News
Estimating rotation periods and magnetic cycle lengths of active stars - PhD thesis by Nigul Olspert
(Nov 16, 2018)
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"
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The Sun shows signatures of turbulent dynamo action
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Surface f-modes indicative of subsurface magnetic fields in the Sun
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ReSoLVE Alumni Dr. Elizabeth Cole wins 2017 ThesisPrize from Helsinki University. (May 2018)
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Rotation-Activity relation in global convection simulations (May 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."
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 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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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
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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