mean-field
models. Here we argue that these difficulties arise owing to the significant
modulation of the solar activity cycle, and that this modulation arises owing
to either stochastic or deterministic processes. We analyse the implications
for predictability in both of these situations by considering two separate
solar dynamo models. The first model represents a stochastically-perturbed flux
transport dynamo. Here even very weak stochastic perturbations can give rise to
significant modulation in the activity cycle. This modulation leads to a loss
of predictability. In the second model, we neglect stochastic effects and
assume that generation of magnetic field in the Sun can be described by a fully
deterministic nonlinear mean-field model -- this is a best case scenario for
prediction. We designate the output from this deterministic model (with
parameters chosen to produce chaotically modulated cycles) as a target
timeseries that subsequent deterministic mean-field models are required to
predict. Long-term prediction is impossible even if a model that is correct in
all details is utilised in the prediction. Furthermore, we show that even
short-term prediction is impossible if there is a small discrepancy in the
input parameters from the fiducial model. This is the case even if the
predicting model has been tuned to reproduce the output of previous cycles.
Given the inherent uncertainties in determining the transport coefficients and
nonlinear responses for mean-field models, we argue that this makes predicting
the solar cycle using the output from such models impossible.Comment: 22 Pages, 5 Figures, Preprint accepted for publication in Ap
In--out intermittency in PDE and ODE models
We find concrete evidence for a recently discovered form of intermittency,
referred to as in--out intermittency, in both PDE and ODE models of mean field
dynamos. This type of intermittency (introduced in Ashwin et al 1999) occurs in
systems with invariant submanifolds and, as opposed to on--off intermittency
which can also occur in skew product systems, it requires an absence of skew
product structure. By this we mean that the dynamics on the attractor
intermittent to the invariant manifold cannot be expressed simply as the
dynamics on the invariant subspace forcing the transverse dynamics; the
transverse dynamics will alter that tangential to the invariant subspace when
one is far enough away from the invariant manifold.
Since general systems with invariant submanifolds are not likely to have skew
product structure, this type of behaviour may be of physical relevance in a
variety of dynamical settings.
The models employed here to demonstrate in--out intermittency are
axisymmetric mean--field dynamo models which are often used to study the
observed large scale magnetic variability in the Sun and solar-type stars. The
occurrence of this type of intermittency in such models may be of interest in
understanding some aspects of such variabilities.Comment: To be published in Chaos, June 2001, also available at
http://www.eurico.web.co
Solar-Cycle Characteristics Examined in Separate Hemispheres: Phase, Gnevyshev Gap, and Length of Minimum
Research results from solar-dynamo models show the northern and southern
hemispheres may evolve separately throughout the solar cycle. The observed
phase lag between the hemispheres provides information regarding the strength
of hemispheric coupling. Using hemispheric sunspot-area and sunspot-number data
from Cycles 12 - 23, we determine how out of phase the separate hemispheres are
during the rising, maximum, and declining period of each solar cycle.
Hemispheric phase differences range from 0 - 11, 0 - 14, and 2 - 19 months for
the rising, maximum, and declining periods, respectively. The phases appear
randomly distributed between zero months (in phase) and half of the rise (or
decline) time of the solar cycle. An analysis of the Gnevyshev gap is conducted
to determine if the double-peak is caused by the averaging of two hemispheres
that are out of phase. We confirm previous findings that the Gnevyshev gap is a
phenomenon that occurs in the separate hemispheres and is not due to a
superposition of sunspot indices from hemispheres slightly out of phase. Cross
hemispheric coupling could be strongest at solar minimum, when there are large
quantities of magnetic flux at the Equator. We search for a correlation between
the hemispheric phase difference near the end of the solar cycle and the length
of solar-cycle minimum, but found none. Because magnetic flux diffusion across
the Equator is a mechanism by which the hemispheres couple, we measured the
magnetic flux crossing the Equator by examining magnetograms for Solar Cycles
21 - 23. We find, on average, a surplus of northern hemisphere magnetic flux
crossing during the mid-declining phase of each solar cycle. However, we find
no correlation between magnitude of magnetic flux crossing the Equator, length
of solar minima, and phase lag between the hemispheres.Comment: 15 pages, 7 figure
Global-Scale Turbulent Convection and Magnetic Dynamo Action in the Solar Envelope
The operation of the solar global dynamo appears to involve many dynamical
elements. Self-consistent MHD simulations which realistically incorporate all
of these processes are not yet computationally feasible, though some elements
can now be studied with reasonable fidelity. Here we consider the manner in
which turbulent compressible convection within the bulk of the solar convection
zone can generate large-scale magnetic fields through dynamo action. We
accomplish this through a series of three-dimensional numerical simulations of
MHD convection within rotating spherical shells using our ASH code on massively
parallel supercomputers. Since differential rotation is a key ingredient in all
dynamo models, we also examine here the nature of the rotation profiles that
can be sustained within the deep convection zone as strong magnetic fields are
built and maintained. We find that the convection is able to maintain a
solar-like angular velocity profile despite the influence of Maxwell stresses
which tend to oppose Reynolds stresses and thus reduce the latitudinal angular
velocity contrast throughout the convection zone. The dynamo-generated magnetic
fields exhibit a complex structure and evolution, with radial fields
concentrated in downflow lanes and toroidal fields organized into twisted
ribbons which are extended in longitude and which achieve field strengths of up
to 5000 G. The flows and fields exhibit substantial kinetic and magnetic
helicity although systematic hemispherical patterns are only apparent in the
former. Fluctuating fields dominate the magnetic energy and account for most of
the back-reaction on the flow via Lorentz forces. Mean fields are relatively
weak and do not exhibit systematic latitudinal propagation or periodic polarity
reversals as in the sun. This may be attributed to the absence of a tachocline.Comment: 55 pages (ApJ refereeing format), 15 figures (low res), published by
ApJ on October 2004 (abstract slightly reduced in order to fit in 24 lines
limit) see also Browning, Miesch, Brun & Toomre 2006, ApJL, 648, 157
(astro-ph/0609153) for the effect of a tachocline in organizing the mean
field
Large-scale dynamos in turbulent convection with shear
(abridged) Aims: Three-dimensional numerical simulations of penetrative
compressible convection with uniform horizontal shear are used to study dynamo
action and the generation of large-scale magnetic fields. Methods: We consider
cases where the magnetic Reynolds number is either marginal or moderately
supercritical with respect to small-scale dynamo action in the absence of shear
and rotation. The effects of magnetic helicity fluxes are studied by comparing
results for the magnetic field with open and closed boundaries. Results:
Without shear no large-scale dynamos are found even if the ingredients
necessary for the alpha-effect (rotation and stratification) are present in the
system. When uniform horizontal shear is added, a large-scale magnetic field
develops, provided the boundaries are open. In this case the mean magnetic
field contains a significant fraction of the total field. For those runs where
the magnetic Reynolds number is between 60 and 250, small-scale dynamo is
expected to be excited, but the field distribution is found to be similar to
cases with smaller magnetic Reynolds number where the small-scale dynamo is not
excited. In the case of closed (perfectly conducting) boundaries, magnetic
helicity fluxes are suppressed and no large-scale fields are found. Similarly,
poor large-scale field development is seen when vertical shear is used in
combination with periodic boundary conditions in the horizontal directions. If,
however, open (normal-field) boundary conditions are used in the x-direction, a
large-scale field develops. These results support the notion that shear not
only helps to generate the field, but it also plays a crucial role in driving
magnetic helicity fluxes out of the system along the isocontours of shear,
thereby allowing efficient dynamo action.Comment: 10 pages, 19 figures, accepted for publication in Astron. Astrophy
Alpha effect and turbulent diffusion from convection
(abridged) Aims: To study turbulent transport coefficients that describe the
evolution of large-scale magnetic fields in turbulent convection. Methods: We
use the test field method together with 3D numerical simulations of turbulent
convection with shear and rotation to compute turbulent transport coefficients
describing the evolution of large-scale magnetic fields in mean-field theory in
the kinematic regime. 1D mean-field models are used with the derived turbulent
transport coefficients to compare with direct simulations. Results: The
alpha-effect increases monotonically as rotation increases. Turbulent
diffusivity, eta_t, is proportional to the square of the turbulent vertical
velocity. Whereas eta_t decreases approximately inversely proportional to the
wavenumber of the field, the alpha-effect and turbulent pumping show a more
complex behaviour. In the presence of shear and no rotation a small
alpha-effect is induced which does not seem to show any consistent trend as a
function of shear. If the shear is large enough, this small alpha is able to
excite a dynamo in the mean-field model. The coefficient responsible for
driving the shear-current effect shows several sign changes as a function of
depth but is also able to contribute to dynamo action in the mean-field model.
The growth rates in these cases are well below those in direct simulations
suggesting that an incoherent alpha-shear dynamo may also act in them. If both
rotation and shear are present, the alpha-effect is more pronounced. The
combination of the shear-current and Omega x J-effects is also stronger than in
the case of shear only, but subdominant to the alpha-shear dynamo. The results
of direct simulations are consistent with mean-field models where all of these
effects are taken into account without the need to invoke incoherent effects.Comment: 14 pages, 14 figures, minor changes to match with the published
versio
MOST detects variability on tau Bootis possibly induced by its planetary companion
(abridged) There is considerable interest in the possible interaction between
parent stars and giant planetary companions in 51 Peg-type systems. We
demonstrate from MOST satellite photometry and Ca II K line emission that there
has been a persistent, variable region on the surface of tau Boo A which
tracked its giant planetary companion for some 440 planetary revolutions and
lies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curves
are folded on a range of periods centered on the planetary orbital period and
phase dependent variability is quantified by Fourier methods and by the mean
absolute deviation (MAD) of the folded data for both the photometry and the Ca
II K line reversals. The region varies in brightness on the time scale of a
rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while
in 2005 it varied between bright and dark. Over the 123 planetary orbits
spanned by the photometry the variable region detected in 2004 and in 2005 are
synchronised to the planetary orbital period within 0.0015 d. The Ca II K line
in 2001, 2002 and 2003 also shows enhanced K-line variability centered on
phi=0.8, extending coverage to some 440 planetary revolutions. The apparently
constant rotation period of the variable region and its rapid variation make an
explanation in terms of conventional star spots unlikely. The lack of
complementary variability at phi=0.3 and the detection of the variable region
so far in advance of the sub-planetary point excludes tidal excitation, but the
combined photometric and Ca II K line reversal results make a good case for an
active region induced magnetically on the surface of tau Boo A by its planetary
companion.Comment: 7 pages, 7 figures; accepted for publication in A&
Hemispheric Sunspot Numbers R_n and R_s: Catalogue and N-S asymmetry analysis
Sunspot drawings are provided on a regular basis at the Kanzelhoehe Solar
Observatory, Austria, and the derived relative sunspot numbers are reported to
the Sunspot Index Data Center in Brussels. From the daily sunspot drawings, we
derived the northern, R_n, and southern, R_s, relative sunspot numbers for the
time span 1975-2000. In order to accord with the International Sunspot Numbers
R_i, the R_n and R_s have been normalized to the R_i, which ensures that the
relation R_n + R_s = R_i is fulfilled. For validation, the derived R_n and R_s
are compared to the international northern and southern relative sunspot
numbers, which are available from 1992. The regression analysis performed for
the period 1992-2000 reveals good agreement with the International hemispheric
Sunspot Numbers. The monthly mean and the smoothed monthly mean hemispheric
Sunspot Numbers are compiled into a catalogue. Based on the derived hemispheric
Sunspot Numbers, we study the significance of N-S asymmetries and the
rotational behavior separately for both hemispheres. We obtain that about 60%
of the monthly N-S asymmetries are significant at a 95% level, whereas the
relative contributions of the northern and southern hemisphere are different
for different cycles. From the analysis of power spectra and autocorrelation
functions, we derive a rigid rotation with about 27 days for the northern
hemisphere, which can be followed for up to 15 periods. Contrary to that, the
southern hemisphere reveals a dominant period of about 28 days, whereas the
autocorrelation is strongly attenuated after 3 periods. These findings suggest
that the activity of the northern hemisphere is dominated by an active zone,
whereas the southern activity is mainly dominated by individual long-lived
sunspot groups.Comment: 9 pages, 8 figures, data catalogue online available at
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/70
Generalized Boltzmann Equation for Lattice Gas Automata
In this paper, for the first time a theory is formulated that predicts
velocity and spatial correlations between occupation numbers that occur in
lattice gas automata violating semi-detailed balance. Starting from a coupled
BBGKY hierarchy for the -particle distribution functions, cluster expansion
techniques are used to derive approximate kinetic equations. In zeroth
approximation the standard nonlinear Boltzmann equation is obtained; the next
approximation yields the ring kinetic equation, similar to that for hard sphere
systems, describing the time evolution of pair correlations. As a quantitative
test we calculate equal time correlation functions in equilibrium for two
models that violate semi-detailed balance. One is a model of interacting random
walkers on a line, the other one is a two-dimensional fluid type model on a
triangular lattice. The numerical predictions agree very well with computer
simulations.Comment: 31 pages LaTeX, 12 uuencoded tar-compressed Encapsulated PostScript
figures (`psfig' macro), hardcopies available on request, 78kb + 52k
The Differential Rotation of Kappa1 Ceti as Observed by MOST
We first reported evidence for differential rotation of Kappa1 Ceti in Paper
I. In this paper we demonstrate that the differential rotation pattern closely
matches that for the Sun. This result is based on additional MOST
(Microvariability & Oscillations of STars) observations in 2004 and 2005, to
complement the 2003 observations discussed in Paper I. Using StarSpotz, a
program developed specifically to analyze MOST photometry, we have solved for
k, the differential rotation coefficient, and P_{EQ}, the equatorial rotation
period using the light curves from all three years. The spots range in latitude
from 10 to 75 degrees and k = 0.090^{+0.006}_{-0.005} -- less than the solar
value but consistent with the younger age of the star. k is also well
constrained by the independent spectroscopic estimate of vsini. We demonstrate
independently that the pattern of differential rotation with latitude in fact
conforms to solar.
Details are given of the parallel tempering formalism used in finding the
most robust solution which gives P_{EQ} = 8.77^{+0.03}_{-0.04} days -- smaller
than that usually adopted, implying an age < 750 My. Our values of P_{EQ} and k
can explain the range of rotation periods determined by others by spots or
activity at a variety of latitudes. Historically, Ca II activity seems to occur
consistently between latitudes 50 and 60 degrees which might indicate a
permanent magnetic feature. Knowledge of k and P_{EQ} are key to understanding
the dynamo mechanism and rotation structure in the convective zone as well
assessing age for solar-type stars. We recently published values of k and
P_{EQ} for epsilon Eri based on MOST photometry and expect to analyze MOST
light curves for several more spotted, solar-type stars.Comment: 16 pages, 7 Figures, published in Ap
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