About Alfvén critical surface:

• Unlike Earth, the Sun doesn’t have a solid surface. But it does have a superheated atmosphere, made of solar material bound to the Sun by gravity and magnetic forces.
• However, as rising heat and pressure push that material away from the Sun, it reaches a point where gravity and magnetic fields are too weak to contain it. This point is known as the Alfvén critical surface.
• This point marks the end of the solar atmosphere and beginning of the solar wind.

About Parker Solar Probe:

• NASA’s Parker Solar Probe has flown through the Sun’s upper atmosphere – the corona – and sampled particles and magnetic fields there. Parker Solar Probe was launched in 2018.
• Its purpose was to explore the Sun.
• It is believed that the probe has crossed the Alfvén critical surface and finally entered the solar atmosphere.

Source: EPIC December 2021