There are better examples in the Rosetta data set but again one must be aware of
optical illusions arising from crossing of diffuse structures along the line of sight.
There are also sharp boundaries that arise from dust being shadowed above the
nightside hemisphere of the nucleus.
In Fig. 4.38 some of the effects are shown more quantitatively. The image on the
right is from the OSIRIS Wide-Angle Camera. The Sun is vertically upwards. In the
centre is a plot extracted from a polar coordinate transform of the image about the
centre of the nucleus showing the brightness distribution at three impact parameters
from the nucleus centre. The curves have been linearised for the distance to the
nucleus by multiplying by the impact parameter, b. Several things are evident.
Firstly, the overall level of each curve decreases slightly as the distance to the
nucleus increases. This is evidence of particle acceleration (although we note that
other processes may also be active as will be discussed below). Secondly, the
nucleus casts a clearly visible shadow on the dust indicating that there is dust
above the nightside hemisphere as a consequence of either nightside activity or
dayside to nightside flow of material (either through gas flow or gravitation).
Thirdly, the spatial distribution suggests that some of the fine structures are curved
Fig. 4.37 Slightly enhanced version of an OSIRIS image acquired on 4 July 2015 close to
perihelion. (Image number: N20150704T180343702ID30F22). The image shows diffuse activity
from the head of the nucleus. On the body, there are narrow, well-defined structures emanating from
the nucleus. Jets appear to be crossing, particularly in the neck region, but line of sight effects here
need to be carefully considered. Evidence of the nucleus casting a shadow on dust above the
nightside hemisphere of the nucleus is also evident
4.9 Observation of Non-uniform Dust Emission
343
optical illusions arising from crossing of diffuse structures along the line of sight.
There are also sharp boundaries that arise from dust being shadowed above the
nightside hemisphere of the nucleus.
In Fig. 4.38 some of the effects are shown more quantitatively. The image on the
right is from the OSIRIS Wide-Angle Camera. The Sun is vertically upwards. In the
centre is a plot extracted from a polar coordinate transform of the image about the
centre of the nucleus showing the brightness distribution at three impact parameters
from the nucleus centre. The curves have been linearised for the distance to the
nucleus by multiplying by the impact parameter, b. Several things are evident.
Firstly, the overall level of each curve decreases slightly as the distance to the
nucleus increases. This is evidence of particle acceleration (although we note that
other processes may also be active as will be discussed below). Secondly, the
nucleus casts a clearly visible shadow on the dust indicating that there is dust
above the nightside hemisphere as a consequence of either nightside activity or
dayside to nightside flow of material (either through gas flow or gravitation).
Thirdly, the spatial distribution suggests that some of the fine structures are curved
Fig. 4.37 Slightly enhanced version of an OSIRIS image acquired on 4 July 2015 close to
perihelion. (Image number: N20150704T180343702ID30F22). The image shows diffuse activity
from the head of the nucleus. On the body, there are narrow, well-defined structures emanating from
the nucleus. Jets appear to be crossing, particularly in the neck region, but line of sight effects here
need to be carefully considered. Evidence of the nucleus casting a shadow on dust above the
nightside hemisphere of the nucleus is also evident
4.9 Observation of Non-uniform Dust Emission
343