neck is shown in Fig. 3.33 (right). Although the basic appearance of the flow field is
similar to the insolation-driven case, there are subtle differences. For example, the
gas speed (bottom right) is appreciably faster close to the nucleus in the region above
the neck. This illustrates that the flow field can be used to establish variations of the
eaf over the nucleus but it is necessary to make precise measurements of the density,
speed and temperature of the gas at many points in the coma as close as possible to
the nucleus surface. Hoang et al. (2017) also concluded that, during northern
summer through to equinox, the H 2 O emission was illumination-driven with maxima
in the neck. This conclusion was reached assuming free molecular flow from the
source to ROSINA which is a less sophisticated model of the outflow than used by,
for example, Fougere et al., Bieler et al. and Marschall et al. although a crude
comparison with the model of Fougere et al. suggested that the different approaches
to determining the spatial distribution of the source can produce broadly similar
results if one avoids looking at the details and limiting the physical constraints.
3.4.5.4 Degeneracy in Surface Activity Distributions
We have seen in Fig. 3.32 that when there is large scale inhomogeneity in the initial
boundary condition, gas flows to equalise the pressure. But above large inactive
regions, the gas flow is less dense and is clearly sub-sonic. This was also seen in
somewhat more complex activity distribution models (Crifo and Rodionov 1999). In
Fig. 3.33, we have seen that regional variations in the eaf can be identified but these
distributions can be considered to be continuous. Are models with strong
sub-regional variations in emission strength consistent with observation?
The most detailed nucleus shape model we have is that of 67P. In the high fidelity
model, we have more than 40 million facets and, in principle, each of these facets can
(and probably has) its own eaf ranging from totally inactive to being fully active at
the level of free sublimation of water ice (or indeed at the level of an even more
volatile species such as CO 2 ). This provides a challenge in that we end up with an
additional 40 million+ free parameters for the initial boundary condition. But is it
really necessary to consider each facet individually? And can measurements of gas
densities in the inner coma be inverted to give source strengths at the surface at facet
dimension scales?
The key property of the flow here is the Knudsen penetration number and,
through that, the mean free path. The variation with time and position plays an
important role. Inhomogeneities in the boundary condition on scales smaller than the
mean free path will not be distinguishable and they should be irrelevant to the flow
field. This is particularly obvious for very low production rate cases when the
Knudsen penetration number is large. Gas emitted in the same direction but from
different parts of the nucleus cannot be distinguished because there is no interaction.
This has been studied by Liao (2017) who looked at changing the activity distribution from insolation-driven to a distribution where only 2% of the surface was active
with these active areas being randomly distributed over the nucleus. The activity was
240
3 Gas Emissions Near the Nucleus
similar to the insolation-driven case, there are subtle differences. For example, the
gas speed (bottom right) is appreciably faster close to the nucleus in the region above
the neck. This illustrates that the flow field can be used to establish variations of the
eaf over the nucleus but it is necessary to make precise measurements of the density,
speed and temperature of the gas at many points in the coma as close as possible to
the nucleus surface. Hoang et al. (2017) also concluded that, during northern
summer through to equinox, the H 2 O emission was illumination-driven with maxima
in the neck. This conclusion was reached assuming free molecular flow from the
source to ROSINA which is a less sophisticated model of the outflow than used by,
for example, Fougere et al., Bieler et al. and Marschall et al. although a crude
comparison with the model of Fougere et al. suggested that the different approaches
to determining the spatial distribution of the source can produce broadly similar
results if one avoids looking at the details and limiting the physical constraints.
3.4.5.4 Degeneracy in Surface Activity Distributions
We have seen in Fig. 3.32 that when there is large scale inhomogeneity in the initial
boundary condition, gas flows to equalise the pressure. But above large inactive
regions, the gas flow is less dense and is clearly sub-sonic. This was also seen in
somewhat more complex activity distribution models (Crifo and Rodionov 1999). In
Fig. 3.33, we have seen that regional variations in the eaf can be identified but these
distributions can be considered to be continuous. Are models with strong
sub-regional variations in emission strength consistent with observation?
The most detailed nucleus shape model we have is that of 67P. In the high fidelity
model, we have more than 40 million facets and, in principle, each of these facets can
(and probably has) its own eaf ranging from totally inactive to being fully active at
the level of free sublimation of water ice (or indeed at the level of an even more
volatile species such as CO 2 ). This provides a challenge in that we end up with an
additional 40 million+ free parameters for the initial boundary condition. But is it
really necessary to consider each facet individually? And can measurements of gas
densities in the inner coma be inverted to give source strengths at the surface at facet
dimension scales?
The key property of the flow here is the Knudsen penetration number and,
through that, the mean free path. The variation with time and position plays an
important role. Inhomogeneities in the boundary condition on scales smaller than the
mean free path will not be distinguishable and they should be irrelevant to the flow
field. This is particularly obvious for very low production rate cases when the
Knudsen penetration number is large. Gas emitted in the same direction but from
different parts of the nucleus cannot be distinguished because there is no interaction.
This has been studied by Liao (2017) who looked at changing the activity distribution from insolation-driven to a distribution where only 2% of the surface was active
with these active areas being randomly distributed over the nucleus. The activity was
240
3 Gas Emissions Near the Nucleus