S. Zharkov

Publications (15)5.55 Total impact

• Article: Modeling the Subsurface Structure of Sunspots

  H. Moradi, C. Baldner, A. C. Birch, D. C. Braun, R. H. Cameron, T. L. Duvall, L. Gizon, D. Haber, S. M. Hanasoge, B. W. Hindman, [......], R. Komm, P. Rajaguru, M. Rempel, M. Roth, R. Schlichenmaier, H. Schunker, H. C. Spruit, K. G. Strassmeier, M. J. Thompson, S. Zharkov
  [show abstract] [hide abstract]
  ABSTRACT: While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.
  Solar Physics 04/2012; 267(1):1-62. · 2.78 Impact Factor
• Source

  Available from: V. Fedun

  Conference Proceeding: Solar-Stellar Dynamos as Revealed by Helio-and Asteroseismology ASP Conference Numerical Simulation of Acoustic Wave Propagation in the Solar Sub-Photosphere with Localized Magnetic Field Concentration

  S Shelyag, S Zharkov, V Fedun, R Erdélyi, M J Thompson
  [show abstract] [hide abstract]
  ABSTRACT: The results of numerical simulations of acoustic wave propaga-tion and dispersion in the solar sub-photosphere with a localised magnetic field concentration are presented. The initial equilibrium density and pressure strat-ifications are derived from a standard solar model and adjusted to maintain magnetohydrostatic and convective stability. Acoustic waves are generated by a perturbation located at the height corresponding to the visible surface of the Sun. The time-distance diagram of the vertical velocity perturbation at the level corresponding to the visible solar surface shows that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet.
  The Second ATST-EAST Meeting: Magnetic Fields from the Photosphere to the Corona; 11/2011
• Source

  Available from: L. Gizon

  Article: Helioseismology of Sunspots: A Case Study of NOAA Region 9787

  L. Gizon, H. Schunker, C. S. Baldner, S Basu, A. C. Birch, R. S. Bogart, D. C. Braun, R. Cameron, T. L. Duvall Jr, S. M. Hanasoge, J. Jackiewicz, M Roth, T. Stahn, M. J. Thompson, S. Zharkov
  [show abstract] [hide abstract]
  ABSTRACT: Various methods of helioseismology are used to study the subsurface properties of the sunspot in NOAA Active Region 9787. This sunspot was chosen because it is axisymmetric, shows little evolution during 20-28 January 2002, and was observed continuously by the MDI/SOHO instrument. (...) Wave travel times and mode frequencies are affected by the sunspot. In most cases, wave packets that propagate through the sunspot have reduced travel times. At short travel distances, however, the sign of the travel-time shifts appears to depend sensitively on how the data are processed and, in particular, on filtering in frequency-wavenumber space. We carry out two linear inversions for wave speed: one using travel-times and phase-speed filters and the other one using mode frequencies from ring analysis. These two inversions give subsurface wave-speed profiles with opposite signs and different amplitudes. (...) From this study of AR9787, we conclude that we are currently unable to provide a unified description of the subsurface structure and dynamics of the sunspot. Comment: 28 pages, 18 figures
  02/2010;
• Source

  Available from: P. Rajaguru

  Article: Modeling the Subsurface Structure of Sunspots

  H. Moradi, C. Baldner, A. C. Birch, D. Braun, R. Cameron, T. L. Duvall Jr, L. Gizon, D. Haber, S. Hanasoge, B.W. Hindman, [......], R. Komm, P. Rajaguru, M. Rempel, M Roth, R. Schlichenmaier, H. Schunker, H. Spruit, K. Strassmeier, M. J. Thompson, S. Zharkov
  [show abstract] [hide abstract]
  ABSTRACT: While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this paper, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out an helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by \citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat. Comment: 73 pages, 19 figures, accepted by Solar Physics
  12/2009;
• Source

  Available from: V. Fedun

  Article: Numerical Simulation of Acoustic Wave Propagation in the Solar Sub-Photosphere with Localized Magnetic Field Concentration

  S. Shelyag, S. Zharkov, V. Fedun, R. Erdélyi, M. J. Thompson
  [show abstract] [hide abstract]
  ABSTRACT: The results of numerical simulations of acoustic wave propagation and dispersion in the solar sub-photosphere with a localised magnetic field concentration are presented. The initial equilibrium density and pressure stratifications are derived from a standard solar model and adjusted to maintain magnetohydrostatic and convective stability. Acoustic waves are generated by a perturbation located at the height corresponding to the visible surface of the Sun. The time-distance diagram of the vertical velocity perturbation at the level corresponding to the visible solar surface shows that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet.
  11/2009; 416:167.
• Source

  Available from: R. Erdélyi

  Article: Evidence of slow magneto-acoustic waves in photospheric observations of a sunspot

  S. Zharkov, S. Shelyag, R. Erdélyi, M. J. Thompson
  [show abstract] [hide abstract]
  ABSTRACT: We show the observational evidence for the presence of MHD waves in the solar photosphere deduced from SOHO MDI Dopplergram velocity observations. The magneto-acoustic oscillations are observed as acoustic power enhancement in the sunspot umbra at high frequency bands in the velocity component transverse to the magnetic field. We use numerical modelling of the wave propagation through localised non-uniform magnetic field concentration along with the same filtering procedure as applied to the observations to identify the observed waves. Underpinned by the results of the numerical simulations we classify the observed oscillations as slow magneto-acoustic waves excited by the trapped sub-photospheric acoustic waves. We consider the potential application of the presented method as a diagnostic tool for magnetohelioseismology. Comment: 9 pages, 5 figures, submitted to Astrophysical Journal Letters
  09/2009;
• Chapter: Helioseismology of Sunspots: A Case Study of NOAA Region 9787

  L. Gizon, H. Schunker, C. S. Baldner, S. Basu, A. C. Birch, R. S. Bogart, D. C. Braun, R. Cameron, T. L. Duvall, S. M. Hanasoge, J. Jackiewicz, M. Roth, T. Stahn, M. J. Thompson, S. Zharkov
  [show abstract] [hide abstract]
  ABSTRACT: Various methods of helioseismology are used to study the subsurface properties of the sunspot in NOAA Active Region9787. This sunspot was chosen because it is axisymmetric, shows little evolution during 20–28 January 2002, and was observed continuously by the MDI/SOHO instrument. AR9787 is visible on helioseismic maps of the farside of the Sun from 15 January, i.e. days before it crossed the East limb. Oscillations have reduced amplitudes in the sunspot at all frequencies, whereas a region of enhanced acoustic power above 5.5 mHz (above the quiet-Sun acoustic cutoff) is seen outside the sunspot and the plage region. This enhanced acoustic power has been suggested to be caused by the conversion of acoustic waves into magneto-acoustic waves that are refracted back into the interior and re-emerge as acoustic waves in the quiet Sun. Observations show that the sunspot absorbs a significant fraction of the incoming p and f modes around 3 mHz. A numerical simulation of MHD wave propagation through a simple model of AR9787 confirmed that wave absorption is likely to be due to the partial conversion of incoming waves into magneto-acoustic waves that propagate down the sunspot. Wave travel times and mode frequencies are affected by the sunspot. In most cases, wave packets that propagate through the sunspot have reduced travel times. At short travel distances, however, the sign of the travel-time shifts appears to depend sensitively on how the data are processed and, in particular, on filtering in frequency-wavenumber space. We carry out two linear inversions for wave speed: one using travel-times and phase-speed filters and the other one using mode frequencies from ring analysis. These two inversions give subsurface wave-speed profiles with opposite signs and different amplitudes. The travel-time measurements also imply different subsurface flow patterns in the surface layer depending on the filtering procedure that is used. Current sensitivity kernels are unable to reconcile these measurements, perhaps because they rely on imperfect models of the power spectrum of solar oscillations. We present a linear inversion for flows of ridge-filtered travel times. This inversion shows a horizontal outflow in the upper 4Mm that is consistent with the moat flow deduced from the surface motion of moving magnetic features. From this study of AR9787, we conclude that we are currently unable to provide a unified description of the subsurface structure and dynamics of the sunspot.
  04/2009: pages 249-273;
• Source

  Available from: R. Erdélyi

  Article: Acoustic wave propagation in the solar sub-photosphere with localised magnetic field concentration: effect of magnetic tension

  S. Shelyag, S. Zharkov, V. Fedun, R. Erdelyi, M. J. Thompson
  [show abstract] [hide abstract]
  ABSTRACT: Aims. In this paper we analyse numerically the propagation and dispersion of acoustic waves in the solar-like sub-photosphere with localised non-uniform magnetic field concentrations, mimicking sunspots with various representative magnetic field configurations. Methods. Numerical simulations of wave propagation through the solar sub-photosphere with a localised magnetic field concentration are carried out using SAC, which solves the MHD equations for gravitationally stratified plasma. The initial equilibrium density and pressure stratifications are derived from a standard solar model. Acoustic waves are generated by a source located at the height approximately corresponding to the visible surface of the Sun. We analyse the response of vertical velocity to changes in the interior due to magnetic field at the level corresponding to the visible solar surface, by the means of local time-distance helioseismology. Results. The results of numerical simulations of acoustic wave propagation and dispersion in the solar sub-photosphere with localised magnetic field concentrations of various types are presented. Time-distance diagrams of the vertical velocity perturbation at the level corresponding to the visible solar surface show that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet. For the weakly magnetised case the effect of magnetic field is mainly thermodynamic, since the magnetic field changes the temperature stratification. However, we observe the signature of slow magnetoacoustic mode, propagating downwards, for the strong magnetic field cases.
  02/2009;
• Article: Recent Developments in Local Helioseismology

  M. J. Thompson, S. Zharkov
  [show abstract] [hide abstract]
  ABSTRACT: Local helioseismology is providing new views of subphotospheric flows from supergranulation to global-scale meridional circulation and for studying structures and dynamics in the quiet Sun and active regions. In this short review we focus on recent developments, and in particular on a number of current issues, including the sensitivity of different measures of travel time and testing the forward modelling used in local helioseismology. We discuss observational and theoretical concerns regarding the adequacy of current analyses of waves in sunspots and active regions, and we report on recent progress in the use of numerical simulations to test local helioseismic methods.
  Solar Physics 08/2008; 251(1):225-240. · 2.78 Impact Factor
• Source

  Available from: ArXiv

  Article: Time-Distance analysis of the Emerging Active Region NOAA 10790

  S. Zharkov, M. J. Thompson
  [show abstract] [hide abstract]
  ABSTRACT: We investigate the emergence of Active Region NOAA 10790 by means of time--distance helioseismology. Shallow regions of increased sound speed at the location of increased magnetic activity are observed, with regions becoming deeper at the locations of sunspot pores. We also see a long-lasting region of decreased sound speed located underneath the region of the flux emergence, possibly relating to a temperature perturbation due to magnetic quenching of eddy diffusivity, or to a dense flux tube. We detect and track an object in the subsurface layers of the Sun characterised by increased sound speed which could be related to emerging magnetic flux and thus obtain a provisional estimate of the speed of emergence of around $1 {\rm km s^{-1}}$. Comment: 12 pages, 6 figures, accepted Solar Physics
  07/2008;
• Source

  Available from: ArXiv

  Article: Time Distance Study of Isolated Sunspots

  S. Zharkov, C. Nicholas, M. J. Thompson
  [show abstract] [hide abstract]
  ABSTRACT: We present a comparative seismic study of conditions around and beneath isolated sunspots. Using the European Grid of Solar Observations' Solar Feature Catalogue of sunspots derived from SOHO/MDI continuum and magnetogram data, 1996-2005, we identify a set of isolated sunspots by checking that within a Carrington Rotation there were no other spots detected in the vicinity. We then use level-2 tracked MDI Dopplergrams available from SOHO website to investigate wave-speed perturbations of such sunspots using time-distance helioseismology. Comment: 5 pages, 5 figures
  02/2008;
• Source

  Available from: articles.adsabs.harvard.edu

  Article: Comparative study of isolated sunspots using time-distance helioseismology

  S. Zharkov, C. J. Nicholas, M. J. Thompson
  09/2006; 624:61.
• Source

  Available from: articles.adsabs.harvard.edu

  Article: On the Choice of Phase-Speed Filters For Helioseismic Travel-Time Measurements

  S. Zharkov, J. M. Jensen, M. J. Thompson
  06/2006; 617:48.
• Source

  Available from: arxiv.org

  Article: Helioseismology of sunspots: a case study of NOAA region 9787

  L. Gizon, H. Schunker, C. S. Baldner, S Basu, A. C. Birch, R. S. Bogart, D. C. Braun, R. Cameron, Duvall, T. L, S. M. Hanasoge, J. Jackiewicz, M Roth, T. Stahn, M. J. Thompson, S. Zharkov
  Space Science Review, v.144 (2009).
• Article: Erratum to: Helioseismology of Sunspots: A Case Study of NOAA Region 9787

  L. Gizon, H. Schunker, C. S. Baldner, S Basu, A. C. Birch, R. S. Bogart, D. C. Braun, R. Cameron, Duvall, T. L, S. M. Hanasoge, J. Jackiewicz, M Roth, T. Stahn, M. J. Thompson, S. Zharkov
  Space Science Review (2010).