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Estimating rotation periods and magnetic cycle lengths of active stars - PhD thesis by Nigul Olspert (Nov 16, 2018)

Nigul's thesis - IIPeg cyclicity
Nigul Olspert, M.Sc., will defend the dissertation
"From periodic to cyclic processes in stellar magnetic activity research: time series analysis methods and their applications"
on Friday 16 November at 12 noon at the Aalto University School of Science, lecture hall AS1, Maarintie 8, Espoo.

In this dissertation we introduce and develop time series analysis methods which are dedicated to period and cycle length estimation of magnetically active stars. Knowing both of these quantities is important as it makes observable reality comparable with predictions from the dynamo theory. Magnetic activity is primarily manifested through dark spots on the surface of the star. However, the rotation of the star is not uniform, but differential, which makes the period estimation not a trivial task. Furthermore, over time the number of spots changes, forming a cyclic process. For instance, for the Sun the activity cycle is known to last approximately 11 years, while both the length of each individual cycle as well as the amplitude is varying. Such a behaviour is called quasi-periodic.

Opponent: Professor Ivan Andronov, Odessa National Maritime University, Ukraine

Kustos: Aki Vehtari, Aalto University School of Science, Department of Computer Science

Supervisors: Maarit Käpylä (Max Planck Institute for Soar System Research) and Jaan Pelt (Tartu Observatory)
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ReSoLVE is hosting Prof. Axel Brandenburg's 60th Birthday Symposium (April 2019)

"Turbulence & magnetic fields - from the early universe to late-type stars"

Turbulence & magnetic fields - from the early universe to late-type stars

April 1st to April 5th 2019. The venue is the magnificent lakeside hotel Gustavelund, lake Tuusula, southern Finland.

Register here

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The Sun shows signatures of turbulent dynamo action

The Sun shows signatures of turbulent dynamo action

We have determined magnetic helicity spectrum from the solar surface observations using the recently developed two-scale formalism. We analyzed synoptic vector magnetograms built with data from the Vector Spectromagnetograph (VSM) instrument on the Synoptic Optical Long-term Investigations of the Sun (SOLIS) telescope during January 2010 - July 2016, hence covering a large fraction of the solar cycle 24. Our study includes the total of 74 synoptic Carrington rotation maps. We recover here bihelical spectra at different phases of solar cycle 24, where the net magnetic helicity in the majority of the data is consistent with a large-scale dynamo with helical turbulence operating in the Sun. More than 20 precent of the analyzed maps, however, show violations of the expected sign rule.
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Surface f-modes indicative of subsurface magnetic fields in the Sun

The Sun shows signatures of turbulent dynamo action

Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the f-mode, which emerges from the density jump at the surface. In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localized bipolar magnetic structures in the middle of the domain. We argue that detections of f-mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photos phere before these are directly detectable as visible manifestations in terms of active regions or sunspots.
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ReSoLVE Alumni Dr. Elizabeth Cole wins 2017 ThesisPrize from Helsinki University. (May 2018)

Thinkwall 2017 Doctoral Prizes.

The thesis title was "Stellar Magnetism in Theory and Observation". There were four winners across the University. The other theses were "Spatial Conservation Planning for Biodiversity and Ecosystem Services: From concepts and methods to policy agendas in the European Union", "Molecular Classification of Uterine Leiomyomas by Genome-wide Methods", and "Imaginaries of Early Childhood Education: Societal roles of early childhood education in an era of accountability".
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Rotation-Activity relation in global convection simulations (May 2018)

Stellar magnetic activity level and rotation are strongly connected. We studied the effect of increasing rotation rate on solar-like stars using magnetohydrodynamic simulations of stars with outer convective envelopes. At around 1.8 times the solar rotation rate, we found a transition point that separates slowly rotating, magnetically inactive stars, with rather axisymmetric large-scale magnetic fields (like our Sun), from more active, rapidly rotating, stars, with nonaxisymmetric large-scale magnetic fields. In the slow rotators we detected latitudinal dynamo waves reminiscent of the butterfly diagram of the Sun, while in the rapid rotators longitudinal dynamo waves were found. This essentially means that the nonaxisymmetric magnetic field modes rotate with a different speed than the stellar surface, that manifests itself as either prograde or retrograde migration of the magnetic structure. Such behaviour has also been observationally seen in active stars.
Stellar magnetic activity level and rotation are strongly connected. We studied the effect of increasing rotation rate on solar-like stars using magnetohydrodynamic simulations of stars with outer convective envelopes. At around 1.8 times the solar rotation rate, we found a transition point that separates slowly rotating, magnetically inactive stars, with rather axisymmetric large-scale magnetic fields (like our Sun), from more active, rapidly rotating, stars, with nonaxisymmetric large-scale magnetic fields. In the slow rotators we detected latitudinal dynamo waves reminiscent of the butterfly diagram of the Sun, while in the rapid rotators longitudinal dynamo waves were found. This essentially means that the nonaxisymmetric magnetic field modes rotate with a different speed than the stellar surface, that manifests itself as either prograde or retrograde migration of the magnetic structure. Such behaviour has also been observationally seen in active stars.
Our results also highlight the importance of maintaining high enough supercriticality of convection, in particular, in the rapid rotation regime, where a too low supercriticality results in axisymmetric field configuration instead of a nonaxisymmetric one.
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Report to the Academy of Finland Scientific Advisory Board 14th March 2018.

Report to the Academy of Finland Scientific Advisory Board 14th March 2018.

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Hidden solar magnetic cycle explained with modifications in turbulent induction and pumping

The Sun, aside from its eleven year sunspot cycle is additionally subject to long term variation in its activity. We make use of a solar-like convective dynamo simulation of Käpylä et al. 2016, exhibiting equatorward propagation of the magnetic field, multiple frequencies, and irregular variability, including a missed cycle and complex parity transitions between dipolar and quadrupolar modes, to study the physical causes of such events. We use the test field analysis tool to measure and quantify the effects of turbulence in the generation and evolution of the large-scale magnetic field. The test-field analysis provides an explanation of the missing surface magnetic cycle in terms of the reduction of part of the alpha effect, the one of the key ingredients for dynamo action. Furthermore, we found an enhancement of downward turbulent pumping during the event to confine some of the magnetic field at the bottom of the convection zone, where local maximum of magnetic energy is observed during the event. At the same time, however, a quenching of the turbulent magnetic diffusivities is observed. For more detailed analysis, we will perform dedicated mean-field modelling with the measured turbulent transport coefficients in the future.

Convection zone surface Bφ (top), base transport coefficient αφφ (middle) and radial turbulent pumping (bottom) at 40oN, where a magnetic field disturbance occurs.

The Sun, aside from its eleven year sunspot cycle is additionally subject to long term variation in its activity. We make use of a solar-like convective dynamo simulation of Käpylä et al. 2016, exhibiting equatorward propagation of the magnetic field, multiple frequencies, and irregular variability, including a missed cycle and complex parity transitions between dipolar and quadrupolar modes, to study the physical causes of such events. We use the test field analysis tool to measure and quantify the effects of turbulence in the generation and evolution of the large-scale magnetic field. The test-field analysis provides an explanation of the missing surface magnetic cycle in terms of the reduction of part of the alpha effect, the one of the key ingredients for dynamo action. Furthermore, we found an enhancement of downward turbulent pumping during the event to confine some of the magnetic field at the bottom of the convection zone, where local maximum of magnetic energy is observed during the event. At the same time, however, a quenching of the turbulent magnetic diffusivities is observed. For more detailed analysis, we will perform dedicated mean-field modelling with the measured turbulent transport coefficients in the future."
The method results are published in Gent, Käpylä & Warnecke (2017)." [more]
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Development of HPC and data analysis tools

We have developed an effective method for accelerating fluid dynamics calculations with high-order precision on graphics processing units (GPUs). This is done by efficient use of GPU memory with cache blocking and by dividing computation algorithms into memory efficient chunks. Our Nvidia CUDA based, proof of concept code Astaroth is able to achieve 3.6 times speedup in comparison to the reference code, which in practice allows for a week-long turbulence simulation to be performed within a couple of days. The method is published in Pekkilä Väisälä et al. (2017).

The power of graphics processing units harnessed for high-accuracy turbulence modelling

We have developed an effective method for accelerating fluid dynamics calculations with high-order precision on graphics processing units (GPUs). This is done by efficient use of GPU memory with cache blocking and by dividing computation algorithms into memory efficient chunks. Our Nvidia CUDA based, proof of concept code Astaroth is able to achieve 3.6 times speedup in comparison to the reference code, which in practice allows for a week-long turbulence simulation to be performed within a couple of days. The method is published in Pekkilä Väisälä et al. (2017)."
The method is published in Pekkilä Väisälä et al. (2017)." [more]
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Jul 2017

Meeting

Pencil Code Meeting 2017

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Past Activites

Jun 2016

Visitor

Nishant Singh from NORDITA will visit us from 13th to 15th June and give a presentation in the MHD group meeting on 13th at 11:00 in A328.

May 2016

Visitor

Prof. Jaan Pelt from Tartu Observatory, Estonia, is visiting us from 23th to 27th May.

Apr 2016

Visitor

Sarah Jabbari from NORDITA will visit us 25th-28th of April, and give a presentation in the MHD group meeting Mon 25th at 13:00 in A328.

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Postdoc Position

Postdoctoral Position in Solar and Stellar Physics at the Max Planck Institute for Solar System Research (in the Solar and stellar magnetic activity - observations and modelling group led by Maarit Käpylä) DL 5th of May 2016! More information.

Visitors

16.9.-5.10.2015 Prof. Nathan Kleeorin from Ben Gurion University, Beer Sheva, Israel, and 21.9.-2.10. 2015 Dr. Jörn Warnecke from MPS, Göttingen, are visiting ReSoLVE. Jörn will give a talk in the CS Forum, Thu 1.10. 14:15-15:00, TUAS Odeion.

Sep 2015

ReSoLVE Autumn Meeting

28.-29.10.2015 in CS Building T4 at Aalto.
Registration now open.

Jan 2015

The group is recruiting a summer intern to work on multi-GPU MHD solvers (Job descriptor MHD)

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Open position

The group is looking for a summer intern (job identifier MHD). The deadline of the call is 26th of January, 2015.

MHD - Multi-GPU Finite-Difference Solvers for Magnetohydrodynamics

Job description: The next generation of supercomputers based on energy-efficient processors is currently being deployed world-wide. New HPC algorithms and solutions based both on CPU and GPU architectures are required to port the existing computational tools to the new supercomputers. During the project, the goal is to develop further the existing stencil-based finite-difference core solvers on accelerator platforms (GPUs) dedicated to hydro- and magnetohydrodynamic applications, especially to explore the extension(s) to multi-GPU computing using asynchronous communication.

Dec 2014

M.Sc. Marjaana Lindborg's thesis defense on the 12th of December in Univ. of Helsinki Main Building: Spot activity of late-type stars: a study of II Pegasi and DI Piscium

Feb 2014

The group was awarded 32 million CPU hours from PRACE, for the investigation of the solar dynamo process.

14 Oct 2013

The group was awarded 3.9 million CPU hours from CSC's supercomputer Sisu for the execution of the Grand Challenge project DYNAMO13 investigating dynamo processes in rapidly rotating young suns.

7-11 Oct 2013

Dr. Jaan Pelt from Tartu Observatory, Estonia, is visiting ICS. He will present a talk titled "Nyquist barrier - not for all!" in the ICS Forum on Monday 7th, 13:15 in T2 (presentation slides).

1-26 Oct 2013

Dr. Jörn Warnecke from Nordita KTH/SU & Max Planck Institute for Solar System Research, Lindau, is visiting UH and ICS groups. He will present a talk titled "Bipolar magnetic structures driven by stratified turbulence with a coronal envelope" on Friday 4th in UH/Physicum, D114, at 10:15.

26-27 Jul 2013

Prof. Axel Brandenburg from KTH, Nordita, is visiting ICS. He will give a talk titled "Self-organized magnetic structures in computational astrophysics" (presentation slides) in the ICS Forum on Monday 26th, 13:15 in T2.

4 Jul 2013

CMDAA group's work towards the understanding of the Sun's butterfly diagram advertised in the Nordita Newsletter 2/2013

4 Jun 2013

Academy of Finland to fund the Center of Excellence ReSoLVE with a participating team from CMDAA (Academy's press release)

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