The role of solar wind heavy ions in the space environment51
D. E. Shemansky
Department of Aerospace and Mechanical Engineering
University of Southern California
18 July 2002
Abstract
The physical processes in the impact of highly charged ions on gaseous or solid targets
appears not to have had the attention of the community involved in spacecraft development,
or of many others in the engineering community concerned with gas-solid impacts. Orbiting
spacecraft at altitudes above the magnetospheric bow-shock, spacecraft traveling through interplanetary space, and any solar system body without a substantial intrinsic magnetic field are
subjected to the impact of the solar wind flux. This is a highly variable phenomenon sourced by
magnetohydrodynamic turbulence in the solar corona at plasma temperatures generally above
106 K. The mean velocity of the outflowing solar wind is about 450 km s"1, but is quite variable, with values as high as 800 km s"1 appearing fairly frequently. The solar wind density at
1 AU is about 10 cm~3. The ions are dominated by protons, with the inclusion of about 4%
He++, and smaller mixing ratios of heavier ions. The effect of the heavy ions can be significant
relative to the protons in spite of low mixing ratios because of the larger kinetic energy of heavy
ions moving at the same velocity as the protons, and the substantial internal energy invested in
multiple charge states created in the solar corona. Solar wind He++, for example, is much more
effective for sputtering than protons.
The characteristics of the heavy stripped ions lead to the following conclusions: Multiple
charge capture near the solid surface leads to very efficient surface sputtering given that molecular ion recombination leads to dissociation with a probability very close to 1.0. The multiply
charged heavy ion can therefore be 10 to 1000 times more efficient per ion than light singly
charged ions in sputtering and damaging solid surfaces. In addition, the multiply charged ion
produces soft X-rays in the capture process, that can penetrate deeply into the solid to produce
multiple damaged sites caused by secondary photoelectrons.
1
Introduction
The assessment of the effects of the solar wind on both natural and artificial exposed bodies have
been mainly limited to consideration of the two most populous components, protons and alpha
particles. In the planetary science community, this limited consideration abruptly changed when
images of near-Earth comets obtained using the ROSAT Satellite [1] revealed halo images in Xray emission from the sun-side coma. Typically the emission appears roughly 50000 km from the
core of the comet as shown in Figure 1 [see 2]. X-ray emission from a cold object, having no
internal means of generating energy on this scale, came as a complete surprise to the comet science
community. The conclusion has now been drawn that the emission is dominated by discrete soft
X-rays generated by the successive capture of electrons from the neutral gas in the comet coma by
* Invited paper: 23rd International Symposium on Rarefied Gas Dynamics
CP663, Rarefied Gas Dynamics: 23rd International Symposium, edited by A. D. Ketsdever and E. P. Muntz
© 2003 American Institute of Physics 0-7354-0124-1/03/$20.00
687
the stripped heavy solar wind ions. This same process will take place whether the impacted object
is in the gaseous or solid state.
The codes TRIM and SRIM, widely used to assess the physical sputtering of ion impact on
solids, does not address the effects of multiply charged ions. It has been found more recently that
even H+ low energy impact on some surfaces (hydrogenated carbon) can produce erosion well above
the effects predicted by TRIM. The mechanism in the case of H+ impact, however, appears to be
basically different [3] from the process of highly charged ion impact described here.
The quantitative understanding of the physical process in highly charged ion reactions comes
mainly from the fusion physics community in work primarily from the late 1960's up to the present
[see 4, 5]. Charge capture reactions of solar wind ions of mass > 1 amu impacting a neutral particle
produce soft X-ray photon emission with high efficiency, and large cross sections.
II
Figure 1: X-ray images of comets Hyakutake and 6P/d'Arrest, from [2]
The effect of multiply charged ion impact on a surface differs from singly charged ions in the
magnitude of the internal energy carried by the ion. The energy loss of the impacting ion is generally
described as three approximately separable components; nuclear, electronic, and exchange, as shown
here in (1).
SdE\
SdE\
SdE\
= SdE\
\dxJioss
\dxjnuc
\dxjeiect
\dxjexch
The exchange loss has been assumed negligible for singly charged ions. The concept of exchange
loss is that at some point in the penetration of the solid, the ion captures an electron from the
solid-state molecule, losing some small amount of kinetic energy. The neutralized impactor can
688
then be ionized again through subsequent reaction in the solid, given that it has sufficient kinetic
energy. The internal energy of the ion will be converted to a combination of energy delivered to
the ionization potential of the solid molecule, and photon emission by the neutralized ion. The
recombination of the ionized solid state molecule with an electron, in the case of semiconductors
or insulators, then produces solid state dissociation products in many cases with enough kinetic
energy to diffuse into the vacuum. The distinction of multiply charged ions is that they carry much
more internal energy than singly charged ions, and most importantly have much larger electron
capture cross sections. As a result, a penetrating multiply charged ion can capture several electrons
along a short path following penetration of the solid. The primary reaction in the capture process
with the greatest probability is a single electron capture from the target as shown here in (2).
Aq + B -> £+1 + Aq~l
(2)
where q is the positive charge on the solar wind ion, and B is the target atom or molecule.
Some reactions, particularly with multiply charged carbon ions, result in the capture of more
than one electron in a single capture encounter. The energy bound internally in a multiply charged
ion is much larger than the energy required for the ionization of the target. The energy released in
the reaction must go somewhere, and the captured electron enters a high level excited state in the
ion, delivering most of the energy to the emission of photons. The following two examples illustrate
the energy budget. We use the ionization potential of H^O as a place- keeper in this case for the
reactions (3) and (4),
54.4 eV + Q
E(He+(2p))
a(He+(1p))
12.61 + 0 eV
= 41.79 eV
= 40.81 eV
w 34 au
552.074 eV + Q
E(N*+(5p))
E(N*+(6p))
a(7V5+(6p))
(3)
12.61+0 eV
=
=
=
w
539.46 eV
532.27 eV
538.35 eV
70 au
(4)
where the capture cross section is designated by a in units of the area of the first Bohr orbit. Some
momentum transfer takes place in the charge capture reactions, and excess energy in the capture
is delivered in this way. In (3) the He+(2p) ion radiates promptly in the 304 A line. In (4) the
N5+ (np) ions radiate several cascaded line emissions with the most energetic lines near 23 A in the
soft X-ray region. N4+ energetic transitions are near 128 A, illustrating the general trend toward
softer emission lines as the charge state declines.
The very large cross sections for the electron capture process raise the probability that solid
molecules will be stripped of electrons very close to the surface layer. Compared to the heavy
stripped ion capture cross sections, proton capture cross sections are at least 3 orders of magnitude
smaller, and for the higher impact energies, effective electron capture cross sections for protons can
be as much as 5 orders of magnitude smaller than the heavy stripped ions.
689
I
I
I
I
I
I
I
ORBIT NUMBER
————
SOLAR WIND MONTHLY AVERAGE FLUX
i
si
SOLAR WIND YEARLY AVER AGE FLUX
§
«&
———— SOLAR LYMAN-o
X
PV UPWIND INTENSITIES - LINE CENTER
————
MONTHLY AVERAGE SOLAR WIND VELOCITY
_._
YEARLY AVERAGE SOLAR WIND VELOCITY
I/I
"E
6
as
SOLAR CYCLE
-20
- MAX
I
:
MARINER X
(360, 100 R)
OG05
(500,275 R)
I T.
1969 70
I
i I
71
72
73
74
75
SOLAR CYCLE
21
START AND WIN
SOLAR
CYCLE 21
MAX
.i, . , . , . 1
76 77 78 79 80
81
82
83
84
85
86
EARTH YEAR
Figure 2: Variation of solar emissions over the period 1969 to 1986. The plots show the flux of
protons and H Lya and solar wind velocity for various averaging periods. After [6].
2
Solar wind properties
The solar wind is a highly variable phenomenon sourced by magnetohydrodynamic turbulence in
the solar corona at plasma temperatures generally above 106 K. The mean velocity of the outflowing
solar wind is about 450 km s"1, but is quite variable, with values as high as 800 km s"1 appearing
fairly frequently. Figure 2 shows a plot of the solar wind flux on the major solar cycle time scale,
in comparison with the H Lya line, indicating that these quantities are out of phase. Note that
as a monthly averaged quantity, in 1982 the solar wind flux was more than a factor of 2 above the
long term mean. It is possible to have episodic events in which the solar wind flux is an order of
magnitude larger than the long term mean.
The solar wind density at 1 AU is about 10 cm~3. The ions are dominated by protons, with the
inclusion of about 4% He++, and smaller mixing ratios of heavier ions. The effect of the heavy ions
can be significant relative to the protons in spite of low mixing ratios because of the larger kinetic
energy of heavy ions moving at the same velocity as the protons, and the substantial internal energy
invested in multiple charge states created in the solar corona. Solar wind He++, for example, is
much more effective for sputtering than protons. Table 1 gives the normal solar wind mixing ratios
for the most abundant ions, along with the charge states. The ion partitioning given in Table 1
is a nominal average. Measurements of the partitioning indicate the presence of substantial short
term variability.
690
Species
qa
X 104)6
a
He
2
400.
0
6-7
7.
Table 1: Solar wind partitioning
N
Mg
C
Ne
Si
S
4-6 8-9 5-6 8-12 9-10 8-10
1.
3.
3.
0.3
0.3 0.16
Fe
9-11
0.08
Ar
8-9
0.04
lon positive charge distribution
Scaled number density partitioning
6
3
The physical effects of stripped ion impacts
Although we do not have specific experimental measurements of substance, the basic properties of
stripped ion charge capture is expected to be dramatically different from singly charged ion impact,
particularly in sputtering yield. The difference lies in the relative contributions to the total energy
loss in the eq 1. The nuclear loss component for the singly or multiply charged ion will be roughly
equal. Multiply charged ions will show moderately higher loss to electronic excitation, but much
higher loss to charge capture effects. Within this last term of eq 1 the most important effect,
however, is not an issue of energy deposition, but the location of the charge capture events in the
solid sub-surface along the ion path. The illustrative reactions 3 and 4 indicate the general behavior
of the charge capture process. Conservation of energy forces the capture of the electron into high
principal quantum number levels. The very large capture cross sections typically of a magnitude
as given in reaction 4, indicates that electron capture will take place in the first molecular layer
of a semiconductor or insulator surface. The lifetime of the captured electron in the excited state
depends primarily on the orbital angular momentum quantum number, but for the major ions to
consider here, O9, C9, and Ne9, the lifetimes for spontaneous emission are in the range 10 - 50 ps.
This indicates that following electron capture, the subsequent emission of X-ray photons will take
place in the range reaching to the stopping depth of the source ion. The stopping range would
typically be 100 A - 1000 A below the solid surface. The photons released in this process would
then have a range of ~ 4/^m - 40 /^m in a semiconductor or insulator solid ( ~ 0.5 /^m in a metal)
[see 7] before conversion to a photoelectron. The photoelectron will then go on to produce multiple
secondary electrons. For an O+r impact on a solid such as SiO2 it is estimated that of order 100
ion pairs would be created, resulting in a combined total of roughly 300 damage sites and sputtered
atoms. The general physical process is then described as an impact of the ion resulting in multiple
electron captures within the first 10 A of the surface with subsequent damage to surrounding ~
300 sites in the range up to 40 /^m. Recombination of the near-surface sites that lost electrons to
the capture process would have a high probability of dissociative loss into the vacuum, with some
probable damage to adjacent sites.
3.1
Properties of the charge capture cross section
Figure 3 shows measured capture cross sections compared to calculations by the author using the
Classical Trajectory Monte Carlo (CTMC) theory [8] for N+5 captures of electrons into excited
levels of N+4, illustrating the general characteristics of these reactions. For N+5 the reaction
with atomic hydrogen is exothermic at all principal quantum numbers below n = 5 where the
reaction is close to energy resonance. The total capture cross section is dominated by the levels
near n = 5. The exothermic levels have essentially flat cross sections asymptotically toward zero
impact energy, from about 50 keV/amu. At higher principal quantum numbers the cross sections
are endothermic, with distinct energy onsets indicated by the beginning of the data points on the
691
N+5
102
• •*!••
• •**••
* ** ¥
10
•
10,-1
Total
n 2
n 3
n 4
n_5
n 6
n_7
n 8
n_9
Phaneuf
Crandall
Gargaud
A
•
<$>
<?
10,-2
0
A
D
O
10-
V
D
A
10
, A -.. i
io
r1
10°
1
10
2
10
103
E (keV/amu)
Figure 3: Charge capture cross sections for N+5 reacting with H, calculated using the (CTMC)
method. The cross sections are given as a function of impacting ion energy, integrated over orbital
angular momentum quantum number, L The total cross section is compared to three experimental
results, as indicated in the legend.
plots in Figure 3. Measured values indicated on the plot, show basically good agreement with the
theoretical calculations.
The emission spectrum from electron capture by the highly charged ions is completely different
from electron impact spectra because of the fact that the initial excitation of the electronic structure
in the case of charge capture takes place at high principal and orbital angular momentum quantum
numbers. In this latter case the spectrum is characterized by multiple cascaded emission transitions,
with very little direct excitation of the first excited electronic state. The photon emission products
of the charge capture process, therefore occur over a broad spectral range. Figure 4 shows the
emission distrbution of photons from O~*~6 for two kinetic energies of the impacting O~*~7 ion. An
impact energy of 0.8 keV/amu is typical of the solar wind. The high quantum number captures
directly produce the low energy discrete transitions shown on the plot, but these transitions cascade
down through the system to produce the strong emissions at the high end of the photon energy
scale. The impact of the ion on a semiconductor or insulator surface would produce a similar
emission spectrum but the capture process would tend to deliver excitation into higher electronic
states than those represented by the emissions shown in Figure 4.
692
O+7 + H -» O+6(nl) + H+
, i
o
•5
m3 IO 1U
————
————
,
i
+
O+6
(nl) -> 0
i i i i i i
,
,
hv
, , , ,i
,
j
830 eV/amu
8 eV/amu
0,
—
•
f- 102 -
-
1
i
|J
0
5 101 -
« HI
0)
c
o 10°Ec 10"1
+6
.
-
h
H^
-
1 L-i
:
[
_
-
:
i
BO
0
lO
i-
q
i
11
7 -jQ-2
[
i i E iH i j j
2
3
j
4 5 6 7 -jg-1
2
j
3
j
i
i i
4 5678-jQO
i
2
i
3
i
4
E (keV)
Figure 4: The emission spectrum of O+6 produced by charge capture by O+r ions is shown here
as a differential probability distribution function of photon energy in units of keV. The spectrum
is shown for two kinetic energies of the ion impacting the atomic hydrogen target, as indicated on
the plot. The heavy line shows the spectrum for a typical O+r ion kinetic energy in the solar wind.
See text.
4
Sputtering efficiencies of by solar wind impact on surfaces
Calculations of the sputtering efficiencies of the solar wind ions on a reference sample of leaded glass
are shown in Figure 5. The H + , He+, and O+ values are calculated using the TRIM [9] code. The
efficiency for O+r is calculated roughly on the basis of the physical description given above, using
estimated optical depth of the X-ray photons produced in the capture process, and an assumed
reciprocal efficiency of 25 eV per ion pair in the production of secondary electrons. The internal 2
keV/ion contained in the O+r ion relative to O+ at the same kinetic energy delivers a sputtering
efficiency enhancement of about two orders of magnitude. Over the range of impact energies
considered, the charge capture contribution to sputtering is constant, while the nuclear collisional
contribution becomes less efficient with increasing kinetic energy. Scaling to the partitioning of the
solar wind ions as given in Table 1, provides an estimate of the integrated competition between the
ion components in the sputtering of semiconductor or insulator surfaces, as shown in Figure 6. Both
O+r and Ne+9 components are more effective than H + , while C+6 shows comparable efficiency. In
this calculation the heavy solar wind ions in total produce about 1 order of magnitude more surface
sputtering than the dominant proton flux component.
693
102
5
3
o
2
O
13
5
3
2
a
.2
10°
5
3
Q.
10'1
5
3
CO
2
1
°00
1.0
2.0
E (keV/amu)
3.0
4.0
Figure 5: The calculated sputtering yields for the impact of ions on leaded glass. The yields for
O + , He+2, H + , are calculated using the TRIM code (see text). The yield for O+r is calculated on
the basis of the physical considerations described in the text.
"O
.+9
JS
o
cu
O)
He«
a,
CO
<D
oc
10
o.o
1.0
E (keV/amu)
3.0
4.0
Figure 6: The relative sputtering yields of solar wind ion impact on leaded glass, based on the
partitioning of the solar wind described in Table 1, and the computational methods described in
the text.
694
5
Discussion and Conclusions
The rough calculations of the impact of the stripped minor solar wind ions on insulators or semiconductors given here predicts that these components are responsible for most of the sputtering yield.
The physical considerations in these calculations are at a first order level. The surface considered
here is an illustrative case, and the reality of actual extended conditions in space can certainly
significantly alter rate processes, particularly when the solid state is involved. Factor of three systematic uncertainty in the efficiencies given here would not be surprising. A more stable quantity
would be the estimated total ionization activity in the solid, with spatial distribution rather more
uncertain. The basic point of this paper is, however, that the energy invested by the sun in stripping
the heavy solar wind components of their electrons, is deposited directly in to the sub-surface of the
impacted solid where it can efficiently ionize and dissociate the molecular structure. The process is
electron capture from the solid into high electronic states that promptly radiate discrete emission
over a range of wavelengths extending to the soft X-ray region. This radiation extends several /^m
thru the solid and produces photoelectrons that generate multiple ionized sites. The recombination
of these sites in the sub-surface then generate the dissociation products that can diffuse into the
vacuum. The physical processes can be more complex than those considered here, and the geometry
of the interaction is certainly in reality much more complex. Many of the effects not considered
in this simple calculation will tend to increase the efficiency of the sputtering and structural damage process. The calculations here are based on normal incidence impact on an unbroken surface.
A complex surface would tend to show more efficient sputtering losses. It was assumed that the
excited state of the captured electron was deactivated by radiation. In the dense solid, however,
radiationless deactivation by the bound electrons in the dense solid needs to be considered. An
estimate of the level of probability for this process is not available. If radiationless deactivation
takes place with significant efficiency, the secondary electron ionization process will occur generally
closer to the top layer of the solid, producing greater sputtering efficiency. A significant fraction of
the secondary electrons generated in the solid will have sufficient energy to diffuse into the vacuum.
The electron excess in the vacuum will tend to lead to a negatively charged surface with immobile
positively charged sites in the sub-surface, further complicating the consideration of the physical
processes in an electric field stressed solid.
Acknowledgment. This work has been supported by a contract with the Space Science Office
of the Lockheed Martin Advanced Technology Center, Palo Alto, CA.
6
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