The Wilson Effect in Solar Astronomy:
The Wilson Effect describes the apparent increase in the ratio of the penumbra width in the direction of the solar limb (the distance measured across the solar disc radius), to that towards the centre of the disc as a sunspot approaches the disc. (Definitions below)
A simple explanation of this is that a sunspot is a depression of the photosphere. In new papers however new approaches have been made to the Wilson effect including a photometric technique that uses the whole sunspot as well as others observed at different angles to get a better answer. The apparent change is due to the rotation of the sun.
Alexander Wilson noticed the shape of sunspots flattened when approaching the limb in 1769; he was the first to notice that a sunspot was a feature of the suns own surface rather than objects above it. The depths of these sunspots differ and the largest can reach over one thousand kilometers.
Other than Wilson, after whom the effect is named, Prokakis in 1974 as well as Henja and Solov'ev in 1985 have conducted further studies into the effect.
A sunspot is a region within the suns photosphere, which holds intense magnetic activity and high levels of magnetic flux and convection; this causes areas of lower surface temperatures (around 4,000 to 4,500 Kelvin as opposed to the normal surface temperature of around 6000 Kelvin). Sunspots tend to occur in bipolar clusters or groups. They appear as black dots on heat signature images of the sun, because of the temperature difference mentioned above.
The penumbra is the sunspot area that can surround the darker umbra or umbrae; it is made up of linear bright and dark elements, which are radial from the sunspots umbra.
The Umbra is the dark core within a sunspot with a penumbra, or a sunspot without a penumbra. A solar limb is the edge of the solar disk: the visible surface of the sun that we see projected against out sky.
There is also a set of classification each sunspot can fall into if you are interested in further research:
MOUNT WILSON MAGNETIC CLASSIFICATIONS.
Alpha. Denotes a unipolar sunspot group.
Beta. A sunspot group having both positive and negative mag-
netic polarities, with a simple and distinct division between
Beta-Gamma. A sunspot group that is bipolar but in which no
continuous line can be drawn separating spots of opposite
Delta. A complex magnetic configuration of a solar sunspot
group consisting of opposite polarity umbrae within the same
Gamma. A complex active in which the positive and nega-
tive polarities are so irregularly distributed as to prevent
classification as a bipolar group.
(some definitions taken from swpc.gov)
For further research just google the Wilson effect' or try looking through libraries of papers, many can be found to purchase online but you can see many at libraries for frees so just go take a look.
Also see here:
And here for more definitions: