surface of Alfvén has been determined.
This is the line that separates the Sun’s
corona from the solar wind’s unfettered movement. This occurred in April of
last year, when the Parker Solar Probe made three passes through the Sun’s
The border is nonspherical, with an average distance of 18.8 times the
Sun’s radii from the star’s centre.
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.