[austincheney]

http://earthsky.org/space/heat-shield-parker-solar-probe

It appears the heat shield is a carbon sheet sandwich. At first I was guessing some form of tungsten-carbine, but that is the traditional material of NASA heat shields.

> Why is the solar wind a breeze closer to the sun but a supersonic torrent farther away? Why is the corona itself millions of degrees hotter than the surface?

I suspect the answer to all those questions is simply gravity, but it will be nice to verify such things with data.

[mturmon]

No. The latter question is the “coronal heating problem “ which is a major unexplained phenomenon in Physics: https://en.m.wikipedia.org/wiki/Corona#Coronal_heating_probl...

[hutzlibu]

"Why is the corona itself millions of degrees hotter than the surface?

I suspect the answer to all those questions is simply gravity, but it will be nice to verify such things with data"

Can you explain your hypothesis a bit?

[austincheney]

Certainly. Heat is an expression of energy. The first gas law explains the relationship between volume, mass, and energy. In this case we are talking about the interchange between a plasma and a gas. While the gas law works well for gasses it is still less uniformly applicable to other states of matter. https://en.wikipedia.org/wiki/Gas_laws

The Sun has enormous gravity. It comprises 99.86% of the solar system's total mass. It would seem heat can be greater expressed where it is more free upon the vacuum of space upon escaping the gravity that confines the high density mass. https://en.wikipedia.org/wiki/Sun

As for solar wind momentum it would make sense that a particle is accelerating away from the Sun at a near constant energy that is less confined by gravity over distance... at least until it hits termination shock at the edge of the solar system.

Of course these are all speculations and hopefully the probe will provide the data to qualify more valid conclusions.

[smueller1234]

Not GP, but here's an article that talks about it. Note that this is relatively recent. It's not something that was thoroughly understood in ancient times exactly.

https://www.nasa.gov/feature/goddard/sounding-rockets/strong...

[superkuh]

> https://www.nasa.gov/feature/goddard/sounding-rockets/strong...

It explains the temporal heating behavior at some scales. But it doesn't give a mechanism of heating. It could be electron beam target heating. But it could also be mediated by plasma waves.

The electron beams need acceleration and the most common suggestion is x-point magnetic reconnection providing up and down voltage gradients due to changing magnetic field. But the amount of electrons needed is unphysically large; the entire electron contents of the relevant volume of the corona.

There are plasma wave models that don't require unphysically large parameters.

These two (and a couple other) options aren't clarified by the observation of heating profiles. With the launch of Parker Solar Probe and the DKIST (diffraction limited solar telescope) the two models above will finally be testable. Spectroscopy of ion species by DKIST will tell what kind of heating is happening and Solar Probe will be there to measure the input from the corona.