The solar wind streams off of the Sun in
all directions at speeds of about 400 km/s
(about 1 million miles per hour). The source
of the solar wind is the Sun's hot corona.
The temperature of the corona is so high
that the Sun's gravity cannot hold on to it. Although we understand why this happens we do not understand the details about how and where the coronal gases are accelerated to these high velocities. This question is related to the question of coronal heating.