Speaker
Rolf Schlichenmaier
(Kiepenheuer-Institut fuer Sonnenphysik)
Description
The various ways of energy transport in radiatively driven
magnetoconvection determine the structure of sunspots. At
first glance, sunspots are composed of a dark umbra and a
brighter penumbra. The darkness of the umbra is readily
explained by the tension of the magnetic field lines that
supresses convection. Yet, it is clear that even in the
darkest part of the umbra, radiative and conductive heat
transport are not sufficient to explain the observed
photospheric temperatures. Efficient convection must be
present everywhere in sunspots. Indeed, with increasing
spatial resolution in the last, say, 30 years the existence
of umbral fine structure became apparent. I will present
some images obtained with GREGOR that show this fine
structure at a spatial resolution of 0.08 arcsec. These
images demonstrate that magneto-convection in the umbra
operates at different intensity levels. In contrast, the
magneto-convective mode in the penumbra seems to be more
uniform. It always produces filaments that look alike and
that lead to the same spatially-averaged brightness. The
penumbra is characterized and defined by its brightness
relative to the umbra. It also always exhibits the Evershed
flow. Yet, the crucial question is: Is there a distinct
magnetic property that makes the difference between umbral
und penumbral mode of magneto-convection? In this talk we
will argue that, yes, there is a canonical value for the
vertical component of the magnetic field that makes the
difference. The formation of the penumbra is triggered by
large inclination, but the penumbra mode of convection can
only prevail, if B(vertical) is smaller than a well defined
canonical value.
Primary author
Rolf Schlichenmaier
(Kiepenheuer-Institut fuer Sonnenphysik)
