Differential Rotation and Magnetism across the HR Diagram

8 Apr 2013, 08:00 → 3 May 2013, 18:00 Europe/Stockholm

132:028 (Nordita)

Maarit Mantere (University of Helsinki), Petri Käpylä (University of Helsinki), Rainer Arlt (AIP Potsdam)

Photo: Atefeh Barekat

Scope

The goal of the programme is to advance our understanding of the physical processes generating differential rotation in various types of stars, and the role that this effect plays for stellar magnetic activity and dynamos. The Sun is the only star for which the internal rotation profile is observationally known thanks to helioseismology – for other stars, only the surface differential rotation can be inferred from photometric or spectroscopic observations. Much relies on numerical models, working either under the mean-field approximation, or alternatively directly modelling rotating turbulent convection. The former approach has developed to the stage at which rotation profiles of different types of stars can be routinely calculated, although the effect of magnetic fields is normally not taken into account. Models of the latter category can quite satisfactorily produce the solar internal rotation profile, but calculations are computationally tremendously expensive, making large parameter studies very difficult. Analysis of observations, on the other hand, seems to indicate somewhat stronger, and sometimes anti-solar, differential rotation, as opposed to the predictions from mean-field models. This leaves quite uncertain grounds for further development of stellar dynamo theory, one of the key ingredients for the operation of the dynamo being the stellar nonuniform rotation. The main goal of the program is to investigate the connection between the theories and observations and obtain better understanding of the generation and role of differential rotation for stellar magnetism, and to formulate new theoretical, modeling and data analysis methods.

[Timetable - with slides of contributions]

Focus event

We are planning a one-week conference in the beginning of the program, which will be tightly focused to the topics addressed in the scientific case, but be open to a much larger number of participants. - Schedule of the conference

The conference takes place in the lecture rooms B2 and B3 of the building Brinellvägen 23 on the campus of Kungliga Tekniska Högskolan (KTH).

Other program activities

The work schedule for the remaining three weeks of the program consists of topical discussion sessions, seminars, and team work on issues raised during the focus event. These activities take place at Nordita and on the Albanova campus (top-left part of this PDF; we will mostly use building 23, but also building 33 "FP").

Time	Activity	Topic
April 8 - April 12	Conference	Differential Rotation and Magnetism across the HR Diagram
April 15 - April 19	Program week	Generation and observation of differential rotation
April 22 - April 26	Program week	Differential rotation interacting with magnetic fields (instabilities, torsional oscillations)
April 29 - May 03	Program week	Dynamo processes in differentially rotating stars

Other activities

During the program, currently on three of the Fridays at 15:15-16:30, there astrobiology lectures, which may be of interest to some of the participants. There may also be some AlbaNova/Nordita colloquia on Thursdays, 15:15.

There is no registration fee.

Accommodation

Nordita provides a limited number of rooms in the Stockholm apartment hotel BizApartments free of charge for program participants.

Sponsored by:

pictures

• group_stockholm_2013_barekat_9609_imp_tiny.jpg
• _MG_9571_tiny.jpg
• st_P1060148_tiny.jpg

Monday, 8 April

07:00 MONDAY

1 Registration

2 BROWNING: Simulations of differential rotation at the bottom of the main sequence

3 MORIN: Differential rotation in very-low-mass stars: a clue to dynamo bistability?

Slides Morin.pdf

11:00 break

4 STRASSMEIER: DI and ZDI of stellar magnetism and surface velocity fields

5 BRUN: Differential rotation and dynamo action in G and K stars

12:30 Lunch

6 BONANNO: Current-driven Instabilities in Stellar Radiation Zones: linear analysis and nonlinear evolution from DNS

7 MOND: Non-dissipative saturation of the magneto-rotational instability

Slides Mond.pdf

8 HYPOLITE: A two-dimensional model of gravitationally contracting rotating star

16:30 break

9 MIESCH: Meridional circulation in solar and stellar convection zones

Slides Miesch.pdf

10 SOOD: Dynamic model of dynamo (magnetic activity) and stellar rotation

Slides Sood.pdf

Tuesday, 9 April

07:00 TUESDAY

11 KITCHATINOV: Mean-field theory of meridional flow and differential rotation

Slides Kitchatinov.pps

12 ZHANG: Consistent Long-term Variation in the Hemispheric Asymmetry of Solar Rotation

Slides Zhang.pdf

13 ARLT: The solar differential rotation in the 18th century

Slides arlt_froehlich_solar_diffrot.pdf

14 FOURNIER: Magnetic flux emergence with differential rotation in compressible shells

Slides Fournier.pdf

11:00 break

15 HACKMAN: Are starspots equivalents to sunspots?

Slides Hackman.pdf

16 KOCHUKHOV: Zeeman Doppler imaging: techniques and limitations

17 BESSOLAZ: Magnetism and differential rotation in pre-main sequence stars

13:00 Lunch

18 BASRI: What fraction of Kepler targets show differential rotation?

Slides Basri.pdf

19 REINHOLD: Rotation and Differential Rotation of the Kepler stars

20 GARCIA: Studying stellar rotation and magnetic activity cycles with asteroseismic measurements

21 KAROFF: Sounding Stellar Cycles with Kepler

16:30 break

22 CHAN: Rotational effects on convection

Slides Chan.pdf

23 LEHTINEN: Viability of using photometric period variations as a proxy for stellar surface differential rotation

Slides Lehtinen.pdf

Wednesday, 10 April

07:00 WEDNESDAY

24 BRANDENBURG: Is solar activity a surface phenomenon?

Slides Brandenburg.ppt

25 BENEVOLENSKAYA: Synoptic solar cycle observed by Solar Dynamics Observatory

Slides Benevolenskaya1.pdf

26 WARNECKE: Solar-like differential rotation in a convective dynamo with a coronal envelope

Slides Warnecke.pdf

11:00 break

27 REINERS: Observational constraints for dynamos in low-mass stars

28 PETITDEMANGE: On the field strength and field topology in anelastic spherical dynamo simulations

29 SCHMITT: Surface constraints on the solar dynamo

Slides Schmitt.pdf

13:00 Lunch

14:30 Free afternoon

Thursday, 11 April

07:00 THURSDAY

30 CANTIELLO: Pulsations and magnetic fields in massive stars

Slides Cantiello.pdf

31 GREGORY: Can we predict the global magnetic topology of a pre-main-sequence star from Its position in the Hertzsprung-Russell diagram?

Slides Gregory.pdf

32 SPADA: Modelling low-mass main, sequence stars: radius discrepancy and magnetic activity

Slides Spada.pdf

11:00 break

33 SCHRöDER: O.C.Wilson's stars: The aging of stellar activity on the MS

Slides Schroeder.pdf

34 KüKER: Meridional flows in stellar convection zones

Slides Kueker.pdf

35 BRAITHWAITE: The Tayler instability in stars

Slides Braithwaite.pdf

13:00 Lunch

36 ROGACHEVSKII: New scaling for the alpha effect in slowly rotating turbulence

37 GUERRERO: Implicit large-eddy simulation of rotating turbulent convection with the EULAG code

Slides Guerrero.pdf

38 KäPYLä: Magnetic cycles and equatorward migration in simulations of turbulent convection

Slides Kapyla.pdf

39 SCHLICKEISER: Cosmic magnetogenesis: from spontaneously emitted aperiodic turbulent to ordered equipartition fields

Slides Schlickeiser.pdf

16:30 break

40 GAILITIS: The Riga dynamo experiment

Slides Gailitis.pdf

41 TRIANA: Inertial waves driven by differential rotation: a laboratory experiment

Slides Triana.pdf

Friday, 12 April

07:00 FRIDAY

42 BENEVOLENSKAYA: Detailed evolution and rotation of the active regions NOAA 11101, 11106 from the SDO/HMI data

Slides Benevolenskaya2.pdf

43 SOKOLOFF: Reversals of the solar dipole

Slides Sokoloff1.pdf

44 ILLARIONOV: A new dynamo pattern revealed by the tilt angle of bipolar sunspot groups

Slides Illarionov.pdf

11:00 break

45 COLE: Differential rotation and dynamo in compressible convection simulations

46 KARAK: Effect of the turbulent pumping of magnetic flux on the predictability of the solar cycle

Slides Karak.pdf

47 Resonances for activity waves in spherical mean field dynamos

Speaker: Dmitry Sokoloff

Slides Sokoloff2.pdf

Monday, 15 April

48 Information on differential rotation from asteroseismology

Speaker: Mikkel Lund (Stellar Astrophysics Centre, Aarhus University)

49 SHUKUROV: Smoothing with Gaussian processes

Tuesday, 16 April

50 Magnetic fields in the early universe

The talk discusses the possibility that cosmic magnetic fields can originate in the early universe. Primordial fields could be generated during one of the early universe phase transitions like, inflation or the elctroweak/QCD phase transitions. We discuss these possibilities, their subsequent evolution, and the possible signals they leave on the Cosmic microwave background and on the high redshift universe. Such primordial fields could also provide seed magnetic fields for the dynamo, and explain the fields which have been recently claimed to be present in the large scale voids.

Speaker: Prof. Kandaswamy Subramanian

51 Baroclinic instability in differentially rotating stars

Speaker: Leonid Kitchatinov (Institute for Solar-Terrestrial Physics)

Slides kitchatinov_barocline.pdf

Wednesday, 17 April

52 Flow induction in MHD turbulence

Effect of cross helicity (velocity--magnetic-field correlation) in the mean momentum equation is investigated. Turbulent cross helicity enters the expression for the mean-field Lorentz force. Combined with the contributions through the Reynolds and turbulent Maxwell stresses, it gives a possibility to induce a flow. Mean momentum equation is examined from this viewpoint. As for some examples, poloidal flow in the reversed-shear (RS) mode in tokamak plasmas, torsional oscillations in the solar convection zone, and explosive magnetic reconnection are argued in this framework.

Speaker: Dr Nobumitsu Yokoi (Institute of Industrial Science, University of Tokyo)

53 MIESCH: A 3D Babcock-Leighton dynamo model with spots

Thursday, 18 April

54 Activity Cycles of the red giant star OP And

Speaker: Ana Borisova (Institute of Astronomy and NAO-Rozhen, Bulgarian Academy of Sciences)

Slides Borisova_poster.pdf

Friday, 19 April

55 BIRZAN: Radio-mode AGN feedback (Astro-seminar)

56 Planet formation and life in the Galaxy

Speaker: Johansen

Monday, 22 April

57 Quasi-geostrophic approximation of anelastic convection

Speaker: Friedrich Busse

Tuesday, 23 April

58 Testing models on data - Bayesian inference

Speaker: Mr Rainer Arlt (Leibniz Institute for Astrophysics Potsdam)

59 The Li-rich single K giant DI Psc

Fast rotating red giant branch stars in the first dredge-up phase provide a unique opportunity to study connections between enhanced Li abundance and activity. DI Psc is (HD 217352) a Li- rich, active single K giant, a new candidate for the rather small group of giants currently in this state. We reconstructed the surface temperature distribution by the means of Doppler Imaging on several lines including Li I 6708 using exceptionally high resolution and signal-to-noise spectra covering two rotational periods. The surface differential rotational pattern was recovered using cross correlation of the consecutive maps to investigate if it is affected by the dredge-up mechanism.

Speaker: Levente Kriskovics (Konkoly Observatory of the Hungarian Academy of Sciences, Budapest)

Wednesday, 24 April

60 Differential rotation in red giants: constraining the physics of angular momentum transport with asteroseismology

Recent observations have revealed the presence of both pulsations and magnetic fields in massive stars. As these phenomena can directly affect the mass-loss and the rotation rate, their impact on stellar evolution is potentially huge, with consequences for the final fate of massive stars. I will review some of the latest observational and theoretical developments in the field, focusing in particular on the impact of subsurface convection zones in hot, massive stars. These convective regions might be responsible for both the presence of stochastically excited pulsations, small scale surface magnetic fields and surface micro/macro-turbulence.

Speaker: Matteo Cantiello (Kavli Institute for Theoretical Physics, University of California)

61 Using the wavelet transform to measure the stellar magnetic fields

The standard method of the magnetic field measurement, based on an analysis of the relation between the Stokes V-parameter and the first derivative of the total line profile intensity, has been modified by applying a linear integral operator L to the both sides of this relation. The wavelet transform with DOG-wavelets as the operator L is used. The advantage of the method is an effective noise suppression for the line profile and the Stokes parameter V. The efficiency of the method was tested for model line profiles with various noise contributions. To test the proposed method, we used the spectropolarimetric observations of the A0-type star alpha2 CVn and the young O-type star theta1 Ori C with well known magnetic field phase curves. The calculated by our method longitudinal magnetic field strengths for these stars are in a good agreement with those determined by other methods.

Speaker: Alexander F. Kholtygin (Astronomical Institute, Saint-Petersburg State University)

Thursday, 25 April

62 New theory of differential rotation in anisotropic turbulent convection

We discuss a new theory of differential rotation in anisotropic density stratified inhomogeneous turbulent convection. A key point of this theory is an effect of the turbulent heat flux on the Reynolds stresses in a rotating turbulent convection. We solved a coupled system of dynamical equations which includes the equations for the Reynolds stresses, the entropy fluctuations and the turbulent heat flux. We used a spectral tau approximation in order to close the system of dynamical equations. The model of the background turbulent convection takes into account an increase of the anisotropy of turbulence with increase of the rate of rotation. We also took into account the effect of rotation on the turbulent correlation time. We found that the ratio of the contributions to the Reynolds stresses caused by the turbulent heat flux and the anisotropic eddy viscosity is much larger than the ratio the density hight scale to the maximum scale of turbulent motions. We demonstrated that the effect of the turbulent heat flux on the Reynolds stresses is crucial for the formation of the differential rotation and should be taken into account in the theories of the differential rotation of the Sun, stars and planets. We found that this effect causes the differential rotation which is comparable with the typical solar differential rotation.

Speaker: Nathan J. Kleeorin (Ben-Gurion University of the Negev, Beer-Sheva)

63 Stellar spot modelling and differential rotation

Speakers: Hans-Erich Fröhlich, Mr Rainer Arlt (Leibniz Institute for Astrophysics Potsdam)

Slides froehlich_arlt_stellar_diffrot.pdf

Friday, 26 April

64 Observing Feedback and the Circumgalactic Medium: Pushing Toward the Norm

Speaker: Kate Rubin

65 Effect of the turbulent pumping of magnetic flux on the predictability of the solar cycle

The irregular nature of the solar cycle makes the prediction of future cycles challenging. However, the prediction of the solar activity is important for several reasons. In the Babcock-Leighton dynamo models, the poloidal field is generated near the solar surface whereas the toroidal field is generated near the base of the convection zone. Therefore a finite time necessary for the poloidal field to reach the base of the convection zone introduces a memory in the dynamo model which allows the dynamo model to predict the future solar cycle. We have shown that this predictability of the solar cycle is strongly affected by the inclusion of the downward turbulent pumping of the magnetic flux which is unavoidable in the convectively unstable solar convection zone. With a significant turbulent pumping in Babcock-Leighton dynamo models, a long term prediction of the solar cycle is impossible; only a short term prediction (of about 5 years) may be possible.

Speaker: Bidya Binay Karak (Department of Physics, Indian Institute of Science, Bangalore)

Monday, 29 April

66 Revisiting the ABC dynamo

Speaker: Emmanuel Dormy

67 Past and present of sunspot observations

Speaker: Mr Rainer Arlt (Leibniz Institute for Astrophysics Potsdam)

Slides arlt_sunspots_past_and_present_pixelized.pdf

68 Role of sunspots in the polar magnetic field reversal on the Sun

Using Greenwich catalogue of sunspots and magnetic field observations it was shown that impulses of sunspot activity during a course of solar cycle are responsible for residual magnetic flux transported by meridional circulation toward the poles. This, in turn, is related to the polarity reversal of the axisymmetric magnetic fields. Single and compound magnetic field reversals at solar poles during 1875-2012 are reconstructed and compared with reversals restored from Hα synoptic maps. Asynchronous reversals in both hemispheres are compared also with the phase differences between northern and southern activities. Relationships between the strong cycles 18 and 19 with the strength of the polar magnetic field at solar minimums are discussed.

Speaker: Nadezhda V. Zolotova (Earth's Physics Department, St.Petersburg State University)

Slides Zolotova_Nordita_2013.pdf

Tuesday, 30 April

69 Anisotropy of helicity in the Sun

We study observational proxies of magnetic helicity (and the alpha effect) in the Sun by analysis of photospheric vector magnetograms. The newest finding is determination of anisotropy of helicity near the solar surface. We can demonstrate that assumptions of local homogeneity and isotropy require serious revision in the light of these findings. Furthermore, we can show that rotation can cause this significant anisotropy. Furthermore, we can speculate how the methods developed for computation of large-scale helicity in the Sun can be applied for computation of helicity proxies in the stars for which magnetic patterns are available with sufficient cadence.

Speaker: Kirill Kuzanyan (IZMIRAN, Russia)

70 Anelastic dynamo models with variable electrical conductivity: an application to gas giants

Observations of the gas giants show that both planets have dipolar magnetic fields: Jupiter's is very similar to the Earth's magnetic field and Saturn's is very axisymmetric. In addition, both gas giants present a very dynamical behaviour of the atmospheric flow, organized in banded structures of east-west flow. Our main goal is to approach more realistic numerical models that explain these features. While the small density gradient across terrestrial iron cores allows the use of the Boussinesq approximation, the picture is different for the gas giants. Here, the density decreases by a factor of around 5000 from the deep interior to the surface (1 bar level). Though most of this density jump is accommodated in the outer molecular envelopes of the planets, it may still be significant in the metallic dynamo region. Among other properties, the electrical conductivity also varies significantly with radius, being roughly constant in the metallic hydrogen region and decaying superexponentially in the molecular envelope. In this work, we solve an anelastic numerical dynamo model (which differs from a fully compressible model by neglecting sound waves) to explore the effects of density stratification and electrical conductivity variation on the magnetic field generation and on the configuration and strength of the surface east-west flow. We use an anelastic version of the MHD code MagIC with inner-to-outer boundary density variation of up to 245 and an electrical conductivity profile that decays exponentially in the outer 5-30% of the simulated shell. Previous simulations using constant conductivity showed that dipole-dominated magnetic fields are only found for weak density variations. The exponential conductivity decrease helps to cancel this effect by separating magnetic field generation from the dominant convective region. For intermediate gradients of the density stratification (6

Speaker: Lúcia D. V. Duarte (Max-Planck-Institut für Sonnensystemforschung, Lindau)

Slides Duarte.pdf

71 Bridging planetary and stellar dynamos: scaling laws for anelastic spherical shell dynamos

Numerical dynamo models always operate at parameters which are many orders of magnitude smaller or larger than the values expected in natural objects. However, numerical modelling has been very successful in reproducing many interesting properties of dynamos existing in nature. This qualitative agreement fuels the idea that both numerical and natural systems are in an asymptotic regime of dynamics where the diffusive processes do not play an important role. Such asymptotic regimes can be probed using scaling studies. In the recent past, scaling laws derived from relatively simple dynamo simulations have proven to be very fruitful: numerical models successfully predict the mean magnetic field strength of a broad range of astrophysical objects encompassing Earth, Jupiter, and some rapidly-rotating fully-convective stars. We study more than 250 new direct numerical simulations of Boussinesq and anelastic dynamos in spherical shell to extend earlier scaling laws derived from only Boussinesq models. We find that the scaling laws for heat transfer, mean kinetic and magnetic energy in these systems are very robust. Our study provides strong support for the hypothesis that both mean kinetic and magnetic energy relate to the power generated by buoyancy forces via a simple power law.

Speaker: Rakesh Yadav (Max-Planck-Institut für Sonnensystemforschung, Lindau)

72 Dynamo efficiency as a constraint for dynamo models?

Speaker: Martin Schrinner

Wednesday, 1 May

73 Direct simulation of helicity and application in mean-field dynamos

Speaker: Dr Gustavo Guerrero (NORDITA)

74 Anelastic approximations

We will present a summary of different versions of the convective approximations of the fully compressible version of the Navier-Stokes equation, and question their numerical implementation.

Speaker: Raynaud Raphael

Thursday, 2 May

75 Stastistics of magnetic fields on high-mass stars

Speaker: Prof. Alexander Kholtygin (Astronomical Institute of Saint-Petersburg University, Russia)

76 Negative effective magnetic pressure instability

Speaker: Illa Rivero Losada

77 Surface flux concentrations in a spherical alpha-square dynamo

In the presence of strong density stratification, turbulence can lead to a large-scale instability of a horizontal magnetic field if its strength is in a suitable range (within a few percent of the turbulent equipartition value). This instability is related to a suppression of the turbulent pressure so that the turbulence contribution to the mean magnetic pressure becomes negative. This results in the excitation of a negative effective magnetic pressure instability (NEMPI). This instability has so far only been studied for an imposed magnetic field. We want to know how NEMPI works when the mean magnetic field is generated self-consistently by an \alpha^2 dynamo, whether it is affected by global spherical geometry, and whether it can influence the properties of the dynamo itself. We adopt the mean-field approach which has previously been shown to provide a realistic description of NEMPI in direct numerical simulations. We assume axisymmetry and solve the mean-field equations with the Pencil-Code for an adiabatic stratification at a total density contrast in the radial direction of approximately 4 orders of magnitude. NEMPI is found to work when the dynamo-generated field is about 4% of the equipartition value, which is achieved through strong \alpha quenching. This instability is excited in the top 5% of the outer radius provided the density contrast across this top layer is at least 10. NEMPI is found to occur at lower latitudes when the mean magnetic field is stronger. For weaker fields, NEMPI can make the dynamo oscillatory with poleward migration. In conclusion, NEMPI is a viable mechanism for producing magnetic flux concentrations in a strongly stratified spherical shell in which a magnetic field is generated by a strongly quenched \alpha effect dynamo.

Speaker: Sarah Jabbari (KTH Royal Institute of Technology, Stockholm)

Friday, 3 May

78 Combining Models of Coronal Mass Ejections and Solar Dynamos

Speaker: Mr Jörn Warnecke (Nordita)

79 Differential rotation in radiative stars

Speaker: Friedrich Busse

80 Appearance of discontinuities in MHD

Speaker: Jonathan Braithwaite

Slides Braithwaite_2.pdf