Home Practical information Registration Payment Abstract submission Participants Programme Important dates Pre-registration deadline: 15/06/2013 Registration and Payment deadline: 15/06/2013 Workshop: 22-27/07/2013

REVISITING THE ABC FLOW DYNAMO I. Bouya, IMJ, joint work with E. Dormy. The ABC flow is a prototype for fast dynamo action, essential to the origin of magnetic field in large astrophysical objects. Probably the most studied configuration is the classical 1:1:1 flow. We investigate its dynamo properties varying the magnetic Reynolds number Rm. We identify two kinks in the growth rate, which correspond respectively to an eigenvalue crossing and to an eigenvalue coalescence. The dominant eigenvalue becomes purely real for a finite value of the control parameter. Finally we show that even for Rm = 25000, the dominant eigenvalue has not yet reached an asymptotic behaviour. It still varies very significantly with the controlling parameter. Even at these very large values of Rm the fast dynamo property of this flow cannot yet be established. ON THE PROBLEM OF LARGE-SCALE MAGNETIC FIELD GENERATION IN ROTATING COMPRESSIBLE CONVECTION P Bushby, Newcastle University, joint work with B Favier. Mean-field dynamo theory suggests that turbulent convection in a rotating layer of electrically-conducting fluid should produce a significant $\alpha$-effect, which is one of the key ingredients in any mean-field dynamo model. Provided that this $\alpha$-effect operates more efficiently than (turbulent) magnetic diffusion, such a system should be capable of sustaining a large-scale dynamo. We consider dynamo action in rotating compressible convection, focusing primarily upon the dependence of the dynamo on the rotation rate. Our simulations indicate that these turbulent compressible flows can drive a small-scale dynamo but, even when the layer is rotating very rapidly, we find no evidence for the generation of a significant large-scale field. As in the Boussinesq model of Cattaneo & Hughes (2006), we measure a negligible time-averaged $\alpha$-effect when a uniform horizontal magnetic field is imposed across the computational domain. However, when fluctuations in the horizontally-averaged magnetic field are artificially suppressed within the code, we do measure a significant mean electromotive force (comparable to that found in related calculations in which the $\alpha$-effect is measured using the test-field method). IN SEARCH OF A STABLE JELLY-FISH LIKE FLYING MACHINE S. Childress, Courant Institute, New York University. Flapping-wing aircraft offer an alternative to helicopters in achieving maneuverable flight at small scales, although stabilizing such aircraft remains a key challenge. Experimental studies of the stable hovering of rigid structures in an oscillating airflow suggest a design which mimics the flapping contractions of a jelly fish. We have constructed a prototype of such a craft, and test flown it in free flight in air with an external power source. Our results indicate the possibility of passively stable hovering of a flapping-wing craft. DYNAMO BIFURCATIONS, FROM THEORY TO NUMERICS E. Dormy, CNRS. We investigate the dynamo bifurcation in numerical models relevant to the Earth's core. We show how the nature of the dynamo bifurcation happens to be very sensitive to the parameter regime. We report supercritical, subcritical and isola modes. We find how the bifurcation depends on the relevant parameters. Finally, we exhibit a weak and a strong field branche of dipolar dynamos, for a given set of parameters, and transitions between these branches. ZONAL JETS, MIXING AND SHEARING FLOW ON A BETA-PLANE S. Durston, University of Exeter, joint work with A. D. Gilbert. In this talk I will discuss the effects of a linear background shear flow on two-dimensional beta-plane turbulence and the zonal jets associated with such flows. In order to implement a strictly linear shear, we make use of a shearing box coordinate system, which is often used in accretion disc models where shear occurs due to rotation. Of interest to us is the mixing of a passive scalar carried by the flow. In particular, the vertical flux of passive scalar across the jets may be used as a diagnostic related to the effective diffusivity of the flow (Moffatt, 1983). I will look at the statistics of this flux as obtained from numerical simulations, and compare with quasi-linear analysis of both standard and sheared cases. THE ARCHONTIS AND OTHER STRONG-FIELD DYNAMOS: SOME BOUNDS AND NEW RESULTS D. Galloway, School of Mathematics and Statistics, University of Sydney, joint work with M. Proctor. The Archontis dynamo is a spatially periodic forced flow with an equilibrium state where the velocity and magnetic fields are asymptotically equal when expressed in Alfvenic units. Its behaviour shows a remarkable robustness over a wide range of magnetic Prandtl numbers, provided both magnetic and viscous diffusivities are low enough. This talk will give numerical illustrations and show that bounds can be derived which yield theoretical insight into how this occurs. Conservation of total energy and cross-helicity also constrain the behaviour and may be useful for understanding other strong-field dynamos. DISSIPATION FOR FLOWS IN CURVED GEOMETRY AND RELATED TOPICS A.D. Gilbert , University of Exeter, joint work with X. Riedinger, J. Thuburn. We consider several issues concerning fluid dynamics in a generalised geometrical setting of fluid flow in a curved space, i.e. manifold. First we discuss the nature of the dissipative, viscous term, for two-dimensional flows on a curved surface, relevant to understanding fluid flow on the surface of a sphere. In this case writing down the usual Laplacian of the flow field or the vorticity field does not give the correct treatment of angular momentum and this may be traced to the fact that derivatives cannot be commuted freely in curved geometry. We also consider the related problem of the nature of dissipation in shallow water systems. In both cases it is argued that the dissipative term should arise from the divergence of a stress tensor, so as to properly dissipate energy and conserve angular momentum. We discuss some recent work on the Hybrid Euler-Lagrange methods for fluid dynamics in a general setting, a key research interest of Andrew Soward, amongst others, since the 1970s. If time permits we will also discuss some instabilities of open shallow water shear flows. SUBCRITICAL DYNAMOS IN THE EARLY MARS' CORE: IMPLICATIONS FOR CESSATION OF THE PAST MARTIAN DYNAMO Kumiko Hori, Earthquake Research Institute, University of Tokyo, joint work with Johannes Wicht. Mars has no active dynamo action at present, but likely had one in the early stage of its history. Clarifying why and how it ceased is a challenging question. Several different scenarios have been proposed so far, here we explore the possibility that the dynamo stopped operating due to its subcritical nature. Former studies suggested that the subcritical regime is rather narrow, indicating that it may not play an important role for the cessation. Here we show that a more appropriate model for the early Martian dynamo, driven exclusively by secular cooling and using heat flux conditions at the outer boundary, yields a much wider subcritical regime than previously reported. This increased extent makes it more likely that subcriticality may have played a role in the shutdown of the early Martian dynamo. The magnetic field may thus have decayed rather quickly from its typical strength within a few thousand years after the heat flux through the core-mantle boundary became too low to support dynamo action. Because of the subcriticality, it would have been very difficult to restart the dynamo after the heat flux recovered, for example, after a major impact. DYNAMOS IN ROTATING CONVECTION D.W. Hughes, University of Leeds. One of the classical models of dynamo action -- though one that is far from being fully understood -- is that driven by plane layer, Boussinesq rotating convection. I shall discuss some of the latest results for this model, including the possibility of two very different types of dynamo action -- essentially large-scale and small-scale dynamos -- and where in parameter space these can occur. I shall then describe how the incorporation of a large-scale shear flow can lead to large-scale field generation in a highly turbulent regime in which, in the absence of a large-scale flow and, despite the flow being helical, only small-scale fields are generated. SPHEROIDAL AND ELLIPSOIDAL MEAN FIELD KINEMATIC DYNAMOS D. Ivers, University of Sydney. The self-exciting kinematic dynamo instability is considered in a uniformly electrically-conducting fluid which occupies a spheroid or tri-axial ellipsoid. The conducting fluid is surrounded by an insulating exterior. Regeneration of the magnetic field is due to a turbulent mean-field alpha-effect. The problem has application to galactic dynamos. The linear instability of the magnetic field is investigated numerically. Homeoidal spheroidal or ellipsoidal toroidal-poloidal solenoidal representations are used for the magnetic field and the velocity. The magnetic induction equation is transformed so that it differs from the spherical case by an anisotropic magnetic diffusion and an anisotropic alpha-effect, even if the original alpha-effect is isotropic. The equations are discretised using spherical harmonic expansions of the magnetic toroidal-poloidal potentials and finite differences in scaled radius. The linearised instability problem then reduces to a generalised eigen- and critical-value problem. Results are presented for several models, including non-axisymmetric solutions. DYNAMO MODELS OF JUPITER'S MAGNETIC FIELD Chris Jones, University of Leeds. Numerical dynamo models have had some success in reproducing important features of the Earth's magnetic field. Here we report on simulations of Jupiter's magnetic field using the anelastic approximation, which takes into account the large density variation across the dynamo region. The reference state used in these models is a Jupiter model taken from ab initio calculations of the physical properties of Jupiter's magnetic field (French et al. 2012), which makes the reasonable assumption that the interior is close to adiabatic. The French et al. work also gives an electrical conductivity profile which is adopted here. Dynamo simulations depend on the dimensionless input parameters, partibularly the Ekman number, Rayleigh number, the Prandtl number and magnetic Prandtl number. Many different types of field have been found, some of which will be described. The most relevant models are those which produce a Jupiter-like strong dipole dominated field. These are found at low Ekman number, Rayleigh numbers well above critical, low Prandtl number and moderate magnetic Prandtl number. Another important issue is the driving heat flux source. Here we assume that Jupiter evolves through a sequence of adiabats, leading to a distributed entropy source throughout the planet, rather than basal heating from the small rocky core. The interaction between the magnetic field, the zonal flow and the convection appears to be crucial in determining the type of magnetic field found. HOW ANISOTROPIC IS CURRENT HELICITY IN THE SUN Kirill Kuzanyan, IZMIRAN, Moscow, Russia, joint work with R. Stepanov, D. Sokoloff, H. Xu, H. Zhang. Magnetic helicity plays a crucial role in construction of self-consistent dynamo models. Electric current helicity (dot product of the magnetic field and electric current vectors) can be computed by systematic reduction of photospheric vector magnetograph data of solar active regions. The spatio-temporal averages of the current helicity can be used as a proxy of magnetic helicity in the Sun. The current helicity is a pseudo-scalar quantity (trace of a tensor). However, the observations which are available over the recent two-three decades allow us to determine only the vertical part of current helicity. Under the hypothesis of isotropy this part is equal to the other two parts. Therefore, one can estimate of the full helicity out of that observational analysis. Our aim is to check this hypothesis. Our 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, mean magnetic field, and possibly stratification of convection can cause this significant anisotropy. We study the variability of these anisotropic parts with the solar cycle. We conclude on importance to include distribution and dynamics of the anisotropic parts of helicity into solar dynamo models. THEORETICAL CHALLENGES FROM INITIAL OBSERVATIONS FROM THE THREE METER DIAMETER GEODYNAMO EXPERIMENT Daniel Lathrop, University of Maryland. A liquid sodium model of the earth's outer core has been fabricated to be able to reach a magnetic Reynold number of Rm=900. The first two years of experiments were done using water as a working fluid and observed precessionally driven flows and turbulent bi-stability in spherical shear flow. Afterward, the initial sodium metal flows are in hand with a few months of trial runs. We have seen significant induction of magnetic fields by the Omega effect and many other induced magnetic field effects. While no dynamo effect has been observed at half speed (Rm<450) we have seen a gain of seven in the Omega effect, but not yet enough conversion of toroidal to poloidal field to self-generate. We have also characterized the power input of the system as a function of Rossby number, observed a dozen different non-dynamo states, and examined the fluctuations in induced magnetic field. For now all of this is at parameters not yet accessible by simulation, but the observations are likely amenable to theory in reduced models. COMPETITION OF ROTATION AND STRATIFICATION IN FLUX CONCENTRATIONS I. R. Losada, Nordita, joint work with A. Brandenburg, N. Kleeorin, I. Rogachevskii. In a strongly stratified turbulent layer, a uniform horizontal magnetic field can become unstable to form spontaneously local flux concentrations due to a negative contribution of turbulence to the large-scale (mean-field) magnetic pressure. This mechanism is of interest in connection with dynamo scenarios in which most of the magnetic field resides in the bulk of the convection zone, and not at the bottom, as is usually assumed. Recent work using the mean-field hydromagnetic equations has shown that this negative effective magnetic pressure instability (NEMPI) becomes suppressed at rather low rotation rates with Coriolis numbers as low as 0.1. Here we extend these earlier investigations by studying the effects of rotation both on the development of NEMPI and on the effective magnetic pressure (turbulent and non-turbulent contributions). We quantify the kinetic helicity resulting from rotation and stratification and compare with earlier work at smaller scale-separation ratios. We also determine the sensitivity of surface diagnostics of magnetic helicity. We use direct numerical simulations (DNS) and mean-field calculations of the three-dimensional hydromagnetic equations in a Cartesian domain and analytical studies using the $\tau$ approach. We find that the growth rates of NEMPI in earlier mean-field calculations are well reproduced with DNS and that the rotational effect on the effective magnetic pressure is negligible as long as the production of flux concentrations is not inhibited by rotation. In that case, kinetic and magnetic helicity are also found to be weak. Production of magnetic flux concentrations through the suppression of turbulent pressure appears to be possible only in the upper-most layers of the Sun, where the convective turnover time is less than 2 hours. SCALING LAWS FOR CONVECTIVE BOUSSINESQ DYNAMOS K.A. Mizerski, Department of Magnetism, Institute of Geophysics, Polish Academy of Sciences, joint work with C. A. Jones. We study magnetic dynamos in developed (turbulent) Boussinesq convection. The scaling laws for the strength of the induced magnetic field measured by the Hartmann number with the Rayleigh number (measuring the strength of the driving force) and the Nusselt number (measuring the superadiabatic convective heat flux) are found. Three cases are studied - two of them covering the range of parameters achievable in numerical and possible future laboratory experiments and the third corresponding to the parameter regime for the Earth's liquid outer core. We find, that at least in the case of Boussinesq dynamos the presence of induced magnetic field tends to decrease (or leave unchanged) the order of magnitude of the convective heat flux in the system. ENERGY DISSIPATION IN AMBIPOLAR DIFFUSION MAGNETOHYDRODYNAMICS G. Momferratos, ENS Paris, joint work with P. Lesaffre E. Falgarone G. Pineau des Forets. We compare the topology and energy content of the fields of extreme dissipation in viscous resistive and ambipolar diffusion MHD turbulence. We quantify the property of intermittency of the total dissipation field (viscous, ohmic and ambipolar diffusion) and measure the scaling exponents of the structures of high dissipation, defined as connected sets of points where the total dissipation is most intense. TURBULENCE IN GEODYNAMO SIMULATIONS Franck Plunian, ISTerre, joint work with Nathanael Schaeffer, Alexandre Fouriner, Julien Aubert. We aim at producing geodynamo simulations in a regime more representative of the Earth's core. A major concern is to obtain a dynamical regime where the magnetic energy $E_m$ is much larger than the kinetic energy $E_k$, as in the Earth's core where $E_m/E_k \sim 10^4$. By using scaling laws, we are able to restart numerical dynamo simulations from different parameters. This allows us to run high resolution spherical simulations with only little transients, which considerably reduces the computing time required to obtain statistical equilibrium. Starting from a well converged simulation at Ekman number $E=10^{-5}$, we are able to compute at $E=10^{-6}$ and $E=10^{-7}$, with a constant magnetic Reynolds number $Rm=680$. At $E=10^{-7}$, we have a magnetic Prandtl number $Pm=0.1$, a Rayleigh number $Ra=2.4 \times 10^{13}$ and $E_m/E_k \sim 10$. We report energy spectra, correlations and observed features in physical space. NUMERICAL VON KARMAN FLOW FORCING BY TWO ROTATING PROPELLER USING PENALIZATION METHOD Y. Ponty, CNRS, Observatoire de la Côte d'Azur, France, joint work with S. Kreuzahler, H. Homann, R Grauer. Simulations of impeller-driven flows in cylindrical geometry are compared to the Von Kàrmàn velocity experiment. The geometry of rotating impellers assembled of several basic objects is modelled via a penalization method and implemented in a massive parallel pseudo-spectral Navier-Stokes solver, called LaTu. Simulations of impellers with different numbers of blades and different curvature radii, especially one resembling the so-called TM28 configuration used in the experiment [1], were performed. Though the obtained Reynolds numbers of about 300 to 400 at a resolution of 256^3 grid points are far smaller than experimental values, DNS offers the possibility of a spatially resolved analysis of the flow structure. Visualizations of the mean velocity fields as well as flow profiles along the symmetry axes of each simulated flow reveal that all considered blade configurations have the same general structure: two flow cells, one on each side of the cylinder, mostly equal to the simple s2t2 flow, which is meant to generate dynamo action. The decomposition into poloidal and toroidal components allows to compare quantitatively DNS with experimental results, especially for the TM28 flow [1]. We analysed the flow structure close to the impeller blades which might lead to the dynamo-relevant alpha effect. Some preliminary dynamo results will be also presented. KINEMATIC DYNAMO ACTION IN SQUARE AND HEXAGONAL PATTERNS M.Proctor, DAMTP, Cambridge,UK, joint work with B.Favier. We consider kinematic dynamo action in rapidly rotating Boussinesq convection just above onset. The velocity is constrained to have either a square or a hexagonal pattern. For the square pattern, large-scale dynamo action is observed at onset, with most of the magnetic energy being contained in the horizontally-averaged component. As the magnetic Reynolds number increases, small-scale dynamo action becomes possible, reducing the overall growth rate of the dynamo. For the hexagonal pattern, the breaking of symmetry between up and down flows results in an effective pumping velocity. For intermediate rotation rates, this additional effect can prevent any mean-field dynamo to grow, so that only a small-scale dynamo is eventually possible at large enough magnetic Reynolds number. For very large rotation rates, this pumping term becomes negligible, and the square and hexagonal patterns are qualitatively similar. These results hold for both perfectly conducting and vertical field bo undary conditions. NEW SCALING FOR THE ALPHA EFFECT IN SLOWLY ROTATING TURBULENCE I. Rogachevskii, Ben-Gurion University of the Negev, joint work with A. Brandenburg, O. Gressel, P.J. K\"apyl\"a, N. Kleeorin and M.J. Mantere. We discuss an analytic theory for a new scaling for the alpha effect in turbulence with large Reynolds numbers and slow rotation. Using this theory and direct numerical simulations of slowly rotating stratified turbulence, we show that the alpha effect responsible for the generation of astrophysical magnetic fields is proportional to the logarithmic gradient of kinetic energy density rather than that of momentum, as was previously thought. Thus, the contribution of density stratification is less important than that of turbulent velocity. The alpha effect and other turbulent transport coefficients are determined by means of the test-field method. In addition to forced turbulence, we also investigate supernova-driven turbulence and stellar convection. In some cases (intermediate rotation rate for forced turbulence, convection with intermediate temperature stratification, and supernova-driven turbulence) we find that the contribution of density stratification might be even less important than suggested by the analytic theory. EXTREME EVENTS FROM ASYMPTOTIC PROBABILITY DISTRIBUTIONS. Alexander Ruzmaikin, Jet Propulsion Laboratory, California Institute of Technology. Rare, extreme events such as large earthquakes, droughts, hurricanes, huge lottery/gambling wins, and active 70-year old scientists are always impressive. In statistics, extremes are defined by strong deviations from the mean value and characterized by the tails of probability distributions. The analysis of extremes is a challenging task since usually there is a small number of extremes recorded, thus a law of large numbers or central limit theorem is not applicable. Fortunately, three great statisticians: Fisher, Tippett, and Gnedenko (FTG) proved that there is an asymptotic (limit) probability distribution of extremes. A practical method of analysis of extremes based on the FTG theorem has been developed by S. Stoev and his collaborators. I will discuss the application of this method to extreme Space Weather events, defined as the disturbances in the space environment that presents hazards to the operation of spacecraft systems, instruments or lives of astronauts. The most surprising finding is that these rare extremes are not independent but arrive in clusters, which can be described by a compound Poisson process. MAGNETIC FIELD GENERATION IN THE SUPERNOVA-REGULATED INTERSTELLAR MEDIUM G.R. Sarson, Newcastle University, joint work with F.A. Gent, A. Shukurov, A. Fletcher, M.J. Mantere. The origin and structure of the magnetic fields in the interstellar medium of spiral galaxies is investigated with 3D, non-ideal, compressible MHD simulations, including stratification in the galactic gravity field, differential rotation and radiative cooling. A rectangular domain, $1\times1\times2\kpc^3$ in size, spans both sides of the galactic mid-plane. Supernova explosions drive transonic turbulence. The thermal structure of the modelled ISM is classified by inspection of the joint probability density of the gas number density and temperature. We confirm that most of the complexity can be captured in terms of just three phases, separated by temperature borderlines at about $10^3\K$ and $5\times10^5\K$. The probability distribution of gas density within each phase is approximately lognormal. The correlation scale of the random flows is calculated from the velocity autocorrelation function; it is of order 100\,pc and tends to grow with distance from the mid-plane. A seed magnetic field grows exponentially to reach a statistically steady state within 1.6\,Gyr. Following Germano (1992), we use volume averaging with a Gaussian kernel to separate magnetic field into a mean field and fluctuations. Such averaging does not satisfy all Reynolds rules, yet allows a formulation of mean-field theory. The mean field thus obtained varies in both space and time. Growth rates differ for the mean-field and fluctuating field and there is clear scale separation between the two elements, whose integral scales are about $0.7\kpc$ and $0.3\kpc$, respectively. FLUCTUATIONS IN THE FRAMEWORK OF MEAN-FIELD DYNAMOS D.Sokoloff, Moscow State University. Mean-field dynamo models provide a reasonable tool for description of magnetic field excitation and evolution in celestial bodies. In some cases however fluctuations of the dynamo governing parameters as well as magnetic field fluctuations give important contributions in the phenomenology under investigation. We consider several astrophysical dynamo problems in which a coexistence of mean-field component and fluctuations is important. THE ASYMPTOTIC SOLUTION OF A KINEMATIC ALPHA-OMEGA DYNAMO WITH MERIDIONAL CIRCULATION A.M. Soward, Newcastle University, joint work with A.P. Bassom, K. Kuzanyan, D. Sokoloff and S.M. Tobias. Many stars exhibit magnetic cycles typified by the butterfly diagram characterising our Sun's 11 year solar activity cycle. Parker explained the phenomenon by an alpha-omega dynamo acting in the star's convection zone causing the equatorial propagation of dynamo waves. In contrast, to the many continuing numerical investigations, we adopt a minimalist approach and expand on Parker's original one-dimensional uniform plane layer model. To apply asymptotic methods, we suppose that the dynamo is confined to a thin shell with latitudinal variations of the alpha-omega sources, whose product the Dynamo number vanishes at the pole and equator. The ensuing linear stability problem is resolved by global stability criteria. Our new results concern the role of meridional circulation. They show that sufficiently large circulation halts the Parker travelling waves leading to non-oscillatory behaviour, a result only predicted previously from numerical integration of the full pde's governing axisymmetric alpha-omega dynamos. CRITICAL STABILITY OF CONVECTION IN RAPIDLY ROTATING PLANETARY INTERIORS S.V. Starchenko, IZMIRAN, Kaluzhskoe Hwy 4, Troitsk, Moscow, 142190 Russia. This overview describes some began dramatically in the last century and happily terminated in this century history of solutions to the classical problem of the critical stability of convection in a rapidly rotating spherical shell that simulates deep liquid interiors of the various planets and their large moons. Particularly affected are close to the author questions: the adequate modeling of those interiors and asymptotic way to gain surprisingly simple or even analytic prescriptions for the critical frequencies, Rayleigh numbers and other characteristics that are important for the study of planetary heat-mass-transfer, hydrodynamic and magnetism. SPONTANEOUS GRAVITY WAVE RADIATION FROM VORTICES IN AN UNBOUNDED F-PLANE SHALLOW WATER SYSTEM N.S. Norihiko Sugimoto, Keio University, joint work with K.I. Keiichi Ishioka, H.K. Hiromichi Kobayashi, Y.S. Yutaka Shimomura. Spontaneous gravity wave radiation from vortical flows is investigated analytically and numerically in an unbounded f-plane shallow water system. It is well known that gravity waves play very important roles on the atmosphere and ocean by driving global circulation, especially in the middle atmosphere, since they propagate far away from the source region and put significant amount of momentum and energy. Recently, observational studies suggest that gravity waves are radiated from vortical flows, such as polar night jet, sub-tropical jet, and tropical cyclone. Since gravity waves are spontaneously radiated from nearly balanced vortical flows, this radiation process is called as a spontaneous gravity wave radiation. Although there are several numerical studies, this process has not been fully understood. In the present study, we use the most simplified system of shallow water that includes both gravity waves and vortical flows. In order to discuss the conditions of gravity wave radiation, we use the analogy with the theory of the aero-acoustic sound wave radiation (Lighthill theory). We derive analytical estimation of far field of gravity waves spontaneously radiated from a co-rotating pair of vortices. Recently, we have also developed a new numerical method to treat an unbounded domain. Thus, numerical simulations of the same experimental setting as the analytical study are performed, too. We will report the results of parameter sweep experiments to focus on the effect of rotation and stratification on spontaneous gravity wave radiation from several configuration of vortical flows, such as merging of two vortices. SHEAR-DRIVEN DYNAMO WAVES AT HIGH Rm S.M. Tobias, University of Leeds, joint work with F. Cattaneo. In this talk I will describe a new mechanism for the formation of systematic magnetic fields at high magnetic Reynolds number. The mechanism involves the suppression of small-scale dynamo action via a large-scale flow. Once the small-scale dynamo has been suppressed, systematic magnetic fields are free to do what they do best - propagate and form cyclic magnetic activity.