3.5 Exploding granules

Particularly undisturbed granules that grow to sufficient size tend to develop dark centers, where the vertical flow reverses. These so called ‘exploding granules’ have been the topic of a number of observational and theoretical papers (e.g., Musman, 1972Jump To The Next Citation PointAllen and Musman, 1973Namba and van Rijsbergen, 1977Jump To The Next Citation PointNamba, 1986Simon and Weiss, 1991Simon et al., 1991a,bRast, 1995Hirzberger et al., 1999Roudier et al., 2001).

As per the discussion in Section 3.3 above, the reason for the phenomenon is the increase with size of the pressure at the center of granules, which is necessary to drive the increasing horizontal flow that results from the requirement of mass conservation. The increase of the density associated with the increasing pressure leads to a negative buoyancy, which eventually reduces the vertical velocity down below the value needed to maintain the surface luminosity. The resulting cooling then accelerates the process, leading to a rapid cooling and reversal of the vertical flow.

Arguments essentially to this effect were given already by Musman (1972) and Namba and van Rijsbergen (1977).

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