Département de Physique ENS

Page personnelle de Steven Balbus

Home page of Steven Balbus


Professeur des Universités

École Normale Supérieure
24, rue Lhomond
Laboratoire de Radioastronomie
75231 PARIS CEDEX 05
FRANCE

email: Steven.Balbus AT lra.ens.fr
Tel.: +33 (0)1 44 32 33 53


I am a theoretical astrophysicist with particular interests in the field of astrophysical gas dynamics. Early in my career I worked on problems of the interstellar medium, clusters of galaxies, and galactic dynamics, but since 1990 I have focused my attention on the behavior of accretion disks. An accretion disk consists of gas in rotation about a central mass, which could be an ordinary star, or a collapsed object such as a white dwarf, neutron star, or black hole. When they are even slightly magnetized, accretion disks are extremely unstable to what is known as the magnetorotational instability. This instability disrupts the gas from smooth laminar flow and renders it turbulent. This is now thought to be the underyling physical mechanism for accretion disk ''viscosity,'' once-upon-a-time a longstanding problem in theoretical astrophysics. I am currently pursuing studies, in a wide variety of fluids, of the consequences of this intrinsically magnetohydrodynamical turbulence.

More recently, I have also become interested in the problem of differential rotation in stellar convective zones, particularly the case of the Sun. I am studying the possibility that at least in their simplest form, these rotation profiles are a consequence of little more than vorticity generation in a nonbarotropic flow (so-called thermal wind balance), and a confluence of surfaces of constant angular velocity and constant ''residual'' entropy. Now, read more below in the section Some Recent Papers...


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BROCHURE DE RENTREE 2009 (M1)

EMPLOI DU TEMPS 2009-10 (M1)

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Course Notes:

M2: Magnetohydrodynamics

M2 sample final problem plus solution.
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1er cours: Introduction à l'Astrophysique.

2me cours: Introduction à l'Astrophysique.

3me cours: Introduction à l'Astrophysique.

4me cours: Introduction à l'Astrophysique.

5me cours: Introduction à l'Astrophysique.

6me cours: Introduction à l'Astrophysique.

7me cours: Introduction à l'Astrophysique.

8me cours: Introduction à l'Astrophysique.

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Hydrodynamics. Updated: 20/01/10. (Thanks to Amandine Le Brun.)

Sample questions for Hydrodynamics Final.



Review Papers:

Instability, turbulence, and enhanced transport in accretion disks. (Balbus, S. A. and Hawley, J. F. 1998, Reviews of Modern Physics, 70, 1)
An technical introduction to the physics of accretion disk turbulence.

Enhanced angular momentum transport in accretion disks. (Balbus, S. A. 2003, Annu. Rev. Astron. Astrophys., 41, 555)
An update of accretion disk physics with a more detailed treatment of wave transport.

Magnetohydrodynamics of Protostellar Disks. (Balbus, S. A. 2009, To be published in Physical Processes in Circumstellar Disks Around Young Stars, ed. P. Garcia, (University of Chicago Press: Chicago )
A review of MHD processes in low ionization disks from a 2006 conference workshop.

Steven A. Balbus (2009) Magnetorotational instability . Scholarpedia, 4(7):2409.
A brief online review of the magnetorotational instability, suitable for advanced undergraduates and beginning graduate students.

Selected Recent Papers:

Magnetostrophic MRI in the Earth's Outer Core. (Petitdemange, L., Dormy, E., and Balbus, S. 2008, Geophysical Research Letters., 35, L15305)
A demonstration of the possibility of magnetorotational instability in a regime appropriate to the geodynamo.

Regulation of Thermal Conductivity in Hot Galaxy Clusters by MHD Turbulence. (Balbus, S. A. and Reynolds, C. S. 2008, Ap. J. (Letters), 681L, 65.)
A calculation outlining how the HBI (Quataert, E. 2008, ApJ, 673, 758) may regulate the evolution of the hot X-ray emitting gas in clusters of galaxies.

A Simple Model for Solar Isorotation Contours. (Balbus, S. A. 2009, MNRAS, 395, 2056.)
An analytic model for the solar convection zone (SCZ) isorotation contours based on the solution of the thermal wind equation and the assumption that constant entropy and constant rotation surfaces coincide. The latter is a requirement for the SCZ to be marginally stable against magneto-baroclinic instability, just as an adiabatic temperature profile is a requirement for marginal stability against a purely convective instability.

On Differential Rotation and Convection in the Sun. (Balbus, S. A., Bonart, J., Latter, H. N., and Weiss, N. O. 2009, MNRAS, 400, 176.)
A detailed comparison between the solution of the thermal wind equation and GONG data reveals a precise quantitative match, away from the boundary layers at the Sun's outer surface and tachocline. As an alternative to MHD, a purely hydrodynamical explanation is put forth for this result, which obviates the need for entropy and angular velocity surfaces to coincide. The figure below shows the fit between theoretical calculation (white curves) and the helioseismology GONG data (dark curves).


Differential Rotation in Fully Convective Stars. Balbus, S. A. and Weiss, N. O. 2010, MNRAS, in press.
A generalization of the mathematical solution for the isorotation contours in the Sun to fully convective stars. The figures below show solutions for a solar-like surface profile (left), and an anti-solar profile (right), which rotates more rapidly at the poles than the equator.




Update: 15 jan 10