[MOVIE] Wilson effect in a sunspot near the solar limb
Wilson effect in a sunspot near the solar limb
Hinode Broadband Filter Imager
This movie shows the disappearance of a fully-fledged sunspot and the associated facular regions behind the solar limb, due to the rotation of the Sun. The observations were taken by the Hinode Broadband Filter Imager using a G-band filter that samples photospheric layers.
As the sunspot approaches the limb, the penumbral side facing the observer (the left one) shrinks and eventually disappear when the spot is right at the limb, while the penumbral side closer to the limb shrinks at a lower pace. This called Wilson effect after its discoverer. The Wilson effect is a projection effect. The umbra of sunspots that we observe is deeper than the penumbra, and the penumbra is slightly deeper than the surrounding solar granulation, like a plate. When viewed at an angle, the side of the penumbra facing the observer is hidden by the higher layer of the granulation and disappear from view.
Note also in this movie that unresolved magnetic fields around the sunspot appear as very bright features near the limb. They are called “plage”. This, again, is due to projection effects of a different type.