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Author name code: petrovay
ADS astronomy entries on 2022-09-14
author:"Petrovay, Kristof"
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Title: Role of observable nonlinearities in solar cycle modulation
Authors: Talafha, M.; Nagy, M.; Lemerle, A.; Petrovay, K.
2022A&A...660A..92T Altcode: 2021arXiv211214465T
Context. Two candidate mechanisms have recently been considered with
regard to the nonlinear modulation of solar cycle amplitudes. Tilt
quenching (TQ) comprises the negative feedback between the cycle
amplitude and the mean tilt angle of bipolar active regions relative
to the azimuthal direction. Latitude quenching (LQ) consists of a
positive correlation between the cycle amplitude and average emergence
latitude of active regions. <BR /> Aims: Here, we explore the relative
importance and the determining factors behind the LQ and TQ effects. <BR
/> Methods: We systematically probed the degree of nonlinearity induced
by TQ and LQ, as well as a combination of both using a grid based
on surface flux transport (SFT) models. The roles played by TQ and
LQ are also explored in the successful 2×2D dynamo model, which has
been optimized to reproduce the statistical behaviour of real solar
cycles. <BR /> Results: The relative importance of LQ versus TQ is
found to correlate with the ratio u<SUB>0</SUB>/η in the SFT model
grid, where u<SUB>0</SUB> is the meridional flow amplitude and η is
the diffusivity. An analytical interpretation of this result is given,
further demonstrating that the main underlying parameter is the dynamo
effectivity range, λ<SUB>R</SUB>, which is, in turn, determined by
the ratio of equatorial flow divergence to diffusivity. The relative
importance of LQ versus TQ is shown to scale as C_1+C_2/λ_R<SUP>2</SUP>
. The presence of a latitude quenching effect is seen in the 2×2D
dynamo, contributing to the nonlinear modulation by an amount that is
comparable to TQ. For other dynamo and SFT models considered in the
literature, the contribution of LQ to the modulation covers a broad
range - from entirely insignificant to serving as a dominant source
of feedback. On the other hand, the contribution of a TQ effect (with
the usually assumed amplitude) is never shown to be negligible.
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Title: The Polar Precursor Method for Solar Cycle Prediction:
Comparison of Predictors and Their Temporal Range
Authors: Kumar, Pawan; Nagy, Melinda; Lemerle, Alexandre; Karak,
Bidya Binay; Petrovay, Kristof
2021ApJ...909...87K Altcode: 2021arXiv210105013K
The polar precursor method is widely considered to be the most robust
physically motivated method to predict the amplitude of an upcoming
solar cycle. It uses indicators of the magnetic field concentrated near
the poles around the sunspot minimum. Here, we present an extensive
analysis of the performance of various such predictors, based on both
observational data (Wilcox Solar Observatory (WSO) magnetograms, Mount
Wilson Observatory polar faculae counts, and Pulkovo A(t) index) and
outputs (polar cap magnetic flux and global dipole moment) of various
existing flux transport dynamo models. We calculate Pearson correlation
coefficients (r) of the predictors with the next cycle amplitude
as a function of time measured from several solar cycle landmarks:
setting r = 0.8 as a lower limit for acceptable predictions, we find
that observations and models alike indicate that the earliest time
when the polar predictor can be safely used is 4 yr after the polar
field reversal. This is typically 2-3 yr before the solar minimum
and about 7 yr before the predicted maximum, considerably extending
the usual temporal scope of the polar precursor method. Reevaluating
the predictors another 3 yr later, at the time of the solar minimum,
further increases the correlation level to r ≳ 0.9. As an illustration
of the result, we determine the predicted amplitude of Cycle 25 based
on the value of the WSO polar field at the now official minimum date
of 2019 December as 126 ± 3. A forecast based on the value in early
2017, 4 yr after the polar reversal would have only differed from this
final prediction by 3.1 ± 14.7%.
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Title: Towards an algebraic method of solar cycle
prediction. I. Calculating the ultimate dipole contributions of
individual active regions
Authors: Petrovay, Kristóf; Nagy, Melinda; Yeates, Anthony R.
2020JSWSC..10...50P Altcode: 2020arXiv200902299P
We discuss the potential use of an algebraic method to compute
the value of the solar axial dipole moment at solar minimum, widely
considered to be the most reliable precursor of the activity level in
the next solar cycle. The method consists of summing up the ultimate
contributions of individual active regions to the solar axial dipole
moment at the end of the cycle. A potential limitation of the approach
is its dependence on the underlying surface flux transport (SFT) model
details. We demonstrate by both analytical and numerical methods that
the factor relating the initial and ultimate dipole moment contributions
of an active region displays a Gaussian dependence on latitude with
parameters that only depend on details of the SFT model through the
parameter η/Δ<SUB>u</SUB> where η is supergranular diffusivity
and Δ<SUB>u</SUB> is the divergence of the meridional flow on the
equator. In a comparison with cycles simulated in the 2 × 2D dynamo
model we further demonstrate that the inaccuracies associated with
the algebraic method are minor and the method may be able to reproduce
the dipole moment values in a large majority of cycles.
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Title: Towards an algebraic method of solar cycle
prediction. II. Reducing the need for detailed input data with ARDoR
Authors: Nagy, Melinda; Petrovay, Kristóf; Lemerle, Alexandre;
Charbonneau, Paul
2020JSWSC..10...46N Altcode: 2020arXiv200902300N
An algebraic method for the reconstruction and potentially prediction
of the solar dipole moment value at sunspot minimum (known to be a
good predictor of the amplitude of the next solar cycle) was suggested
in the first paper in this series. The method sums up the ultimate
dipole moment contributions of individual active regions in a solar
cycle: for this, detailed and reliable input data would in principle
be needed for thousands of active regions in a solar cycle. To reduce
the need for detailed input data, here we propose a new active region
descriptor called ARDoR (Active Region Degree of Rogueness). In a
detailed statistical analysis of a large number of activity cycles
simulated with the 2 × 2D dynamo model we demonstrate that ranking
active regions by decreasing ARDoR, for a good reproduction of the solar
dipole moment at the end of the cycle it is sufficient to consider
the top N regions on this list explicitly, where N is a relatively
low number, while for the other regions the ARDoR value may be set
to zero. For example, with N = 5 the fraction of cycles where the
dipole moment is reproduced with an error exceeding ±30% is only 12%,
significantly reduced with respect to the case N = 0, i.e. ARDoR set to
zero for all active regions, where this fraction is 26%. This indicates
that stochastic effects on the intercycle variations of solar activity
are dominated by the effect of a low number of large "rogue" active
regions, rather than the combined effect of numerous small ARs. The
method has a potential for future use in solar cycle prediction.
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Title: The determination of stellar temperatures from Baron
B. Harkányi to the Gaia mission
Authors: Petrovay, Kristof
2020arXiv200308092P Altcode:
The first determination of the surface temperature of stars other than
the Sun is due to the Hungarian astrophysicist Béla Harkányi. Prompted
by the recent unprecedented increase in the availability of stellar
temperature estimates from Gaia, coinciding with the 150th anniversary
of Harkányi's birth, this article presents the life and work of this
neglected, yet remarkable figure in the context of the history of
stellar astrophysics.
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Title: Solar cycle prediction
Authors: Petrovay, Kristóf
2020LRSP...17....2P Altcode: 2019arXiv190702107P
A review of solar cycle prediction methods and their performance is
given, including early forecasts for Cycle 25. The review focuses
on those aspects of the solar cycle prediction problem that have a
bearing on dynamo theory. The scope of the review is further restricted
to the issue of predicting the amplitude (and optionally the epoch)
of an upcoming solar maximum no later than right after the start of
the given cycle. Prediction methods form three main groups. Precursor
methods rely on the value of some measure of solar activity or magnetism
at a specified time to predict the amplitude of the following solar
maximum. The choice of a good precursor often implies considerable
physical insight: indeed, it has become increasingly clear that the
transition from purely empirical precursors to model-based methods
is continuous. Model-based approaches can be further divided into
two groups: predictions based on surface flux transport models and
on consistent dynamo models. The implicit assumption of precursor
methods is that each numbered solar cycle is a consistent unit in
itself, while solar activity seems to consist of a series of much less
tightly intercorrelated individual cycles. Extrapolation methods, in
contrast, are based on the premise that the physical process giving
rise to the sunspot number record is statistically homogeneous, i.e.,
the mathematical regularities underlying its variations are the same
at any point of time, and therefore it lends itself to analysis and
forecasting by time series methods. In their overall performance during
the course of the last few solar cycles, precursor methods have clearly
been superior to extrapolation methods. One method that has yielded
predictions consistently in the right range during the past few solar
cycles is the polar field precursor. Nevertheless, some extrapolation
methods may still be worth further study. Model based forecasts are
quickly coming into their own, and, despite not having a long proven
record, their predictions are received with increasing confidence by
the community.
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Title: Optimization of surface flux transport models for the solar
polar magnetic field
Authors: Petrovay, K.; Talafha, M.
2019A&A...632A..87P Altcode: 2019arXiv190906125P
Context. The choice of free parameters in surface flux transport (SFT)
models describing the evolution of the large-scale poloidal magnetic
field of the Sun is critical for the correct reproduction of the polar
magnetic flux built up during a solar cycle, which is known to be a
good predictor of the amplitude of the upcoming cycle. <BR /> Aims:
For an informed choice of parameters it is important to understand the
effects of and interplay among the various parameters and to optimize
the models for the polar magnetic field. <BR /> Methods: Here we present
the results of a large-scale systematic study of the parameter space in
an SFT model where the source term representing the net effect of tilted
flux emergence was chosen to represent a typical, average solar cycle
as described by observations. <BR /> Results: Comparing the results
with observational constraints on the spatiotemporal variation of the
polar magnetic field, as seen in magnetograms for the last four solar
cycles, we mark allowed and excluded regions in the 3D parameter space
defined by the flow amplitude u<SUB>0</SUB>, the magnetic diffusivity
η and the decay time scale τ, for three different assumed meridional
flow profiles. <BR /> Conclusions: Without a significant decay term in
the SFT equation (i.e., for τ > 10 yr) the global dipole moment
reverses too late in the cycle for all flow profiles and parameters,
providing independent supporting evidence for the need of a decay term,
even in the case of identical cycles. An allowed domain is found
to exist for τ values in the 5-10 yr range for all flow profiles
considered. Generally higher values of η (500-800 km<SUP>2</SUP>
s<SUP>-1</SUP>) are preferred though some solutions with lower η are
still allowed.
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Title: Precursors of an upcoming solar cycle at high latitudes from
coronal green line data
Authors: Petrovay, K.; Nagy, M.; Gerják, T.; Juhász, L.
2018JASTP.176...15P Altcode: 2018arXiv180205628P
After reviewing potential early indicators of an upcoming solar
cycle at high latitudes, we focus attention on the rush-to-the-poles
(RTTP) phenomenon in coronal green line emission. Considering various
correlations between properties of the RTTP with the upcoming solar
cycle we find a correlation between the rate of the RTTP and the time
delay until the maximum of the next solar cycle. On the basis of this
correlation and the known internal regularities of the sunspot number
series we predict that, following a minimum in 2019, cycle 25 will
peak in late 2024 at an amplitude of about 130 (in terms of smoothed
monthly revised sunspot numbers). This slightly exceeds the amplitude
of cycle 24 but it would still make cycle 25 a fairly weak cycle.
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Title: Rogue Active Regions and the Inherent Unpredictability of
the Solar Dynamo
Authors: Petrovay, Kristóf; Nagy, Melinda
2018IAUS..340..307P Altcode: 2018arXiv180403427P
New developments in surface flux transport modeling and theory of flux
transport dynamos have given rise to the notion that certain large
active regions with anomalous properties (location, tilt angle and/or
Hale/non-Hale character) may have a major impact on the course of solar
activity in subsequent years, impacting also on the amplitude of the
following solar cycles. Here we discuss our current understanding of
the role of such “rogue” active regions in cycle-to-cycle variations
of solar activity.
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Title: The Effect of "Rogue" Active Regions on the Solar Cycle
Authors: Nagy, Melinda; Lemerle, Alexandre; Labonville, François;
Petrovay, Kristóf; Charbonneau, Paul
2017SoPh..292..167N Altcode: 2017arXiv171202185N
The origin of cycle-to-cycle variations in solar activity is currently
the focus of much interest. It has recently been pointed out that
large individual active regions with atypical properties can have a
significant impact on the long-term behavior of solar activity. We
investigate this possibility in more detail using a recently developed
2 ×2 D dynamo model of the solar magnetic cycle. We find that even
a single "rogue" bipolar magnetic region (BMR) in the simulations
can have a major effect on the further development of solar activity
cycles, boosting or suppressing the amplitude of subsequent cycles. In
extreme cases, an individual BMR can completely halt the dynamo,
triggering a grand minimum. Rogue BMRs also have the potential to
induce significant hemispheric asymmetries in the solar cycle. To
study the effect of rogue BMRs in a more systematic manner, a series
of dynamo simulations were conducted, in which a large test BMR
was manually introduced in the model at various phases of cycles of
different amplitudes. BMRs emerging in the rising phase of a cycle
can modify the amplitude of the ongoing cycle, while BMRs emerging
in later phases will only affect subsequent cycles. In this model,
the strongest effect on the subsequent cycle occurs when the rogue
BMR emerges around cycle maximum at low latitudes, but the BMR does
not need to be strictly cross-equatorial. Active regions emerging as
far as 20<SUP>∘</SUP> from the equator can still have a significant
effect. We demonstrate that the combined effect of the magnetic flux,
tilt angle, and polarity separation of the BMR on the dynamo is via
their contribution to the dipole moment, δ D<SUB>BMR</SUB>. Our
results indicate that prediction of the amplitude, starting epoch,
and duration of a cycle requires an accurate accounting of a broad
range of active regions emerging in the previous cycle.
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Title: Magnetic cycles at different ages of stars
Authors: Oláh, K.; Kővári, Zs.; Petrovay, K.; Soon, W.; Baliunas,
S.; Kolláth, Z.; Vida, K.
2016A&A...590A.133O Altcode: 2016arXiv160406701O
<BR /> Aims: We study the different patterns of interannual magnetic
variability in stars on or near the lower main sequence, approximately
solar-type (G-K dwarf) stars in time series of 36 yr from the Mount
Wilson Observatory Ca II H&K survey. Our main aim is to search
for correlations between cycles, activity measures, and ages. <BR
/> Methods: Time-frequency analysis has been used to discern and
reveal patterns and morphology of stellar activity cycles, including
multiple and changing cycles, in the datasets. Both the results from
short-term Fourier transform and its refinement using the Choi-Williams
distribution, with better frequency resolution, are presented in this
study. Rotational periods of the stars were derived using multifrequency
Fourier analysis. <BR /> Results: We found at least one activity cycle
on 28 of the 29 stars we studied. Twelve stars, with longer rotational
periods (39.7 ± 6.0 days), have simple smooth cycles, and the remaining
stars, with much faster rotation (18.1 ± 12.2 days) on average, show
complex and sometimes vigorously changing multiple cycles. The cycles
are longer and quite uniform in the first group (9.7 ± 1.9 yr), while
they are generally shorter and vary more strongly in the second group
(7.6 ± 4.9). The clear age division between stars with smooth and
complex cycles follows the known separation between the older and
younger stars at around 2 to 3 Gyr of age.
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Title: Introduction to the Solar Activity Cycle: Overview of Causes
and Consequences
Authors: Balogh, A.; Hudson, H. S.; Petrovay, K.; von Steiger, R.
2015sac..book....1B Altcode:
No abstract at ADS
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Title: The Solar Activity Cycle
Authors: Balogh, André; Hudson, Hugh; Petrovay, Kristóf; von
Steiger, Rudolf
2015sac..book.....B Altcode:
No abstract at ADS
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Title: Oscillator Models of the Solar Cycle
Authors: Lopes, Ilídio; Passos, Dário; Nagy, Melinda; Petrovay,
Kristof
2015sac..book..535L Altcode:
No abstract at ADS
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Title: Solar Polar Fields and the 22-Year Activity Cycle: Observations
and Models
Authors: Petrie, G. J. D.; Petrovay, K.; Schatten, K.
2015sac..book..325P Altcode:
No abstract at ADS
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Title: Introduction to the Solar Activity Cycle: Overview of Causes
and Consequences
Authors: Balogh, A.; Hudson, H. S.; Petrovay, K.; von Steiger, R.
2014SSRv..186....1B Altcode: 2014SSRv..tmp...60B
The 11-year activity cycle is a dominant characteristic of the Sun. It
is the result of the evolution in time the solar dynamo that generates
the solar magnetic field. The nearly periodic variation in the sunspot
number has been known since the mid-1800s; as the observations of
the Sun broadened to cover an increasing number of phenomena, the
same 11-year periodicity was noted in most of them. The discovery of
solar magnetic fields introduced a 22-year periodicity, as the magnetic
polarities of the polar regions change sign every 11 years. Correlations
have been identified and quantified among all the measured parameters,
but in most cases such correlations remain empirical rather than
grounded in physical processes. This introductory paper and the reviews
in the volume describe and discuss the current state of understanding
of the causal chains that lead from the variable nature of the solar
magnetic fields to the variability of solar phenomena. The solar
activity cycle is poorly understood: predictions made for the current
Cycle 24 have proved to be generally wrong. However, the re-evaluation
of the relationships in the light of unexpected shortcomings is likely
to lead to a better physical understanding of solar physics. This
will help in the systematic reassessment of solar activity indices and
their usefulness in describing and predicting the solar activity cycle.
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Title: Oscillator Models of the Solar Cycle. Towards the Development
of Inversion Methods
Authors: Lopes, Ilídio; Passos, Dário; Nagy, Melinda; Petrovay,
Kristof
2014SSRv..186..535L Altcode: 2014arXiv1407.4918L; 2014SSRv..tmp...32L
This article reviews some of the leading results obtained in solar
dynamo physics by using temporal oscillator models as a tool to
interpret observational data and dynamo model predictions. We discuss
how solar observational data such as the sunspot number is used to
infer the leading quantities responsible for the solar variability
during the last few centuries. Moreover, we discuss the advantages
and difficulties of using inversion methods (or backward methods)
over forward methods to interpret the solar dynamo data. We argue
that this approach could help us to have a better insight about the
leading physical processes responsible for solar dynamo, in a similar
manner as helioseismology has helped to achieve a better insight on
the thermodynamic structure and flow dynamics in the Sun's interior.
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Title: Solar Polar Fields and the 22-Year Activity Cycle: Observations
and Models
Authors: Petrie, G. J. D.; Petrovay, K.; Schatten, K.
2014SSRv..186..325P Altcode: 2014SSRv..tmp...28P
We explore observations and models of the interacting, cyclical behavior
of the active regions and the polar magnetic fields of the Sun. We
focus on observational evidence of these fields interacting across the
corridor between active and polar latitudes. We present observations of
diverse magnetic signatures on, above and beneath the solar surface, and
find much evidence of phenomena migrating in both directions across this
corridor in each hemisphere, including photospheric fields, ephemeral
bipoles, interior torsional oscillations, high-latitude filaments,
and coronal green line intensity. Together these observations produce
a complex physical picture of high-latitude solar magnetic field
evolution in the photosphere, atmosphere and interior, and demonstrate
their essential role in the solar cycle. The picture presented by
these collected observations is consistent with the Babcock-Leighton
phenomenological model for the cycle, and we discuss related efforts to
predict cycle amplitudes based on polar field strengths and on combining
activity and polar-field information in a single phase-independent,
slowly-evolving index. We also briefly review related work on magnetic
flux transport models for the solar cycle, with particular reference
to the interaction between flux emergence patterns and meridional flows.
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Title: Turbulent magnetic energy spectrum and the cancellation
function of solar photospheric magnetic fields
Authors: Marschalkó, G.; Petrovay, K.; Petrovay, K.
2013AN....334..952M Altcode: 2014arXiv1404.1772M
A simple analytical relation of form α=2κ-1 between the magnetic
energy spectral exponent α of the turbulent magnetic field in the
solar photosphere and its magnetic flux cancellation exponent κ,
valid under certain restrictive assumptions, is tested and extended
outside its range of validity in a series of Monte Carlo simulations. In
these numerical tests artificial “magnetograms” are constructed in 1D
and 2D by superposing a discrete set of Fourier modes of the magnetic
field distribution with amplitudes following a power law spectrum and
measuring the cancellation function on these simulated magnetograms. Our
results confirm the validity of the analytical relation and extend it to
the domain α<-1 where κ-> 0 as α-> -∞. The observationally
derived upper limit of 0.38 on κ implies α<-0.24 in the granular
size range, apparently at odds with a small scale dynamo driven in
the inertial range.
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Title: Oscillator models of the solar cycle and the Waldmeier effect
Authors: Nagy, M.; Petrovay, K.
2013AN....334..964N Altcode: 2014arXiv1404.3668N
We study the behaviour of the van der Pol oscillator when either its
damping parameter μ or its nonlinearity parameter ξ is subject to
additive or multiplicative random noise. Assuming various power law
exponents for the relation between the oscillating variable and the
sunspot number, for each case we map the parameter plane defined by
the amplitude and the correlation time of the perturbation and mark
the parameter regime where the sunspot number displays solar-like
behaviour. Solar-like behaviour is defined here as a good correlation
between the rise rate and cycle amplitude and the lack of a good
correlation between the decay rate and amplitude, together with
significant ({⪆ 10} %) r.m.s. variation in cycle lengths and cycle
amplitudes. It is found that perturbing μ alone the perturbed van der
Pol oscillator does not show solar-like behaviour. When the perturbed
variable is ξ, solar-like behaviour is displayed for perturbations with
a correlation time of about 3-4 years and significant amplitude. Such
studies may provide useful constraints on solar dynamo models and
their parameters.
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Title: Flux transport dynamo coupled with a fast tachocline scenario
Authors: Karak, Bidya Binay; Petrovay, Kristof
2013IAUS..294..427K Altcode: 2012arXiv1211.0140K
The tachocline is important in the solar dynamo for the generation and
the storage of the magnetic fields. A most plausible explanation for the
confinement of the tachocline is given by the fast tachocline model in
which the tachocline is confined by the anisotropic momentum transfer by
the Maxwell stress of the dynamo generated magnetic fields. We employ
a flux transport dynamo model coupled with the simple feedback formula
of this fast tachocline model which basically relates the thickness
of the tachocline to the Maxwell stress. We find that this nonlinear
coupling not only produces a stable solar-like dynamo solution but
also a significant latitudinal variation in the tachocline thickness
which is in agreement with the observations.
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Title: Structures in compressible magnetoconvection and the nature
of umbral dots
Authors: Tian, C.; Petrovay, K.
2013A&A...551A..92T Altcode: 2011arXiv1105.5590T
Context. Structures seen in idealized numerical experiments on
compressible magnetoconvection in an imposed strong vertical magnetic
field show important differences from those detected in observations
or realistic numerical simulations of sunspot umbrae. <BR /> Aims:
To elucidate the origin of these discrepancies, we present a series
of idealized 3D compressible magnetoconvection experiments that differ
from previous such experiments in several details, bringing them closer
to realistic solar conditions. <BR /> Methods: An initially vertical
magnetic field B<SUB>0</SUB> is imposed on a time snapshot of fully
developed solar-like turbulent convection in a layer bounded by a stable
layer from above. Upon relaxation to a statistically steady state,
the structure of the flow field and magnetic field is examined. <BR />
Results: Instead of the vigorous granular convection (GRC) well known
to take place in magnetized or weakly magnetized convection, for high
values of B<SUB>0</SUB> heat is transported by small-scale convection
(SSC) in the form of narrow, persistent convective columns consisting
of slender upflows accompanied by adjacent downflow patches, which are
reminiscent of the "convectons" identified in earlier semianalytic
models. For moderate field strengths, flux separation (FXS) is
observed: isolated field-free inclusions of GRC are embedded in a
strongly magnetized plasma with SSC. Between the SSC and FXS regimes,
a transitional regime (F/S) is identified where convectons dynamically
evolve into multiply segmented granular inclusions and back. <BR />
Conclusions: Our results agree in some aspects more closely with
observed umbral structures than earlier idealized models, because
they do reproduce the strong localized, patchy downflows immediately
adjacent to the narrow convective columns. Based on recent observations
of umbral dots, we suggest that in some cases the conditions in sunspot
umbræ correspond to the newly identified F/S transitional regime.
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Title: On the Compatibility of a Flux Transport Dynamo with a Fast
Tachocline Scenario
Authors: Karak, Bidya Binay; Petrovay, Kristof
2013SoPh..282..321K Altcode: 2012arXiv1209.0319K
The compatibility of the fast-tachocline scenario with a flux-transport
dynamo model is explored. We employ a flux-transport dynamo model
coupled with simple feedback formulae relating the thickness of the
tachocline to the amplitude of the magnetic field or to the Maxwell
stress. The dynamo model is found to be robust against the nonlinearity
introduced by this simplified fast-tachocline mechanism. Solar-like
butterfly diagrams are found to persist and, even without any parameter
fitting, the overall thickness of the tachocline is well within the
range admitted by helioseismic constraints. In the most realistic case
of a time- and latitude-dependent tachocline thickness linked to the
value of the Maxwell stress, both the thickness and its latitudinal
dependence are in excellent agreement with seismic results. In
nonparametric models, cycle-related temporal variations in tachocline
thickness are somewhat larger than admitted by helioseismic constraints;
we find, however, that introducing a further parameter into our feedback
formula readily allows further fine tuning of the thickness variations.
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Title: Solar Cycle Prediction
Authors: Petrovay, Kristóf
2010LRSP....7....6P Altcode: 2010arXiv1012.5513P
A review of solar cycle prediction methods and their performance is
given, including forecasts for cycle 24. The review focuses on those
aspects of the solar cycle prediction problem that have a bearing
on dynamo theory. The scope of the review is further restricted to
the issue of predicting the amplitude (and optionally the epoch) of
an upcoming solar maximum no later than right after the start of the
given cycle.
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Title: Turbulence in the Solar Atmosphere and Solar Wind
Authors: Petrosyan, A.; Balogh, A.; Goldstein, M. L.; Léorat, J.;
Marsch, E.; Petrovay, K.; Roberts, B.; von Steiger, R.; Vial, J. C.
2010SSRv..156..135P Altcode: 2010SSRv..tmp..117P
The objective of this review article is to critically analyze turbulence
and its role in the solar atmosphere and solar wind, as well as to
provide a tutorial overview of topics worth clarification. Although
turbulence is a ubiquitous phenomenon in the sun and its heliosphere,
many open questions exist concerning the physical mechanisms of
turbulence generation in solar environment. Also, the spatial and
temporal evolution of the turbulence in the solar atmosphere and solar
wind are still poorly understood. We limit the scope of this paper
(leaving out the solar interior and convection zone) to the magnetized
plasma that reaches from the photosphere and chromosphere upwards to
the corona and inner heliosphere, and place particular emphasis on
the magnetic field structures and fluctuations and their role in the
dynamics and radiation of the coronal plasma. To attract the attention
of scientists from both the fluid-dynamics and space-science communities
we give in the first two sections a phenomenological overview of
turbulence-related processes, in the context of solar and heliospheric
physics and with emphasis on the photosphere-corona connection and
the coupling between the solar corona and solar wind. We also discuss
the basic tools and standard concepts for the empirical analysis and
theoretical description of turbulence. The last two sections of this
paper give a concise review of selected aspects of oscillations and
waves in the solar atmosphere and related fluctuations in the solar
wind. We conclude with some recommendations and suggest topics for
future research.
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Title: An analytic interface dynamo over a shear layer of finite depth
Authors: Petrovay, K.; Kerekes, A.; Erdelyi, R.
2010GApFD.104..619P Altcode: 2010arXiv1007.1634P
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 α-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
behavior, 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.
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Title: The Magnetic Sun: Reversals and Long-Term Variations
Authors: Petrovay, K.; Christensen, U. R.
2010SSRv..155..371P Altcode: 2010SSRv..tmp...64P; 2010arXiv1004.5102P
A didactic introduction to current thinking on some aspects of the
solar dynamo is given for geophysicists and planetary scientists.
---------------------------------------------------------
Title: Harmonic analysis approach to solar cycle prediction and the
Waldmeier effect
Authors: Petrovay, K.
2010IAUS..264..150P Altcode:
It is demonstrated that, in addition to the precursor method, harmonic
analysis approach to solar cycle prediction is also strongly conditioned
by the Waldmeier effect.
---------------------------------------------------------
Title: The Magnetic Sun: Reversals and Long-Term Variations
Authors: Petrovay, K.; Christensen, U. R.
2010tema.book..371P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The importance of facular and plage data for the understanding
of the solar cycle
Authors: Petrovay, Kristof
2010cosp...38.2885P Altcode: 2010cosp.meet.2885P
Most global analyses of the solar activity cycle are based on sunspot
data as other indicators such as F10.8 or flare data only cover the
last few solar cycles. The only solar activity param-eter covering
a historically significant time span is the facular area data set
in the Greenwich catalogue. In this poster we first demonstrate the
use of such data for the study of the solar cycle by considering
similarities and differences in the Waldmeier effect in faculae as
opposed to sunspots. The Greenwich catalogue was discontinued in 1976,
so the extension of such studies to more recent times relies on "proxy"
data such as plage areas; however, due to the short overlap in time,
cross-calibration of these data has not been consistently made. We
discuss the possibilities offered by SDO to cross-correlate facular
areas with their proxies and to set up a new systematic data set of
these activity indices.
---------------------------------------------------------
Title: Solar and planetary dynamos: comparison and recent developments
Authors: Petrovay, K.
2009IAUS..257...71P Altcode: 2009arXiv0901.0839P
While obviously having a common root, solar and planetary dynamo
theory have taken increasingly divergent routes in the last two or
three decades, and there are probably few experts now who can claim
to be equally versed in both. Characteristically, even in the fine and
comprehensive book “The magnetic Universe” (Rudiger & Hollerbach
2004), the chapters on planets and on the Sun were written by different
authors. Separate reviews written on the two topics include Petrovay
(2000), Charbonneau (2005), Choudhuri (2008) on the solar dynamo and
Glatzmaier (2002), Stevenson (2003) on the planetary dynamo. In the
following I will try to make a systematic comparison between solar
and planetary dynamos, presenting analogies and differences, and
highlighting some interesting recent results.
---------------------------------------------------------
Title: “Black Star” or Astrophysical Black Hole?
Authors: Petrovay, K.
2008AIPC..968..426P Altcode: 2007arXiv0707.2450P
Recently wide publicity has been given to a claim by T. Vachaspati that
“black holes do not exist”, that the objects known as black holes in
astrophysics should rather be called “black stars” and they not only
do not have event horizons but actually can be the source of spectacular
gamma ray bursts. In this short essay (no flimsier than the original
preprint where these extravagant claims appeared) I demonstrate that
these ill-considered claims are clearly wrong. Yet they present a good
occasion to reflect on some well known but little discussed conceptual
difficulties which arise when applying relativistic terminology in an
astrophysical context.
---------------------------------------------------------
Title: A theoretical model for the magnetic helicity of solar
active regions
Authors: Chatterjee, Piyali; Choudhuri, Arnab Rai; Petrovay, Kristof;
Nandy, Dibyendu
2008AdSpR..41..893C Altcode:
Active regions on the solar surface are known to possess magnetic
helicity, which is predominantly negative in the northern hemisphere
and positive in the southern hemisphere. Choudhuri et al. [Choudhuri,
A.R. On the connection between mean field dynamo theory and flux
tubes. Solar Phys. 215, 31 55, 2003] proposed that the magnetic helicity
arises due to the wrapping up of the poloidal field of the convection
zone around rising flux tubes which form active regions. Choudhuri
[Choudhuri, A.R., Chatterjee, P., Nandy, D. Helicity of solar active
regions from a dynamo model. ApJ 615, L57 L60, 2004] used this idea to
calculate magnetic helicity from their solar dynamo model. Apart from
getting broad agreements with observational data, they also predict
that the hemispheric helicity rule may be violated at the beginning
of a solar cycle. Chatterjee et al. [Chatterjee, P., Choudhuri, A.R.,
Petrovay, K. Development of twist in an emerging magnetic flux tube
by poloidal field accretion. A&A 449, 781 789, 2006] study the
penetration of the wrapped poloidal field into the rising flux tube
due to turbulent diffusion using a simple 1-d model. They find that
the extent of penetration of the wrapped field will depend on how
weak the magnetic field inside the rising flux tube becomes before
its emergence. They conclude that more detailed observational data
will throw light on the physical conditions of flux tubes just before
their emergence to the photosphere.
---------------------------------------------------------
Title: From solar cycle to solar cycle
Authors: Petrovay, Kristof
2008cosp...37.2423P Altcode: 2008cosp.meet.2423P
The start of the new solar cycle prompts us to wonder what it will be
like? Strong or weak, long or short? There are sharply conflicting
predictions concerning the behaviour of solar activity in the next
few years, based on different models of the solar dynamo and on
empirical relationships. To what extent is the long-term variation of
solar activity characterized by regularities, chaotic or stochastic
behaviour, and to what extent can it be predicted? The talk reviews
this subject, focusing on model-based prediction methods and also
presenting a possible new mechanism for the origin of grand minima of
solar activity within the framework of the interface dynamo scenario.
---------------------------------------------------------
Title: On the possibility of a bimodal solar dynamo
Authors: Petrovay, K.
2007AN....328..777P Altcode: 2007arXiv0708.2131P
A simple way to couple an interface dynamo model to a fast tachocline
model is presented, under the assumption that the dynamo saturation
is due to a quadratic process and that the effect of finite shear
layer thickness on the dynamo wave frequency is analogous to the
effect of finite water depth on surface gravity waves. The model
contains one free parameter which is fixed by the requirement that a
solution should reproduce the helioseismically determined thickness
of the tachocline. In this case it is found that, in addition to this
solution, another steady solution exists, characterized by a four
times thicker tachocline and 4-5 times weaker magnetic fields. It
is tempting to relate the existence of this second solution to the
occurrence of grand minima in solar activity.
---------------------------------------------------------
Title: Molecular cloud abundances and anomalous diffusion
Authors: Marschalkó, G.; Forgács-Dajka, E.; Petrovay, K.
2007AN....328..871M Altcode: 2008arXiv0801.0510M
The chemistry of molecular clouds has been studied for decades,
with an increasingly general and sophisticated treatment of the
reactions involved. Yet the treatment of turbulent diffusion has
remained extremely sketchy, assuming simple Fickian diffusion
with a scalar diffusivity D. However, turbulent flows similar to
those in the interstellar medium are known to give rise to anomalous
diffusion phenomena, more specifically superdiffusion (increase of the
diffusivity with the spatial scales involved). This paper considers
to what extent and in what sense superdiffusion modifies molecular
abundances in interstellar clouds. For this first exploration of the
subject we employ a very rough treatment of the chemistry and the effect
of non-uniform cloud density on the diffusion equation is also treated
in a simplified way. The results nevertheless clearly demonstrate that
the effect of superdiffusion is quite significant, abundance values
at a given radius being modified by order of unity factors.
---------------------------------------------------------
Title: The origin of magnetic helicity in solar active regions
Authors: Petrovay, K.
2007AIPC..934....3P Altcode:
The magnetic fields in solar active regions are known to be helical. The
current helicity, as measured from magnetograms, is negative on the
northern and positive in the southern hemisphere, its normalized mean
value being α<SUB>p</SUB>≡B.(∇×B)/B<SUP>2</SUP>~10<SUP>-8</SUP>
m<SUP>-1</SUP>. Recent observations have convincingly demonstrated
that the helicity originates in subsurface regions and the flux tubes
emerge in an already helical form. One possible contribution to this
helicity is the accretion of the weak general poloidal magnetic field
of the Sun by the rising toroidal flux loop. For this process to work,
the field lines of the background field first need to be wrapped around
the rising flux tube, which assumes that freezing-in holds to a good
degree of approximation. The wrapped-up field then has to diffuse into
the flux tube, giving rise to a twisted magnetic field structure. The
conflict between the requirements of freezing-in (for wrapup) and
diffusion (for penetration into the tube) is only apparent. The role
of diffusion is likely to be limited during most of the rise of the
tube, allowing a significant amount of flux to be swept up; then, in
the uppermost 20-30% of the tube's path through the convective zone
diffusion prevails, allowing penetration of the wrapped up flux.
---------------------------------------------------------
Title: The Effect of Abnormal Granulation on Acoustic Wave Travel
Times and Mode Frequencies
Authors: Petrovay, K.; Erdélyi, R.; Thompson, M. J.
2007SoPh..240..197P Altcode: 2007astro.ph..2076P
Observations indicate that in plage areas (i.e. in active regions
outside sunspots) acoustic waves travel faster than in the quiet Sun,
leading to shortened travel times and higher p-mode frequencies. Coupled
with the 11-year variation of solar activity, this may also explain the
solar cycle variation of oscillation frequencies. While it is clear
that the ultimate cause of any difference between the quiet Sun and
plage is the presence of magnetic fields of order 100 G in the latter,
the mechanism by which the magnetic field exerts its influence has
not yet been conclusively identified. One possible such mechanism is
suggested by the observation that granular motions in plage areas tend
to be slightly "abnormal", dampened compared to the quiet Sun.
---------------------------------------------------------
Title: Helical Magnetic Fields in Solar Active Regions: Theory
vs. Observations
Authors: Petrovay, K.; Chaterjee, P.; Choudhuri, A.
2007astro.ph..2073P Altcode:
The mean value of the normalized current helicity in solar active
regions is on the order of 1e-8 1/m, negative in the northern
hemisphere, positive in the southern hemisphere. Observations indicate
that this helicity has a subsurface origin. Possible mechanisms leading
to a twist of this amplitude in magnetic flux tubes include the solar
dynamo, convective buffeting of rising flux tubes, and the accretion
of weak external poloidal flux by a rising toroidal flux tube. After
briefly reviewing the observational and theoretical constraints on
the origin of helicity, we present a recently developed detailed model
for poloidal flux accretion.
---------------------------------------------------------
Title: Helical Magnetic Fields in Solar Active Regions: Theory
vs. Observations
Authors: Petrovay, K.; Chaterjee, P.; Choudhuri, A.
2006PADEU..17....5P Altcode:
The mean value of the normalized current helicity
α_p=čB\cdot(nabla×čB)/B^2 in solar active regions is on the order
of 10^{-8} m^{-1}, negative in the northern hemisphere, positive in
the southern hemisphere. Observations indicate that this helicity
has a subsurface origin. Possible mechanisms leading to a twist
of this amplitude in magnetic flux tubes include the solar dynamo,
convective buffeting of rising flux tubes, and the accretion of weak
external poloidal flux by a rising toroidal flux tube. After briefly
reviewing the observational and theoretical constraints on the origin
of helicity, we present a recently developed detailed model for poloidal
flux accretion.
---------------------------------------------------------
Title: Sound Wave Travel Times in Plage Areas - The Effect of
Abnormal Granulation
Authors: Petrovay, K.; Erdélyi, R.; Thompson, M. J.
2006ESASP.617E..73P Altcode: 2006soho...17E..73P
No abstract at ADS
---------------------------------------------------------
Title: On the Origin of Current Helicity in Active Regions
Authors: Petrovay, K.; Chatterjee, P.; Choudhuri, A.
2006ESASP.617E..67P Altcode: 2006soho...17E..67P
No abstract at ADS
---------------------------------------------------------
Title: Development of twist in an emerging magnetic flux tube by
poloidal field accretion
Authors: Chatterjee, P.; Choudhuri, A. R.; Petrovay, K.
2006A&A...449..781C Altcode: 2005astro.ph.12472C
Aims.Following an earlier proposal for the origin of twist in the
magnetic fields of solar active regions, we model the penetration of
a wrapped up background poloidal field into a toroidal magnetic flux
tube rising through the solar convective zone. Methods.The rise of the
straight, cylindrical flux tube is followed by numerically solving the
induction equation in a comoving Lagrangian frame, while an external
poloidal magnetic field is assumed to be radially advected onto the
tube with a speed corresponding to the rise velocity. Results.One
prediction of our model is the existence of a ring of reverse current
helicity on the periphery of active regions. On the other hand, the
amplitude of the resulting twist depends sensitively on the assumed
structure (diffuse vs. concentrated/intermittent) of the active
region magnetic field right before its emergence, and on the assumed
vertical profile of the poloidal field. Nevertheless, in the model
with the most plausible choice of assumptions a mean twist comparable
to the observations results. Conclusions.Our results indicate that the
contribution of this mechanism to the twist can be quite significant,
and under favourable circumstances it can potentially account for most
of the current helicity observed in active regions.
---------------------------------------------------------
Title: A History of the Department of Astronomy
Authors: Petrovay, K.
2006PADEU..16...69P Altcode:
The head of the observatory at Nagyszombat, founded in in 1755,
already had the privileges of a full professor, so the foundation
of the observatory can be regarded as the beginning of the Chair
of Astronomy. Both the university and its department of astronomy
have been moved and renamed on several occasions. Nevertheless,
the institutional continuity can be traced down to the contemporary
Department of Astronomy of Eötvös University.
---------------------------------------------------------
Title: A theoretical model for the magnetic helicity of solar
active regions
Authors: Choudhuri, A. R.; Chatterjee, P.; Petrovay, K.; Nandy, D.
2006cosp...36..714C Altcode: 2006cosp.meet..714C
Active regions on the solar surface are known to possess magnetic
helicity which is predominantly negative in the northern hemisphere
and positive in the southern hemisphere Choudhuri 2003 Sol Phys 123 217
proposed that the magnetic helicity arises due to the wrapping up of the
poloidal field of the convection zone around rising flux tubes which
form active regions Choudhuri Chatterjee and Nandy 2004 ApJ 615 L57
used this idea to calculate magnetic helicity from their solar dynamo
model and found broad agreements with observational data Chatterjee
Choudhuri and Petrovay 2006 A A in press have studied the penetration
of the wrapped poloidal field into the rising flux tube and concluded
that more detailed observational data will throw light on the physical
conditions of flux tubes just before their emergence to the photosphere
---------------------------------------------------------
Title: British-Romanian-Hungarian N+N+N Workshop for Young Researchers
on Plasma- and Astrophysics: from laboratory to outer space
Authors: Ballai, I.; Forgács-Dajka, E.; Marcu, A.; Petrovay, K.
2005PADEU..15.....B Altcode:
The International Networking for Young Scientists (INYS), initiated
and funded by the British Council, models itself on the N+N concept,
where N+N refers to a workshop involving a number of researchers from
the UK and an equal number of local researchers; ideally eight or more
from each country. INYS is an initiative that supports the British
Council's purpose of "nurturing mutually beneficial relationships with
other countries", by encouraging and facilitating the mobility of, and
direct contact between, young researchers (and their supervisors). It
supports face-to-face meetings between young scientists and engineers
from the UK and other countries, for the exchange of ideas, knowledge
and information and the building of international connections that
assist the innovation process. The meeting was an extended N+N
workshop, where young scientist from UK, Romania and Hungary (N+N+N)
got together to discuss their results, to initiate further real and
virtual networking and to identy common research areas. It was the
first time that Babes-Bolyai University (Cluj, Romania) has hosted such
a meeting and the support by the University (Department of Physics,
in particular) gives us hope that there will be other events to
follow. The topic of the meeting was "Plasma- and astrophysics: from
laboratory to outer space". Plasma is a gaseous state, commonly known
as the fourth state of matter and it is made of electrically charged
particles where the dynamics of particles is controlled by ambient
magnetic fields. Plasmas make up almost 99 per cent of astrophysical
objects such as galaxies, stars and supernovae. On Earth, plasmas
exist naturally as lightning bolts and the bath of charged particles
in the Earth's upper atmosphere. In the high-tech electronics industry,
beams of artificially created plasmas engrave sophisticated patterns on
computer chips. In an attempt to provide mankind with an abundant source
of cheap clean energy, scientists work hard to make artificial suns:
plasmas so hot and so dense that their particles fuse to release free
energy. This pursuit of nuclear fusion, as a practical energy source,
is another major branch of plasma physics research. The meeting was
hosted by the Babes-Bolyai University and took place from 17th to
19th January, 2005. The Host provided all the technical support and
the infrastructure needed for a smooth and successful meeting. The
event was opened by Prof. Laszlo Nagy (Pro-Vice Chancellor) and Mrs
Monica Marasescu (British Council).
---------------------------------------------------------
Title: The Sun as a laboratory for turbulence theory: the problem
of anomalous diffusion
Authors: Petrovay, K.
2005PADEU..15...53P Altcode:
The solar atmosphere offers a unique possibility to study tuubulent
motions under conditions presently unattainable in laboratory experiment
or even numerical simulations. This short review will focus on one
controversial issue in turbulence theory, on which some light can be
shed by solar observations: anomalous turbulent diffusion.
---------------------------------------------------------
Title: The effect of a meridional flow on Parker's interface dynamo
Authors: Petrovay, K.; Kerekes, A.
2004MNRAS.351L..59P Altcode: 2004astro.ph..4607P; 2004MNRAS.tmp..150P
Parker's interface dynamo is generalized to the case when a
homogeneous flow is present in the high-diffusivity (upper) layer in
the lateral direction (i.e. perpendicular to the shear flow in the
lower layer). This is probably a realistic first representation of
the situation near the bottom of the solar convective zone, as the
strongly subadiabatic stratification of the tachocline (lower layer in
the interface dynamo) imposes a strong upper limit on the speed of any
meridional flow there. <P />Analytic solutions to the eigenvalue problem
are presented for the cases of vanishing diffusivity contrast and
infinite diffusivity contrast. Unlike the trivial case of a homogeneous
system, the ability of the meridional flow to reverse the propagation
of the dynamo wave is strongly reduced in the interface dynamo. In
particular, in the limit of high diffusivity contrast relevant to the
solar case it is found that a meridional flow of realistic amplitude
cannot reverse the direction of propagation of the dynamo wave. The
implications of this result for the solar dynamo problem are discussed.
---------------------------------------------------------
Title: British-Hungarian N+N Workshop for Young Researchers On
Computer processing and use of satellite data in astronomy and
astrophysics and 3rd Workshop of Young Researchers in Astronomy
& Astrophysics
Authors: Forgács-Dajka, E.; Petrovay, K.; Erdélyi, R.
2004PADEU..14.....F Altcode:
The N+N Young Researchers' Workshop scheme, initiated and funded by the
British Council, aims to provide an opportunity for young researchers
to exchange ideas, knowledge and information by coming together in the
form of N+N workshops and meetings. Here the term N+N workshop refers
to a workshop involving a number of researchers from the UK and an
equal number of local researchers. The workshops are followed by real
and virtual networking to sustain the contacts made with a view to
produce a proposal for longer term externally funded collaboration or
applications for further funding. As at the Department of Astronomy
of the Eötvös University we have had a tradition of national
workshops of young researchers in astronomy and astrophysics with
similar characteristics, the call for meeting proposals by the British
Council seemed an ideal way to extend the geographical scope of our
meeting, while still maintaining its general format and spirit. Thus,
this year's Hungarian Young Researchers' Workshop in Astronomy and
Astrophysics was held in tandem with the British-Hungarian N+N workshop,
and the contributions are also presented together in this volume. For
the topic of this year's meeting we chose "Computer processing and use
of satellite data in astronomy and astrophysics". The reason for this
is that, thanks to a high number of space probes, in the past decades
a vast amount data has been collected from the extraterrestrial world,
from the magnetosphere to the most distant galaxies and beyond. We are
now in a situation where the amount data grows much faster than the
speed by which they can be processed and duly analyzed. The workshop was
devoted to methods aimed at improving on this situation, as well as to
scientific results born out of the use of space data. The workshop was
open to post-doctoral scientists and engineers and those tenured for
five years or less. PhD students in an advanced phase of their project
were also admitted. The number of participants from the U.K. was nine,
while a total of 13 applications were accepted from Hungary for the
N+N workshop (with many more participants at the adjoining national
workshop). It is to be noted that from the nine UK participants, only
four were British citizens, the rest hailing from Armenia, Germany,
Hungary, and Italy. Nevertheless, all have been residing in the UK for
years, and this composition reflects well the international character
and openness of current British academic research.
---------------------------------------------------------
Title: Past and future of scientific space missions: an overview
Authors: Petrovay, K.
2004PADEU..14....5P Altcode:
After briefly mentioning a few highlights of recent, current and future
scientific space missions, an extensive list of all the most important
such missions, together with web links, is given.
---------------------------------------------------------
Title: Turbulence, Waves and Instabilities in the Solar Plasma
Authors: Erdélyi, R.; Petrovay, K.; Roberts, B.; Aschwanden, M.
2003twis.book.....E Altcode:
Significant advances have been made recently in both the theoretical
understanding and observation of small-scale turbulence in different
layers of the Sun, and in the instabilities that give rise to them. The
general development of solar physics, however, has led to such a
degree of specialization as to hinder interaction between workers in
the field. This book therefore presents studies of different layers
and regions of the Sun, but from the same aspect, concentrating on
the study of small-scale motions. The main emphasis is on the common
theoretical roots of these phenomena, but the book also contains
an extensive treatment of the observational aspects. <P />Link: <A
href="http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&changeHeader=true">http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&changeHeader=true</A>
---------------------------------------------------------
Title: A Consistent one-Dimensional Model for the Turbulent Tachocline
Authors: Petrovay, K.
2003SoPh..215...17P Altcode: 2003astro.ph..3034P
The first consistent model for the turbulent tachocline is presented,
with the turbulent diffusivity computed within the model instead of
being specified arbitrarily. For the origin of the 3D turbulence a new
mechanism is proposed. Owing to the strongly stable stratification,
the mean radial shear is stable, while the horizontal shear is expected
to drive predominantly horizontal, quasi-2D motions in thin slabs. Here
I suggest that a major source of 3D overturning turbulent motions in
the tachocline is the secondary shear instability due to the strong,
random vertical shear arising between the uncorrelated horizontal flows
in neighboring slabs. A formula for the vertical diffusivity due to
this turbulence, Equation (9), is derived and applied in a simplified
1D model of the tachocline. It is found that Maxwell stresses due to
an oscillatory poloidal magnetic field of a few hundred gauss are able
to confine the tachocline to a thickness less than 5 Mm. The integral
scale of the 3D overturning turbulence is the buoyancy scale, on the
order of 10 km, and its velocity amplitude is a few m s<SUP>−1</SUP>,
yielding a vertical turbulent diffusivity on the order of 10<SUP>8</SUP>
cm<SUP>2</SUP> s<SUP>−1</SUP>.
---------------------------------------------------------
Title: Contributions to NATO Advanced Research Workshop Turbulence,
Waves, and Instabilities in the Solar Plasma
Authors: Forgács-Dajka, E.; Petrovay, K.; Erdélyi, R.
2003PADEU..13.....F Altcode:
This volume contains focus reviews, oral contributions and poster
papers presented at the NATO Advanced Research Workshop “Turbulence,
Waves, and Instabilities in the Solar Plasma”, held at Hotel Normafa,
Budapest, 16-20 September, 2002. The more exensive invited reviews
presented at the same meeting are published by Kluwer in a companion
volume, with the same title as that of the meeting. The purpose of the
workshop was to facilitate interchange and communication between diverse
groups studying different layers and regions of the Sun but from the
same aspect, concentrating on the study of small-scale motions. While
the emphasis was on the common theoretical roots of these phenomena,
observational aspects were not excluded either. The selection of
invited speakers concentrated on the researchers currently most
active in the field, mostly on a post-doctoral/tenure/fresh faculty
position level. A number of senior experts and PhD students were
also invited. Scientists from NATO partner countries were especially
encouraged to apply. Altogether, 50 scientists from 11 different
countries participated in the workshop. The relative isolation of the
venue, as well as the fact that the participants all lived at the same
place, where the conference was also held, contributed to the success
of the meeting, offering plenty of opportunities to meet and exchange
ideas. We are convinced that many of the papers in the present volume
will prove to be a very useful reference for some rarely discussed
chapters of solar physics.
---------------------------------------------------------
Title: A new model for the lower overshoot layer in the Sun
Authors: Marik, D.; Petrovay, K.
2002A&A...396.1011M Altcode: 2003astro.ph..7534M
We present a model for the lower overshoot layer of the Sun,
based on the realistic solar stratification, without the use of a
“mixing-length” parameter, by solving the system of Reynolds momentum
equations using the closure formalism of Canuto & Dubovikov
(\cite{Canuto+Dubov:1,Canuto+Dubov:2}). A fixed value of velocity
anisotropy is assumed, and the local convection model is assumed
to be valid for the convectively unstable layer. In accordance with
seismic constraints, overshoot (defined as the amount by which the
convectively mixed zone extends beyond its boundary in local theory)
is found to be as low as about 6 percent of the pressure scale height,
and it is not bounded by a discontinuity from below.
---------------------------------------------------------
Title: Dynamics of the fast solar tachocline. I. Dipolar field
Authors: Forgács-Dajka, E.; Petrovay, K.
2002A&A...389..629F Altcode: 2002astro.ph..1241F
One possible scenario for the origin of the solar tachocline, known as
the “fast tachocline”, assumes that the turbulent diffusivity exceeds
eta >~ 10<SUP>9</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP>. In this case the
dynamics will be governed by the dynamo-generated oscillatory magnetic
field on relatively short timescales. Here, for the first time, we
present detailed numerical models for the fast solar tachocline with all
components of the magnetic field calculated explicitly, assuming axial
symmetry and a constant turbulent diffusivity eta and viscosity nu . We
find that a sufficiently strong oscillatory poloidal field with dipolar
latitude dependence at the tachocline-convective zone boundary is able
to confine the tachocline. Exploring the three-dimensional parameter
space defined by the viscosity in the range log nu =9-11, the magnetic
Prandtl number in the range Pr<SUB>m</SUB>=0.1-10, and the meridional
flow amplitude (-3 to +3 cm s<SUP>-1</SUP>), we also find that the
confining field strength B<SUB>conf</SUB>, necessary to reproduce the
observed thickness of the tachocline, increases with viscosity nu ,
with magnetic Prandtl number nu /eta , and with equatorward meridional
flow speed. Nevertheless, the resulting B<SUB>conf</SUB> values
remain quite reasonable, in the range 10<SUP>3</SUP>-10<SUP>4</SUP> G,
for all parameter combinations considered here. The thickness of the
tachocline shows a marked dependence on both time and latitude. The
latitude dependence is similar to that inferred by helioseismology,
while the time dependence is within the observational errors.
---------------------------------------------------------
Title: How Turbulent is the Tachocline?
Authors: Petrovay, Kristóf
2002smra.progE..20P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Role of Active Regions in the Generation of Torsional
Oscillations
Authors: Petrovay, K.; Forgács-dajka, E.
2002SoPh..205...39P Altcode: 2001astro.ph..8297P
We present a model for torsional oscillations where the inhibiting
effect of active region magnetic fields on turbulence locally reduces
turbulent viscous torques, leading to a cycle- and latitude-dependent
modulation of the differential rotation. The observed depth dependence
of torsional oscillations as well as their phase relationship with
the sunspot butterfly diagram are reproduced quite naturally in
this model. The resulting oscillation amplitudes are significantly
smaller than observed, though they depend rather sensitively on model
details. Meridional circulation is found to have only a weak effect
on the oscillation pattern.
---------------------------------------------------------
Title: Tachocline Confinement by an Oscillatory Magnetic Field
Authors: Forgács-dajka, E.; Petrovay, K.
2001SoPh..203..195F Altcode: 2001astro.ph..6133F
Helioseismic measurements indicate that the solar tachocline is very
thin, its full thickness not exceeding 4% of the solar radius. The
mechanism that inhibits differential rotation to propagate from the
convective zone to deeper into the radiative zone is not known, though
several propositions have been made. In this paper we demonstrate
by numerical models and analytic estimates that the tachocline can
be confined to its observed thickness by a poloidal magnetic field
B<SUB>p</SUB> of about one kilogauss, penetrating below the convective
zone and oscillating with a period of 22 years, if the tachocline region
is turbulent with a diffusivity of η∼10<SUP>10</SUP> cm<SUP>2</SUP>
s<SUP>−1</SUP> (for a turbulent magnetic Prandtl number of unity). We
also show that a similar confinement may be produced for other pairs
of the parameter values (B<SUB>p</SUB>, η). The assumption of the
dynamo field penetrating into the tachocline is consistent whenever
η≳10<SUP>9</SUP> cm<SUP>2</SUP> s<SUP>−1</SUP>.
---------------------------------------------------------
Title: Can an inverse Λ-effect explain the thin tachocline?
Authors: Forgács-Dajka, E.; Petrovay, K.
2001ESASP.464..301F Altcode: 2001soho...10..301F
Helioseismic measurements indicate the existence of a transition layer
(the tachocline) between the differentially rotating convection zone and
the rigidly rotating radiative interior. According to the theoretical
models, the differential rotation should penetrate into the deep layers
of the radiative interior, but, based on helioseismic measurements, the
tachocline is thin. This is known as the "Thin Tachocline Problem". In
this paper we examine the influence of an assumed inverse Λ-effect
on the radial spreading of the differential rotation. This effect may
contribute to horizontal angular momentum transport below the convection
zone. Our results indicate that an inverse Λ-effect can only reduce the
thickness of the tachocline to the observed value if an unrealistically
high amplitude is assumed. Thus, our results practically exclude the
possibility that the thin tachocline is a consequence of a Λ-effect.
---------------------------------------------------------
Title: Numerical solutions of the momentum equations for the lower
overshoot layer
Authors: Marik, D.; Petrovay, K.
2001ESASP.464..657M Altcode: 2001soho...10..657M
The lower overshooting layer, which plays an important role in the
dynamo mechanism, is one of the least known regions of the Sun. The most
promising way to model this region is the Reynolds-stress method. In
this paper we determine the radial distribution of the turbulent
kinetic energy k, the mean square relative temperature fluctuation
q, the normalized energy flux J, and the energy dissipation rate
ɛ. We present solutions in the case of a simple k-ɛ model and in
the case of solving all four differential equations using various
Δ∇ distributions (temperature stratifications). We use a diffusive
approximation for the nonlocal fluxes ("Xiong's closure"), considering
the case of both strong and weak nonlocality. The resulting profiles
of k and ɛ are found to be approximately linear and the profiles of
the turbulent lengths and time scales l and τ are also similar for
different cases. The shapes of these profiles thus seem to be robust
properties of the solution, with little sensitivity to the particular
parameter values and background stratification assumed. In contrast,
we find that the penetration depth depends rather sensitively on the
slope of the Δ∇ curve and on the strength of nonlocality assumed.
---------------------------------------------------------
Title: Turbulence in the Solar Photosphere
Authors: Petrovay, K.
2001SSRv...95....9P Altcode: 2000astro.ph..5409P
The precise nature of photospheric flows, and of the transport
effects they give rise to, has been the subject of intense debate in
the last decade. Here we attempt to give a brief review of the subject
emphasizing interdisciplinary (solar physics-turbulence theory) aspects,
key open questions, and recent developments.
---------------------------------------------------------
Title: What Makes the Sun Tick? The Origin of the Solar Cycle
Authors: Petrovay, K.
2000ESASP.463....3P Altcode: 2000sctc.proc....3P; 2000astro.ph.10096P
In contrast to the situation with the geodynamo, no breakthrough
has been made in the solar dynamo problem for decades. Since the
appearance of mean-field electrodynamics in the 1960's, the only
really significant advance was in the field of flux tube theory
and flux emergence calculations. These new results, together with
helioseismic evidence, have led to the realization that the toroidal
magnetic flux giving rise to activity phenomena must be stored and
presumably generated below the convection zone proper, in what I will
call the DOT (Dynamo-Overshoot-Tachoclyne) layer. The only segment of
the problem we can claim to basically understand is the transport of
flux from this layer to the surface. On the other hand, as reliable
models for the DOT layer do not exist we are clueless concerning the
precise mechanisms responsible for toroidal/poloidal flux conversion
and for characteristic migration patterns (extended butterfly diagram)
and periodicities. Even the most basic result of mean-field theory,
the interpretation of the butterfly diagram as an alpha-omega dynamo
wave, has been questioned. This review therefore will necessarily ask
more questions than give answers. Some of these key questions are -
Structure of the DOT layer - alpha-quenching and distributed dynamo -
High-latitude migration patterns and their interpretation - The ultimate
fate of emerged flux
---------------------------------------------------------
Title: Making sense of sunspot decay - II. Deviations from the Mean
Law and Plage Effects
Authors: Petrovay, K.; Martínez Pillet, V.; van Driel-Gesztelyi, L.
1999SoPh..188..315P Altcode: 1999astro.ph..6258P
In a statistical analysis of Debrecen Photoheliographic Results sunspot
area data we find that the logarithmic deviation (log D)' of the area
decay rate D from the parabolic mean decay law (derived in the first
paper in this series) follows a Gaussian probability distribution. As
a consequence, the actual decay rate D and the time-averaged decay
rate are also characterized by approximately lognormal distributions,
as found in an earlier work. The correlation time of (log D)' is about
3 days. We find a significant physical anticorrelation between (log
D)' and the amount of plage magnetic flux of the same polarity in an
annulus around the spot on Kitt Peak magnetograms. The anticorrelation
is interpreted in terms of a generalization of the turbulent erosion
model of sunspot decay to the case when the flux tube is embedded in
a preexisting homogeneous `plage' field. The decay rate is found to
depend inversely on the value of this plage field, the relation being
very close to logarithmic, i.e., the plage field acts as multiplicative
noise in the decay process. A Gaussian probability distribution of
the field strength in the surrounding plage will then naturally lead
to a lognormal distribution of the decay rates, as observed. It is
thus suggested that, beside other multiplicative noise sources, the
environmental effect of surrounding plage fields is a major factor
in the origin of lognormally distributed large random deviations from
the mean law in the sunspot decay rates.
---------------------------------------------------------
Title: Transport Effects in the Evolution of the Global Solar
Magnetic Field
Authors: Petrovay, K.; Szakály, G.
1999SoPh..185....1P Altcode: 1998astro.ph.12464P
The axisymmetric component of the large-scale solar magnetic fields
has a pronounced poleward branch at higher latitudes. In order
to clarify the origin of this branch we construct an axisymmetric
model of the passive transport of the mean poloidal magnetic field
in the convective zone, including meridional circulation, anisotropic
diffusivity, turbulent pumping and density pumping. For realistic values
of the transport coefficients we find that diffusivity is prevalent,
and the latitudinal distribution of the field at the surface simply
reflects the conditions at the bottom of the convective zone. Pumping
effects concentrate the field to the bottom of the convective zone; a
significant part of this pumping occurs in a shallow subsurface layer,
normally not resolved in dynamo models. The phase delay of the surface
poloidal field relative to the bottom poloidal field is found to be
small. These results support the double dynamo wave models, may be
compatible with some form of a mixed transport scenario, and exclude
the passive transport theory for the origin of the polar branch.
---------------------------------------------------------
Title: Science and Astronomy Education in Hungary: A Personal View
Authors: Petrovay, K.
1999arse.conf...67P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Close Encounters of the Fourth Kind: Getting Astrophysics to
the People of the Canary Islands
Authors: Petrovay, K.
1999arse.conf...72P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Origin and Propagation of Fluctuations of Turbulent Magnetic
Fields
Authors: Petrovay, K.
1999ASPC..183...70P Altcode: 1998astro.ph.12466P; 1999hrsp.conf...70P
The degree of linear polarization has recently been found to show wide
random variations over the solar disk. These variations are presumably
at least partly due to fluctuations in the flux density of turbulent
photospheric magnetic fields and associated variations in the degree of
Hanle depolarization. In order to understand the origin of such large
scale fluctuations of the turbulent magnetic flux density we develop a
phenomenological model to calculate the spatial Fourier spectrum of the
fluctuations of turbulent magnetic fields in the solar photosphere and
convective zone. It is found that if the model parameters are fitted to
turbulence closure models and numerical experiments the characteristic
scale of the fluctuations is by about an order of magnitude larger than
the turbulence scale (the scale of the granulation), owing to the more
effective quenching of small-scale fluctuations by turbulent diffusion.
---------------------------------------------------------
Title: On the validity of quasi-linear kinematic mean-field
electrodynamics in astrophysical flows
Authors: Petrovay, K.; Zsargo, J.
1998MNRAS.296..245P Altcode:
Mean-field theory in its kinematic form with the quasi-linear
approximation is widely used for the modelling of the transport of
weak magnetic fields in turbulent media. The validity of this approach
to real astrophysical flows is discussed. Numerically evaluating the
turbulent electromotive force using Lagrangian analysis for a set of
simple, prescribed 2D flow patterns with a wide range of parameters,
we find that quasi-linear expressions for the turbulent diffusivities
and for the pumping velocities are correct within a factor of 2 for
a wide variety of flow types with order of unity (or even higher)
effective Strouhal numbers. The degree of the non-linear quenching of
turbulent transport by a weak magnetic field is also discussed. We
argue that, owing to the intermittency and small filling factors of
magnetic fields in realistic astrophysical media, diffusivity and
pumping effects are not quenched to order of magnitude, while a more
moderate quenching of order 10 per cent is still present.
---------------------------------------------------------
Title: Limits on Anomalous Diffusion in the Solar Photosphere from
Sunspot Decay
Authors: Petrovay, K.
1998ESASP.417..273P Altcode: 1998cesh.conf..273P
No abstract at ADS
---------------------------------------------------------
Title: K-epsilon models of the lower overshoot layer
Authors: Petrovay, K.
1998IAUS..185..121P Altcode:
Apart from the solar core, the overshoot layer below the convective
zone is the layer where some discrepancies between the standard and
seismic solar models continue to exist. Non-local mixing length
models are in clear conflict with the seismic evidence. A major
difficulty for the development of more sophisticated models is the
large degree of arbitrariness in formulating expressions for input
parameters like the length scale. We propose that the application of the
k-epsilon modelling approach, widely used in physics and engineering,
could help in alleviating these difficulties. We present some simple
overshoot models calculated with this approach, and we point out that
the resulting expression of the length scale naturally reduces to the
usual mixing length expression (proportional to the pressure scale
height) well inside the convectively unstable region. Implications
for the solar Li problem and for dynamo theory will also be discussed.
---------------------------------------------------------
Title: Making Sense of Sunspot Decay. I. Parabolic Decay Law and
Gnevyshev-Waldmeier Relation
Authors: Petrovay, K.; van Driel-Gesztelyi, L.
1997SoPh..176..249P Altcode: 1997astro.ph..6029P
In a statistical study of the decay of individual sunspots based on
DPR data we find that the mean instantaneous area decay rate is related
to the spot radius r<SUB>o</SUB> and the maximum radius r<SUB>o</SUB>
as D = C<SUB>D</SUB> r/r<SUB>o</SUB>, C<SUB>D</SUB> = 32.0±0.26 MSH
day <SUP>-1</SUP>. This implies that sunspots on the mean follow a
parabolic decay law; the traditional linear decay law is excluded
by the data. The validity of the Gnevyshev-Waldmeier relationship
between the maximum area A <SUB>0</SUB> and lifetime T of a spot group,
A<SUB>0</SUB>/T ≃10 MSH day<SUP>-1</SUP>, is also demonstrated for
individual sunspots. No evidence is found for a supposed supergranular
`quantization' of sunspot areas. Our results strongly support the
recent turbulent erosion model of sunspot decay while all other models
are excluded.
---------------------------------------------------------
Title: Turbulent Erosion of Magnetic Flux Tubes
Authors: Petrovay, K.; Moreno-Insertis, F.
1997ApJ...485..398P Altcode: 1997astro.ph..3152P
Results from a numerical and analytical investigation of the solution of
a nonlinear axisymmetric diffusion equation for the magnetic field are
presented for the case when the nonlinear dependence of the diffusivity
ν(B) on the magnetic field satisfies basic physical requirements. We
find that for sufficiently strong nonlinearity (i.e., for sufficiently
strong reduction of ν inside the tube) a current sheet is spontaneously
formed around the tube within one diffusion timescale. This sheet
propagates inward with a velocity inversely proportional to the
ratio of the field strength just inside the current sheet to the
equipartition field strength B<SUB>0</SUB>/B<SUB>e</SUB>, so the
lifetime of a tube with constant internal flux density is increased
approximately by a factor not exceeding B<SUB>0</SUB>/B<SUB>e</SUB>,
even for infinitely effective inhibition of turbulence inside the
tube. Among the applications of these results, we point out that
toroidal flux tubes in the solar convective zone are subject to
significant flux loss owing to turbulent erosion on a timescale of ~1
month and that turbulent erosion may be responsible for the formation
of a current sheet around a sunspot. It is further proposed that,
despite the simplifying assumptions involved, our solutions correctly
reflect the essential features of the sunspot decay process.
---------------------------------------------------------
Title: Theory of Passive Magnetic Field Transport
Authors: Petrovay, Kristof
1997astro.ph..3154P Altcode:
In recent years, our knowledge of photospheric magnetic fields
went through a thorough transformation--nearly unnoticed by dynamo
theorists. It is now practically certain that the overwhelming
majority of the unsigned magnetic flux crossing the solar surface
is in turbulent form (intranetwork and hidden fields). Furthermore,
there are now observational indications (supported by theoretical
arguments discussed in this paper) that the net polarity imbalance of
the turbulent field may give a significant or even dominant contribution
to the weak large-scale background magnetic fields outside unipolar
network areas. This turbulent magnetic field consists of flux tubes with
magnetic fluxes below 1e10 Wb (1e18 Mx). The motion of these thin tubes
is dominated by the drag of the surrounding flows, so the transport of
this component of the solar magnetic field must fully be determined by
the kinematics of the turbulence (i.e. it is "passive"), and it can
be described by a one-fluid model like mean-field theory (MFT). This
paper reviews the theory of passive magnetic field transport using
mostly first (and occasionally higher) order smoothing formalism; the
most important transport effects are however also independently derived
using Lagrangian analysis for a simple two-component flow model. Solar
applications of the theory are also presented. Among some other novel
findings it is proposed that the observed unsigned magnetic flux density
in the photosphere requires a small-scale dynamo effect operating in
the convective zone and that the net polarity imbalance in turbulent
(and, in particular, hidden) fields may give a major contribution to
the weak large-scale background magnetic fields on the Sun.
---------------------------------------------------------
Title: Sunspot Decay as Turbulent Erosion of a Magnetic Flux Tube
Authors: Petrovay, K.; van Driel-Gesztelyi, L.
1997ASPC..118..145P Altcode: 1997fasp.conf..145P
Decisive evidence is presented against the validity of the conventional
linear decay law for sunspots. Preliminary results from a statistical
analysis of Debrecen Photoheliographic Results data show that the decay
rate of sunspots is significantly slower in late phases of the decay
than in earlier phases. Our findings are compatible with the predictions
of a recently developed turbulent erosion model for sunspot decay.
---------------------------------------------------------
Title: Turbulent magnetic fields in the solar photosphere: diagnostics
and interpretation.
Authors: Faurobert-Scholl, M.; Feautrier, N.; Machefert, F.; Petrovay,
K.; Spielfiedel, A.
1995A&A...298..289F Altcode:
Turbulent magnetic fields in the solar photosphere may be determined
by their depolarizing Hanle effect on the linear polarization of
some solar absorption lines formed outside active regions (Stenflo
1982). In Faurobert-Scholl (1993) the center-to-limb variation of
the linear polarization in the SrI 4607 A line was analyzed. It was
shown that it is affected by the Hanle effect due to a weak magnetic
field with mixed polarity at small scales. The accurate determination
of the turbulent magnetic field strength requires radiative transfer
calculations taking into account the effect of collisions and magnetic
fields on the frequency and angular redistribution of the light. Here
we present the different steps in the theory of redistribution,
together with the first precise quantum calculations of the collisional
cross-sections between hydrogen and SrI and CaI atoms. These new results
are then used to perform a more accurate determination of the turbulent
magnetic field strength in the solar photosphere. It is shown that the
center-to-limb variation of the linear polarization in the SrI line
is mainly sensitive to the average value of the field strength between
the altitudes 200 and 400km. According to the observations this average
value is between 20 and 10G. A theoretical interpretation is given in
terms of a one-dimensional passive magnetic field transport equilibrium
model with first-order smoothing. The resulting <|B_x_|>(z)
equilibrium distribution reproduces the observations rather well, but
the uncertainty in the amplitude, height-dependence and interpretation
of microturbulence suggests that the present model could be brought to
an even better agreement with the observations if a microturbulence
different from the VAL3C model is used. It is finally pointed out
that in the upper photosphere the turbulent field becomes force-free
instead of being passively transported. It is found that this does
not seriously modify our conclusions.
---------------------------------------------------------
Title: Numerical Tests of the Quasilinear Approximation of Mean-field
Electrodynamics
Authors: Zsargo, J.; Petrovay, K.
1995AAS...186.0209Z Altcode: 1995BAAS...27..814Z
It is widely known that a sufficient condition for the applicability
of quasilinear-type approximations (e.g. the second-order correlation
approximation or SOCA) in mean-field electrodynamics is that Utau
<< min {l, H} where l, H, U and tau are characteristic horizontal
and vertical scale lengths, velocity, and time, respectively. A
necessary condition for their validity is however not known. In order
to check the validity of the quasilinear results in cases where the
above condition is not satisfied, as well as to study qualitative
and quantitative differences between the quasilinear results and the
actual solutions, we numerically solve the MHD induction equation
for the kinematical case in a series of simplified "toy" model flows
and then compare the results with the corresponding quasilinear
solutions. Our model flows are two-dimensional two-component flows
with simple (exponential or linear) stratifications. For conceptual
clarity, in each model only one independent physical quantity (initial
magnetic field, density, or velocity amplitude, respectively) has an
inhomogeneous distribution. Solutions are computed for several widely
differing values of the l/H horizontal/vertical scale length ratio. In
all cases we find that the computed turbulent electromotive force does
not differ from the quasilinear value by more than an order-of-unity
factor, as long as Utau does not greatly exceed min {l, H}.
---------------------------------------------------------
Title: On the Existence of a Discontinuity at the Lower Boundary of
the Solar Convective Zone
Authors: Petrovay, K.; Marik, M.
1995ASPC...76..216P Altcode: 1995gong.conf..216P
No abstract at ADS
---------------------------------------------------------
Title: Theory of passive magnetic field transport
Authors: Petrovay, K.
1994ASIC..433..415P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Density pumping of magnetic fields in the solar convective zone
Authors: Petrovay, K.
1994smf..conf..146P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The origin of intranetwork fields: a small-scale solar dynamo
Authors: Petrovay, K.; Szakaly, G.
1993A&A...274..543P Altcode:
The intranetwork magnetic fields observed on the solar surface
consist of flux tubes that are thin enough for their motion to be fully
determined by the drag forces exerted on them by turbulent motions. The
equations governing such a passive transport of the mean magnetic
flux density <B> and of the unsigned flux density <|B|> in
the convective zone of the Sun (assuming a one-dimensional geometry)
are derived and discussed: turbulent diffusion and turbulent pumping
are found to be the main transport effects. As the timescale of the
transport is much shorter than the solar cycle, the flux density at
any instant is given by an equilibrium solution of the transport
equations. These solutions are computed and presented. The main
conclusions are the following. 1. If no source terms are included in
its transport equation, the mean flux density increases by 4 orders
of magnitude from the surface to the bottom of the convective zone,
showing that turbulent pumping is one of the main mechanisms confining
the global dynamo to the bottom of the convective zone. 2. The observed
emergence rate of magnetic flux in active regions is not sufficient
to sustain the observed mean magnetic fields which must therefore
be sustained and continuously refreshed on a time scale of ∼20
days by the emergence of statistically aligned smaller bipolar flux
concentrations (intranetwork fields or small ephemeral regions) from
below. 3. The observed unsigned flux density of intranetwork fields
is only consistent with the model if a small-scale dynamo mechanism is
operating in the convective zone continuously producing unsigned flux;
no other likely sources of flux of the correct order of magnitude are
known. 4. Modelling the source term corresponding to the small-scale
dynamo on the basis of numerical simulation results, the hidden magnetic
flux density is predicted to lie between about 2.5 and 7.5 mT. This
prediction may be tested by observations in the not too distant future.
---------------------------------------------------------
Title: Local and non-local contributions to horizontal motions in
stellar convective zones
Authors: Petrovay, K.
1993ASPC...40..293P Altcode: 1993IAUCo.137..293P; 1993ist..proc..293P
No abstract at ADS
---------------------------------------------------------
Title: The Sizes of Active Regions and Convective Triggering of
Buoyant Loop Instability
Authors: Petrovay, K.; Szakaly, G.
1993ASPC...46..108P Altcode: 1993IAUCo.141..108P; 1993mvfs.conf..108P
No abstract at ADS
---------------------------------------------------------
Title: Area-Weighting of Sunspot Group Positions and Proper Motion
Artifacts
Authors: Petrovay, K.
1993ASPC...46..123P Altcode: 1993mvfs.conf..123P; 1993IAUCo.141..123P
No abstract at ADS
---------------------------------------------------------
Title: The Small-Scale Photospheric Magnetic Field as an Indicator
of the Dynamo
Authors: Petrovay, K.; Szakaly, G.
1993ASPC...46..143P Altcode: 1993mvfs.conf..143P; 1993IAUCo.141..143P
No abstract at ADS
---------------------------------------------------------
Title: The anisotropy of low prandtl number turbulent convection
Authors: Petrovay, K.
1992GApFD..65..183P Altcode: 1997astro.ph..3155P
A model for homogeneous anisotropic incompressible turbulence
is proposed. The model generalizes the GISS model of homogeneous
isotropic turbulence; the generalization involves the solution of the
GISS equations along a set of integration paths in wavenumber (k-)
space. In order to make the problem tractable, these integration paths
("cascade lines") must be chosen in such a way that the behaviour
of the energy spectral function along different cascade lines should
be reasonably similar. In practice this is realized by defining the
cascade lines as the streamlines of a cascade flow; in the simplest
case the source of this flow may be identified with the source function
of the turbulence. Owing to the different approximations involved,
the resulting energy spectral function is not exact but is expected to
give good approximative values for thebulk quantities characterising
the turbulent medium, and for the measure of the anisotropy itself
in particular. The model is then applied to the case of low Prandtl
number thermal convection. The energy spectral function and the bulk
quantities characterizing the flow are derived for different values
of the parameter S = Rao. The most important new finding is that
unlike the anisotropy of the most unstable mode in linear stability
analysis the anisotropy of the turbulence doesnot grow indefinitely
with increasingS but it rather saturates to a relatively moderate
finite asymptotic value.
---------------------------------------------------------
Title: On the Properties of Toroidal Flux Tubes in the Solar Dynamo
Authors: Petrovay, K.
1991SoPh..134..407P Altcode:
Prompted by a recently revived debate concerning the structure of the
toroidal magnetic field of the Sun, this letter points out that the
observational evidence now suggests that the toroidal flux tubes have
fluxes of 10<SUP>14</SUP> Wb (10<SUP>22</SUP> Mx) and flux densities
of about 10 T (10<SUP>5</SUP> G). It is proposed that such high flux
densities may be produced by the work done on the flux tubes by the
drag force due to differential rotation.
---------------------------------------------------------
Title: Cluster analysis of the space-time distribution of sunspot
groups during solar cycle no. 20
Authors: Petrovay, Kristof; Abuzeid, Bashir K.
1991SoPh..131..231P Altcode:
Cluster analysis (a Bayesian iteration procedure) was used to study the
space-time distribution of sunspot groups in the time interval from 1965
to 1977. (Data were taken from the Greenwich and Debrecen Heliographic
Results.) The distribution proved to be significantly non-random for
the 8-10 groups cluster<SUP>−1</SUP> (gr cl<SUP>−1</SUP>) level of
clustering. Convincing evidence also favours non-random behaviour for
other levels of clustering from the lowest (3-4 gr cl<SUP>−1</SUP>)
up to the highest (∼ 150 gr cl<SUP>−1</SUP>) level. The rotation
rate of the non-random pattern is generally slightly lower than the
Carrington rate.
---------------------------------------------------------
Title: Topological Pumping in the Lower Overshoot Layer
Authors: Petrovay, Kristóf
1991LNP...380...67P Altcode: 1991sacs.coll...67P; 1991IAUCo.130...67P
Problems associated with topological pumping in the lower overshoot
layer suggest a strongly turbulent and strongly differentially rotating
upper radiative zone as the seat of the dynamo and as flux reservoir.
---------------------------------------------------------
Title: Morphology of Convection and Mixing-Length Theory
Authors: Petrovay, Kristof G.
1990ApJ...362..722P Altcode:
It is pointed out that observations and numerical experiments are
not the only way to determine the morphological characteristics of
convection in different layer of stars. It is demonstrated that a
sufficiently general formulation of mixing length theory (MLT) that
incorporates the kinetic energy flux and the anisotropy of turbulence
can be used to give reliable predictions concerning the morphology. Such
an MLT, applied to a recent model of the solar convective zone (SCZ),
shows that the morphology in the bulk of the SCZ is characterized by
isolated fibrillar downflows. A topology reversal occurs a few hundred
km below the photosphere, and the outer layers are characterized by
isolated upflows and a cellular structure. If the SCZ has a thin lower
boundary layer, then near it the structure becomes cellular again, but
with isolated downflows. Unlike solar-type stars, convective stellar
cores are probably not dominated by fibrillar isolated downflows,
but rather by isolated upflows.
---------------------------------------------------------
Title: Asymmetric Flux Loops in Active Regions - Part Two
Authors: Petrovay, K.; Brown, J. C.; van Driel-Gesztelyi, L.; Fletcher,
L.; Marik, M.; Stewart, G.
1990SoPh..127...51P Altcode:
We propose that magnetic flux loops in the subphotospheric layers
of the Sun are seriously asymmetrical as a consequence of the drag
force exerted on them because of the different rotational rate of the
surrounding plasma. In numerical models of stationary slender flux
loops in the plane parallel approximation we show that a serious
tilt is both possible and probable. Observational facts (see van
Driel-Gesztelyi and Petrovay, 1989; Paper I) strongly support the case
for high asymmetry. The different stability of p and f spots may also
be related to such an asymmetry.
---------------------------------------------------------
Title: Asymmetric flux loops in active regions, I
Authors: van Driel-Gesztelyi, L.; Petrovay, K.
1990SoPh..126..285V Altcode: 1990SoPh..126..285G
We investigate asymmetries of bipolar sunspot groups. We find that
the magnetic field distribution of simple bipolar sunspot groups is
significantly asymmetrical: the polarity inversion line is usually
nearer to the main following polarity spot than to the main preceding
one. This asymmetry grows with the age of the sunspot group. We suggest
that this asymmetry has a causal link with two long-established
asymmetries- the one in the proper motions of young sunspots, the
other in the relative stability of p and f spots.
---------------------------------------------------------
Title: Asymmetry of Emerging Flux Loops Caused by Radial Differential
Rotation
Authors: Marik, M.; Petrovay, K.
1990IAUS..138..321M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Investigation of Sunspot Nests during Solar Cycle No.20
Authors: Abuzeid, B. K.; Petrovay, K.
1990PDHO....7...98A Altcode: 1990dysu.conf...98A; 1990ESPM....6...98A
The space-time distribution of sunspot groups in the time interval
1965 - 1977 was studied using cluster analysis. Results for the 8 -
10 groups/cluster level of clustering are presented.
---------------------------------------------------------
Title: Properties of a Spherical Galaxy with Exponential Energy
Distribution
Authors: Petrovay, K.
1987Ap&SS.138..323P Altcode:
Some analytical relations for the phase space functions of a
self-consistent spherical stellar system are derived. The integral
constraints on the distribution function by imposing a given ϱ(r)
density distribution andN(E) fractional energy distribution are
determined. For the case of radially-anisotropic velocity distribution
in theE→0 limit the constraint by an exponentialN(E) implies thatf(E,
J <SUP>2</SUP>) tends to zero in the order (-E)<SUP>3/2</SUP>. This
lends analytical support to the use of the Stiavelli and Bertin (1985)
distribution function for modeling elliptical galaxies. Maximum phase
space density constraint confirms the necessity of high collapse factors
to produce such a distribution function. Limits on the steepness of
an exponentialN(E) for the case when ϱ(r) resembles the emissivity
law of ellipticals are also derived.