<p>I don't think it means anything special.  Take, for example, the claim  at  <a href="https://en.wikipedia.org/wiki/Interstellar_cloud" rel="nofollow noreferrer">Wikipedia</a></p>

<blockquote>
  <p>An interstellar cloud is generally an accumulation of gas, plasma, and
  dust in our and other galaxies. Put differently, an interstellar cloud
  is a denser-than-average region of the interstellar medium, (ISM), the
  matter and radiation that exists in the space between the star systems
  in a galaxy. Depending on the density, size, and temperature of a
  given cloud, its hydrogen can be neutral, making an H I region;
  ionized, or plasma making it an H II region; or molecular, which are
  referred to simply as molecular clouds, or sometimetimes dense clouds.
  Neutral and ionized clouds are sometimes also called diffuse clouds.
  An interstellar cloud is formed by the gas and dust particles from a
  red giant in its later life.</p>
</blockquote>

<p>As well as another <a href="https://en.wikipedia.org/wiki/Interstellar_medium" rel="nofollow noreferrer">statement</a></p>

<blockquote>
  <p>In all phases, the interstellar medium is extremely tenuous by
  terrestrial standards. In cool, dense regions of the ISM, matter is
  primarily in molecular form, and reaches number densities of <span class="math-container">$10^6$</span>
  molecules per <span class="math-container">$cm^3$</span> (1 million molecules per cm3). In hot, diffuse
  regions of the ISM, matter is primarily ionized, and the density may
  be as low as <span class="math-container">$10^{−4}$</span> ions per <span class="math-container">$cm^3$</span>. Compare this with a number density of
  roughly <span class="math-container">$10^{19}$</span> molecules per cm3 for air at sea level, and <span class="math-container">$10^{10}$</span>
  molecules per <span class="math-container">$cm^3$</span> (10 billion molecules per <span class="math-container">$cm^3$</span>) for a laboratory
  high-vacuum chamber.</p>
</blockquote>

CC BY-SA 4.0
2018-10-16T04:09:29.483Z
Carl, you need to fix the numbers - e.g. 10^19 rather than 1019.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-10-16T10:31:24.113Z
@Chappo thanks - I didn't pay attention when I cut/pasted
Carl Witthoft
CC BY-SA 4.0
2018-10-15T17:28:58.507Z
2018-10-16T10:30:15.467Z
2018-10-16T10:30:15.467Z
Carl Witthoft
<p>The "diffuse ionized gas" (DIG) is another term for the phase of the interstellar medium (ISM) usually called the <strong>warm ionized medium (WIM)</strong>. With a temperature of the order <span class="math-container">$10^4\,\mathrm{K}$</span>, but extenting to lower and higher temperatures, it is hot enough to keep hydrogen ionized, and various metals exist as low-ionization species, such as S II, N II, and O II, and even (weak) O III (e.g. <a href="https://arxiv.org/abs/1211.0369" rel="noreferrer">Hill et al. 2012</a>; <a href="https://arxiv.org/abs/1612.02000" rel="noreferrer">Zang et al. 2017</a>; <a href="https://arxiv.org/abs/1712.04450" rel="noreferrer">Weilbacher et al. 2018</a>).</p>

<p>Rough pressure equilibrium with the other phases of the ISM results in characteristic densities of <span class="math-container">$0.1\,\mathrm{cm}^{-1}$</span>, but typical ranges are an order of magnitude to both sides.</p>

<p>A review of the DIG/WNM can be found in <a href="https://arxiv.org/abs/0901.0941" rel="noreferrer">Haffner et al. (2009)</a>.</p>

CC BY-SA 4.0
2018-10-15T22:16:48.460Z
2018-10-15T22:16:48.460Z
pela
<p>I've been trying to find a clean definition what people mean by when they talk about <em>diffuse ionized gas in the interstellar medium</em>, but I couldn't find anything so far.
Apparently it's supposed to be trivial.</p>

<p>Is it simply ionized gas, mixed with non-ionized gas in the ISM through the process of diffusion?</p>

CC BY-SA 4.0
2018-10-15T17:26:35.280Z
PLease post links to examples of this usage.
Carl Witthoft
CC BY-SA 4.0
2018-10-16T12:39:36.797Z
It's not mixed via diffusion, it is diffuse, as in rarefied, as in the opposite of dense.
FJC
CC BY-SA 4.0
2018-10-16T16:25:27.623Z
@FJC that cleared it up! Didn't know that was a synonym for rarefied.
mivkov
<interstellar-medium><gas>
What is the diffuse ionized gas?
mivkov
# What is the diffuse ionized gas?

(Asked by: username_0 on 2018-10-15)

I've been trying to find a clean definition what people mean by when they talk about *diffuse ionized gas in the interstellar medium*, but I couldn't find anything so far.
Apparently it's supposed to be trivial.

Is it simply ionized gas, mixed with non-ionized gas in the ISM through the process of diffusion?

**Tags:** interstellar-medium, gas

> **Comments:**
> **username_1**: PLease post links to examples of this usage.  
> **username_2**: It's not mixed via diffusion, it is diffuse, as in rarefied, as in the opposite of dense.  
> **username_0**: @username_2 that cleared it up! Didn't know that was a synonym for rarefied.  

---

## 2 answers

### Answer by username_4 on 2018-10-15. Score: 5 | Accepted answer

The "diffuse ionized gas" (DIG) is another term for the phase of the interstellar medium (ISM) usually called the **warm ionized medium (WIM)**. With a temperature of the order $10^4\,\mathrm{K}$, but extenting to lower and higher temperatures, it is hot enough to keep hydrogen ionized, and various metals exist as low-ionization species, such as S II, N II, and O II, and even (weak) O III (e.g. [Hill et al. 2012](https://arxiv.org/abs/1211.0369); [Zang et al. 2017](https://arxiv.org/abs/1612.02000); [Weilbacher et al. 2018](https://arxiv.org/abs/1712.04450)).

Rough pressure equilibrium with the other phases of the ISM results in characteristic densities of $0.1\,\mathrm{cm}^{-1}$, but typical ranges are an order of magnitude to both sides.

A review of the DIG/WNM can be found in [Haffner et al. (2009)](https://arxiv.org/abs/0901.0941).


---

### Answer by username_1 on 2018-10-15. Score: 0

I don't think it means anything special. Take, for example, the claim at [Wikipedia](https://en.wikipedia.org/wiki/Interstellar_cloud)

> An interstellar cloud is generally an accumulation of gas, plasma, and
> dust in our and other galaxies. Put differently, an interstellar cloud
> is a denser-than-average region of the interstellar medium, (ISM), the
> matter and radiation that exists in the space between the star systems
> in a galaxy. Depending on the density, size, and temperature of a
> given cloud, its hydrogen can be neutral, making an H I region;
> ionized, or plasma making it an H II region; or molecular, which are
> referred to simply as molecular clouds, or sometimetimes dense clouds.
> Neutral and ionized clouds are sometimes also called diffuse clouds.
> An interstellar cloud is formed by the gas and dust particles from a
> red giant in its later life.

As well as another [statement](https://en.wikipedia.org/wiki/Interstellar_medium)

> In all phases, the interstellar medium is extremely tenuous by
> terrestrial standards. In cool, dense regions of the ISM, matter is
> primarily in molecular form, and reaches number densities of $10^6$
> molecules per $cm^3$ (1 million molecules per cm3). In hot, diffuse
> regions of the ISM, matter is primarily ionized, and the density may
> be as low as $10^{−4}$ ions per $cm^3$. Compare this with a number density of
> roughly $10^{19}$ molecules per cm3 for air at sea level, and $10^{10}$
> molecules per $cm^3$ (10 billion molecules per $cm^3$) for a laboratory
> high-vacuum chamber.

> **Comments:**
> **username_3**: Carl, you need to fix the numbers - e.g. 10^19 rather than 1019.  
> **username_1**: @Chappo thanks - I didn't pay attention when I cut/pasted  

---

