Isotopic Composition Measured In-Situ in Different Solar
Wind Regimes by CELIAS/MTOF on board SOHO
R. Kallenbach
International Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland
Abstract. The Sun is the largest reservoir representing the matter of the early solar nebula. Its isotopic
composition for the elements N, Ne, Mg, Si, Ar, and Fe has been determined by measuring solar wind abundances
with the CELIAS experiment on board SOHO and other spacecraft instruments. These measurements and
theoretical considerations indicate that the solar wind, in particular the coronal-hole type solar wind, is much
less isotopically fractionated than solar energetic particles. The data give evidence that the isotopic abundance
ratios typically vary by only ~ 1 - 2% per amu in different solar wind regimes such as coronal-hole type and
streamer-belt associated solar wind, ejecta of coronal mass ejections (CMEs), or so-called 'blobs'.
INTRODUCTION
The elemental and isotopic composition of solar system
samples is usually compared to the composition of terrestrial material as a reference. However, due to the distribution of mass, any non-nuclear fractionation process
during the formation of the inner solar system from the
early solar nebula could have had impact on the terrestrial composition, but practically not on the solar composition. The relevant fractionation processes are related to
the typical size-scale of the dominating bodies present in
the protosolar disk at different times [1]:
1. The first /mi- to mm-sized solids, such as dust
grains, chondrules, and chondrites, found today in
meteoritical inclusions, formed within about ten
million years in the protosolar disk. Due to heterogeneities in pressure and temperature or because
of the interaction of the accretion flow with the
magnetosphere of the young Sun, isotopic heterogeneities have been created by reprocessing and
redistribution of solids. These heterogeneities are
much less pronounced for refractory elements than
for volatiles. Isotopic fractionation of the total matter in the protosolar disk with respect to the molecular cloud is assumed to be minor. The protosolar
disk collapsed from the molecular cloud within only
a few hundred thousand years. Most of the disk accretion occured in even shorter episodes of a few ten
thousand years with rates up to 10~5 M© y"1.
2. The planetesimals, up to several hundred km in size,
accreted from smaller bodies and differentiated into
core, mantle, and crust within one to two million
years. The differentiation has not influenced very
much the mean composition of the planetesimals,
which is basically given by the assemblage of chondritic types.
3. The final accretion of planets, hundreds to thousands of kilometers in size, occurred by collisions
between planetesimals and 'planetary embryos' on
timescales often to hundred million years. Impacts
play a crucial role in the formation of the terrestrial planets' atmospheres as they trigger losses of
volatiles. Additionally, hydrodynamic escape continued during later phases of the inner solar system's
history.
This scenario creates Earth with an isotopic composition
of refractory elements similar to that of the Sun, but with
compositional deviations for the volatile isotopes.
The goal of this summary is to constrain isotopic fractionation processes in the inner corona to infer the solar
isotopic composition from solar wind isotopic abundance
ratios. The variations of these ratios have been observed
in different types of solar wind streams such as streamerbelt ('slow'), coronal-hole associated ('fast'), and CMErelated solar wind, as well as in 'blobs', which are radially elongated structures of high density, forming near
the heliospheric current sheet at the tip of helmet streamers beyond heliocentric distances of ~2.5 R® and passively tracing the outflow of the 'slow' solar wind [2].
The isotopic abundance variations in the solar wind have
been compared to theoretical considerations on variable
Coulomb friction and wave heating in the solar wind,
and on gravitational settling in closed coronal magnetic
structures [3].
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
113
24
Mg/ 26 Mg vs. Solar Wind Bulk Velocity
SOHO/CELIAS/MTOF
50HO CELIAS MTOF
|.|1.04H
«*
£§1.00-
95%-confidence limits
day 21 of 1996-day 115 of 1997
450
500
550
Solar Wind Bulk Velocity [kmIs]
FIGURE 2. Solar wind 24Mg/26Mg ratio divided by the terrestrial 24Mg/26Mg value versus the solar wind bulk speed. A
trend can be seen in the sense that the heavier isotope is depleted by a few percents in the slow solar wind. Data from [7].
FIGURE 1. Schematic view of the MTOF sensor. Highly
charged solar wind ions enter the instrument through the socalled WAVE (Wide Angle Variable Energy) entrance system
that has an energy-per-charge (E/q) acceptance bandwidth of
about half a decade, and a conic field of view of ±25° width.
When passing through the thin (~3 jug cm~2) carbon foil of the
V-shaped 'VMASS' spectrometer, the ions release secondary
electrons to trigger a start signal on a micro channel plate
detector and exchange charge. The fractions of charge states
have been calibrated for 16 solar wind elements as a function
of their speed when leaving the foil [4]. Typically, the ions
leave the foil as neutrals and singly charged, but with increasing
speed also multiply charged. After a time of flight proportional
to <\/M/Q* (M: mass, g*: charge after passage through the
carbon foil), the ions hit a position sensing stop detector.
15N/HN ratio rgj because of the interference of 30Si2+
with 15N+ in the VMASS and because of the very low
abundance of 15N in the solar wind. Precise measurements have been made for Mg, as the minor isotopes
25
Mg and 26Mg are fairly abundant in the solar wind and
do not suffer interferences in the VMASS. Therefore, the
Mg isotopic abundances were mainly used to constrain
fractionation processes in the inner corona.
OBSERVATIONS
INSTRUMENTATION
Figure 2 shows the solar wind 24Mg/26Mg ratio divided
by the terrestrial 24Mg/26Mg value versus the solar wind
bulk speed [7]. Time-of-flight spectra of the time period
from day 21 in 1996 to day 115 in 1997 have been filtered to restrict the instrumental isotopic fractionation
to at most ±15%. As mentioned above, this procedure
leads to 24Mg/26Mg ratios with an absolute experimental uncertainty of about 3%. Evaluating the variation of
24
Mg/26Mg with solar wind speed, the experimental uncertainty is mainly statistical. Most of the instrumental
fractionation arises from the energy-per-charge (E/q) analyzer WAVE. Its voltage U is stepped to cover the E/q
range of all solar wind ions for any bulk speed. For longtime accumulations with variable solar wind speeds the
measured data are spread in a similar way into the different classes of instrument fractionation as the ion optics of the WAVE only depend on E/(qU). Thus, systematic instrumental uncertainties cancel out to some extent. The data suggest a variation of the 24Mg/26Mg ratio as a function of solar wind bulk speed with a slope
of (-2.7±2.0)x 10~5s/amu/km. Similar trends have been
observed for the solar wind Ne and Si isotopes [6, 9].
The data reported in this work mainly originate from
the Charge, Element, and Isotope Analysis System
(CELIAS) [5] on board SOHO, and in particular from the
MTOF sensor (Figure 1). MTOF is an isochronous timeof-flight mass spectrometer with a resolution M/AM of
about 100. It provides the possibility of resolving the
different isotopes in the mass range 3 to 60 amu of
the elements in the solar wind at bulk speeds of 300 to
1000 km/s. Mass spectra, instrument settings, and solar
wind parameters from the proton monitor, a subsystem
of MTOF, are available every five minutes. Therefore,
the flight data can reliably be classified according to solar wind parameters, detection efficiencies of the timeof-flight (VMASS) spectrometer, and the ion optical instrument discrimination of the entrance system. Besides
statistical uncertainties, the isotopic abundance measurements have a precision of ~1.5% per amu in the mass
range above 20 amu, as evaluated in detail for the Ne isotopes [6]. Additional uncertainties arise for some isotopic
species due to interferences with isotopes that have the
same time of flight, i.e. the same M/Q*9 in the VMASS
spectrometer. In particular, it was difficult to evaluate the
114
ization Potential (FIP) effect [10]. Low-FIP elements are
enriched by up to a factor four in the streamer-belt solar wind but to much less extent in the coronal-hole type
solar wind. Furthermore, streamer-belt solar wind has
typically higher density, lower temperature, and a lower
4
He/1H flux ratio than coronal-hole type solar wind. The
4
He/]H ratio may vary from typically ~0.05 in the 'fast'
solar wind to ~0.02 in the 'slow' solar wind, because
in the strongly superradially expanding source regions of
the 'slow' wind with a rapidly decreasing flux density of
the protons, the Coulomb friction with the protons is less
efficient in dragging away the 4He2+ ions from the solar surface than in the 'fast' wind [11]. Approximately
the same variation in the solar wind 24Mg/26Mg ratio has
been observed in correlation to the 4He/]H ratio as in
correlation to the bulk speed [12].
Recently, time series on the solar wind abundance ratios at 1 AU of low- and high-FIP elements have become available, such as Fe/O from the Charge Time-ofFlight (CTOF) sensor of the CELIAS experiment [13]
and Mg/O from MTOF (this work). Figure 3 shows preliminary results from a particular time period in May and
June 1996 on the correlation of the 24Mg/26Mg ratio with
several solar wind parameters. The solar wind has been
divided into two classes, one with rather high Mg/O ratio, higher density, but lower speed and temperature, and
one with the opposite characteristics. At least one of the
parameters shows a step-like transition when switching
from one class to the other. For each of the time intervals the average isotopic abundance ratio 24Mg/26Mg
is shown. Unlike for Figure 2, the instrumental uncertainties do not cancel out because no data with large instrument fractionation (more than ±15%) have been discarded, and because solar wind and instrument parameters are not evenly distributed in E/(qU) for the short
time period evaluated.
Nonetheless, the data of Figure 3 confirm the result of
Figure 2 that there is a fractionation of about 1.5% per
amu, depleting the heavier isotopes in the streamer-belt
solar wind relative to their abundance in the coronal-hole
associated solar wind. Similar results have been obtained
when correlating 24Mg/26Mg with the Fe/O elemental
abundance ratio in the solar wind [14].
The May 29 data show a particular structure with very
high density resembling the so-called 'blobs' [2], which
have initial sizes of ~1 R® in the radial direction and
0.1 RQ in the transverse direction. For this structure, the
MTOF data show low Mg/O, low speed, and low temperature. The very low 26Mg/24Mg ratio may indicate
that the 'blobs' are released from gravitationally stratified helmet streamers. It is necessary to search the full
MTOF data set to possibly identify more of these structures and to exclude them from the analysis.
The CME ejecta belong to a similar class of special
plasma. Isotopic fractionation of up to a factor 2 - 3 has
Days in May and June 1996
I
10.2
*J
"fast" I
os
.
8
w
8-
.
|
O)
OJ
J
M
I
i
+
J--^±
I
,
"slow"
I
,
T
-H————— ~"
1
i-
6-
i
^
Mg/26Mg:
Fast
Solar Wind: 7.21 ±0.1 5
4Slow Solar Wind: 7.69 + 0.28
. Terrestrial Value: 7.1 7
-
S4
,
24
1 2- Isotopic Fractionation: {3.3+2.8)% per amu
g
CD
0
CO
- SOHO/CELIAS/MTOF
0-
May 10 -June 8, 1996
•
I
i
I
i
0
0.04
0.08
I
0.12
i
I
0.16
i
0.
Solar Wind Mg/O Abundance Ratio
FIGURE 3. Time series of solar wind speed, density, and
temperature. The Mg/O ratio and the 24Mg/26Mg ratio are
averaged over time periods during which the solar wind was
either of 'slow' or of 'fast' type. The width of the center lines
for the Mg/O and the 24Mg/26Mg data indicate the statistical
uncertainty, and the additional lines show the uncertainty in the
instrument efficiencies.
This result has been cross-checked by analyzing the
correlation of the variation in the 24Mg/26Mg isotopic
ratio with other indicators to distinguish streamer-belt
('slow') and coronal-hole type ('fast') solar wind. A better indicator than the solar wind speed is the First lon-
115
TABLE 1. Average solar wind isotopic abundance ratios from in-situ spacecraft measurements on board SOHO,
WIND, and ULYSSES, compared to solar system values
[15]. If not noted otherwise, values are measured with
SOHO/CELIAS/MTOF [3]. For the 3He/4He ratio the values [16] for the 'fast' (f) and the 'slow' (s) solar wind are
given separately, as the fractionation appears to be much
larger than the experimental uncertainty. See also the review, Ref. [17].
Ratio
3
4
s
He/ He( )
He/4He(f)
15
N/ 14 N
3
180/160
21
Ne/20Ne
Ne/ 20 Ne
25
Mg/24Mg
26
Mg/24Mg
29
Si/28Si
30
Si/28Si
22
36Ar/38Ar
42
Ca/40Ca
Ca/40Ca
44
54Fe/56pe
Solar wind
Solar System
0.00041 ±0.000025
0.00033±0.000027
0.005±0.0014
0.0022±0.0006[18]
0.0023±0.0006
0.0728±0.0013
0.1260d=0.0014
0.1380±0.0031
0.03344±0.00024
0.05012±0.00072
0.183±0.008[19]
0.00657±0.00017
0.0209±0.0011
0.000488
0.000488
0.00368
0.0020
0.0024±0.0003
0.073±0.00007
0.12658
0.13947
0.033612
0.050634
0.1880
0.006621
0.021208
0.063236
°-085-0 022 t2°]
is based on the idea that the protons, and the alphaparticles, are the fastest and dominant species in the hot
corona and drag the heavier elements away from the solar
surface. The parameter
2Aj-Zj-l
Z?
orders the fractionation strength of elements and isotopes depending on their atomic mass number AI and
charge number Z/ in the solar wind [25]. The strongest
fractionation occurs for 4He2+ and 3He2+, which is
taken to calibrate typical fractionation strengths for other
ions from observations on the solar wind 4He/3He ratio [25]. These observations constrain the typical ratio (4He/3He)s ratio in the 'slow' wind to be larger
than 0.75 times the typical ratio (4He/3He)f in the 'fast'
wind. This leads to typical fractionation strengths of
( 13 C/ 12 C) S / (13C/12C)f = 0.984, (15N/14N)S/ (15N/14N)f
= 0.985, (18O/16O)S/ (18O/16O)f = 0.969, (22Ne/20Ne)s /
(22Ne/20Ne)f = 0.981, (26Mg/24Mg)s / (26Mg/24Mg)f 0.989, (30Si/28Si)s / (30Si/28Si)f = 0.992, and (34S/32S)S /
(34S/32S)f - 0.989, comparing 'slow' and 'fast' wind
with about equal temperatures of heavy ions in the 'slow'
wind (see Tab. 4 of Ref. [25]; their results are not very
sensitive to the ion temperatures in the 'fast' wind).
This appears to be compatible with the measurements reported above.
Wave heating in a strong form, ion cyclotron damping of Alfven waves, has been observed with the Ultraviolet Coronal Spectrograph (UVCS) in the 'fast' solar
wind [26]. As lower frequencies dominate the Alfven
wave spectrum, ions with low Z///4/ such as O5+ are
heated more strongly than ions with higher Z/A4, ratio
such as protons. Therefore, heavier elements are sufficiently heated to leave the solar surface without help of
the Coulomb drag. In the steady flow of the fast solar
wind not much fractionation of any kind in the total flux
of elemental or isotopic species is expected or observed,
e.g. in form of the FIP effect [27], so that wave-heating
mainly has influence on the temperatures of ion species.
At 1 AU the temperatures are observed to be proportional
to the ion mass [28]. The effect of the ion temperatures
on the fractionation due to variable Coulomb friction has
been evaluated [25]. Isothermal ion species experience
the fractionation summarized above, whereas ion species
with temperatures proportional to the ion mass experience stronger fractionation in the 'slow' solar wind and
still fairly weak fractionation in the 'fast' wind. In the
source regions of the streamer-belt ('slow') solar wind
the species appear to be rather isothermal [29, 30].
Gravitational settling in closed coronal magnetic
loops, however, may cause quite strong depletions or enrichments of heavy isotopes in CMEs and in 'blobs', if
statically stratified closed loop material is fed to the solar
been observed in solar energetic particles associated with
the November 1997 events [21]. Data from ACE/SWIMS
have constrained the isotopic fractionation in the bulk
solar wind during the same events to less than 5% per
amu [22]. This implies that the energetic particles are
fractionatied by injection or acceleration mechanisms.
However, as the isotopic composition in CME ejecta
has not yet been studied rigorously, any identified CMErelated solar wind has been excluded from the MTOF
data analysis. It remains uncertain, though, how many
'blobs' have contributed to the analysis. Despite this uncertainty, the solar wind isotopic composition of elements heavier than ~20 amu can be identified with the
solar isotopic composition within ~ 1% precision. Table I
summarizes the solar wind isotopic composition measured by spacecraft instruments. Note, that concordant
values for the He and Ne isotopic composition of the solar wind have been measured before with the Apollo Solar Wind foil Collection (SWC) experiment [23].
THEORETICAL CONSIDERATIONS
As potential processes that fractionate isotopes in the
solar wind, variable Coulomb friction and wave heating
in the inner corona, as well as gravitational settling in
closed magnetic coronal loops are discussed.
Variable Coulomb friction in the inner corona has
been studied in detail [24, 25]. The fractionation model
116
wind. In hydrostatic and thermal equilibrium in a coronal loop, the pressure of a species with atomic mass M
scales as [31]
r\
0
with the scale height AQ,M = 5QT/M in meters, T the
temperature in K, r0 = 6.96 x 108 m the solar radius,
TO the closest distance from the center of the Sun where
hydrostatic and thermal equilibrium is reached, and r\
the typical height of the loop. This can be rewritten as
"5
Pipo
with r$ the temperature in MK and p = r/r0. For example, in the core of a helmet streamer observed with the
UVCS spectrometer on board SOHO, at pi — 1 .7 and at a
measured temperature 7$ « 1.3, the elemental abundance
of O was reduced by one order of magnitude compared to
its typical 'slow' solar wind abundance [32]. This means
Pi - Po ~ 0.04. A fairly strong depletion of the heavier
isotopes of Ne, Mg, and Si of up to 20% per amu may be
associated with the largest coronal loops. However, the
effect on the mean isotopic abundance ratio in the 'slow'
solar wind is weaker because not all of the 'slow' solar
wind is fed by closed loop material. Furthermore, the elements Ne, Mg, and Si are presumably also depleted in
these 'blobs' so that their contribution to the long-time
averages of the 'slow' solar wind isotopic abundances remains small. Assuming an upper limit of 20% of 'slow'
solar wind flux originating from closed coronal magnetic
field structures an upper limit of 0.5% per amu is estimated here for the depletion of the heavier isotopes of
Ne, Mg, and Si in the 'slow' solar wind. Coronal mass
ejections may carry material, which either is depleted,
enriched, or not fractionated in heavy isotopes, depending on the altitude of the material in the pre-CME loop,
and on the temperature profile and other physical properties of the loop. In any case, it is wise to exclude the
CME-related solar wind from the analysis during the recently launched GENESIS mission.
Ne isotopes in Earth's atmosphere are strongly fractionated - most likely due to hydrodynamic escape - with
respect to solar system (solar wind) Ne isotopes. The differences between the solar wind 4He/3He ratios and the
solar system 4He/3He ratio [15] are well explained by
processes in the interior of the Sun and by fractionation
through variable Coulomb friction in the source regions
of the solar wind [16].
However, the solar wind 15N/14N ratio of this work,
although rather uncertain, needs to be discussed in more
detail. The value derived from MTOF data is compatible
with measurements on young lunar soils [33] reflecting
the solar N/Ar elemental abundance ratio and with the
15
N/14N ratio in Earth's atmosphere. It is also compatible with the N isotopic composition in very pristine solar
system material, namely in chondrites [34, 35], where
'heavy' N is associated with Ne of solar isotopic composition. It deviates from the values measured in lunar
ilmenites [36] and in the Jovian troposphere [37]. A recent publication [38] suggests a protosolar isotopic ratio
15N/14N = 3<1+o.5 x 1Q-3? inferred from HCN in comet
Hale-Bopp [39]. Although the latter value and the value
of this work, 15N/14N = (5 ± 1.4) x 10~3, are quite different, their la-margins still overlap. In the past, it was
discussed that recent solar wind N may not reflect the N
in the early solar nebula because 15N may be enriched
due to spallation of 16O by energetic flare protons in the
solar atmosphere [40]. The solar wind 7Li/6Li ratio inferred from measurements on lunar samples [41] indicates a sufficient energetic proton flux to produce significant amounts of spallation 15N over the Sun's lifetime. On the other hand, this appears to be incompatible with observed amounts of H B from spallation [42].
It should also be mentioned that 15N is depleted in the
Sun's interior because of the nuclear CNO cycle, but it is
assumed that this depletion is not observable at the Sun's
surface due to insufficient diffusion [42]. As predicted in
the SOHO/CELIAS proposal, the present-day solar wind
15
N/14N ratio cannot be determined unambigously from
MTOF data, so that the discussion may remain open until a more precise value is available from the GENESIS
mission.
CONCLUSIONS
DISCUSSION
The data in Table I demonstrate that the solar isotopic
composition inferred from the in-situ solar wind measurements, averaged over long-time periods but excluding CME events, agrees well, except for He, with the solar system isotopic composition inferred from meteorites
and planetary samples [15]. The values for the Ne isotopic composition in Ref. [15] already include the Apollo
SWC result [23], confirmed by MTOF data [6], that the
117
The solar wind, in particular the coronal-hole associated
solar wind, is an authentic witness of the isotopic composition of the early solar nebula for elements not influenced by processes in the Sun's interior. The interpretation of in-situ measurements of solar wind abundances
is free of the pecularities to interprete abundances derived from planetary and meteoritic samples. Present insitu solar wind measurements are limited by instrumental
uncertainties. To identify Galactic nuclear processes, the
precision of in-situ measurements should be increased to
the permil level. This is planned for the GENESIS mission. However, pre-cautions must be taken to eliminate
biases due to isotopic fractionation in the solar wind by
recording its parameters.
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ACKNOWLEDGMENTS
The experimental work was supported by the Swiss National Science Foundation, by the PRODEX programme
of ESA, by NASA grant NAG5-2754, and by DARA,
Germany, with grants 50 OC 89056 and 50 OC 9605.
The MTOF sensor was developed in the CELIAS consortium [5], in a project led by the University of Maryland
space physics group. The WAVE entrance system was
built under the guidance of the Physics Institute of the
University of Bern at INTEC, Bern. The integrated sensor and the charge exchange of solar wind ions in thin
carbon foils were calibrated at the University of Bern, at
the Strahlenzentrum of the University of Giessen, Germany, and at the Centre d'Etudes Nucleaires de Grenoble, France. The Technical University of Braunschweig,
Germany, contributed the Data Processing Unit.
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