in which neutrals, ions and electrons were treated as separate fluids but with the same
bulk velocity. (This is, in itself, a significant simplication.)
One of the basic simplifying assumptions in the Haser model is that the outflow of
both parent and daughter species is purely radial. However, it was recognized in the
late 1970s that suprathermal velocities of daughter products would arise from the
photodissociation reaction (R1) because there is excess energy of the order of 2 eV
available after overcoming the chemical bond energy and that these velocities could
be significantly higher than the velocities of the parent species. Combi et al. (2004)
give the exothermic velocities of dissociation products of H 2 O which shows that the
daughter product velocities strongly depend upon the wavelength of the incoming
photon. For the two most probable outcomes (arising from the reaction H 2 O + hϑ !
H + OH which occurs in around 70% of cases), the velocity of the OH product will
be around 1.05 km s
À1 whereas the velocity of the H product will be around 18 km
s
À1 . For higher energy photons initiating the dissociation, the hydrogen product
velocity can reach 37 km s
À1 . The high velocity of the neutral hydrogen arising from
photodissociation combined with the relatively long lifetime of hydrogen before
ionization occurs, leads to the hydrogen coma of comets being extremely large
covering a vast volume of space. The excess energy has the effect of increasing
the velocity of the daughter products far above the outflow speed of the parent and
the direction of motion is clearly no longer necessarily radial with respect to the
nucleus. Hence, this is no longer a “free molecular flow”.
Festou (1981) introduced the “vectorial model” to account for the excess energy
and thereby correct OH (and thereby H 2 O) production rates from ground-based
observations. At the time of writing a web version of this model (the Web Vectorial
Model) is still available
10 and an example calculation is shown in Fig. 3.47. This
shows a predicted OH column density along the Sun-comet line for 67P at 1.3 AU
using an H 2 O production rate of 3.3 10
28 molecule s
À1 in steady-state with isotropic
emission.
An alternative is to use a Monte Carlo approach (Combi and Delsemme 1980;
Combi et al. 2004) which offers a great deal more flexibility in treating specific cases
such as non-isotropic outgassing and a larger reaction network.
The hydrogen coma has been studied through observations of Lyman-α as far
back as 1972. Observations of 1P/Halley using sounding rockets (McCoy et al.
1992) showed that multiple populations of hydrogen were needed to fit the data
corresponding to the different velocities arising from different parents (OH and
H 2 O). However, another slow component was also needed which was attributed to
the thermalization of fast hydrogen atoms arising from collisions near the nucleus.
Obviously, the presence of this component would strongly depend upon the total
comet H 2 O production rate.
Measurements of Lyman-α have been performed frequently by the Solar Wind
ANisotropies (SWAN) instrument onboard the Solar and Heliospheric Observatory
spacecraft, SOHO, from which water production rates and their variation with
10 http://www.boulder.swri.edu/wvm/
258
3 Gas Emissions Near the Nucleus
bulk velocity. (This is, in itself, a significant simplication.)
One of the basic simplifying assumptions in the Haser model is that the outflow of
both parent and daughter species is purely radial. However, it was recognized in the
late 1970s that suprathermal velocities of daughter products would arise from the
photodissociation reaction (R1) because there is excess energy of the order of 2 eV
available after overcoming the chemical bond energy and that these velocities could
be significantly higher than the velocities of the parent species. Combi et al. (2004)
give the exothermic velocities of dissociation products of H 2 O which shows that the
daughter product velocities strongly depend upon the wavelength of the incoming
photon. For the two most probable outcomes (arising from the reaction H 2 O + hϑ !
H + OH which occurs in around 70% of cases), the velocity of the OH product will
be around 1.05 km s
À1 whereas the velocity of the H product will be around 18 km
s
À1 . For higher energy photons initiating the dissociation, the hydrogen product
velocity can reach 37 km s
À1 . The high velocity of the neutral hydrogen arising from
photodissociation combined with the relatively long lifetime of hydrogen before
ionization occurs, leads to the hydrogen coma of comets being extremely large
covering a vast volume of space. The excess energy has the effect of increasing
the velocity of the daughter products far above the outflow speed of the parent and
the direction of motion is clearly no longer necessarily radial with respect to the
nucleus. Hence, this is no longer a “free molecular flow”.
Festou (1981) introduced the “vectorial model” to account for the excess energy
and thereby correct OH (and thereby H 2 O) production rates from ground-based
observations. At the time of writing a web version of this model (the Web Vectorial
Model) is still available
10 and an example calculation is shown in Fig. 3.47. This
shows a predicted OH column density along the Sun-comet line for 67P at 1.3 AU
using an H 2 O production rate of 3.3 10
28 molecule s
À1 in steady-state with isotropic
emission.
An alternative is to use a Monte Carlo approach (Combi and Delsemme 1980;
Combi et al. 2004) which offers a great deal more flexibility in treating specific cases
such as non-isotropic outgassing and a larger reaction network.
The hydrogen coma has been studied through observations of Lyman-α as far
back as 1972. Observations of 1P/Halley using sounding rockets (McCoy et al.
1992) showed that multiple populations of hydrogen were needed to fit the data
corresponding to the different velocities arising from different parents (OH and
H 2 O). However, another slow component was also needed which was attributed to
the thermalization of fast hydrogen atoms arising from collisions near the nucleus.
Obviously, the presence of this component would strongly depend upon the total
comet H 2 O production rate.
Measurements of Lyman-α have been performed frequently by the Solar Wind
ANisotropies (SWAN) instrument onboard the Solar and Heliospheric Observatory
spacecraft, SOHO, from which water production rates and their variation with
10 http://www.boulder.swri.edu/wvm/
258
3 Gas Emissions Near the Nucleus