|Parker suggested that the corona, the exterior part of the solar atmosphere is continuously expanding. This
spherically symmetric expansion is called the solar wind. This wind is an ever lasting plasma stream.
By the time the wind reaches Earth, it has become supersonic and leads to the typical form of the dipolar Earth magnetosphere,
our magnetic protection with the stretched tail at the night-side and compressed bulb at the day-side.
The solar wind is associated with open magnetic field lines. Coronal
holes are solar structures with this typical open magnetic field configuration. The wind drags with it
the solar magnetic field lines. The plasma and the magnetic field stick together. Thinking further, Parker understood how the Sun's magnetic
field should look in space. He came to the concept of a spiral, the famous Parker spiral. The spiral is a way to picture the open
magnetic field lines emanating from the Sun, how they reach out in space and how they are bended because of the solar rotation.