explanation      blue bibcodes open ADS page with paths to full text
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 &gt; 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&amp;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&lt;-1 where κ-&gt; 0 as α-&gt; -∞. The observationally
  derived upper limit of 0.38 on κ implies α&lt;-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.

---------------------------------------------------------
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.

---------------------------------------------------------
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 &amp; 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&amp;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
    &amp; 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&amp;changeHeader=true">http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&amp;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 &amp; 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 &gt;~ 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 &lt;|B_x_|&gt;(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
  &lt;&lt; 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 &lt;B&gt; and of the unsigned flux density &lt;|B|&gt; 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.

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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.

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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.

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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

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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.

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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.