14 research outputs found
Convection and the Origin of Evershed Flows
Numerical simulations have by now revealed that the fine scale structure of
the penumbra in general and the Evershed effect in particular is due to
overturning convection, mainly confined to gaps with strongly reduced magnetic
field strength. The Evershed flow is the radial component of the overturning
convective flow visible at the surface. It is directed outwards -- away from
the umbra -- because of the broken symmetry due to the inclined magnetic field.
The dark penumbral filament cores visible at high resolution are caused by the
'cusps' in the magnetic field that form above the gaps. Still remaining to be
established are the details of what determines the average luminosity of
penumbrae, the widths, lengths, and filling factors of penumbral filaments, and
the amplitudes and filling factors of the Evershed flow. These are likely to
depend at least partially also on numerical aspects such as limited resolution
and model size, but mainly on physical properties that have not yet been
adequately determined or calibrated, such as the plasma beta profile inside
sunspots at depth and its horizontal profile, the entropy of ascending flows in
the penumbra, etc.Comment: 13 pages, 7 figures. To appear in "Magnetic Coupling between the
Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten,
Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg,
Berlin, 200
An approximate self-consistent theory of the magnetic field of fluted penumbrae.
A self-consistent mathematical description of the magnetic field of fluted sunspot penumbrae is presented. This description is based on an expansion of the nonlinear force-free magnetohydrostatic equations written in cylindrical coordinates. The lowest order solutions are mathematically equivalent to laminated force-free equilibria in Cartesian geometry. The lowest order solutions have no toroidal component of the magnetic field and the magnetic pressure does not vary with azimuth but the solutions allow arbitrary variations of the magnetic field components with azimuth. Explicit solutions are presented which have a realistic radial profile of the magnetic field strength and reproduce the basic features of the observations.</p