H

Fig. 25. Dust column density distribution for a jet expanding into a background gas for different viewing angles, $ [132]. Upper left: $ = 107°; Upper right: $ = 120°; $ = 135° ; $ = 150°. The background sublimation is set to 1% of the production rate at the active region

• These features are much narrower than expected for the free expansion of a jet into vacuum.

• Whether such filaments are indeed seen in images depends strongly on the viewing geometry (Fig. 25).

Interaction regions between streaming gas flows are not only seen for a single jet expanding into a background gas, but also for neighboring active regions. In such case, two gas jets interact, again producing shock systems with related gas and dust density enhancements (Figs. 26 and 27).

There are several possible reasons, why some areas on the surface are more active than others. An effect studied intensively in the past is the different solar illumination on an irregularly shaped nucleus. The differences in solar energy received by the various parts of the surface result in differences of surface sublimation and finally in interacting gas jet flows. Again shock systems form, giving the appearance of multiple filaments in the coma. A summary of the gas flow field around an irregular nucleus is given in [52].

Observations of Jets and Filaments Jets

The lateral expansion of gas jets, excess energies for daughter molecules and nucleus rotation altogether lead to a relatively isotropic gas coma on a large scale (in comparison to the innermost coma), with asymmetries because

Fig. 27. Dust column densities corresponding to the flow region of Fig. 26 for different dust particle classes a. The large particles are focussed into a narrow straight filament-like structure [132]

of external forces, such as radiation pressure. Nevertheless, ground-based images of comets often show gas and dust jets. For example, in Fig. 28 two gas jets are clearly seen in an image of comet C/2004 Q2 Machholz taken through a CN filter. The continuum image shows a sun-tail asymmetry, indicating preferred sunward outgassing of the nucleus. The spatial distribution of the jets observed for gas molecules and dust particles is usually not the same. This is not surprising because gas and dust decouple after a few kilometers in the coma, as described above. Gas molecules and dust particles then move outward with different velocity. On a rotating nucleus, their spatial distribution is then expected to be different. The gas jets are often broader than the dust in agreement with a more efficient lateral expansion of the gas in comparison to the visible dust. Furthermore, jets observed in light of daughter radicals that receive isotropic excess velocities after photodissociation of their parent also appear broader than parent molecule jets. A nice illustration of the fast lateral expansion from localized surface regions is the appearance of the ejecta cloud after impact of the Deep Impact probe in comet Tempel 1. About 17 h after impact, the ejecta cloud of CN is already visible all around the nucleus, whereas the dust cloud still expands mainly in the sunward direction with a much narrower opening angle (e.g., [180]).

To distinguish jets from narrow filaments, it is helpful to plot the azimuthal intensity profile in the coma, as shown in Fig. 29. Broad jet structures with intensity enhancements of several 10%, as in the example shown here, are obviously jets related to active surface regions on the nucleus surface. Filaments produced by gas flow interaction in the coma would be much narrower than the broad features seen in the azimuthal profiles.

Fig. 28. Jets in comet C/2004 Q2 Machholz observed on Dez. 8/9, 2004. The comet was at rh = 1.4 AU and A = 0.5 AU. The field-of-view is 4.5arcmin. The solar direction is at the bottom of each frame. At each radial distance, a mean coma intensity has been subtracted to enhance non-isotropic structures in the coma. Left: CN at 385 nm, the gray scale indicates variation of ± 25% from the mean value; right: dust continuum at 443 nm, the gray scale corresponds to ± 15% intensity deviation from the mean. Two gas jets are clearly visible in the CN frame. The dust image shows no clear jets, but enhanced intensity toward the Sun

Fig. 28. Jets in comet C/2004 Q2 Machholz observed on Dez. 8/9, 2004. The comet was at rh = 1.4 AU and A = 0.5 AU. The field-of-view is 4.5arcmin. The solar direction is at the bottom of each frame. At each radial distance, a mean coma intensity has been subtracted to enhance non-isotropic structures in the coma. Left: CN at 385 nm, the gray scale indicates variation of ± 25% from the mean value; right: dust continuum at 443 nm, the gray scale corresponds to ± 15% intensity deviation from the mean. Two gas jets are clearly visible in the CN frame. The dust image shows no clear jets, but enhanced intensity toward the Sun

Fig. 29. Azimuthal intensity profile in the coma of comet C/2004 Q2 Machholz. The profiles correspond to the images shown in Fig. 28. The profiles have been averaged over a nucleocentric distance of 1800-3600 km. Gray line: continuum, black: CN

Filaments

Narrow straight filaments are common in the close vicinity of nuclei, as can be seen in the images of comets Halley (Fig. 30), Borrelly (Fig. 31), and Wild 2 (Fig. 32). Such filaments are narrow and faint structures, clearly different to the broad jets with relatively strong contrast to the mean coma density.

It is not obvious how to relate jets and filaments to active regions on the nucleus surface. How can we find out what we are looking at? In general, gas and dust jets produced by an isolated active region on the nucleus surface are expected to have the following appearance:

- wide opening angle, sometimes curved appearance

- relatively high contrast to the background gas and dust

- observed on a large spatial scale

- observed over long time periods

Narrow straight filaments are characterized by:

- narrow structures, straight

- low contrast to the background gas and dust

- observed in the near-nucleus region

Fig. 30. Filaments seen close to the nucleus of comet Halley [203]

Cim of mar ]« laçina fit)

Coirrated Beta Jet

Si*i-f«0«m»d ten

Sun-ported far*

Fig. 31. Filaments seen close to the nucleus of comet Borrelly [198]

Fig. 31. Filaments seen close to the nucleus of comet Borrelly [198]

In summary, for ground-based as well as in situ images obtained in fly-bys, it is important to look at the contrast and opening angle of the jets and filaments seen before drawing conclusions on their relation to the location of active areas on a nucleus surface.

3.2 Dynamics in the Outer Coma and Neutral Gas Tails

On a larger scale, beyond about 104 km, the gas is accelerated again by heating because of photoprocesses (Fig. 12). The dominant photo reaction for heating is photodissociation of H2O to OH and H (e.g., [47]). Further out, radiative cooling of the OH molecules begins to dominate. However, on this large scale collisions are rare and the molecules move in free flow.

In this regime, radiation pressure from the Sun is important for the gas flow. Absorption and re-emission of solar photons causes an acceleration of atoms and gas molecules in the anti-solar direction. The acceleration is small for coma molecules, but can be large for atoms with high fluorescence efficiency factors, such as hydrogen and sodium atoms. The acceleration by solar radiation pressure is given by:

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