For the first time in history, the position of the critical surface of Alfvén has been determined.
The magnetic field of the Sun controls the behaviour of charged particles around it, and the magnetic pressure in the corona exceeds the thermal pressure, allowing magnetohydrodynamic Alfvén waves to propagate far faster than sound waves.
You can learn more about this in the video below by NASA:
Why Didn’t The Spacecraft Melt:
Parker Solar Probe is built to resist the mission’s extreme climate conditions and wide temperature swings. With its unique heat shield and an autonomous mechanism that protects the probe yet allows coronal material to “touch” the spacecraft is the most critical feature.
Moreover, the corona through which Parker Solar Probe passes has a very high temperature but a low density. As an illustration, think about the difference between placing your hand in an oven and a pot of boiling water (don’t do this at home!). Hands can survive far higher temperatures in the oven for longer periods of time than they can in boiling water since they must deal with much more particles.
Similarly, the corona is less dense than the visible surface of the Sun, so the spacecraft encounters fewer hot particles and does not get as much heat as it would on the visible surface. As a result, the heat shield facing the Sun on Parker Solar Probe will only be heated to roughly 2,500 F (1,400 C) while it travels through the corona atmosphere with temperatures of several million degrees.