Implement capability to ascertain whether an astrophysical fluid is plasma or gas #3271

Description

Add functionality that would use plasma parameters of an astrophysical fluid to determine whether it is in the gas or plasma states of matter.

Motivation

In astrophysics, the term "gas" has two definitions: (1) plasma, and (2) gas. This ambiguity makes it hard to ascertain the state of matter that the fluid is in when reading a research article, going to a research seminar, or even reading a Wikipedia article. This functionality would make disambiguation straightforward when plasma properties like the electron/neutral number densities and the temperature are roughly known.

Implementation strategy

There are multiple quantitative criteria for whether a fluid meets the definition of a plasma.

1. The size of the system is larger than the Debye length.
2. The Debye number is ≫ 1.
3. The electron plasma frequency is faster than the frequency that an electron collides with other particles (so that at least one plasma oscillation can occur before an electron bumps into something).

The first two are straightforward, but the third depends on the electron-neutral collision cross section. That cross section depends on which neutrals that electrons are most likely to collide with. In astrophysics, the most common neutrals are generally either $\mathrm{H}^{0+}$ or H₂⁰⁺, though there are likely some situations where other molecules become important (such as possibly gas giants, substellar objects, maybe the photospheres of cool stars). These cross sections are a function of temperature (and maybe number density?), and it is likely necessary to extrapolate to the coldest temperatures in molecular clouds (e.g., Pinto & Galli 2008). It would likely also work to put bounds on the cross-section to determine whether a fluid is a plasma or not.

This capability should also handle the situation where a fluid is right on the edge between gas and plasma, since the phase transition between gas and plasma is gradual (rather than abrupt, like boiling or freezing).

Test cases would include the Cold Neutral Medium in the ISM (which is actually a partially ionized plasam due to cosmic rays and/or photoionization) and molecular clouds (which are weakly ionized plasma due to cosmic ray ionization).