Granulation

Viewed in white light or in the wings of strong Fraunhofer lines the photosphere is covered in an irregular polygonal pattern known as granulation (Figure 1.7). The granular formation is due to the convective motion of the gas in the sub-photospheric layers, with the bright centre as upward flows and the dark lanes as downward flows. This overturning fluid carries heat bodily to the solar surface. As the hot opaque gas approaches the photosphere it is exposed to the interstellar vacuum, cools suddenly and becomes transparent. This loss in energy corresponds with a rise in density and the vertical motion is decelerated and diverted into a horizontal motion. Further radiation of energy leads to further increases in density and eventually gravity pulls the parcel back down, along the intergranular lanes. In response to this, hot gas flows up to replace it, and so the convective cycle is complete. This convective flow is dominated by the cooling at the photosphere. The increased density at the bottom of the convective zone, 10⁶ greater than at the photosphere, means the acceleration of convective flow here is 10⁶ times less than at the photosphere. This is unlike normal convection (e.g., boiling water), where the top and bottom of the flow contributes equally.

Granules vary in size from 150 km (current ground based limit of observation) to 2500 km, with the majority around 1100 km diameter. Ground based observations require careful selection of observation sights, using realtime frame selection (taking a burst of hundreds of images and selecting the best images) or by speckle interferometry. The convective motion exhibits vertical flows (measured by Doppler shifts) of around 2 km s peak to peak. The centre of the granules are typically 30% brighter than the intergranular lanes, which corresponds to ~400 K temperature difference by the Stefan-Boltzmann law. Granules are born from fragments of other larger granules or from the merging of smaller granules. After expansion, they die either by fragmentation, merging, shrinking or dissolution. and have a typical lifetime, birth to death, of around 8-20 minutes (Spruit, Nordlund, & Title 1990).

Larger scale convective cells have been the topic of much debate (Hathaway et al. 2000). A second, larger, supergranular cell was discovered by Hart (1954) and later associated with convection by Leighton, Noyes, & Simon (1962). It was discovered in the Doppler shifted velocity images of Fraunhofer lines. These images suggested horizontal velocities parallel to the surface of around 0.4km/s, leading to large cells of around 30,000 km with a lifetime of about one day. Recent work by Hagenaar, Schrijver, & Title (1997) suggest this may be overestimated, and propose a typical cell diameter of around 15,000 km. These larger cells carry the smaller granules like flotsam carried along a river, and hence the horizontal flows can be viewed by measuring the granular motion. This motion also carries magnetic flux tubes, which meet at the supergranular boundaries and fall. Some flux remains which moves along the boundaries to vertices, where the downflow is concentrated into small plumes, often with a vortex motion. Due to this concentration of magnetic flux, supergranular cells are spatially related to high chromospheric structure (Section 1.5.3), but theory places their origins below that of granulation. Simon & Weiss (1968) suggested different cell sizes on the Sun are each excited by various ionisation/recombination layers. Granulation was attributed to recombination of hydrogen at a depth of 1,000 km below the surface and supergranules attributed to recombination of helium at a depth of 15,000 km. Two further cells sizes were predicted, a 5000 km cell due to recombination of helium at 5,000 km depth and a much larger cell due to recombination of metals at 200,000 km depth (the base of the convection zone). These were later found as mesogranules (November et al. 1981) and giant cells (Beck, Duvall & Scherrer 1998). The mesogranular flow can be observed as regions of converging and diverging horizontal flow of around 5,000 to 10,000 km. Bright, long-lived, rapidly expanding granules, normally with dark centres, are preferentially found in upflowing mesogranular centres, where horizontal flows diverge. Small, faint, short-lived granules are preferentially found in downflowing mesogranular boundaries, where horizontal flows converge. New observations and simulations suggest cells are more 'pancake shaped' with structure depth to around 1/4 to 1/2 of their diameter (Simon 2001). As such, granules are formed around 100 km depth and supergranules around 5000 - 10,000 km. Some authors (e.g., Roudier et al. 1999) argue that the the fact that mesogranular parameters (e.g., velocity, lifetime) are dependant on the chosen filters suggests that mesogranulation may purely be an artifact of the data reduction. Hathaway et al. (2000) have further suggested that mesogranules are simply large granules (or small supergranules) and giant cells are large supergranules. The driving mechanism behind these large convection cells, and factors determining their size (or various sizes) remain unanswered.