The corona is so hot that the Sun's gravity cannot hold all of it back. Some of the plasma escapes and flows outwards. This is the solar wind. The speed of the wind is related to the temperature of the corona, which is about 1-2 million degrees. How the solar wind is accelerated to speeds of up to 800km/s is still not fully understood. We believe it happens within a few thousand miles of the Sun's surface, and it is very hard to observe what's going on there.
Here's an image of the Sun obtained with SOHO in the ultraviolet. The small rectangle shows a part of the coronal hole region. The red and blue represent the speeds at which the gas (at a temperature of 800,000 degrees) is flowing in different parts of the coronal hole. Blue means that the gas is flowing towards us (away from the surface of the Sun) out into the solar wind, at about 10 km/s, and red means that it is flowing away from us.
It looks like hot gas is shooting out from 'cracks' in the solar surface!
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