Issue |
A&A
Volume 461, Number 3, January III 2007
|
|
---|---|---|
Page(s) | 1163 - 1171 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20066390 | |
Published online | 24 October 2006 |
The origin of the reversed granulation in the solar photosphere
1
Max Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany e-mail: [cheung;msch]@mps.mpg.de
2
Institúto de Astrofísica de Canarias, 38200 La Laguna (Tenerife), Spain e-mail: fmi@ll.iac.es
3
Dept of Astrophysics, Faculty of Physics, University of La Laguna, 38200 La Laguna (Tenerife), Spain
Received:
12
September
2006
Accepted:
19
October
2006
Aims.We study the structure and reveal the physical nature of the reversed granulation pattern in the solar photosphere by means of 3-dimensional radiative hydrodynamics simulations.
Methods.We used the MURaM code to obtain a realistic model of the near-surface layers of the convection zone and the photosphere.
Results.The pattern of horizontal temperature fluctuations
at the base of the photosphere consists of relatively hot granular
cells bounded by the cooler intergranular downflow network. With
increasing height in the photosphere, the amplitude of the
temperature fluctuations diminishes. At a height of –140 km
in the photosphere, the pattern of horizontal temperature
fluctuations reverses so that granular regions become relatively
cool compared to the intergranular network. Detailed analysis of
the trajectories of fluid elements through the photosphere reveal
that the motion of the fluid is non-adiabatic, owing to strong
radiative cooling when approaching the surface of optical depth
unity followed by reheating by the radiation field from below. The
temperature structure of the photosphere results from the
competition between expansion of rising fluid elements and
radiative heating. The former acts to lower the temperature of the
fluid whereas the latter acts to increase it towards the radiative
equilibrium temperature with a net entropy gain. After the fluid
overturns and descends towards the convection zone, radiative
energy loss again decreases the entropy of the fluid. Radiative
heating and cooling of fluid elements that penetrate into the
photosphere and overturn do not occur in equal amounts. The
imbalance in the cumulative heating and cooling of these fluid
elements is responsible for the reversal of temperature
fluctuations with respect to height in the photosphere.
Key words: convection / radiative transfer / Sun: atmosphere / Sun: photosphere / Sun: granulation / hydrodynamics
© ESO, 2007
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