Astrophysical dynamos

Until 1 Dec. 2005 this research was conducted at the Kiepenheuer Institute for Solar Physics (Freiburg i. Br., Germany)

Topics of interest: theory and numerics of dynamo action in the Sun, stars and Earth. Below you can see a glimpse of recent 3D simulations of solar magnetoconvection. I'm also interested in the reversals of the geodynamo, statistical aspects of dynamo theory, and mean-field methods.

Publications

Refereed papers, invited reviews

• Dynamo Action in Fully Convective Rotating Spheres, M. Ossendrijver, M. Stix, A&A (in preparation)
• Aspects of the Dynamo Layer in Solar-Type Stars, M. Ossendrijver, A&A (in preparation)
• Local Simulations of Stellar Convection III: The Strouhal Number, P.J. Käpylä, M.J. Korpi, M. Ossendrijver, I. Tuominen, A&A 448, 433-438 (2006)
• Magnetoconvection and Dynamo Coefficients III: Alpha-Effect and Magnetic Pumping in the Rapid Rotation Regime, P.J. Käpylä, M.J. Korpi, M. Ossendrijver, M. Stix, A&A 455, 401-412 (2006)
• Approaching the Solar Dynamo or (local link), M. Ossendrijver, Adv. in Space Res. 38, Issue 5, 832-838 (2006)
• Estimates of the Strouhal Number from Numerical Models of Convection, P.J. Käpylä, M.J. Korpi, M. Ossendrijver, I. Tuominen, AN 326, 186-189 (2005)
• The Dynamo Layer in Solar-Type Stars, M. Ossendrijver, AN 326, 166-169 (2005)
• Crisis-Induced Intermittency due to Attractor-Widening in a Buoyancy-Driven Solar Dynamo, M. Ossendrijver, E. Covas, IJBC 13, 2327-2333 (2003)
• The Solar Dynamo (or here), M. Ossendrijver, A&ARev 11, 287-367 (2003); Errata (2005)
• Do Spherical Alpha2-Dynamos Oscillate?, G. Rüdiger, D. Elstner, M. Ossendrijver, A&A 406, 15-21 (2003)
• The Solar Dynamo: a Challenge for Theory and Observations, M. Ossendrijver, in: A. Wilson (ed.), Solar variability: from core to outer frontiers, ESA SP-506, 797-804 (2002)
• Magnetoconvection and dynamo coefficients: II. Field-Direction Dependent Pumping of Magnetic Field, M. Ossendrijver, M. Stix, A. Brandenburg, A&A 394, 735-745 (2002)
• A Theoretical Analysis of the Observed Variability of the Geomagnetic Dipole Field, P. Hoyng, D. Schmitt, M.A.J.H. Ossendrijver, Phys. of the Earth and Planet. Interiors, 130, 143-157 (2002)
• Magnetic Field Reversals and Secular Variation in a Bistable Geodynamo Model, D. Schmitt, M.A.J.H. Ossendrijver, P. Hoyng, Phys. of the Earth and Planet. Interiors, 125, 119-124 (2001)
• The Geodynamo as a Bistable Oscillator, P. Hoyng, M.A.J.H. Ossendrijver, D. Schmitt, GAFD 94, 263-314 (2001)
• Magnetoconvection and Dynamo Coefficients: Dependence of the Alpha Effect on Rotation and Magnetic Field, M. Ossendrijver, A. Brandenburg, M. Stix, A&A 376, 713-726 (2001)
• The Dynamo Effect of Magnetic Flux Tubes, M. Ossendrijver, A&A 359, 1205-1210 (2000)
• Grand Minima in a Buoyancy-Driven Solar Dynamo, M. Ossendrijver, A&A 359, 364-372 (2000)
• Mean Magnetic Field and Energy Balance of Parker's Surface-Wave Dynamo, M.A.J.H. Ossendrijver, P. Hoyng, A&A 324, 329-343 (1997)
• On the Cycle Periods of Stellar Dynamos, M.A.J.H. Ossendrijver, A&A 323, 151-157 (1997)
• Stochastic and Nonlinear Fluctuations in a Mean Field Dynamo, M.A.J.H. Ossendrijver, P. Hoyng, A&A 313, 959-970 (1996)
• Stochastic Excitation and Memory of the Solar Dynamo, M.A.J.H. Ossendrijver, P. Hoyng, D. Schmitt, A&A 313, 938-948 (1996)
• Lattice Gases with Static Disorder: Renormalization of Mean Field Theory, M.A.J.H. Ossendrijver, A. Santos, M.H. Ernst, J. Stat. Phys. 71, 1015-1042 (1993)

Thesis

• Fluctuations and Energy Balance in Solar and Stellar Dynamos, M. Ossendrijver, ISBN 90-393-1345-8, thesis Rijksuniversiteit Utrecht (1996)

Other publications

Transport Coefficients for Solar and Stellar Dynamos. M. Ossendrijver, P.J. Käpylä, in: Solar Activity and its Magnetic Origin, IAU Symposium 233, Cambridge University Press, 3-8 (2006)

Book review: A. Ferriz-Mas, M. Nunez, Advances in Nonlinear Dynamos, The Fluid Mechanics of Astrophysics and Geophysics, M. Ossendrijver, Sol. Phys. 218, 359 (2003)

Solar Cycle (or here), M. Ossendrijver, P. Hoyng, in: P. Murdin (ed.), Encyclopedia of Astronomy and Astrophysics, Bristol: Institute of Physics Publishing (2002)

The Alpha-Effect of Toroidal Fluxtubes and Its Application in a Mean-Field Solar Dynamo, M. Ossendrijver, in: M. Nunez, A. Ferriz-Mas, Proceedings of the Workshop "Stellar dynamos: nonlinearity and chaotic flows" held at Medina del Campo, Valladolid, Spain, 28-30 September 1998, ASP Conf. Series 178, 127-137 (1999)

Does the Sun Have a Memory?, M.A.J.H. Ossendrijver, D. Schmitt, Nachr. d. Akademie d. Wiss. in Göttingen, II. Math.-phys. Kl., 4, 247-253 (1996)

Other items

• Dynamotheorie, M. Ossendrijver, Lecture notes, Freiburg University (WS 1997/1998)

Magnetoconvection and dynamo coefficients

Snapshot of the flow (u). The vertical velocity is indicated by the coloring (red: upward, blue: downward), the direction of the flow by the arrows.

Simultaneous snapshot of the magnetic field (B). The strength of the magnetic field is indicated by the coloring (blue: low, red: high), and its orientation by the arrows. The white curve denotes the zero level of the vertical velocity.

The large-scale magnetic field of the Sun is produced by dynamo action in the convection zone. In order to understand the dynamo process, knowledge of the dynamo coefficients is crucial. By means of 3D magnetohydrodynamic (MHD) simulations, these are calculated from the electromotive force, uxb, where u and b are the disturbances of the velocity and the magnetic field, respectively. The simulation domain shown here represents a section of the solar convection zone, sandwiched between a thin cooling layer on top and a stably stratified layer underneath. From below the box is supplied with a constant heat flux; at the top the temperature is held fixed. The axis of rotation points downward, as is the case on the south pole of the Sun. Initially, the magnetic field is vertical and homogeneous.

Three snapshots of the magnetic field (B). The strength of the magnetic field is indicated by the coloring (green: low, red: high), and its orientation by the arrows. The white curve denotes the zero level of the vertical velocity. It separates isolated upwelling regions from downflowing lanes. Initially, a weak magnetic field with a horizontal orientation and a Gaussian dependence on depth is introduced into the middle of the convectively unstable layer. Subsequently, this layer is modified by advection.

On average, this results in a spreading of the magnetic field equivalent to turbulent diffusion as well as a net downward advection that can be attributed to magnetic pumping.