[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.