The non-radial nature of the flow is well illustrated by the third row from the top in
Fig. 3.32. The colour-coding indicates that close to the terminator and within
1 nucleus radius of the surface, the radial component of the velocity changes
markedly as a percentage of the whole with the remaining motion being lateral to
the surface. This percentage is rather insensitive to the production rate and hence
strong production rate gradients will result in a significant component of the flow
being lateral to the surface as was deduced from near-surface observations of dust at
1P/Halley by Keller and Thomas (1989).
3.4.5.2 Cases with Realistic Shapes
The deviations of cometary nuclei from spherical symmetry influence the flow field.
However, the effects can be subtle. In Fig. 3.33, we see a DSMC calculation of the
gas flow field around the nucleus of 67P at one instant representative of a point in
November 2014 when the total gas production rate was around 2 kg/s. The two plots
to the left show the gas density (top) and gas speed (bottom) for a model where the
production rate at each facet of the nucleus shape model was set according to
Eq. (2.101) (and therefore insolation-driven) and multiplied by a single effective
active fraction (Sect. 2.9.3.1) to provide consistency with the observed water
production rate.
It is evident that although there are two regions of higher production rate (one on
the head of the nucleus and one on the body), the gas flow smooths this inhomogeneity out so that by the time the gas reaches the edge of the domain, 10 km from the
centre of the nucleus, the distribution of gas with azimuth is smooth and shows only
one maximum in (roughly) the sunward direction. At the edge of the domain, the
flow appears quite similar to that seen in an insolation-driven spherical case. This
illustrates an important point. For modelling of observations at large (>10 R N )
distances from the nucleus, the irregular shape of the nucleus plays only a modest
role in defining the flow field and hence, if the gas distribution is observed to be
strongly deviating from that arising from an insolation-driven sphere, this implies
that major surface heterogeneity in the source distribution must be suspected.
3.4.5.3 Deviations from Insolation-Driven Activity
The effective active fraction (eaf) was introduced in Sect. 2.9.3.1 as an hoc means of
reducing the strength of the source because free sublimation from a water ice surface
Fig. 3.32 (continued) from the top, the log number density, the speed, the magnitude of the radial
component of the speed, and the temperature anisotropy. The colour-coding is the same for all
panels in a row. Note that this has been achieved for the number density by using the scaling factor
indicated in the plot. (Credit: Marschall, PhD. thesis, 2017)
238
3 Gas Emissions Near the Nucleus
Fig. 3.32. The colour-coding indicates that close to the terminator and within
1 nucleus radius of the surface, the radial component of the velocity changes
markedly as a percentage of the whole with the remaining motion being lateral to
the surface. This percentage is rather insensitive to the production rate and hence
strong production rate gradients will result in a significant component of the flow
being lateral to the surface as was deduced from near-surface observations of dust at
1P/Halley by Keller and Thomas (1989).
3.4.5.2 Cases with Realistic Shapes
The deviations of cometary nuclei from spherical symmetry influence the flow field.
However, the effects can be subtle. In Fig. 3.33, we see a DSMC calculation of the
gas flow field around the nucleus of 67P at one instant representative of a point in
November 2014 when the total gas production rate was around 2 kg/s. The two plots
to the left show the gas density (top) and gas speed (bottom) for a model where the
production rate at each facet of the nucleus shape model was set according to
Eq. (2.101) (and therefore insolation-driven) and multiplied by a single effective
active fraction (Sect. 2.9.3.1) to provide consistency with the observed water
production rate.
It is evident that although there are two regions of higher production rate (one on
the head of the nucleus and one on the body), the gas flow smooths this inhomogeneity out so that by the time the gas reaches the edge of the domain, 10 km from the
centre of the nucleus, the distribution of gas with azimuth is smooth and shows only
one maximum in (roughly) the sunward direction. At the edge of the domain, the
flow appears quite similar to that seen in an insolation-driven spherical case. This
illustrates an important point. For modelling of observations at large (>10 R N )
distances from the nucleus, the irregular shape of the nucleus plays only a modest
role in defining the flow field and hence, if the gas distribution is observed to be
strongly deviating from that arising from an insolation-driven sphere, this implies
that major surface heterogeneity in the source distribution must be suspected.
3.4.5.3 Deviations from Insolation-Driven Activity
The effective active fraction (eaf) was introduced in Sect. 2.9.3.1 as an hoc means of
reducing the strength of the source because free sublimation from a water ice surface
Fig. 3.32 (continued) from the top, the log number density, the speed, the magnitude of the radial
component of the speed, and the temperature anisotropy. The colour-coding is the same for all
panels in a row. Note that this has been achieved for the number density by using the scaling factor
indicated in the plot. (Credit: Marschall, PhD. thesis, 2017)
238
3 Gas Emissions Near the Nucleus