Recent Knowledge: Ionosphere of Titan
A body with such a dense atmosphere like Titan should also possess a significant ionosphere. Voyager 1, Cassini and Huygens detected three separate ionospheric layers, which were postulated by theoretical models.
Ionosphere of Titan
| Height (km) | Electron Number Density (cm-3) | Mechanism | Detection |
|---|---|---|---|
| 1200 | 1500-2400 | solare EUV, magnetospheric electrons | Voyager 1, Cassini |
| 500 | 400-1600 | meteor ablation | Cassini |
| 60 | 450 | galactic cosmic rays | Huygens |
The main ionosphere in 1200 km is caused by photoionization (solare EUV radiation) and shock-ionization by magnetospheric electrons. The electron number density undergoes diurnal variation, but the ionosphere is clearly present on the nightside as well. In 500 km altitude there is a second ionosphere probably generated by metallic ions ablated by infalling meteoroids. Furthermore, there is a third ionosphere in 60 km altitude caused by galactic cosmic rays.
The upper boundary of the ionosphere is the ionopause. At this boundary pressure equilibrium between Titan's inner ionosphere and the arriving magnetospheric plasma is established. Therefore, mixing of two plasmas is mostly ruled out, and the magnetospheric plasma flows around Titan. The formation of the ionopause strongly depends on the ionospheric density. If this is too low, the pressure will not be sufficient to resist the impact pressure of the arriving plasma. In such a case, the magnetospheric plasma can penetrate deep into the ionosphere, and is slowly braked. Due to the strong variation of Titan's plasma environment, this can indeed occur. You find more about this phenomenon on our plasma interaction page.
In the upper ionosphere the chemical reaction paths of ions and neutral particles contain reactions for the formation of H2CN+, which is the main species in the peak region of the ionosphere (about 1000 - 1200 km height). The H2CN+ detected in the plasma tail could originate from this region when it diffuses through the ionopause and reaches the exosphere.
In the lower ionosphere produced by cosmic rays cluster ions and long chain hydrocarbon ions dominate. The electron distribution in the lower ionosphere, especially in the troposphere, will ultimately depend on the abundance of electrophilic species. Photochemical models do predict the existence of such molecules. If, however, their abundance is negligible, the electron number density should have to be much higher than on Earth because there is no oxygen, which would immediately bind free electrons.