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It was realised in the 1950's that the solar corona could not be in static equilibrium. Parker (1953) realised that the hot corona must be in a continual state of expansion. Parker's solar wind solution is described in Solar wind theory. But where does this continual outflow of plasma come from? The main source of the solar wind are the large coronal holes that are shown in the X-ray pictures of the Sun as the black regions. Here the magnetic field is open to the interplanetary space and the ionised plasma flows freely along the field lines.
The hot, bright plasma is contained by closed magnetic fields that have both feet anchored in the photosphere. The plasma is unable to move easily across the magnetic field lines and so there is no contribution to the solar wind from these regions.
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