Abstract The temporal variation of the mean size of granules recently discovered (Macris et al., 1983; Macris et al. 1983; Muller and Roudier, 1984) is confirmed using a new set of photographs of higher quality and homogeneity : granules are smaller at sunspot maximum. However the amplitude of the variation is found to be much smaller : 5 % instead of 10 to 20 %. Moreover it is found that the number of granules is highly dependent of the image quality; this has to be taken into account when counting granules. In this paper, as well as in the previous ones, granules were identified visually, making it difficult to maintain the same granule definition for every analysed frame. Two computer image processings are presented and discussed, namely an auto-correlation analysis and an isophote countour display in order to try to overcome this difficulty. However both processing methods appear to be inappropriate. The first process fails because at very high spatial resolution (0.25), the granulation has no longer a characteristic size (its size histogram showing an increasing number of small granules, Roudier and Muller, 1986). Granules appear to have more and more fine structure and they are not homogeneously distributed at the surface of the Sun : the larger (> 1.4, the critical size), as well as the smaller granules form a pattern of mesogranulation scale. The variation of the granule number density (or the granule average size) is well correlated not only with the variation of the annual mean number of sunspots, but also with the number of Network Bright Points. NBPs are the tiny bright points forming the photospheric network and associated with magnetic flux tubes. Their number, which is proportional to the magnetic flux outside of active regions, varies nearly in antiphase with the sunspot number by a factor of 3 or so. The variation of the size of granules probably results from the interaction between the convection zone and the magnetic field at the global scale of the Sun. However, from the correlation with the NBP variation, a local interaction between granules and flux tubes cannot be excluded.
Variability of solar granulation
Advances in Space Research ; 8 , 7 ; 159-167
1989-01-01
9 pages
Aufsatz (Zeitschrift)
Elektronische Ressource
Englisch
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