INTRODUCTION
The Sun is a totally gaseous body - a star at the centre of our solar system. It consists of an interior and an atmosphere. Both of these are layered. The interior of the Sun consists of the core which at a temperature of about 15 million Kelvin is the regions where all the energy is produced in a series of nuclear reactions. Above this is the radiative zone where the density is so large that it can take up to a million years for the core energy to make it through this region to the convection zone where connective movements in a series of cells will transport the interior heat to the 'surface' in a month or so..
There is no solid boundary at the layer we call the surface of the Sun. It is simply the level at which the density of the gas becomes low enough that the layers overlying this level are 'transparent' in that they allow most of the solar radiation to pass through them. It is this level, that we call the photosphere, that is the level (or layer) that we see from the Earth when we observe the Sun with only a neutral density filter for protection.
At times the photosphere appears featureless, with the only variation being a darkening of the light toward the limb of the Sun. At other times we can small areas or spots which we call sunspots. At these times we might also notice brighter areas near the solar equatorial limb that we call faculae..
However, if we have a large enough telescope and observe carefully at times of low atmospheric turbulence we can also see a very fine mottled pattern all over the solar surface. This is called the solar granulation.
HISTORY
It appears that William Herschel first observed and noted the phenomenon of solar granulation in 1801. Numerous other observations were made in the 19th century by, among others, Naysmith, Dawes, Janssen and Langley. In fact, at times of sunspot minimum the solar granulation is the only feature on the photosphere so it is not surprising that it was studied at this time.
In the middle of the 19th century a controversy arose as to the form of the granules. One camp claimed they were in the shape of 'willow leaves, whereas the other thought they were similar to rice grains. Eventually they called in the clergy for clarification and Secchi produced a beautiful drawing of solar granulation from his observations with a small telescope. He published this in a book 'Le Soleil' in 1875. This was probably one of the best images of solar granulation, even well into the era of photographic images of the photosphere.
The first photograph of solar granulation was made by Pierre Jannsen in 1878. He used refractor with an aperture of 135 mm and enlarged the solar image to a diameter of 300 mm. Twelve of the original photographs were published in a book in 1896.
It was not until the late 1950's and early 1960's that the first high resolution observations of solar granulation was made using a telescope carried aloft by a balloon (project Stratoscope).
OBSERVATIONS
It was not until the late 1950's and early 1960's that the first high resolution observations of solar granulation were made using a telescope carried aloft by a balloon (project Stratoscope). Images taken from space get above the Earth's atmosphere and the associated turbulence.
Ground observations would initially rely on taking a high frame rate video movie of the Sun and then selecting the individual images where the atmospheric turbulence was low producing the best contrast images.
In the last few decades, the process of 'stacking' digital images can provide a much superior image, and even small telescopes with digital camera can provide high contrast images of the granulation. It is found best to use a green filter for these images. This provides a higher contrast than images in the red or blue.
Nowadays, any serious solar telescope is evacuated to eliminate turbulence inside the telescope tube, and then placed on a high mountain to minimise the atmosphere above the telescope. The best current example of this is the Daniel K Inouye solar telescope (DKIST) built by the US National Solar Observatory (with funding from the US National Science Foundation) at the peak of Mount Haleakala on the Hawaiian island of Maui. It is currently the world's largest solar telescope with an aperture of four metres.
Below is one of the superb images of solar granulation taken by the DKIST.
ORIGIN
Although it was probably realised by some earlier, it was in the 1930's that the physics was detailed indicating that solar granulation is in fact a manifestation of convective motions in the Sun. The granules being bubbles of hot gas pushing their way upward through cooler descending material.
It is now well recognised that the bright granules are hot ascending gas, whereas the much narrower dark lanes delineating the granules are cooler descending gas as shown below.
CHARCTERISTICS
The solar granulation extends over the entire photospheric 'surface'. A total of around four million granules covers the entire photosphere. It is only suppressed where sunspots appear, their strong magnetic field inhibiting the convection. However the granules appear little changed right up to the sunspot penumbra - maybe showing a slight elongation in shape near the boundary.
Granules are polygonal in shape with relatively straight edges where they are adjacent to other granules.
The average granule angular dimension as seen from the Earth is 1.3 seconds of arc. This corresponds to a distance of 1000 km on the Sun. The variation in size ranges from about 0.3 to 3 arcseconds (200 to 2300 linear kilometres). An average granule thus has roughly the area of the state of New South Wales.
Individual granules have a short lifetime ranging from 6 to 16 minutes with the average around 10 minutes. Small granules tend to fade away whereas large granules tend to fragment or occasionally merge. New granules are born from previous granular fragments.
The upwelling speed in granules averages around one km/sec but can reach supersonic speeds of seven km/sec creating a sonic boom and setting off other oscillations of gas in the photospheric plasma.
REFERENCES
RJ Bray, RE Loughhead & CJ Durrant, The Solar Granulation, Cambridge (2009)
AJ Meadows, Early Solar Physics, Pergamon (1970)
Harold Zirin, Astrophysics of the Sun, Cambridge (1988)
Arvind Bhatnagar & William Livingston, Fundamentals of Solar Astronomy, World Scientific (2005)
Edward Gibson, The Quiet Sun, NASA (1973)
Australian Space Academy