### Browsing by Author "Olspert, Nigul"

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Item Common dynamo scaling in slowly rotating young and evolved stars(Nature Publishing Group, 2020-07) Lehtinen, Jyri J.; Spada, Federico; Käpylä, Maarit J.; Olspert, Nigul; Käpylä, Petri J.; Department of Computer Science; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Professorship Korpi-Lagg Maarit; Professorship Vehtari Aki; Max Planck Institute for Solar System ResearchOne interpretation of the activity and magnetism of late-type stars is that these both intensify with decreasing Rossby number up to a saturation level(1-3), suggesting that stellar dynamos depend on both rotation and convective turbulence(4). Some studies have claimed, however, that rotation alone suffices to parametrize this scaling adequately(5,6). Here, we tackle the question of the relevance of turbulence to stellar dynamos by including evolved, post-main-sequence stars in the analysis of the rotation-activity relation. These stars rotate very slowly compared with main-sequence stars, but exhibit similar activity levels(7). We show that the two evolutionary stages fall together in the rotation-activity diagram and form a single sequence in the unsaturated regime in relation only to Rossby numbers derived from stellar models, confirming earlier preliminary results that relied on a more simplistic parametrization of the convective turn-over time(8,9). This mirrors recent results of fully convective M dwarfs, which likewise fall on the same rotation-activity sequence as partially convective solar-type stars(10,11). Our results demonstrate that turbulence plays a crucial role in driving stellar dynamos and suggest that there is a common turbulence-related dynamo mechanism explaining the magnetic activity of all late-type stars. Uniform analysis of both main-sequence and evolved, post-main-sequence stars shows that a common, turbulence-dependent, dynamo mechanism operates throughout these stages of stellar evolution.Item Detecting Large-Scale Vortices from Fluid Simulations with Deep Learning(2022-04-15) Prasad, Sayoojya; Olspert, Nigul; Insinööritieteiden korkeakoulu; St-Pierre, LucItem Extended Subadiabatic Layer in Simulations of Overshooting Convection(2017-08-20) Käpylä, Petri J.; Rheinhardt, Matthias; Brandenburg, Axel; Arlt, Rainer; Käpylä, Maarit J.; Lagg, Andreas; Olspert, Nigul; Warnecke, Jörn; Department of Computer Science; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Professorship Vehtari Aki; Stockholm University; University of Potsdam; Max Planck Institute for Solar System ResearchWe present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stratified according to the Schwarzschild criterion while the enthalpy flux is outward directed. This occurs when the heat conduction profile at the bottom of the CZ is smoothly varying, based either on a Kramers-like opacity prescription as a function of temperature and density or a static profile of a similar shape. We show that the subadiabatic layer arises due to nonlocal energy transport by buoyantly driven downflows in the upper parts of the CZ. Analysis of the force balance of the upflows and downflows confirms that convection is driven by cooling at the surface. We find that the commonly used prescription for the convective enthalpy flux being proportional to the negative entropy gradient does not hold in the stably stratified layers where the flux is positive. We demonstrate the existence of a non-gradient contribution to the enthalpy flux, which is estimated to be important throughout the convective layer. A quantitative analysis of downflows indicates a transition from a tree-like structure where smaller downdrafts merge into larger ones in the upper parts to a structure in the deeper parts where a height-independent number of strong downdrafts persist. This change of flow topology occurs when a substantial subadiabatic layer is present in the lower part of the CZ.Item From periodic to cyclic processes in stellar magnetic activity research: time series analysis methods and their applications(Aalto University, 2018) Olspert, Nigul; Käpylä, Maarit, Prof., Max Planck Institute for Solar System Research, Germany; Pelt, Jaan, Prof., Tartu Observatory, Estonia; Tietotekniikan laitos; Department of Computer Science; Perustieteiden korkeakoulu; School of Science; Vehtari, Aki, Prof., Aalto University, Department of Computer Science, FinlandOne of the unanswered questions in stellar activity research is how the rotation period and the magneticcycle period of a star are related. A prerequisite to answering this question is being able to estimate both of these quantities as reliably as possible. Throughout the years, the prevailing methods have mostly been based on the well-known Lomb-Scargle periodogram. However, such a periodogram and its analogues are hard to interpret, when the input signal is not fully periodic. Observations of the solar cycle properties through factors, such as, the sunspot number over time, and non-linear dynamo models both clearly indicate that the stellar dynamo process is indeed quasi-periodic and non-stationary. Hence, a more correct approach is to relax the assumption of periodicity. The development and application of such methods is the main aim of this thesis. To investigate stellar cycles theoretically, the most advanced approach is to use global 3D magnetoconvection models solving the full MHD equations. These have only recently started to show similar quasi-periodic behaviour as the observed datasets. Real and simulated data pose completely different requirements for the analysis methods. While the former are unevenly sampled and sparse, the latter contain vast amounts of multidimensional data. For the estimation of magnetic cycles, an additional problem with observational data is their relative shortness. Throughout the thesis I will thoroughly address the above aspects. In this work, several methods have been developed for analysing time series of active stars. Carrier fit (CF)method is a simple and efficient way for fitting a continuous model into the time series of active stars. Side by side with this method a visualisation technique is used, which allows deviations from strict periodicity at different times to be easily detected, revealing the quasi-periodic and non-stationary effects. Another method, called D2 phase dispersion statistic is a robust tool for estimating periods of a quasi-periodic time series. It allows a simple generalisation to multiple dimensions, which is useful when analysing datasets of 3D magnetoconvection simulations. We also use probabilistic models for period estimation. For short datasets, the period estimates can become sensitive to the ways the linear trend in the data is handled. We show that for proper treatment one needs to include the trend component in the model, while using prior distributions for regularisation. Other probabilistic models, which have been used in the study include Gaussian processes (GPs) with periodic and quasi-periodic covariance functions. From the toolbox of methods suitable for non-stationary data, we have used ensemble empirical mode decomposition (EEMD). Our applications involve a young solar analogue LQ Hya, 3D magnetoconvection simulation called PENCILMillennium and a Mount Wilson (MW) stellar chromospheric activity dataset. For LQ Hya, we estimated the mean rotation period, surface differential rotation coefficient and fitted a continuous light curve model using the CF method. In the case of PENCIL-Millennium simulation data, we used both EEMD and the D2 statistic to extract the different dynamo modes with their locations in the convection zone. These modes include a five-year cycle, which is an analogue of the 22-year magnetic cycle of the Sun, and two much longer cycles. Furthermore, with the help of the D2 statistic, we were able to find a very incoherent short cycle with a period around half a year, which resembles the quasi-biennial oscillations of the Sun. In the analysis of the MW dataset, the main aim was to repeat the cycle length estimation with a simple harmonic model while properly handling trends, but also trying out periodic and quasi-periodic GP models. All three methods led to quite similar results, however, the reliability of the quasi-periodic model remained questionable due to the shortness of the datasets.We confirmed the existence of two different star populations in the activity diagram. However, as opposed to the formerly known positive correlations within both of these branches, we confirmed only a positive correlation within the inactive branch. The results were also compared to the recent 3D magnetoconvection simulations.Item A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions(IOP Publishing Ltd., 2021-04-01) Lehtinen, Jyri J.; Käpylä, Maarit J.; Olspert, Nigul; Spada, Federico; Department of Computer Science; Professorship Korpi-Lagg Maarit; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Max Planck Institute for Solar System ResearchThe magnetic activity of late-type stars is correlated with their rotation rates. Up to a certain limit, stars with smaller Rossby numbers, defined as the rotation period divided by the convective turnover time, have higher activity. A more detailed look at this rotation-activity relation reveals that, rather than being a simple power law relation, the activity scaling has a shallower slope for the low-Rossby stars than for the high-Rossby ones. We find that, for the chromospheric CaII H&K activity, this scaling relation is well modelled by a broken two-piece power law. Furthermore, the knee-point of the relation coincides with the axisymmetry to non-axisymmetry transition seen in both the spot activity and surface magnetic field configuration of active stars. We interpret this knee-point as a dynamo transition between dominating axi- and non-axisymmetric dynamo regimes with a different dependence on rotation and discuss this hypothesis in the light of current numerical dynamo models.Item Method of frequency dependent correlations: Investigating the variability of total solar irradiance(2017-04-01) Pelt, Jaan; Käpylä, Maarit; Olspert, Nigul; Department of Computer Science; Professorship Karhunen J.; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Tartu ObservatoryThis paper contributes to the field of modeling and hindcasting of the total solar irradiance (TSI) based on different proxy data that extend further back in time than the TSI that is measured from satellites. We introduce a simple method to analyze persistent frequency-dependent correlations (FDCs) between the time series and use these correlations to hindcast missing historical TSI values. We try to avoid arbitrary choices of the free parameters of the model by computing them using an optimization procedure. The method can be regarded as a general tool for pairs of data sets, where correlating and anticorrelating components can be separated into non-overlapping regions in frequency domain. Our method is based on low-pass and band-pass filtering with a Gaussian transfer function combined with de-trending and computation of envelope curves. We find a major controversy between the historical proxies and satellite-measured targets: a large variance is detected between the low-frequency parts of targets, while the low-frequency proxy behavior of different measurement series is consistent with high precision. We also show that even though the rotational signal is not strongly manifested in the targets and proxies, it becomes clearly visible in FDC spectrum. The application of the new method to solar data allows us to obtain important insights into the different TSI modeling procedures and their capabilities for hindcasting based on the directly observed time intervals.Item Multiperiodicity, modulations, and flip-flops in variable star light curves - III. Carrier fit analysis of LQ Hydrae photometry for 1982-2014(2015) Olspert, Nigul; Käpylä, Maarit J.; Pelt, Jaan; Cole, Elizabeth M.; Hackman, Thomas; Lehtinen, Jyri; Henry, Gregory W.; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Department of Computer ScienceAims. We study LQ Hya photometry for 1982–2014 with the carrier fit (CF) method and compare our results to earlier photometric analysis and recent Doppler imaging maps. Methods. As the rotation period of the object is not known a priori, we utilize different types of statistical methods first (least-squares fit of harmonics, phase dispersion statistics) to estimate various candidates for the carrier period for the CF method. Secondly, a global fit to the whole data set and local fits to shorter segments are computed with the period that is found to be optimal. Results. The harmonic least-squares analysis of all the available data reveals a short period, of close to 1.6 days, as a limiting value for a set of significant frequencies. We interpret this as the rotation period of the spots near the equatorial region. In addition, the distribution of the significant periods is found to be bimodal, hinting of a longer-term modulating period, which we set out to study with a two-harmonic CF model. A weak modulation signal is, indeed retrieved, with a period of roughly 6.9 yr. The phase dispersion analysis gives a clear symmetric minimum for coherence times lower than and around 100 days. We interpret this as the mean rotation pattern of the spots. Of these periods, the most significant and physically most plausible period statistically is the mean spot rotation period 1 60514, which is chosen to be used as the carrier period for the CF analysis. With the CF method, we seek any systematic trends in the spot distribution in the global time frame, and locally look for previously reported abrupt phase changes in rapidly rotating objects. During 2003–2009, the global CF reveals a coherent structure rotating with a period of 1 6037, while during most other times the spot distribution appears somewhat random in phase. Conclusions. The evolution of the spot distribution of the object is found to be very chaotic, with no clear signs of an azimuthal dynamo wave that would persist over longer timescales, although the short-lived coherent structures occasionally observed do not rotate with the same speed as the mean spot distribution. The most likely explanation of the bimodal period distribution is attributed to the high- and low-latitude spot formation regions confirmed from Doppler imaging and Zeeman Doppler imaging.Item Multiple dynamo modes as a mechanism for long-term solar activity variations(2016-05) Käpylä, Maarit; Käpylä, Petri; Olspert, Nigul; Brandenburg, A.; Warnecke, J.; Karak, B. B.; Pelt, J.; Department of Computer Science; Max-Planck-Institut für Sonnensystemforschung; Tartu ObservatoryContext. Solar magnetic activity shows both smooth secular changes, such as the modern Grand Maximum, and quite abrupt drops that are denoted as grand minima, such as the Maunder Minimum. Direct numerical simulations (DNS) of convection-driven dynamos off er one way of examining the mechanisms behind these events. Aims. In this work, we analyze a solution of a solar-like DNS that was evolved for roughly 80 magnetic cycles of 4.9 years and where epochs of irregular behavior are detected. The emphasis of our analysis is to find physical causes for such behavior. Methods. The DNS employed is a semi-global (wedge-shaped) magnetoconvection model. For the data analysis tasks we use Ensemble Empirical Mode Decomposition and phase dispersion methods, as they are well suited for analyzing cyclic (non-periodic) signals. Results. A special property of the DNS is the existence of multiple dynamo modes at different depths and latitudes. The dominant mode is solar-like (equatorward migration at low latitudes and poleward at high latitudes). This mode is accompanied by a higher frequency mode near the surface and at low latitudes, showing poleward migration, and a low-frequency mode at the bottom of the convection zone. The low-frequency mode is almost purely antisymmetric with respect to the equator, while the dominant mode has strongly fluctuating mixed parity. The overall behavior of the dynamo solution is extremely complex, exhibiting variable cycle lengths, epochs of disturbed and even ceased surface activity, and strong short-term hemispherical asymmetries. Surprisingly, the most prominent suppressed surface activity epoch is actually a global magnetic energy maximum; during this epoch the bottom toroidal magnetic field obtains a maximum, demonstrating that the interpretation of grand minima-type events is non-trivial. The hemispherical asymmetries are seen only in the magnetic field, while the velocity field exhibits considerably weaker asymmetry. Conclusions. We interpret the overall irregular behavior as being due to the interplay of the different dynamo modes showing different equatorial symmetries, especially the smoother part of the irregular variations being related to the variations of the mode strengths, evolving with different and variable cycle lengths. The abrupt low-activity epoch in the dominant dynamo mode near the surface is related to a strong maximum of the bottom toroidal field strength, which causes abrupt disturbances especially in the differential rotation profile via the suppression of the Reynolds stresses.Item Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars(2018-08-01) Viviani, Mariangela; Warnecke, Jörn; Käpylä, Maarit J.; Käpylä, Petri J.; Olspert, Nigul; Cole-Kodikara, Elizabeth M.; Lehtinen, Jyri J.; Brandenburg, Axel; Department of Computer Science; Centre of Excellence Research on Solar Long-Term Variability and Effects, ReSoLVE; Professorship Vehtari Aki; Max Planck Institute for Solar System Research; University of Helsinki; NorditaContext. Both dynamo theory and observations of stellar large-scale magnetic fields suggest a change from nearly axisymmetric configurations at solar rotation rates to nonaxisymmetric configurations for rapid rotation. Aims. We seek to understand this transition using numerical simulations. Methods. We use three-dimensional simulations of turbulent magnetohydrodynamic convection in spherical shell wedges and considered rotation rates between 1 and 31 times the solar value. Results. We find a transition from axi- to nonaxisymmetric solutions at around 1.8 times the solar rotation rate. This transition coincides with a change in the rotation profile from antisolar- to solar-like differential rotation with a faster equator and slow poles. In the solar-like rotation regime, the field configuration consists of an axisymmetric oscillatory field accompanied by an m = 1 azimuthal mode (two active longitudes), which also shows temporal variability. At slow (rapid) rotation, the axisymmetric (nonaxisymmetric) mode dominates. The axisymmetric mode produces latitudinal dynamo waves with polarity reversals, while the nonaxisymmetric mode often exhibits a slow drift in the rotating reference frame and the strength of the active longitudes changes cyclically over time between the different hemispheres. In the majority of cases we find retrograde waves, while prograde waves are more often found from observations. Most of the obtained dynamo solutions exhibit cyclic variability either caused by latitudinal or azimuthal dynamo waves. In an activity-period diagram, the cycle lengths normalized by the rotation period form two different populations as a function of rotation rate or magnetic activity level. The slowly rotating axisymmetric population lies close to what in observations is called the inactive branch, where the stars are believed to have solar-like differential rotation, while the rapidly rotating models are close to the superactive branch with a declining cycle to rotation frequency ratio and an increasing rotation rate. Conclusions. We can successfully reproduce the transition from axi- to nonaxisymmetric dynamo solutions for high rotation rates, but high-resolution simulations are required to limit the effect of rotational quenching of convection at rotation rates above 20 times the solar value.