For the first time in history, the position of the criticalsurface of Alfvén has been determined.
This is the line that separates the Sun’scorona from the solar wind’s unfettered movement. This occurred in April oflast year, when the Parker Solar Probe made three pᴀsses through the Sun’scorona.
The magnetic field of the Sun controls the behaviour ofcharged particles around it, and the magnetic pressure in the corona exceedsthe thermal pressure, allowing magnetohydrodynamic Alfvén waves to propagatefar 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 extremeclimate conditions and wide temperature swings. With its unique heat shield andan autonomous mechanism that protects the probe yet allows coronal material to“touch” the spacecraft is the most critical feature.
Moreover, the coronathrough which Parker Solar Probe pᴀsses has a very high temperature but a lowdensity. As an illustration, think about the difference between placing yourhand in an oven and a pot of boiling water (don’t do this at home!). Hands cansurvive far higher temperatures in the oven for longer periods of time thanthey can in boiling water since they must deal with much more particles.
Similarly, the corona is less dense than the visiblesurface of the Sun, so the spacecraft encounters fewer H๏τ particles and doesnot get as much heat as it would on the visible surface. As a result, the heatshield facing the Sun on Parker Solar Probe will only be heated to roughly2,500 F (1,400 C) while it travels through the corona atmosphere withtemperatures of several million degrees.
Reference(s): NASA
