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What are the conditions for an ionosphere to form on a given planet?

Factors I could think of are

  • Class and age of the star the planet orbits
  • Distance to said star
  • Density and composition of the planet's atmosphere
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According to the paper "Simultaneous ionospheric variability on Earth and Mars" (Mendillo et al. 2003), the overall structure of any ionised region of a planetary atmosphere depends on

a blend of in situ production and loss processes, plus effects of transport of ionization into or out of the local region of interest

Specifically, according to Mendillo et al., the only significant influence is from the photon flux from the parent star - so, the further the planet is from the star, the less the incoming photon flux. The other mechanisms are suggested to be planetary-based, including (from the article):

  • planetary rotation rate

  • orbital obliquity

which both also affect the photon flux reaching that particular part of the planetary atmosphere. Other mechanisms are to do with the planetary atmospheres themselves, with factors including:

  • the thermal structure, chemistry and dynamics of the planetary 'neutral' atmosphere itself.

  • by how much energetic particles (from the stellar wind and/or magnetosphere) affects the planetary atmosphere.

  • diffusion and electrodynamics from coupling from above the layer; and tides, waves and electrodynamics from coupling with below the layer.

A further study in the paper "Ionospheric photoelectrons: Comparing Venus, Earth, Mars and Titan" (Coates et al. 2011) compared the sunlit portion of planetary ionospheres, which is sustained by photoionization of primarily neutral atmospheric constituents by solar EUV (Extreme UV), a process originally identified for Earth, but recently:

the Mars Express, Venus Express and Cassini-Huygens missions have revealed the importance of this process at Mars, Venus and Titan, respectively.

It should be noted, that for planets without magnetic fields (Venus and Mars for example), the Max Planck Institute for Solar System Research page "Research about the planetary plasma environment",

The ionosphere is assumed to display two distinct states, "magnetized" and "unmagnetized", depending on the penetration depth of the solar wind magnetic field.

One more factor is suggested - dust, theorised from observations reported in the paper "On the role of dust in the lunar ionosphere" (Stubbs et al. 2011). According to the NASA page summarising the paper "Mystery of the Lunar Ionosphere", ubiquitous grains of dust from the Lunar surface may be the source of the Lunar ionosphere. The research from Stubbs et al. is summarised as:

UV rays from the sun hit the grains and ionize them. According to their calculations, this process produces enough charge (positive grains surrounded by negative electrons) to create the observed ionosphere.

The process with the lunar ionosphere is illustrated below (from the NASA page linked):

enter image description here

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@Hackworth has also asked this question on the Geosciences SX site (link to question), where I provided an answer (link to my answer). I'm including a link to my answer there from here in case it proves useful to somebody.

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