However, it is probable that one of these processes must have been influencing the
inner coma of 1P/Halley as all the other processes that are thought to be of possible
significance produce decreasing values of A as the impact parameter increases. A
schematic summary of this is shown in Fig. 4.48.
In Fig. 4.49, a plot is shown from data acquired at 67P close to perihelion. The
data set was selected because the phase angle is 90
and therefore dust above the
dayside and nightside hemispheres are clearly separable. It is to be noted that the
image from which this plot was extracted shows only weak evidence of shadowing
of dust by the nucleus. The left panel of Fig. 4.38 shows the same type of plot but
there the shadowing is more prominent (see the image in Fig. 4.35 right). Shadowing
can lead to A initially decreasing and then slightly increasing with impact parameter
before stabilizing at the expected constant value. The evidence of the importance of
shadowing can be seen when comparing the contribution to A from the dayside
(dashed line) and the nightside (dot-dashed line) (compare with Fig. 4.38). On the
dayside, unlike the observation at 1P/Halley, A reaches an almost constant value at
<5 nucleus radii from the surface. This suggests that the processes itemized above
have ceased having any influence on the dust flow <10 km from the surface. (The
alternative is that the effects of the processes are cancelling each other out which
would be contrary to Ockham’s razor). Furthermore, the product rises as the nucleus
is approached indicating a steeper profile than 1/r close to the nucleus. Several of the
itemized effects could be responsible for this behaviour. The nightside shows a
steady increase in both our examples which is partially attributable to the decrease in
the solid angle subtended by the nucleus shadowing the nightside dust as the impact
parameter increases. Further effects of non-radial outflow are also likely to be
present.
Dust acceleration as illustrated in Fig. 4.45 must occur and hence this is the prime
candidate to explain the observation steeper profile than 1/r close to the nucleus.
However, non-radial flow from and close to the nucleus produces a similar effect.
Some combination of these two processes is highly probable. Particle fragmentation
can also be responsible. If particles split into sub-particles that are no longer optically
active (i.e. < 100 nm in the case of observations at visible wavelengths; see
Fig. 4.30) then you lose scattering efficiency and A drops with distance from the
nucleus. Sublimation of particles produces a similar effect. The cross-section may
reduce as the particles lose volatiles.
As we shall see below, there is strong evidence that 67P emits a significant
number of large, slow moving, possibly non-escaping large particles. Large particles
in gravitationally-bound orbits are clearly not undergoing force-free radial outflow
and would appear as a change in A as one approaches the nucleus as observed. An
increase or a decrease in A can occur depending upon the dominant particle
population (escaping or orbiting) as we shall discuss later. If the innermost coma
region cannot be sampled by, for example, multi-point in situ investigation, the only
way to tackle the problem is through detailed modelling. However, we can already
see by comparison of Fig. 4.45 with Fig. 4.49 that the percentage increase in A from
10 km inwards to 3.5 km from the centre of the nucleus is almost identical in the
4.10 Processes in the Innermost Coma
355
inner coma of 1P/Halley as all the other processes that are thought to be of possible
significance produce decreasing values of A as the impact parameter increases. A
schematic summary of this is shown in Fig. 4.48.
In Fig. 4.49, a plot is shown from data acquired at 67P close to perihelion. The
data set was selected because the phase angle is 90
and therefore dust above the
dayside and nightside hemispheres are clearly separable. It is to be noted that the
image from which this plot was extracted shows only weak evidence of shadowing
of dust by the nucleus. The left panel of Fig. 4.38 shows the same type of plot but
there the shadowing is more prominent (see the image in Fig. 4.35 right). Shadowing
can lead to A initially decreasing and then slightly increasing with impact parameter
before stabilizing at the expected constant value. The evidence of the importance of
shadowing can be seen when comparing the contribution to A from the dayside
(dashed line) and the nightside (dot-dashed line) (compare with Fig. 4.38). On the
dayside, unlike the observation at 1P/Halley, A reaches an almost constant value at
<5 nucleus radii from the surface. This suggests that the processes itemized above
have ceased having any influence on the dust flow <10 km from the surface. (The
alternative is that the effects of the processes are cancelling each other out which
would be contrary to Ockham’s razor). Furthermore, the product rises as the nucleus
is approached indicating a steeper profile than 1/r close to the nucleus. Several of the
itemized effects could be responsible for this behaviour. The nightside shows a
steady increase in both our examples which is partially attributable to the decrease in
the solid angle subtended by the nucleus shadowing the nightside dust as the impact
parameter increases. Further effects of non-radial outflow are also likely to be
present.
Dust acceleration as illustrated in Fig. 4.45 must occur and hence this is the prime
candidate to explain the observation steeper profile than 1/r close to the nucleus.
However, non-radial flow from and close to the nucleus produces a similar effect.
Some combination of these two processes is highly probable. Particle fragmentation
can also be responsible. If particles split into sub-particles that are no longer optically
active (i.e. < 100 nm in the case of observations at visible wavelengths; see
Fig. 4.30) then you lose scattering efficiency and A drops with distance from the
nucleus. Sublimation of particles produces a similar effect. The cross-section may
reduce as the particles lose volatiles.
As we shall see below, there is strong evidence that 67P emits a significant
number of large, slow moving, possibly non-escaping large particles. Large particles
in gravitationally-bound orbits are clearly not undergoing force-free radial outflow
and would appear as a change in A as one approaches the nucleus as observed. An
increase or a decrease in A can occur depending upon the dominant particle
population (escaping or orbiting) as we shall discuss later. If the innermost coma
region cannot be sampled by, for example, multi-point in situ investigation, the only
way to tackle the problem is through detailed modelling. However, we can already
see by comparison of Fig. 4.45 with Fig. 4.49 that the percentage increase in A from
10 km inwards to 3.5 km from the centre of the nucleus is almost identical in the
4.10 Processes in the Innermost Coma
355