respect to the nucleus where such a balance occurs is then a function of the gas
production rate. Secondly, as they expand, the neutrals are ionized by solar EUV
photons. The ions are initially moving at the velocities of their parent neutrals (~
0.8 km s
À1 relative to the nucleus) but this velocity is negligible compared to the
relative velocity with respect to the solar wind and the interplanetary magnetic field.
Consequently, the ions are “picked up” by the solar wind and accelerated. However,
this pick-up requires a momentum transfer from the solar wind to the newly created
ions. This process of mass loading the flow, which is not only significant because of
the numbers of created ions but also because the masses of species such as O
+ are
heavier than solar protons, acts to progressively reduce the flow velocity as the
cometary ion density increases. It is the combination of the two mechanisms that
generates most of the observed phenomena. We start by looking at the ionization
process.
5.2 The Production of Cometary Ions
We have seen a long list of reaction types that can occur within the coma in
Table 3.9. The main sources that are sufficient to explain the observed plasma
densities in the vicinity of comets appear to be photo-ionization and electron-impact
ionization although the influence of charge-exchange can be seen in some phenomena (see, for example, Heritier et al. 2018 for 67P).
Consequently, the three major reaction types of interest are
• Photoionization (R2)
• Charge exchange (R8)
• Electron impact (dissociative) ionization (R5, R6)
5.2.1 Photoionization
Photoionization cross-sections for several cometary species in computer readable
form can be found online.
1 Some examples of relevance are shown in Fig. 5.2. The
photo-ionization of atomic hydrogen (indicated by H ! H+ in the diagram) has a
threshold at 91.175 nm (Table 5.2). The cross-section is at a maximum at this
wavelength and drops rapidly with increasing photon energy so that higher energy
photons will penetrate a hydrogen coma further than photons closer to threshold. The
other lines in Fig. 5.2 show branches where ionized products from photo-absorption
by the water molecule arise. There are, of course, dissociation branches where only
neutrals are produced but they are not shown here to limit the details in the diagram.
1 https://phidrates.space.swri.edu/
402
5 The Plasma Environment
production rate. Secondly, as they expand, the neutrals are ionized by solar EUV
photons. The ions are initially moving at the velocities of their parent neutrals (~
0.8 km s
À1 relative to the nucleus) but this velocity is negligible compared to the
relative velocity with respect to the solar wind and the interplanetary magnetic field.
Consequently, the ions are “picked up” by the solar wind and accelerated. However,
this pick-up requires a momentum transfer from the solar wind to the newly created
ions. This process of mass loading the flow, which is not only significant because of
the numbers of created ions but also because the masses of species such as O
+ are
heavier than solar protons, acts to progressively reduce the flow velocity as the
cometary ion density increases. It is the combination of the two mechanisms that
generates most of the observed phenomena. We start by looking at the ionization
process.
5.2 The Production of Cometary Ions
We have seen a long list of reaction types that can occur within the coma in
Table 3.9. The main sources that are sufficient to explain the observed plasma
densities in the vicinity of comets appear to be photo-ionization and electron-impact
ionization although the influence of charge-exchange can be seen in some phenomena (see, for example, Heritier et al. 2018 for 67P).
Consequently, the three major reaction types of interest are
• Photoionization (R2)
• Charge exchange (R8)
• Electron impact (dissociative) ionization (R5, R6)
5.2.1 Photoionization
Photoionization cross-sections for several cometary species in computer readable
form can be found online.
1 Some examples of relevance are shown in Fig. 5.2. The
photo-ionization of atomic hydrogen (indicated by H ! H+ in the diagram) has a
threshold at 91.175 nm (Table 5.2). The cross-section is at a maximum at this
wavelength and drops rapidly with increasing photon energy so that higher energy
photons will penetrate a hydrogen coma further than photons closer to threshold. The
other lines in Fig. 5.2 show branches where ionized products from photo-absorption
by the water molecule arise. There are, of course, dissociation branches where only
neutrals are produced but they are not shown here to limit the details in the diagram.
1 https://phidrates.space.swri.edu/
402
5 The Plasma Environment