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I already asked this on Astronomy SE but did not receive any answer so far:

What are the prerequisites for an ionosphere to form?
For example, can a planet without any atmosphere have an ionosphere?
What about a planet that is very far from its sun, is there a point where the solar wind is too weak to create an ionosphere?

In one sentence, what are the conditions for an ionosphere to form on a given planet?

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  • $\begingroup$ What type of answer are you interested in? In my opinion every planet must have an ionosphere, do you then want to know why this is, or something else? $\endgroup$ – AtmosphericPrisonEscape Apr 18 '16 at 11:27
  • $\begingroup$ I want to know the prerequisites for an ionosphere to form, as my question reads. For example, can a planet without any atmosphere have an ionosphere? What about a planet that is very far from its sun, is there a point where the solar wind is too weak to create an ionosphere? In one sentence, what are the conditions for an ionosphere to form on a given planet? Also I would be interested in why my question got downvoted. $\endgroup$ – Hackworth Apr 18 '16 at 11:59
  • $\begingroup$ I didn't downvote you but I would assume that the fact that you linked to your question on Astronomy.SE instead of actually asking your question here might be one possible reason behind those downvotes. Could you edit the body of your question to include what you asked on Astronomy.SE (and what you added in your previous comment)? $\endgroup$ – plannapus Apr 18 '16 at 13:24
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    $\begingroup$ No I think I'm already done with an SE that just downvotes first time questions without explanation. Thanks for your effort though. $\endgroup$ – Hackworth Apr 18 '16 at 13:28
  • $\begingroup$ @Hackworth - it was one downvote and two upvotes. In other words 2/3 which is 66.666666 %. You have made your point as 2 out of 3 people think this is a good question. So I wouldn't take this the wrong way at all. $\endgroup$ – gansub Apr 18 '16 at 15:04
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Most of the visible matter in the universe consists of ionized matter (I'm citing thise from the publicly available lectures from R. Fitzpatrick and the monograph from J.A. Bittencourt - Introduction to plasma physics). This is true independent whether you measure it by mass or by number of atoms and molecules.

Why is this important?
As I see it the ionosphere is the part of the atmosphere that connects the electrically neutral atmosphere at high pressures and the low-pressure ionized regions of the universe.
Thus, any planet will have firstly an ionisphere and secondly maybe an electrically neutral region that we call "atmosphere". There is a caveat here, as the ionosphere doesn't have to be fully ionized, just have enough electrons and ions floating around to act as a plasma.

Why is gas ionized at low pressures?
The amount of ions that fly around in a certain volume is not only a function of the amount of freed electrons, but also of the amount of electrons that recombine.
The strength of ionization will be a function of the process under consideration. However recombination will be independent of this and be governed by a $\sim n^2$-dependency on the density $n$.
This will generally result in a disproportionally weak recombination as pressure / density decreases high up in the atmosphere. Or in other terms: At low pressures, you can get ionized thermally, by UV-photons, [...] but you don't find a partner anymore to recombine.

Concluding I've presented a case why every planet must have an ionosphere. Details like the solar wind, distance from the star, it's class and atmospheric variables then control the shape of the entailing magnetosphere and exact height-profiles of ionized species.

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ionosphere - geophysics in short.

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km (31 mi) to more than 1,000 km (620 mi). It owes its existence primarily to ultraviolet radiation from the Sun.

Thus there must be a source of gas that can form a plasma and a sun active enough to cause the atmosphere to ionize (hence the name) into a plasma. If there is no source of gas to ionize or not enough radiation to cause ionization, there will be no ionosphere.

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  • $\begingroup$ Read between the lines of the question (and that at Astronomy SE), the questioner wants to know How much solar radiation is necessary? and How much atmosphere needs to be around a planet? to lead to an ionosphere. $\endgroup$ – daniel.neumann Apr 19 '16 at 9:22
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    $\begingroup$ @daniel.neumann Then he should have asked that explicitly. I just answered the question as asked. $\endgroup$ – sabbahillel Apr 19 '16 at 9:31
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In a Sentence:

For a planet to have an ionosphere, we would expect it to have a pressure of at least 1 bar ("thick enough") and to have a host star producing sufficient high-energy photons.

Background

Generation of an Ionosphere

The ionosphere is a balance between plasma production and plasma loss. The primary source of ionization (photons getting absorbed by an atom or molecule, knocking off an electron), is usually high-energy photons. The primary source of recombination (ionized atom or molecule picks up an electron, causing the particle to be charge-neutral), is usually collisions between particles. The chemical composition of the atmosphere definitely influences this balance, too. When there's more ionization, the ionosphere grows. When there's more recombination, the ionosphere shrinks. It can be shrinking at one location and growing in another (see Planetary Rotation, below, for an example).

Location of an Ionosphere

High above the ionosphere, the gas density is low enough that most of the light from a planet's host star travels through the atmosphere without interacting with (i.e. ionizing) the atmosphere. As the light travels downward through the atmosphere, the atmospheric density increases, which causes more high-energy radiation --- particularly extreme ultraviolet (EUV) --- to be absorbed by the atmosphere, causing ionization (plasma generation). Travelling deeper still, the density increases exponentially, but at this depth, most of the high-energy photons have already been absorbed, so there's little ionization happening. Moreover, at this depth, any ions are short-lived due to frequent collisions with the neutral atmosphere that lead to recombination (plasma depletion).

The ionosphere is therefore a "Goldilocks" region in a planet's atmosphere: low enough in the atmosphere for there to be plenty of molecules to absorb high-energy photons, but still high enough that there's still plenty of high-energy radiation from the host star and tenuous enough for recombination to not dominate.

As a very general rule of thumb, roughly $1/e$ of incoming light gets absorbed above the 1 bar pressure level in an atmosphere. (Of course, this varies with wavelength, chemistry, and more....) This is a relatively high fraction of light that has been absorbed, but it's deep enough that collisions between particles are frequent enough for recombination to be more frequent than ionization. Any particles that get ionized quickly become neutralized. Based on this alone (but also substantiated by studies of Earth and other planets), we may conclude that the ionosphere is somewhere above the 1 bar pressure level, but this can vary.

Criteria

An Atmosphere

Without an atmosphere, there aren't any molecules to ionize. Therefore, one needs an atmosphere to have an ionosphere.

Since ionospheres tend to be above the 1 bar pressure level (at least in our Solar System), we may guess that a planet needs to have at least 1 bar of pressure in order to have an ionosphere.

A Consistent Source of Radiation

Without high-energy radiation, the atmosphere will not naturally ionize. Usually, this is a host star. (One could also imagine far-fetched sci-fi scenarios with artificially-generated ionospheres....) The host star has to produce sufficient amounts of high-energy photons to irradiate the atmosphere.

I added the word "consistent" to this section. The reason is because if the star "turned off" (or went through a phase where it produced insufficient EUV photons), then charge recombination would slowly dissipate the ionosphere.

Other Factors

Proximity to Host Star / Star Type

Either a cooler host star or the planet being further from the star will cause it to receive less of the high-energy required for ionization. The opposite is also true.

Receiving less radiation means that the fraction of molecules in the ionosphere that get ionized is smaller, leading to a "weak ionosphere" (i.e. one that is weakly ionized).

For a planet receiving more radiation, the ionosphere will be stronger and thicker for two reasons. First, the higher irradiation means that the few molecules at higher altitudes have more chances to ionize, meaning the ionosphere can be taller than it would otherwise be. Second, because there's more radiation, even though $1/e$ gets absorbed above the 1 bar pressure level, there's still a ton of radiation that hasn't been absorbed. Because of this, the ionosphere can be deeper than it would otherwise be.

Chemical Composition

Different chemical composition could allow an ionosphere to be different than what I've described above. For example, if a planet's atmosphere is primarily composed of a gas that doesn't absorb EUV well, then much of the EUV will penetrate deeper into the atmosphere and would cause the ionosphere to be weaker and/or located deeper in the atmosphere. Or, for a planet with an atmosphere that absorbs EUV extremely well, the ionosphere might be higher in the atmosphere.

In short, the chemistry of the atmosphere can make the ionosphere be located higher or lower in the atmosphere, or cause it to be more strongly- or weakly-ionized.

Planetary Rotation

Earth's ionosphere varies from day to night. At local noon, the ionosphere facing the host star receives the maximum incident radiation, causing more ionization and a thicker/stronger ionosphere. At night, the atmosphere receives essentially no ionizing photons, so recombination makes the ionosphere weaker and thinner. (For the same reason, a planet can have seasonal variations as well.)

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