Breaking the symmetry of emission generates a number of interesting effects. The
simplest and most realistic method to break the symmetry is to use an insolationdriven model for the initial condition on the dayside, i.e. using Eq. (2.101) with no
emission from the nightside. For the purposes of this illustration (Fig. 3.32), the local
production rates have been scaled to reach total production rates of 2, 20, and 200 kg
s
À1 . In the absence of emission from the nightside, flow is no longer radial close to
the nucleus as the gas tries to balance pressure gradients. The top row shows the log
of the number density for the three different production rates. Note that the colourcoding of the flow field has been scaled using the factors that are indicated in the
plots. The three results appear rather similar. However, the lower production rate
case has a lower relative density above the nightside of the sphere indicating that,
relatively speaking, fewer molecules are being scattered from the dayside into the
nightside hemisphere. A better impression of the effects is given by looking at the
other three rows and in particular the magnitude of the non-radial component of the
velocity.
The gas emitted from the dayside expands not merely radially but also laterally
with a component parallel to the surface. Close to the nightside surface, the speed of
the gas flow may be low but the non-radial component of the velocity is high. This
concept of non-radial flow was used as a potential explanation for the ripples seen in
Fig. 2.95. The speed of the (radial) outflow above the dayside is similar to that shown
in Fig. 3.31 and shows the same (small) production rate dependence. However, close
to the terminator, the gas speed drops off much more quickly in the low production
rate case as the number of collisions experienced reduces quickly in the more
rarefied flow. This results in a relatively lower number density above the nightside
hemisphere in the low production rate case. Further evidence for the absence of
collisions on the nightside in the lower production rate case can be seen in the
temperature anisotropy plots (bottom row) which show the ratio of the rotational
temperature to the translational temperature (T rot /T trans ). In LTE this value should be
1 as collisions transfer energy from the rotational to the translational degrees of
freedom. In the high production rate case, this is seen to be the case everywhere
except in a thin layer directly above the surface of the nightside. For the low
production rate case, almost the entire hemisphere above the nightside has T rot /
T trans ! 3 indicating that there are insufficient collisions to produce equilibrium and
to transfer energy from the rotational degrees of freedom into speed. Lower radial
outflow speeds above the nightside are a direct consequence of this. It is also evident
that the condition required for the fluid solution (namely that Ma ¼ 1 at the surface)
is not met at or just above the nightside surface.
What we see here is that, even in a relatively simple case, the outflow speed of the
gas is dependent upon position in the inner coma and that while there is a small speed
dependence on production rate in the isotropic case, the insolation-driven case
results in large speed dependencies on production rate away from the main direction
of outflow. Of particular importance for the interpretation of Rosetta data is that
near-nucleus dayside to nightside flows significantly affect the inner coma properties
near the terminator. (Rosetta was frequently in terminator orbits about the nucleus
and hence its in situ experiments were often sampling this region of the inner coma.)
236
3 Gas Emissions Near the Nucleus
simplest and most realistic method to break the symmetry is to use an insolationdriven model for the initial condition on the dayside, i.e. using Eq. (2.101) with no
emission from the nightside. For the purposes of this illustration (Fig. 3.32), the local
production rates have been scaled to reach total production rates of 2, 20, and 200 kg
s
À1 . In the absence of emission from the nightside, flow is no longer radial close to
the nucleus as the gas tries to balance pressure gradients. The top row shows the log
of the number density for the three different production rates. Note that the colourcoding of the flow field has been scaled using the factors that are indicated in the
plots. The three results appear rather similar. However, the lower production rate
case has a lower relative density above the nightside of the sphere indicating that,
relatively speaking, fewer molecules are being scattered from the dayside into the
nightside hemisphere. A better impression of the effects is given by looking at the
other three rows and in particular the magnitude of the non-radial component of the
velocity.
The gas emitted from the dayside expands not merely radially but also laterally
with a component parallel to the surface. Close to the nightside surface, the speed of
the gas flow may be low but the non-radial component of the velocity is high. This
concept of non-radial flow was used as a potential explanation for the ripples seen in
Fig. 2.95. The speed of the (radial) outflow above the dayside is similar to that shown
in Fig. 3.31 and shows the same (small) production rate dependence. However, close
to the terminator, the gas speed drops off much more quickly in the low production
rate case as the number of collisions experienced reduces quickly in the more
rarefied flow. This results in a relatively lower number density above the nightside
hemisphere in the low production rate case. Further evidence for the absence of
collisions on the nightside in the lower production rate case can be seen in the
temperature anisotropy plots (bottom row) which show the ratio of the rotational
temperature to the translational temperature (T rot /T trans ). In LTE this value should be
1 as collisions transfer energy from the rotational to the translational degrees of
freedom. In the high production rate case, this is seen to be the case everywhere
except in a thin layer directly above the surface of the nightside. For the low
production rate case, almost the entire hemisphere above the nightside has T rot /
T trans ! 3 indicating that there are insufficient collisions to produce equilibrium and
to transfer energy from the rotational degrees of freedom into speed. Lower radial
outflow speeds above the nightside are a direct consequence of this. It is also evident
that the condition required for the fluid solution (namely that Ma ¼ 1 at the surface)
is not met at or just above the nightside surface.
What we see here is that, even in a relatively simple case, the outflow speed of the
gas is dependent upon position in the inner coma and that while there is a small speed
dependence on production rate in the isotropic case, the insolation-driven case
results in large speed dependencies on production rate away from the main direction
of outflow. Of particular importance for the interpretation of Rosetta data is that
near-nucleus dayside to nightside flows significantly affect the inner coma properties
near the terminator. (Rosetta was frequently in terminator orbits about the nucleus
and hence its in situ experiments were often sampling this region of the inner coma.)
236
3 Gas Emissions Near the Nucleus