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mr h

Fig. 33. The sodium tail (left) in comet Hale-Bopp and a comparison to its H2 O+ tail (right) [51]

The effective acceleration of the molecules and atoms is then given by the balance of the solar radiation pressure force with solar gravitation. This is usually characterized by the parameter:

Fgrav GM0m^ Ai

Here, gi is the g-factor for fluorescence at rh = 1 AU (scales with r-2) for a transition at wavelength Ai, h is the Planck constant, G the gravitational constant, Mq the solar mass, and m the molecular mass.

Taking into account solar radiation pressure is important when computing the dynamics of cometary sodium tails (Fig. 33) and the hydrogen coma (Fig. 42) of comets. Hydrogen forms mainly by photodissociation of H2O and OH. Its large-scale lengths and efficient acceleration leads to an extend of the coma up to several 107 km, turning comets into the largest objects of the solar system when taking their H-coma into account. The dynamics of H atoms is complex and needs to consider their excess energy obtained during formation in addition to solar radiation pressure. [47] provide a detailed review of the dynamics of H atoms and observations of cometary hydrogen.

3.3 Dynamics of Dust Tails

At nucleocentric distances beyond about 103 km, radiation pressure and solar gravity determine the motion of cometary dust particles. The solar gravity force can be expressed as:

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