Sol88
Philosopher
- Joined
- Mar 23, 2009
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The Plasma Environment of Comet 67P/Churyumov-Gerasimenko
JD116, would you like to have a rethink on your position of a diamagnetic cavity caused by expanding gas?
Double layer....
From wake effects it can also be deduced that the bulk plasma flow is radial and supersonic. An ion velocity higher than the neutral velocity is in direct contradiction with the assumption that ions and neutrals are collisionally coupled
and thus in direct contradiction with the model suggested by Cravens (1987).
Vigren and
Eriksson (2017) showed that such a discrepancy can be explained by low collision rates due to low neutral gas densities and propose that an ambipolar electric field (see also Sect. 4.1) can accelerate ions to the observed velocities.
Fuselier et al. (2016) also pointed out that the unexpected H2O+ /H3O+ ratio can be explained by an ambipolar field inside the diamagnetic cavity. It should also be noted that the definition of the collisionopause as a sharp boundary that separates a region where neutrals and ions are coupled from a region where they are not coupled is inappropriate, since collisions are a process that occurs on length scales comparable with the diamagnetic cavity size (Vigren and Eriksson 2019).
As of now, it is unclear which process is responsible for deterring the magnetic field from diffusing into the diamagnetic cavity.
JD116, would you like to have a rethink on your position of a diamagnetic cavity caused by expanding gas?
Double layer....
The same ambipolar field that accelerates the ions retards the electrons and couples them more efficiently to the neutral gas via collisions
Whereas solar wind electrons are typically warm (around 10 eV),at large heliocentric distances (low outgassing) they are accelerated by an ambipolar electric field in the vicinity of the nucleus and reach energies of a few tens of eV responsible for ionisation and excitation of the cometary neutrals.
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