Registration Room: B3

BROWNING: Simulations of differential rotation at the bottom of the main sequence Room: B3

MORIN: Differential rotation in very-low-mass stars: a clue to dynamo bistability? Room: B3

STRASSMEIER: DI and ZDI of stellar magnetism and surface velocity fields Room: B3

BRUN: Differential rotation and dynamo action in G and K stars Room: B3

BONANNO: Current-driven Instabilities in Stellar Radiation Zones: linear analysis and nonlinear evolution from DNS Room: B3

MOND: Non-dissipative saturation of the magneto-rotational instability Room: B3

HYPOLITE: A two-dimensional model of gravitationally contracting rotating star Room: B2

MIESCH: Meridional circulation in solar and stellar convection zones Room: B2

SOOD: Dynamic model of dynamo (magnetic activity) and stellar rotation Room: B3

KITCHATINOV: Mean-field theory of meridional flow and differential rotation Room: B2

ZHANG: Consistent Long-term Variation in the Hemispheric Asymmetry of Solar Rotation Room: B2

ARLT: The solar differential rotation in the 18th century Room: B2

FOURNIER: Magnetic flux emergence with differential rotation in compressible shells Room: B2

HACKMAN: Are starspots equivalents to sunspots? Room: B2

KOCHUKHOV: Zeeman Doppler imaging: techniques and limitations Room: B2

BESSOLAZ: Magnetism and differential rotation in pre-main sequence stars Room: B2

BASRI: What fraction of Kepler targets show differential rotation? Room: B2

REINHOLD: Rotation and Differential Rotation of the Kepler stars Room: B2

GARCIA: Studying stellar rotation and magnetic activity cycles with asteroseismic measurements Room: B2

KAROFF: Sounding Stellar Cycles with Kepler Room: B2

CHAN: Rotational effects on convection Room: B2

LEHTINEN: Viability of using photometric period variations as a proxy for stellar surface differential rotation Room: B2

BRANDENBURG: Is solar activity a surface phenomenon? Room: B2

BENEVOLENSKAYA: Synoptic solar cycle observed by Solar Dynamics Observatory Room: B2

WARNECKE: Solar-like differential rotation in a convective dynamo with a coronal envelope Room: B2

REINERS: Observational constraints for dynamos in low-mass stars Room: B2

PETITDEMANGE: On the field strength and field topology in anelastic spherical dynamo simulations Room: B2

SCHMITT: Surface constraints on the solar dynamo Room: B2

CANTIELLO: Pulsations and magnetic fields in massive stars Room: B2

GREGORY: Can we predict the global magnetic topology of a pre-main-sequence star from Its position in the Hertzsprung-Russell diagram? Room: B2

SPADA: Modelling low-mass main, sequence stars: radius discrepancy and magnetic activity Room: B2

SCHRöDER: O.C.Wilson's stars: The aging of stellar activity on the MS Room: B2

KüKER: Meridional flows in stellar convection zones Room: B2

BRAITHWAITE: The Tayler instability in stars Room: B3

ROGACHEVSKII: New scaling for the alpha effect in slowly rotating turbulence Room: B2

GUERRERO: Implicit large-eddy simulation of rotating turbulent convection with the EULAG code Room: B2

KäPYLä: Magnetic cycles and equatorward migration in simulations of turbulent convection Room: B2

SCHLICKEISER: Cosmic magnetogenesis: from spontaneously emitted aperiodic turbulent to ordered equipartition fields Room: B3

GAILITIS: The Riga dynamo experiment Room: B2

TRIANA: Inertial waves driven by differential rotation: a laboratory experiment Room: B2

BENEVOLENSKAYA: Detailed evolution and rotation of the active regions NOAA 11101, 11106 from the SDO/HMI data Room: B2

SOKOLOFF: Reversals of the solar dipole Room: B2

ILLARIONOV: A new dynamo pattern revealed by the tilt angle of bipolar sunspot groups Room: B2

COLE: Differential rotation and dynamo in compressible convection simulations Room: B2

KARAK: Effect of the turbulent pumping of magnetic flux on the predictability of the solar cycle Room: B2

Resonances for activity waves in spherical mean field dynamos

• Dmitry Sokoloff

Room: B2

Information on differential rotation from asteroseismology

• Mikkel Lund (Stellar Astrophysics Centre, Aarhus University)

Room: FP41

SHUKUROV: Smoothing with Gaussian processes Room: FP41

Magnetic fields in the early universe

• Kandaswamy Subramanian

Room: Nordita west 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.

Baroclinic instability in differentially rotating stars

• Leonid Kitchatinov (Institute for Solar-Terrestrial Physics)

Room: FP41

Flow induction in MHD turbulence

• Nobumitsu Yokoi (Institute of Industrial Science, University of Tokyo)

Room: Nordita east 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.

MIESCH: A 3D Babcock-Leighton dynamo model with spots Room: FP41

Activity Cycles of the red giant star OP And

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

Room: FP41

BIRZAN: Radio-mode AGN feedback (Astro-seminar) Room: FB54

Planet formation and life in the Galaxy

• Johansen

Room: NORDITA 122.026

Quasi-geostrophic approximation of anelastic convection

• Friedrich Busse

Room: Nordita east

Testing models on data - Bayesian inference

• Rainer Arlt (Leibniz Institute for Astrophysics Potsdam)

Room: 132:028

The Li-rich single K giant DI Psc

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

Room: 132:028 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.

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

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

Room: 132:028 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.

Using the wavelet transform to measure the stellar magnetic fields

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

Room: 132:028 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.

New theory of differential rotation in anisotropic turbulent convection

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

Room: Nordita east 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.

Stellar spot modelling and differential rotation

• Rainer Arlt (Leibniz Institute for Astrophysics Potsdam)
• Hans-Erich Fröhlich

Room: Nordita east

Observing Feedback and the Circumgalactic Medium: Pushing Toward the Norm

• Kate Rubin

Room: Albanova, FA31

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

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

Room: Nordita east 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.

Revisiting the ABC dynamo

• Emmanuel Dormy

Room: Nordita east

Past and present of sunspot observations

• Rainer Arlt (Leibniz Institute for Astrophysics Potsdam)

Room: Nordita east

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

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

Room: Nordita east 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.

Anisotropy of helicity in the Sun

• Kirill Kuzanyan (IZMIRAN, Russia)

Room: Nordita east 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.

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

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

Room: Nordita east 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<density variation<148), the dipole component clearly dominates during short periods. Stable and strong dipolar solutions are found either when the density variation is larger than 148 at E=10e^(-4), which results in a more clear separation between the dynamo and the dominant convective region, or when the Ekman number considered is lower than 10e^(-4). Concerning the flow, we were unable to reproduce the banded structure of the surface zonal flow, with the exception of the strong equatorial jet that is always present. The strength of the equatorial surface flow seems to approach planetary values with the increase of the Rayleigh number and the density gradient.

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

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

Room: Nordita east 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.

Dynamo efficiency as a constraint for dynamo models?

• Martin Schrinner

Room: Nordita east

Direct simulation of helicity and application in mean-field dynamos

• Gustavo Guerrero (NORDITA)

Room: Nordita east

Anelastic approximations

• Raynaud Raphael

Room: Nordita east 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.

Stastistics of magnetic fields on high-mass stars

• Alexander Kholtygin (Astronomical Institute of Saint-Petersburg University, Russia)

Room: Nordita east

Negative effective magnetic pressure instability

• Illa Rivero Losada

Room: Nordita east

Surface flux concentrations in a spherical alpha-square dynamo

• Sarah Jabbari (KTH Royal Institute of Technology, Stockholm)

Room: Nordita east 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.

Combining Models of Coronal Mass Ejections and Solar Dynamos

• Jörn Warnecke (Nordita)

Room: Albanova, FA31

Differential rotation in radiative stars

• Friedrich Busse

Room: Nordita east

Appearance of discontinuities in MHD

• Jonathan Braithwaite

Room: Nordita east