I. Tuominen

Publications (84)112.81 Total impact

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  Available from: Maarit. J. Mantere

  Article: Multiperiodicity, modulations and flip-flops in variable star light curves I. Carrier fit method

  J. Pelt, N. Olspert, M. J. Mantere, I. Tuominen
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  ABSTRACT: The light curves of variable stars are commonly described using simple trigonometric models, that make use of the assumption that the model parameters are constant in time. This assumption, however, is often violated, and consequently, time series models with components that vary slowly in time are of great interest. In this paper we introduce a class of data analysis and visualization methods which can be applied in many different contexts of variable star research, for example spotted stars, variables showing the Blazhko effect, and the spin-down of rapid rotators. The methods proposed are of explorative type, and can be of significant aid when performing a more thorough data analysis and interpretation with a more conventional method.Our methods are based on a straightforward decomposition of the input time series into a fast "clocking" periodicity and smooth modulating curves. The fast frequency, referred to as the carrier frequency, can be obtained from earlier observations (for instance in the case of photometric data the period can be obtained from independently measured radial velocities), postulated using some simple physical principles (Keplerian rotation laws in accretion disks), or estimated from the data as a certain mean frequency. The smooth modulating curves are described by trigonometric polynomials or splines. The data approximation procedures are based on standard computational packages implementing simple or constrained least-squares fit-type algorithms.
  09/2011;
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  Available from: Maarit. J. Mantere

  Article: Surface temperature maps for II Peg during 1999-2002

  M. Lindborg, M. J. Korpi, I. Tuominen, T. Hackman, I. Ilyin, N. Piskunov
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  ABSTRACT: The active RS CVn star II Peg has been spectroscopically monitored for almost 18 years with the SOFIN spectrograph at NOT, La Palma, Spain. In this paper we present five new surface temperature maps of the object for the years 1999 (two maps), 2001 (one map) and 2002 (two maps).
  11/2009;
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  Available from: Maarit. J. Mantere

  Article: Solar active regions: a nonparametric statistical analysis

  J. Pelt, M. J. Korpi, I. Tuominen
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  ABSTRACT: The sunspots and other solar activity indicators tend to cluster on the surface of the Sun.These clusters very often occur at certain longitudes that persist in time.It is of general interest to find new and simple ways to characterize the observed distributions of different indicators and their behaviour in time. In the present work we use Greenwich sunspot data to evaluate statistical but not totally coherent stability of sunspot distribution along latitudes as well as longitudes. The aim was to obtain information on the longitudinal distribution of the underlying spot-generating mechanism rather than on the distribution and migration of sunspots or sunspot groups on the solar surface. Therefore only sunspot groups were included in the analysis, and only the time of their first appearance was used. We use simple nonparametric approach to reveal sunspot migration patterns and their persistency. Our analysis shows that regions where spots are generated tend to rotate differentially as the spots and spot groups themselves do. The activity areas, however, tend to break down relatively fast, during 7-15 solar rotations. This study provides a challenge for solar dynamo models, as our results are consistent with the presence of a non-axisymmetric spot-generating mechanism experiencing differential rotation (known as phase mixing in dynamo theory). The new nonparametric method introduced here, completely independent of the choice of the longitudinal distribution of sunspots, was found to be a very powerful tool for spatio-temporal analysis of surface features. Comment: 8 pages, 11 figures, submitted to Astronomy and Astrophysics
  08/2009;
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  Available from: articles.adsabs.harvard.edu

  Article: A New Zeeman-Doppler Imaging Code for Active Late Type-stars. An Application to II Peg

  M. Kopf, T. A. Carroll, I. Ilyin, K. G. Strassmeier, I. Tuominen
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  ABSTRACT: The fact, that late-type stars exhibit complex and small scale surface magnetic fields, imposes special requirements on their observation as well as on their modeling and reconstruction. Our new Zeeman-Doppler imaging code iMap, which we present here, was particularly designed for the application to late type stars. It does full radiative transfer calculations and utilizes a regularization scheme which is based on local maximum entropy. Furthermore a new multi-line cross-correlation technique by means of a Principal Component Analysis is used to enhance the quality of individual observed polarized line profiles. In a first application we present Zeeman-Doppler images of II Pegasi, which reveal a surprisingly large scale surface structure with one predominant magnetic longitude, containing a mainly radially oriented field.
  05/2009; 405:517.
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  Available from: ArXiv

  Article: Zeeman-Doppler Imaging of II Peg - Magnetic field restructuring from 2004 to 2007

  T. A. Carroll, M. Kopf, K. G. Strassmeier, I. Ilyin, I. Tuominen
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  ABSTRACT: We present Zeeman-Doppler images of the active K2 star II Peg for the years 2004 and 2007. The surface magnetic field was reconstructed with our new ZDI code "iMap" which provides a full polarized radiative transfer driven inversion to simultaneously reconstruct the surface temperature and magnetic vector field distribution. II Peg shows a remarkable large scale magnetic field structure for both years. The magnetic field is predominantly located at high latitudes and is arranged in active longitudes. A dramatic evolution in the magnetic field structure is visible for the two years, where a dominant and largely unipolar field in 2004 has developed into two distinct and large scale bipolar structures in 2007.
  04/2009;
• Article: Magnetic Doppler imaging of II Peg

  Oleg Kochukhov, N. Piskunov, I. Ilyin, I. Tuominen
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  ABSTRACT: Rotational modulation of the intensity and polarization spectra of magnetic stars offers a unique possibility to reconstruct the structure of surface magnetic fields and to investigate their relation to cool starspots. We have developed a new magnetic Doppler imaging code which aims at self-consistent temperature and magnetic mapping of cool active stars. Here we present magnetic Doppler imaging analysis of high-resolution circular polarization observations of the active star II Peg. We demonstrate that a self-consistent approach to magnetic inversion unveils stronger magnetic fields than found previously through disjoint analyses of polarization and intensity observations of active stars.
  Proceedings of the International Astronomical Union 10/2008; 4:439 - 440.
• Article: Does the Sun have a face?

  I. Tuominen, J. Pelt, J. Brooke, M. Korpi
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  ABSTRACT: We analyse the Greenwich sunspot data with methods using kinematic frames, which allow to detect and filter off any systematic motion, such as differential rotation, of the longitudinal activity traces. The aim is to check the recent claim of the existence of century-scale persistent solar active longitudes exhibiting antisolar differential rotation. As a result, we find no evidence for such features. Nevertheless, as is well known, the sunspot distribution is highly clustered in longitude (activity nests); the simple cell-counting statistics allows us to estimate the coherence time of these features, giving roughly10 Carrington rotations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Astronomische Nachrichten 12/2007; 328(10):1020 - 1022. · 1.01 Impact Factor
• Article: Study of FK Comae Berenices. V. Spot evolution and detection of surface differential rotation

  H. Korhonen, S. V. Berdyugina, T. Hackman, I. V. Ilyin, K. G. Strassmeier, I. Tuominen
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  ABSTRACT: Aims:We investigate the spot evolution and the surface differential rotation of the single late-type giant FK Com. Methods: A total of 18 new surface temperature maps of FK Com are calculated with the Doppler imaging technique for the years 1993-2003. Photometric observations from 2002-2004 are also given. The new and previously published spectroscopic and photometric observations are used to study the spot locations and lifetimes, and to estimate the value of the surface differential rotation. Results: The phases of the active regions determined from the Doppler images follow closely the active longitudes determined earlier from the long-term photometric observations. One active longitude can remain active for several years, but the exact spot configuration within the active longitude changes on much shorter time scales, indicating that the spot lifetime is months instead of years. There are periods during which the spot configuration changes even within days. Measurements using spot latitudes from the Doppler images and spot rotation periods from the photometric observations yield a surface differential rotation law of Ω=(151.30°/day ± 0.09°/day)-(1.78°/day ± 0.12°/{day})sin2ψ and the relative differential rotation coefficient α=0.012± 0.002 for FK Comae. Based on observations obtained at the Nordic Optical Telescope, Observatorio Roque de los Muchachos, La Palma, Canary Islands, Spain; the Kitt Peak National Observatory, USA; the National Astronomical Observatory, Rozhen, Bulgaria; Automatic Photometric Telescopes Phoenix 10, Wolfgang and Amadeus, Arizona, USA. Tables 2a-c are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/476/881 Figures 2 and 3 are only available in electronic form at http://www.aanda.org
  Astronomy and Astrophysics 11/2007; 476:881-891. · 4.59 Impact Factor
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  Available from: Maarit. J. Mantere

  Article: Effects of Rotation and Input Energy Flux on Convective Overshooting

  P. J. Käpylä, M. J. Korpi, M. Stix, I. Tuominen
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  ABSTRACT: We study convective overshooting by means of local 3D convection calculations. Using a mixing length model of the solar convection zone (CZ) as a guide, we determine the Coriolis number (Co), which is the inverse of the Rossby number, to be of the order of ten or larger at the base of the solar CZ. Therefore we perform convection calculations in the range Co = 0...10 and interpret the value of Co realised in the calculation to represent a depth in the solar CZ. In order to study the dependence on rotation, we compute the mixing length parameters alpha_T and alpha_u relating the temperature and velocity fluctuations, respectively, to the mean thermal stratification. We find that the mixing length parameters for the rapid rotation case, corresponding to the base of the solar CZ, are 3-5 times smaller than in the nonrotating case. Introducing such depth-dependent alpha into a solar structure model employing a non-local mixing length formalism results in overshooting which is approximately proportional to alpha at the base of the CZ. Although overshooting is reduced due to the reduced alpha, a discrepancy with helioseismology remains due to the steep transition to the radiative temperature gradient. In comparison to the mixing length models the transition at the base of the CZ is much gentler in the 3D models. It was suggested recently (Rempel 2004) that this discrepancy is due to the significantly larger (up to seven orders of magnitude) input energy flux in the 3D models in comparison to the Sun and solar models, and that the 3D calculations should be able to approach the mixing length regime if the input energy flux is decreased by a moderate amount. We present results from local convection calculations which support this conjecture.
  10/2006;
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  Available from: Maarit. J. Mantere

  Article: Kinematic frames and "active longitudes": does the Sun have a face?

  J. Pelt, J. M. Brooke, M. J. Korpi, I. Tuominen
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  ABSTRACT: It has recently been claimed that analysis of Greenwich sunspot data over 120 years reveals that sunspot activity clusters around two longitudes separated by 180 degrees (``active longitudes'') with clearly defined differential rotation during activity cycles.In the present work we extend this critical examination of methodology to the actual Greenwich sunspot data and also consider newly proposed methods of analysis claiming to confirm the original identification of active longitudes. Our analysis revealed that values obtained for the parameters of differential rotation are not stable across different methods of analysis proposed to track persistent active longitudes. Also, despite a very thorough search in parameter space, we were unable to reproduce results claiming to reveal the century-persistent active longitudes. We can therefore say that strong and well substantiated evidence for an essential and century-scale persistent nonaxisymmetry in the sunspot distribution does not exist. Comment: 14 pages, 1 table, 21 figures, accepted in A&A
  09/2006;
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  Available from: Maarit. J. Mantere

  Article: Solar dynamo models with alpha-effect and turbulent pumping from local 3D convection calculations

  P. J. Käpylä, M. J. Korpi, I. Tuominen
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  ABSTRACT: (abridged) Results from kinematic solar dynamo models employing alpha-effect and turbulent pumping from local convection calculations are presented. We estimate the magnitude of these effects to be around 2-3 m/s. The rotation profile of the Sun as obtained from helioseismology is applied. We obtain an estimate of the ratio of the two induction effects, C_alpha/C_Omega \approx 10^-3, which we keep fixed in all models. We also include a one-cell meridional circulation pattern having a magnitude of 10-20 m/s near the surface and 1-2 m/s at the bottom of the convection zone. The model essentially represents a distributed turbulent dynamo, as the alpha-effect is nonzero throughout the convection zone, although it concentrates near the bottom of the convection zone obtaining a maximum around 30 degrees of latitude. Turbulent pumping of the mean fields is predominantly down- and equatorward. We find that, when all these effects are included in the model, it is possible to correctly reproduce many features of the solar activity cycle, namely the correct equatorward migration at low latitudes and the polar branch at high latitudes, and the observed negative sign of B_r B_phi. Although the activity clearly shifts towards the equator in comparison to previous models due to the combined action of the alpha-effect peaking at midlatitudes, meridional circulation and latitudinal pumping, most of the activity still occurs at too high latitudes (between 5-60 degrees). Other problems include the relatively narrow parameter space within which the preferred solution is dipolar (A0), and the somewhat too short cycle lengths of the solar-type solutions. The role of the surface shear layer is found to be important only in the case where the alpha-effect has an appreciable magnitude near the surface.
  07/2006;
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  Available from: articles.adsabs.harvard.edu

  Article: Surface differential rotation on FK Com

  H. Korhonen, S. V. Berdyugina, I. Tuominen
  02/2005; 560:719.
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  Available from: Maarit. J. Mantere

  Article: Estimates of the Strouhal number from numerical models of convection

  P. J. Käpylä, M. J. Korpi, M. Ossendrijver, I. Tuominen
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  ABSTRACT: We determine the Strouhal number (hereafter St), which is essentially a nondimensional measure of the correlation time, from numerical calculations of convection. We use two independent methods to estimate St. Firstly, we apply the minimal tau-approximation (MTA) on the equation of the time derivative of the Reynolds stress. A relaxation time is obtained from which St can be estimated by normalising with a typical turnover time. Secondly, we calculate the correlation and turnover times separately, the former from the autocorrelation of velocity and the latter by following test particles embedded in the flow. We find that the Strouhal number is in general of the order of 0.1 to 1, i.e. rather large in comparison to the typical assumption in the mean-field theories that St << 1. However, there is a clear decreasing trend as function of the Rayleigh number and increasing rotation. Furthermore, for the present range of parameters the decrease of St does not show signs of saturation, indicating that in stellar convection zones, where the Rayleigh numbers are much larger, the Strouhal number may indeed be significantly smaller.
  11/2004;
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  Available from: Maarit. J. Mantere

  Article: Local models of stellar convection II: Rotation dependence of the mixing length relations

  P. J. Käpylä, M. J. Korpi, M. Stix, I. Tuominen
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  ABSTRACT: We study the mixing length concept in comparison to three-dimensional numerical calculations of convection with rotation. In a limited range, the velocity and temperature fluctuations are linearly proportional to the superadiabaticity, as predicted by the mixing length concept and in accordance with published results. The effects of rotation are investigated by varying the Coriolis number, Co = 2 Omega tau, from zero to roughly ten, and by calculating models at different latitudes. We find that \alpha decreases monotonically as a function of the Coriolis number. This can be explained by the decreased spatial scale of convection and the diminished efficiency of the convective energy transport, the latter of which leads to a large increase of the superadibaticity, \delta = \nabla - \nabla_ad as function of Co. Applying a decreased mixing length parameter in a solar model yields very small differences in comparison to the standard model within the convection zone. The main difference is the reduction of the overshooting depth, and thus the depth of the convection zone, when a non-local version of the mixing length concept is used. Reduction of \alpha by a factor of roughly 2.5 is sufficient to reconcile the difference between the model and helioseismic results. The numerical results indicate reduction of \alpha by this order of magnitude.
  11/2004;
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  Available from: Maarit. J. Mantere

  Article: Local models of stellar convection III: The Strouhal number

  P. J. Käpylä, M. J. Korpi, M. Ossendrijver, I. Tuominen
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  ABSTRACT: Aims: The Strouhal number (St), which is a nondimensional measure of the correlation time, is determined from numerical models of convection. The Strouhal number arises in the mean-field theories of angular momentum transport and magnetic field generation, where its value determines the validity of certain widely used approximations, such as the first order smoothing (hereafter FOSA). More specifically, the relevant transport coefficients can be calculated by means of a cumulative series expansion if St < St_crit \approx 1. Methods: We define the Strouhal number as the ratio of the correlation and turnover times, which we determine separately, the former from the autocorrelation of velocity, and the latter by following test particles embedded in the flow. Results: We find that the Strouhal numbers are, generally, of the order of 0.1 to 0.4 which is close to the critical value above which deviations from FOSA become significant. Increasing the rotational influence tends to shorten both timescales in such a manner that St decreases. However, we do not find a clear trend as function of the Rayleigh number for the parameter range explored in the present study.
  11/2004;
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  Available from: arxiv.org

  Article: Century scale persistence in longitude distribution: in the Sun and in silico

  J. Pelt, I. Tuominen, J. Brooke
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  ABSTRACT: Using Greenwich sunspot data for 120 years it was recently observed that activity regions on the Sun's surface tend to lie along smoothly changing longitude strips 180 degrees apart from each other. However, numerical experiments with random input data show that most, if not all, of the observed longitude discrimination can be looked upon as an artifact of the analysis method. Comment: 4 pages, 12 figures, accepted for publication in Astronomy and Astrophysics
  10/2004;
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  Available from: Maarit. J. Mantere

  Article: Magnetic cycles and rotation in active late-type stars

  I. Tuominen, S. V. Berdyugina, M. J. Korpi
  Proceedings of the International Astronomical Union 05/2004; 215:289.
• Article: Spots on FK Com: active longitudes and “flip‐flops”

  H. Korhonen, S.V. Berdyugina, I. Tuominen
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  ABSTRACT: We have earlier investigated the surface structures of a late-type, single, giant FK Com for the years 1994–1998 using Doppler imaging. These surface temperature maps revealed long-lived active regions at high latitudes. Long-term photometric observations also show that these active regions tend to occur at two permanent active longitudes which are 180 degrees apart from each other, and that the activity switches the longitude with an average period of about 3 years (the “flip-flop” phenomenon). In this work we present new Doppler maps of FK Com obtained 1998-2003 and light-curve maps obtained 2002–2003. These new maps are investigated together with the earlier temperature maps and light-curve maps, with an aim of further studying the active longitudes, “flip-flop” phenomenon and surface differential rotation on FK Com. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Astronomische Nachrichten 05/2004; 325(5):402 - 407. · 1.01 Impact Factor
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  Available from: S. P. Järvinen

  Article: Active Longitudes and Magnetic Cycles on AB Dor

  S. V. Berdyugina, S. P. Järvinen, I. Tuominen
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  ABSTRACT: We analyze photometric observations of the young active dwarf AB Dor spanning almost 20 years. Similar to the young solar analogue LQ Hya and the Sun AB Dor shows long-lived non-axisymmetric spot distribution - active longitudes in opposite hemispheres. We find indications of two activity cycles. One cycle originates from repeating switches of the activity between the two active longitudes in about (2.5-3.5)-year intervals. This results in the ""flip-flop"" cycle of 5-7 years which includes two consecutive switches. The second cycle is seen in mean brightness variations of the star on the time-scale 19-22 years and is accompanied by migration of the active longitudes due to differential rotation.
  Proceedings of the International Astronomical Union 12/2003; 219:848.
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  Available from: Maarit. J. Mantere

  Article: Local models of stellar convection: Reynolds stresses and turbulent heat transport

  P. J. Käpylä, M. J. Korpi, I. Tuominen
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  ABSTRACT: We study stellar convection using a local three-dimensional MHD model, with which we investigate the influence of rotation and large-scale magnetic fields on the turbulent momentum and heat transport. The former is studied by computing the Reynolds stresses, the latter by calculating the correlation of velocity and temperature fluctuations, both as functions of rotation and latitude. We find that the horisontal correlation, Q_(theta phi), capable of generating horisontal differential rotation, is mostly negative in the southern hemisphere for Coriolis numbers exceeding unity, corresponding to equatorward flux of angular momentum in accordance with solar observations. The radial component Q_(r phi) is negative for slow and intermediate rotation indicating inward transport of angular momentum, while for rapid rotation, the transport occurs outwards. Parametrisation in terms of the mean-field Lambda-effect shows qualitative agreement with the turbulence model of Kichatinov & R\"udiger (1993) for the horisontal part H \propto Q_(theta phi)/cos(theta), whereas for the vertical part, V \propto Q_(r phi)/sin(theta), agreement only for intermediate rotation exists. The Lambda-coefficients become suppressed in the limit of rapid rotation, this rotational quenching being stronger for the V component than for H. We find that the stresses are enhanced by the presence of the magnetic field for field strengths up to and above the equipartition value, without significant quenching. Concerning the turbulent heat transport, our calculations show that the transport in the radial direction is most efficient at the equatorial regions, obtains a minimum at midlatitudes, and shows a slight increase towards the poles. The latitudinal heat transport does not show a systematic trend as function of latitude or rotation. Comment: 26 pages, 20 figures, final published version. For a version with higher resolution figures, see http://cc.oulu.fi/~pkapyla/publ.html
  12/2003;