Astron. Astrophys. 344, 317-321 (1999)
We have investigated the effect of the electron impact ionization
on the filtration of the interstellar oxygen in the heliospheric
interface. On the basis of the investigation we conclude:
Electron impact ionization acts on the interstellar oxygen
penetrating in the heliosphere and leads to non-negligible additional
filtration of the oxygen atoms in the region of the compressed and
heated solar wind.
In our model, about 70% of interstellar oxygen penetrate in the
heliosphere through the heliospheric interface. For the same solar and
interstellar parameters only 47.5% of interstellar hydrogen penetrate
in the heliosphere. Thus we conclude that the heliospheric relative
abundance of OI to HI is larger than the interstellar abundance.
As it has been discussed in Sect. 3, the electron impact ionization
leads to the increase of the number density of hot energetic neutrals
(population 2) in the compressed solar wind (comparison of curves 1
and 2 in Fig. 2c). This is a consequence of the reverse
charge-exchange reaction ( and could
be important for the heliopause mapping in the oxygen ion
resonance line recently proposed by
Gruntman & Fahr (1998).
Only 30% of interstellar O atoms in the heliosphere are secondary
atoms, while for hydrogen they are about 70%. These secondary
interstellar atoms are created in the region between the BS and the TS
by charge-exchange with the heated and stopped interstellar protons.
The secondary atoms have a smaller velocity and a larger temperature
compared with the primary atoms. As a consequence, in the heliosphere,
interstellar oxygen atoms have a larger temperature and a smaller
velocity than interstellar hydrogen atoms.
The comparison of the HST-GHRS spectroscopic data of Capella
(Linsky et al., 1995) with heliospheric pick-up ion measurements shows
a rather good agreement between data and theory. However, the accuracy
of the filtration factor ratio derived from the experimental data is
still too uncertain to allow a determination of the interstellar
plasma density in an independent way.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999