Atoms, molecules, and atomic and molecular ions generated by solar wind interactions with dust grains of the zodiacal cloud are discussed. The main processes leading to the generation of these particles are described and relevant methods of observation are examined

Ripken, H. W.

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MINORCONSTITUENTSIN THESOLARWINDORIGINATINGFROM
PLASMA-DUSTINTERACTIONS
Hartmut W. Ripken
Institut fur Astrophysik und Extraterrestrische Forschung
Universit_t Bonn
Auf demHHgel 71
5300 Bonn i, Fed. Rep. Germany
It has long been known that high-energy keV neutral hydrogen and helium
atoms are regular but very minor constituents of the solar wind (see, e.g.,
Gabriel, 1971; Holzer, 1977; Withbroe et al., 1982; and references therein).
Ionization processes acting on these constituents in interplanetary space,
mainly charge-exchange reactions, yield protons and singly ionized helium.
While these "solar-wind-originating" minor constituents H, He and He+ are
generally taken into consideration in model calculations, as well as the
heavy multiply-charged ions of coronal origin, a second class of minor
constituents needs to be recalled and accounted for. This group of atoms,
molecules, and atomic and molecular ions is generated by solar wind interac-
tions with dust grains of the zodiacal cloud (Banks, 1971; Paresce and
Bowyer, 1973; Holzer, 1977; Fahr et al., 1981). The main processes leading to
the generation of these particles will briefly be described in the first part
of this comment, while relevant methods of observation are discussed in the
second part. A detailed analysis is given in Fahr et al. (1981).
Solar wind ions with energies in the keV range impinge on zodiacal dust
grain surfaces and penetrate about i0 to i00 nm into the mineral. At the end
of the particle trajectory, neutralization via electron capture will occur.
The newly created neutrals are temporarily retained in the dust grain,
undergoing diffusion processes and chemical reactions. Depending on the type
of mineral, hydrogen atoms react to form OH, H_O, H_, and various carbon and
silicon hydrides Helium, of course, Will stay L"• .... in i_s atomlc form. The dust
grain cannot retain implanted particles indefinitely; saturation occurs on
very short time scales compared to the typical residence times of the grains
in a solar orbit. After saturation is reached, on the average one molecule
or atom will be released from the dust grain surface for each impinging solar
wind ion. For particles released from the surface by an evaporative process,
the dynamics essentially are the same as those of zodiacal dust grains and
can be calculated in an analogous manner. These particles are subject to
strong interaction processes with the solar wind and with the solar radiation
field. Most become dissociated and/or ionized and eventually form the singly-
+
charged secondary solar wind constituents H+, He + and H2 . In addition
singly-charged hydroxyl, carbon and silicon hydrides, and possibly other ions
are formed in this process, depending on the type of parent grain. Only very
few dust-generated neutrals will reach the orbit of the Earth as low-energy
neutral particles.
Besides these surface-released particles, the processes of erosion, sput-
tering, vaporization, and sublimation of zodiacal dust grains generate atoms
and molecules not containing original solar wind material, which in their
singly-charged ionic forms subsequently are also incorporated into the solar
wind and can be traced as very minor but heavy constituents. The most
important ones should be ions of silicon and carbon compounds.
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https://ntrs.nasa.gov/search.jsp?R=19840005048 2020-03-22T07:43:22+00:00Z
In principle, these solar wind-incorporated ions can be detected in-situ
by mass spectrometers. To identify the dust-generated secondary ions in the
background of the primary solar wind, the charge state of the heavy ions will
be the most important factor. Ionization cross sections indicate that singly-
charged ions incorporated into the solar wind flow will predominantly stay in
this initial charge state. Since the zodiacal cloud source region is
strongly compressed into the ecliptic plane (Figure i), the secondary ion
flow markedly decreases with heliographic latitude. If direct detection of
the minor ions in the ecliptic plane can be accomplished, the spatial
variation of this constituent monitored by spacecraft reaching higher helio-
graphic latitudes (e.g. ISPM) would yield important information on the
spatial distribution of zodiacal dust.
ZODIACAL DUST
0.5
R (AU)
-0.4 0.5
. 2.0 _1. 0.8 1.20.2_..._&_O4/'0.6 0 R(AU)/
Figure i. Isodensity lines of interplanetary zodiacal dust, according
to a model derived from Helios measurements (Leinert et al., 1978).
Complementary observations of dust-deionized atomic hydrogen and helium
can be accomplished by monitoring resonantly scattered solar photons at
121.6 nm (hydrogen) and 58.4 nm (helium). Due to sharp intensity increases of
this resonance luminescence radiation with decreasing solar impact altitudes
of the line-of-sight brought about by both density increases of the zodiacal
cloud and increasing solar illumination with decreasing solar distances,
observations are most promising in the immediate solar vicinity (line-of-
sight solar offset angles =6°). For details see Fahr et al. (1981). Measure-
ments of this EUV radiation are planned (Fahr et al., 1980), and if
successful, would unambiguously point towards a relatively high dust density
and thus to the actual existence of singly-charged minor ions in the solar
wind which have not yet been reported in the literature. Possibly, some
unexplained features in recent solar wind mass spectrograms could be reinter-
preted taking into account the dust-generated ions.
644
References
Banks, P.M., Interplanetary hydrogen and helium from cosmic dust and the
solar wind, J. GeophFs. Res., 76, 4341, 1971.
Fahr, H.J., H.W. Ripken, and G. Lay, Inter-Zodiak: A high-apogee rocket
experiment for the observation of dust-generated neutrals in the solar
vicinity, ESA SP-152, 449, 1980.
Fahr, H.J., H.W. Ripken, and G. Lay, Plasma-dust interactions in the solar
vicinity and their observational consequences, Astron. Astrophys., 102,
359, 1981.
Gabriel, A.H., Measurements of the Lyman-alpha corona, Solar Phys., 21, 392,
1971.
Holzer, T.E., Neutral hydrogen in interplanetary space, Rev. Geophys. Space
Phys., 4, 467, 1977.
Leinert, C., M. Hanner, and E. Pitz, On the spatial distribution of inter-
planetary dust near 1AU, Astron. Astrophys., 63, 183, 1978.
Paresce, F., and S. Bowyer, Resonance scattering from interstellar and inter-
planetary helium, Astron. Astrophys., 27, 399, 1973.
Withbroe, G.L., J.L. Kohl, and H. Weiser, Probing the solar wind acceleration
region using spectroscopic techniques, Space Sci. Rev., 33, 17, 1982.
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Minor constituents in the solar wind originating from plasma-dust interactions

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