SOLAR WIND IRON ISOTOPIC ABUNDANCES:
RESULTS FROM SOHO/CELIAS/MTOF
KM. Ipavich*, J.A. Paquette*, P. Bochsler1^, S.E. Lasley* and P. Wurzf
"Department of Physics, University of Maryland, College Park, MD, USA 20742
^ Physikalisches Institut der Universitdt Bern, Switzerland, CH3012
Abstract. The MTOF sensor uses time of flight measurements in a harmonic potential region to identify elements
and isotopes in the solar wind with excellent mass resolution. The combination of MTOF's large bandwidth
electrostatic deflection system and the 3-axis stabilized orientation of SOHO results in excellent counting
statistics. We report relative abundances of the iron isotopes with mass 54, 56 and 57 amu. Since these isotopes
are chemically identical, we expect little fractionation either in the solar wind or in the instrument, resulting in
relatively small estimated uncertainties. Our results agree, within the measurement uncertainties, with terrestrial
values.
INTRODUCTION
(CELIAS) Experiment [4] on the Solar Heliospheric Observatory (SOHO) spacecraft. CELIAS uses three timeof-flight (TOF) sensors to make composition measurements. MTOF has unprecedented mass resolution and
collection power for solar wind composition studies, and
can identify rare elements and isotopes that were previously not resolvable from more abundant neighboring
species, or were not previously observable at all.
Observations of rare elements and isotopes in the solar wind are important because they help determine how
neutral atoms in the relatively cool photosphere become
the highly ionized atoms in the corona and then ultimately the supersonic plasma we call the solar wind. The
results will also help establish the isotopic composition
of the primordial solar nebula.
The good agreement of photospheric and meteoritic
abundances [1] for a wide range of refractory and moderately volatile elements are consistent with a common
origin of solar and planetary matter. The Sun is considered to contain a largely unfractionated sample of matter from the protosolar nebula; hence knowledge of solar composition can reveal the composition of the local
interstellar medium some 4.6 Gy ago, and as well shed
light on the chemical and isotopic evolution of planetary
matter [2]. The solar wind composition is expected to
be that of the solar outer convective zone, differing only
as a result of possible fractionation processes. Elemental
composition is subject to the well-known fractionation
process ordered by the first ionization potential [3], reflecting the separation of neutrals and ions in the upper
chromosphere; the magnitude of this fractionation is of
order 3 or so in the slow solar wind. Isotopic composition is, however, not subject to this fractionation process;
typical isotopic fractionation is expected to be at the percent level.
The Mass Time-of-Flight Spectrometer (MTOF) is
one of four sensors (CTOF, MTOF, STOP, SEM) comprising the Charge, Element, Isotope Analysis System
INSTRUMENTATION
The Mass Time-Of-Flight (MTOF) sensor of the
CELIAS investigation is actually two subsensors. The
MTOF/Main is the primary unit, providing elemental
and isotopic abundance measurements of heavy ions
in the solar wind. The MTOF/Proton Monitor is an
auxiliary unit designed to measure the solar wind proton
parameters. Both units are housed within a common
structure which also contains the low voltage power
converter, the high voltage power supplies, the analog
electronics, and the digital electronics.
The MTOF Main Sensor
The MTOF Main Sensor (illustrated in Figure 1) is
a high-mass (M/AM w 100) resolution system that provides unprecedented solar wind composition data over a
wide range of solar wind conditions. The high sensitivity
of MTOF allows for the first time an accurate determi-
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
121
WAVE
E/Q
selection
SOLAR
SOLAE WIND
WIND ION
E,
E Q, M
techniques. MTOF also generates a 2 to 3 order of magnitude improvement in counting statistics, thanks to: (a)
the fact that SOHO always looks in the
the solar
solar direction
direction
(unlike the spinning WIND spacecraft);
spacecraft); and (b) a novel
entrance deflection system with a very wide bandwidth
(400% vs. the more typical 5% in other solar wind instruments), requiring only a few voltage steps to
to cover
cover the
the
entire energy-per-charge distribution of the solar wind.
As a consequence of the very wide deflection
deflection system
dispassband, MTOF cannot determine the charge state distribution of solar wind ions.
The MASS and MTOF sensors were designed
designed and
and
fabricated by the University of Maryland; their entrance
deflection systems were built by the
the University
University of
of Bern,
Bern,
which also provided the primary calibration facility
facility for
these instruments.
UV Trap
Carbon
Foil
CP
n
Io
M
e-
ns
Io
TOF α M/Q*
a
St
M
rt
CP
THE
THE MTOF
MTOF TOF
TOF SYSTEM
SYSTEM
The heart of the MTOF sensor is the time-of-flight
time-of-flight
High Mass Resolution Spectrometer (HMRS). The prinprincipal of operation of the HMRS is based on the fact
fact that
the time of flight t of an ion of mass M is proportional to
^M/q*
M q in the presence of an electric field
field that
that increases
increases
linearly with distance. Here q*
q is
is the
the ion's
ion’s charge
charge after
after
penetrating a thin carbon foil. Hence, the measurement
for individual
individual
of t gives unambiguous values of M/q*
M q for
field is produced by a combiions. The required electric field
nation of
of aa hyperbolic
set at
at aa large
nation
hyperbolic plate
plate set
large positive
positive voltage
voltage
VHH (typically
(typically 20
V
20 to
to 25
25 kV)
kV) and
and aa V-shaped
V-shaped plate
plate at
at ground
ground
potential.
The high
potential. The
high mass
mass resolution
resolution of
of the
the MTOF
MTOF sensor
sensor
is due
due to
is
to the
the fact
fact that
that tt is
is independent
independent of
of the
the ion
ion energy
energy
and the
the angle
angle at
at which
and
which the
the ion
ion enters
enters the
the HMRS
HMRS electric
electric
field.
The
value
of
VH
determines
the
maximum
ion enenfield. The value of VH determines the maximum ion
ergy
that
can
be
deflected
by
the
electric
field,
but
ergy that can be deflected by the electric field, but does
does
not affect
affect the
not
the mass
mass resolution.
resolution. Since
Since tt can
can be
be measured
measured
with the
fraction of
nanosecond, high
with
the precision
precision of
of aa fraction
of aa nanosecond,
high
mass resolution
mass
resolution is
is achieved.
achieved. Figure
Figure 22 displays
displays aa specspectrum, where
where the
trum,
the TOF
TOF has
has been
been converted
converted into
into mass
mass units,
units,
near the
the mass
near
mass range
range of
of iron.
iron.
The practical
The
practical implementation
implementation of
of the
the HMRS
HMRS requires
requires
an
accurate
measurement
of
t
and
the
an accurate measurement of t and the conversion
conversion of
of
multiply-charged solar
solar wind
multiply-charged
wind ions
ions into
into singly-charged
singly-charged
ions. This
This is
accomplished with
ions.
is accomplished
with aa combination
combination of
of mimicrochannel
plate
(MCP)
detectors
and
a
thin
carbon
crochannel plate (MCP) detectors and a thin carbon foil
foil
at the
the entrance
at
entrance to
to the
the HMRS
HMRS electric-field
electric-field region.
region. Ions
Ions
passing
through
the
foil
undergo
a
large
number
passing through the foil undergo a large number of
of colcollisions with
lisions
with the
the carbon
carbon atoms,
atoms, resulting
resulting in
in some
some energy
energy
loss, moderate
scattering, and
loss,
moderate scattering,
and charge
charge exchange.
exchange. As
As aa reresult
of
the
charge
exchange,
ions
with
initial
charge
sult of the charge exchange, ions with initial charge state
state
emerge from
from the
the foil
foil with
with charge
charge state
state q*,
qq emerge
q , where
where q*
q isis
typically
0
or
+1.
As
the
ions
leave
the
foil
they
also
typically 0 or +1. As the ions leave the foil they also proproduce secondary
secondary electrons
electrons that
duce
that are
are deflected
deflected to
to aa "Start"
“Start”
MCP assembly
assembly that
MCP
that generates
generates aa start
start signal
signal for
for the
the timetime-
Neutrals
TOFa/E*/M
α E*/M
TOF
N
eutral M
Neutral
MCP
CP
F.I. / UMD
MTOF
FIGURE 1.1. Schematic
Schematic of
of the
the MTOF
MTOF (Mass
(Mass Time of Flight)
FIGURE
Instrument. Solar
Solar wind
wind ions
ions enter
enter through
through an
an energy/charge
energy/charge
Instrument.
filter (the
(the WAVE)
WAVE) and
and pass
pass through
through aa carbon
carbon foil,
foil, leaving
leaving them
them
filter
. They
withaacharge
charge state
state qcf.
They then
then enter
enter aa region
region where
where aa special
special
with
electric field
field bends
bends those
those with
with qq*
> 00 down
down to
to aa microchannel
microchannel
electric
plate. For
For an
an ion
ion of
of mass
mass M,
M, the
the time
time of
of flight
flight in
in this
this region
plate.
region is
is
∝ M q .
nation of
of the
the abundances
abundances of
of many
many of
of the
the elements
elements and
and
nation
isotopes
in
the
solar
wind.
The
MTOF
sensor
consists
of
isotopes in the solar wind. The MTOF sensor consists of
the
Wide-Angle,
Variable
Energy/charge
(WAVE)
passthe Wide-Angle, Variable Energy/charge (WAVE) passbanddeflection
deflection system
system and
and the
the time-of-flight
time-of-flight High-Mass
High-Mass
band
Resolution
Spectrometer
(HMRS).
Resolution Spectrometer (HMRS).
MTOF owes
owes its
its excellent
excellent mass
mass resolution
resolution capability
capability
MTOF
to aa specially
specially designed
designed electric
electric field
field configuration
configuration in
in
to
its time-of-flight
time-of-flight region.
region. An
An ion’s
ion's time-of-flight
time-of-flight through
through
its
this region
region isis independent
independent of
of its
its initial
initial energy
energy or
or angle.
angle.
this
Sub-nanosecond
TOF
measurements
translate
into
mass
Sub-nanosecond TOF measurements translate into mass
resolutions
of
a
fraction
of
an
AMU.
resolutions of a fraction of an AMU.
A prototype
prototype of
of the
the MTOF
MTOF sensor,
sensor, known
known as
as MASS,
MASS,
A
was
flown
on
the
WIND
spacecraft
(launched
about
was flown on the WIND spacecraft (launched about aa
year before
before SOHO),
SOHO), and
and was
was able
able to
to identify
identify aa number
number
year
of
elements
and
isotopes
for
the
first
time.
The
MTOF
of elements and isotopes for the first time. The MTOF
sensor
incorporates
a
number
of
enhancements
over
its
sensor incorporates a number of enhancements over its
prototype,
including
position-sensing
and
signal
ampliprototype, including position-sensing and signal amplitude measurements,
measurements, and
and improved
improved background-rejection
background-rejection
tude
122
16Feb96 to 19Feb96 HPS=18.1 kV
16Feb96
to 19Feb96 HPS=18.1kV
11 ch/bin,
eh/bin, Pos>0.5,
Pos>0.5, Weq=9.8kV,
Weq=9.8kV, Vf=0
V£=0kV
kV
MTOF
The
The MTOF
MTOFProton
ProtonMonitor
MonitorSubSensor
SubSensor
The
TheMTOF
MTOFProton
ProtonMonitor
Monitor(PM)
(PM)isisdesigned
designedtotomeameasure
the
solar
wind
proton
bulk
speed,
density
sure the solar wind proton bulk speed, densityand
andtherthermal
mal speed.
speed. The
The bulk
bulkspeed
speed(and,
(and,totoa alesser
lesserextent,
extent,the
the
thermal
thermalspeed)
speed)isisnecessary
necessarytotothe
theevaluation
evaluationofofdata
datafrom
from
the
the main
main MTOF
MTOFsensor.
sensor.The
ThePM
PMisisdescribed
describedinindetail
detail
elsewhere
elsewhere[5].
[5].
SOHO/CELIAS
1000
1000-
counts
MAXIMUM
MAXIMUMLIKELIHOOD
LIKELIHOODTECHNIQUE
TECHNIQUE
I
The
Thecounting
countingrates
ratesininaaseries
seriesofofTime
TimeofofFlight
FlightChannels
Channels
were
simulated
using
a
maximum
likelihood
were simulated using a maximum likelihoodtechnique.
technique.
In
Inthis
thistechnique,
technique,the
thesimulated
simulatedcount
countrate
rateinina aparticular
particular
channel was taken as the mean, µ i , of a Poisson distribuchannel was taken as the mean, ///, of a Poisson distribution. Assuming that this Poisson distribution is the parent
tion. Assuming that this Poisson distribution is the parent
distribution from which the measurements were derived,
distribution from which the measurements were derived,
the probability that any given number of counts N i was
the probability that any given number of counts Nj was
observed in that channel is
100
observed in that channel is
51
52
53
54
55
56
57
58
59
60
61
60
61
PPi
i
µNi i
e
Ni ! e
µi
m
w
~
The probability that the measured TOF spectrum was
Mass
The probability that the measured TOF spectrum was
observed, given the assumptions in the simulation, is
observed, given the assumptions in the simulation, is
simply the product of the probabilities for each channel:
simply the product of the probabilities for each channel:
FIGURE 2. Observed solar wind mass spectrum accumuFIGURE
Observed
solar
lated over a2.3 day
period in
1996wind mass spectrum accumu-
lated over a 3 day period in 1996
PTot
of-flight analysis. The “stop” signal for particles with q of-flight
analysis.atThe
"stop" large
signalarea
for particles
withasq*
> 0 is generated
a second
“Ion” MCP
>sembly
0 is generated
at
a
second
large
area
"Ion"
MCP
aslocated behind the ground surface of the electric
sembly
located
ground
of thesignal
electric
0, the “stop”
is
field region;
for behind
particlesthewith
q = surface
field
region;
with
q* = 0, the "stop" signal is
generated
byfor
theparticles
“Neutral”
MCP.
generated
by thefor"Neutral"
The anodes
all three MCP.
MCPs provide not only the
The
anodes
for
all three
provide not
onlythat
the
required timing signals,
but MCPs
also amplitude
signals
required
timing
signals,
but
also
amplitude
signals
that
are pulse height-analyzed by the electronics. These amare
pulsesignals
height-analyzed
by information
the electronics.
These
amplitude
provide some
about
particle
plitude
signals
provide
some
information
about
particle
mass and energy. In addition, the Neutral and Ion MCP
mass
and
energy.
In addition, position
the Neutral
and Ion MCP
anodes
provide
1-dimensional
information.
The
anodes
The
Neutralprovide
position1-dimensional
is a measure ofposition
the solarinformation.
wind ion’s flow
Neutral
is a measure
of the useful
solar wind
ion's flow
directionposition
in the solar
ecliptic plane,
for interpretadirection
the solar
ecliptic
plane,
usefulusfor
interpretation of theindata.
The Ion
position
allows
to determine
tion
of
the
data.
The
Ion
position
allows
us
to
determine
the E q of an ion, and is also useful for rejecting
backthe
E/q
of
an
ion,
and
is
also
useful
for
rejecting
background events.
ground
events.
The entire HMRS is maintained at a ‘floating’ voltage,
entire
HMRS
at asolar
'floating'
VFThe
, in the
range
-5 tois+5maintained
kV. For high
wind voltage,
speeds,
VF,
in the range
-5 to +5 kV.
Forions
highsosolar
a positive
VF decelerates
heavy
thatwind
they speeds,
can be
adeflected
positive by
VF the
decelerates
heavy
ionsand
so detected
that they by
canthe
be
hyperbola
voltage
deflected
hyperbola
voltage
detectedVby
the
Ion MCP.by
Forthe
low
solar wind
speedsand
a negative
F will
Ion
MCP. For
low before
solar wind
speedsthe
a negative
VF and
will
accelerate
the ions
they reach
carbon foil,
accelerate
the ionsthe
before
they reach
thedetection
carbon foil,
and
thereby improve
efficiency
of their
in the
thereby
improve
the
efficiency
of
their
detection
in
the
HMRS.
HMRS.
∏ Pi
The goal of is to maximize this probability by altering
goal of is
maximize
this probability
by altering
theThe
parameters
oftothe
model function,
thus altering
the
the
parameters
of
the
model
function,
thus
altering
the
µi . In practice, it is easier and equivalent to deal with the
fa.
In
practice,
it
is
easier
and
equivalent
to
deal
with
the
natural log of this expression:
natural log of this expression:
lnPTot ∑ Ni lnµi µi lnNi !
i
A constrained minimization routine (E04KDF from
constrained
minimization
routine
(E04KDF
from
theA
NAG
Library; see
[6]) was used
to minimize
the negthe
NAG
Library;
see
[6])
was
used
to
minimize
the
negative logarithm of the probability (equivalent to maxiative
logarithm
of
the
probability
(equivalent
to
maximizing the positive logarithm.)
mizing
the positive
logarithm.)
The model
function
used had 24 parameters. Fourteen
The model
function
used
24 parameters.
Fourteen
of these
were the
heights
of had
the peaks
corresponding
to
of these48,
were
of the
corresponding
masses
49, the
andheights
52-63. Of
the peaks
remaining
10, two de-to
masses a48,
49, background,
and 52-63. Of
thewere
remaining
two described
linear
two
needed 10,
to convert
scribed
a
linear
background,
two
were
needed
to
convert
from mass to time of flight channel, two were associfrom
mass
to
time
of
flight
channel,
two
were
associated with peaks caused by electronic “ringing” and
the
ated
with
peaks
caused
by
electronic
"ringing"
and
the
remaining 4 described the shape and width of the peaks.
remaining
described
shape was
and awidth
of theasympeaks.
The
form of4 the
model the
function
modified
The form
of the model function was a modified asymmetric
Lorentzian:
metric Lorentzian:
A
1 X X0 Γn
where A is the amplitude (in counts) for each of the 14
where
A isand
theXamplitude
(inlocation
counts) (in
for time
each of
of flight
the 14
mass
values,
0 is the peak
mass values, and XQ is the peak location (in time of flight
123
SOHO/CELIAS/MTOF
SOHO/CELIAS/MTOF
SOHO/CELIAS/MTOF
channel
channelnumber)
number)that
thatisisisderived
derivedfrom
fromthe
themass
massvalues.
values.
channel
number)
that
derived
from
the
mass
values.
The
exponent,
n,
and
the
half
width
at
half
The
exponent,
n,
and
the
half
width
at
half
maximum,
The exponent, n, and the half width at half maximum,
maximum,
Γ,
arethe
thesame
samefor
forall
allspecies,
species,but
butare
areallowed
allowedto
to be
be
F,Γ,are
are
the
same
for
all
species,
but
are
allowed
to
be
different
on
either
side
of
the
peak.
different
on
either
side
of
the
peak.
different on either side of the peak.
Interstream
Interstream---DOY
DOY304
304to
to308(08UT)
308(08UT)1998
1998
Interstream
DOY
304
to
308(08UT)
1998
Positions
Positions>>>0.4
0.4 Vsw:
Vsw:400
400km/s
km/s
Positions
0.4
Vsw:
400
km/s
Measured
Measured Counts
Counts
Simulated
Simulated Counts
Counts
1000
1000
OBSERVATIONS
OBSERVATIONS
OBSERVATIONS
700
700
SoHO/CELIAS/MTOF
SoHO/CELIAS/MTOF
SoHO/CELIAS/MTOF
500
500
300
300
15
15
10
10
5
100
100
10
10
46
46
48
48
50
50
52
52
54
54
56
56
58
58
60
60
62
62
64
64
66
66
68
68
Mass(amu)
(amu)
Mass
Mass
(amu)
FIGURE4.
4. Mass
Mass spectrum
spectrum accumulated
accumulated during
during the
the shaded
shaded
FIGURE
FIGURE
4.
Mass
spectrum
accumulated
time
period
indicated
in
Figure
3.
The
fit
from
the
maximum
time
period
indicated
in
Figure
3.
The
fit
from
the
maximum
time period indicated in
3. The fit
likelihoodtechnique
techniqueisisindicated
indicated by
by the
the solid
solid line.
line.
likelihood
likelihood
technique
ProtonMonitor
Monitor
Proton
Proton
Monitor
Interstream
Interstream
Time
Time
Period
Period
600
600
400
400
Counts
Counts
The
Themass
massspectrum
spectrumdisplayed
displayedin
inFigure
Figure222was
wasaccumuaccumuThe
mass
spectrum
displayed
in
Figure
was
accumulated
latedover
overaaa333day
daytime
timeperiod
periodin
inFebruary
February1996
1996during
lated
over
day
time
period
in
February
1996
during
whichtime
timethe
thesolar
solarwind
windwas
wasof
ofaaanondescript
nondescriptinterinterwhich
which
time
the
solar
wind
was
of
nondescript
interstreamtype.
type.The
Themaximum
maximumlikelihood
likelihoodtechnique
techniquewas
wasapapstream
stream
type.
The
maximum
likelihood
technique
was
appliedto
derivethe
theabundance
abundanceratios
ratiosof
ofthe
theFe
Feisotopes.
isotopes.
plied
plied
totoderive
derive
the
abundance
ratios
of
the
Fe
isotopes.
Theseabundance
abundanceratios
ratiosare
arethen
thencorrected
correctedby
bythe
theratios
ratiosof
of
These
These
abundance
ratios
are
then
corrected
by
the
ratios
of
instrument
efficiencies
for
these
isotopes.
The
efficiency
instrument
efficiencies
for
these
isotopes.
The
efficiency
instrument efficiencies for these isotopes. The efficiency
correctionsdepend
dependweakly
weaklyon
onthe
thesolar
solarwind
windspeed
speedand
and
corrections
corrections
depend
weakly
on
the
solar
wind
speed
and
temperature,
the
deflection
system
and
hyperbolic
plate
temperature,
the
deflection
system
and
hyperbolic
plate
temperature, the deflection system and hyperbolic plate
voltagesettings,
settings,and
andthe
theincident
incidentcharge
chargestate
statedistribudistribuvoltage
voltage
settings,
and
the
incident
charge
state
distributionsof
theFe
Feions,
ions,and
andwere
werein
therange
rangeof
of1%
1%to
to4%
4%
tions
tions
ofofthe
the
Fe
ions,
and
were
ininthe
the
range
of
1%
to
4%
forthe
thetime
timeintervals
intervalsselected
selectedfor
forthis
thispaper.
paper.
for
for
the
time
intervals
selected
for
this
paper.
Thesolar
solarwind
windbehavior
behaviorduring
duringanother
anotherinterstream
interstream
The
The
solar
wind
behavior
during
another
interstream
time
period
is
shown
in
Figure
3.
The
mass
spectrum
time
time period
period isis shown
shown inin Figure
Figure 3.3. The
The mass
mass spectrum
spectrum
accumulated
during
this
time
interval
is
displayed
in
accumulated
accumulated during
during this
this time
time interval
interval isis displayed
displayed inin
Figure
4.
Figure
Figure4.4.
larwind
windspeed
speedwas
was between
between 380
380 and
and 400
400 km/s
km/s
were
inlar
lar
wind
speed
was
between
380
and
400
km/s were
were inincluded.
This
was
done
in
order
to
minimize
the
variation
cluded.
cluded. This
This was
was done
done in
in order
order to
to minimize
minimize the
the variation
variation
ofthe
therelative
relativeefficiencies.
efficiencies.
of
of
the
relative
efficiencies.
Solar Wind Speed
Solar Wind Speed
(km/s)
(km/s)
Proton Density
Proton Density
(1/cm^3)
(1/cm^3)
RESULTS
RESULTS
RESULTS
5
54
56
54Fe56
Theefficiency-corrected
efficiency-correctedratios
ratiosderived
derivedfor
for
Fe
and
The
/Fe
Fe and
The
efficiency-corrected
ratios
derived
forFe
Fe54
57
56
57 Fe56
56 in the 4 intervals discussed above are preFe
57
Fe
Fe in
Fe /Fe
in the
the 44 intervals
intervals discussed
discussed above
above are
are presented in
in Figure
Figure 555 and
and Figure
Figure 6,
6, respectively.
respectively. In
In
each
sented
sented
in
Figure
and
Figure
6,
respectively.
In each
each
figure
the
terrestrial
value
from
[7]
is
indicated
by
the
figure
figure the
the terrestrial
terrestrial value
value from
from [7]
[7] isis indicated
indicated by
by the
the
solid
horizontal
line.
solid
solid horizontal
horizontal line.
line.
Theresults
resultsaveraged
averagedover
overall
all 44
4 time
time
periods
are
shown
The
The
results
averaged
over
all
time periods
periods are
areshown
shown
in
Table
1,
along
with
the
accepted
terrestrial
values
in
in Table
Table 1,1, along
along with
with the
the accepted
accepted terrestrial
terrestrial values
values
from [7].
[7]. Oetliker
Oetliker et
et al.
al. [8]
[8]
derived
solar
wind
Fe
isofrom
from
[7].
Oetliker
et
al.
[8] derived
derived solar
solar wind
wind Fe
Fe isoisotopic ratios using the Wind/MASS sensor. Their results
topic ratios
ratios using
using the
the Wind/MASS
Wind/MASS sensor.
sensor. Their
Their results
topic
have larger uncertainties but are in agreement with the
have larger
larger uncertainties
uncertainties but
but are
are in
in agreement
agreement with
with the
the
have
present work. They obtained a value (in percent) of 8.5
present work.
work. They
They54obtained
obtained
value (in
(in percent)
percent) of
of 8.5
8.5
present
aa value
56
(+0.5, -2.2) for Fe54
Fe56
and an upper limit of 5% for
(+0.5,
-2.2)
for Fe
Fe54/Fe
limit of 5%
5% for
for
(+0.5,
-2.2)
for
Fe 56 and an upper limit
57
56
Fe Fe56
56 .
Fe5757/Fe
Fe
Fe ..
The Fe isotope ratios in Table 1 are, within the meaThe Fe
Fe isotope
isotope ratios
ratios in
in Table
Table 11 are,
are, within
within the
the meameaThe
surement uncertainties, consistent with no fractionation
surement uncertainties,
uncertainties, consistent
consistent with
with no
no fractionation
fractionation
surement
of Fe isotopes in the solar wind. They are however also
of Fe
Fe isotopes
isotopes in
in the
the solar
solar wind.
wind. They
They are
are however also
also
of
consistent with a modest fractionation effect. A mass deconsistent with
with aa modest
modest fractionation
fractionation effect.
effect. A
A mass
mass dedeconsistent
pendent isotopic fractionation has been reported for Ne,
pendent isotopic
isotopic fractionation
fractionation has
has been
been reported
reported for
for Ne,
Ne,
pendent
Mg and Si [9], with the heavier isotope depleted in the
Mg and
and Si
Si [9],
[9], with
with the
the heavier
heavier isotope
isotope depleted
depleted in
in the
the
Mg
slow solar
wind. The
average magnitude
of this effect
slow solar
solar wind.
wind. The
The average
average magnitude
magnitude of
of this
this effect
effect
slow
0
0
100
100
80
80
60
60
40
Thermal Speed
40
Thermal
Speed
(km/s)
20
(km/s)
20
4
4
2
2
0
0
-2
-2
N/S Flow Angle (deg)
-4 N/S
Flow Angle (deg)
-4 301
302
303
304
305
306
307
301
302
303
304 (28
305Oct306
306
307
301
302
303
305
307
DOY 304
1998
to 5 Nov)
DOY 1998
1998 (28
(28 Oct
Oct to
to 55 Nov)
Nov)
DOY
308
308
308
309
309
309
FIGURE 3. Solar wind behavior during a typical interstream
FIGURE
Solarwind
windbehavior
behaviorduring
during aatypical
typical interstream
interstream
FIGURE
3.3. Solar
time period.
timeperiod.
period.
time
In addition to the two individual time periods disIn addition
addition toto the
the two
two individual
individual time
time periods
periods disdisIn
cussed above, mass spectra have also been obtained for
cussed above,
above,mass
mass spectra
spectra have
have also
also been
been obtained
obtained for
for
cussed
two 12-month time periods (April 1996 to March 1997
two 12-month
12-month time
time periods
periods (April
(April 1996
1996 to
to March
March 1997
1997
two
and March 1997 to March 1998). For both 12-month
and March
March 1997
1997 toto March
March 1998).
1998). For
For both
both 12-month
12-month
and
periods, only
those 5-minute
intervals in
which the soperiods, only
only those
those 5-minute
5-minute intervals
intervals inin which
which the
the sosoperiods,
124
SOHO/CELIAS/MTOF
SOHO/CELIAS/MTOF
SOHO/CELIAS/MTOF
SOHO/CELIAS/MTOF
8.0
4.0
4.0
8.0-
Fe57/Fe56
Fe57/Fe56
Fe54/Fe56
Fe54/Fef>6
7.5
7.5-
Fe57/Fe56 (%)
Fe54/Fe56 (%)
3.0
3.0-
7.0
7.0-
I
Terrestrial
Terrestrial
2.0
2
-°
6.5
6.5Terrestrial
Terrestrial
1.0
1.0-
6.0
6.01996
1996
047-050
047-050
Vsw=380
5.5
Vsw=380
5.5-
IT
Apr 1996Apr 1996Mar
1997
Mar 1997
Vsw=390
Vsw=390
I
Mar 1997
Mar 1998
1997
Mar
Mar 1998
Vsw=390
1998
1998
304-308
304-308
Vsw=400
I
IT
Vsw=390
Fe54
54
1996
1996
047-050
047-050
Vsw=380
Vsw=400
0.0
0.0
Fe56
56
Fe
Fe Isotopic
Isotopic Ratio
Ratio (in
(inpercent)
percent)
Fe /Fe
57 Fe56
Fe
56
Fe57/Fe
This
This Work
Work
66.8
8 ±0.4
04
Terrestrial
Terrestrial*
6.37
6.37
22.5
5 ±0.5
05
2.31
2.31
Vsw=390
1998
1998
304-308
304-308
Vsw=400
Vsw=390
Vsw=400
I
I
Fe56
56
FIGURE
FIGURE 6.
6. The
The derived
derived ratios
ratios of
of Fe to
to Fe obtained
obtained
during
during 44 distinct
distinct time
time intervals.
intervals. The
The error
error bars
bars denote
denote 1-σ
1-a
uncertainties.
uncertainties. The
The terrestrial
terrestrial value
value isis indicated
indicated by
by the
the solid
solid
horizontal
horizontal line.
line.
TABLE
TABLE 1.
1. Iron
Iron Isotopic
Isotopic Abundance
Abundance Ratios
Ratios
Fe56
56
IT
Mar 1997
Mar 1998
1997
Mar
Mar 1998
Vsw=390
Fe5757
FIGURE
FIGURE 5.
5. The
The derived
derived ratios
ratios of
of Fe to
to Fe obtained
obtained
during
during 44 distinct
distinct time
time intervals.
intervals. The
The error
error bars
bars denote
denote 1-σ
1-a
uncertainties
uncertainties and
and are
are dominated
dominated by
by systematic
systematic effects.
effects. The
The
terrestrial
terrestrial value
value isis indicated
indicated by
by the
the solid
solid horizontal
horizontal line.
line.
Fe5454
Vsw=380
Apr 1996Apr 1997
1996Mar
Mar 1997
Vsw=390
Beard and Johnson, 1999
Beard and Johnson, 1999
was
was reported
reported to
to be
be approximately
approximately 1.4%
1.4% per
per amu.
amu.The
TheFe
Fe
isotopic
measurements
reported
here
were
all
obtained
isotopic measurements reported here were all obtained
from
from such
such slow
slow solar
solar wind
windflows.
flows. The
Thefractionation
fractionationeffect
effect
from
[9]
would
predict
an
enhancement
ofofsome
2.8%
for
from
[9]
would
predict
an
enhancement
some
2.8%
for
54
54Fe56
56 relative to the unfractionated solar composiFe
Fe /Fe relative to the unfractionated solar composition,
tion, aa result
result also
also consistent
consistent with
with our
our observed
observed enhanceenhance
6
%
above
the
terrestrial
ment
of
about
(7
ment of about (7 ± 6)% above the terrestrialvalue.
value.
The
The current
current uncertainties
uncertainties in
in the
the isotope
isotope ratios
ratios prepresented
in
this
paper
are
dominated
by
systematic
sented in this paper are dominated by systematiceffects.
effects.
One
of
One such
such effect
effect isis the
the lack
lack of
of knowledge
knowledge
of the
the precise
precise
56
56 at TOF values that
shape
of
the
TOF
spectrum
of
Fe
shape of the TOP spectrum of Fe at TOP values that
are
are far
far from
from the
the most
most probable
probable value.
value. We
Weare
are attempting
attempting
to
improve
our
knowledge
of
the
response
to improve our knowledge of the response in
in the
the wings
wings
of
the
TOF
distribution
by
calibrating
the
MTOF
of the TOP distribution by calibrating the MTOF spare
spare
instrument
instrument at
at an
an accelerator
accelerator facility
facility atat the
the University
University of
of
Bern
Bern [10].
[10]. The
The derived
derived mass
mass distribution
distributioncan
canalso
alsobe
beimimproved
proved by
by carefully
carefully correcting
correcting for
for the
the effects
effects caused
caused by
by
125
“electronic
"electronic walk”,
walk", in
in which
which different
different amplitude
amplitudesignals
signals
create
slightly
different
times
of
flight.
create slightly different times of flight. There
There isis also
also aa
very
small
but
statistically
significant
variation
in
very small but statistically significant variation in the
the
TOF
TOP with
with the
the residual
residual ion
ion energy
energy (which
(which can
can be
be deterdetermined
mined from
from the
the MCP
MCP position
position measurement)
measurement) and
and also
also
with
instrument
temperature.
We
estimate
that
correcting
with instrument temperature. We estimate that correcting
for
for all
all of
of the
the above
aboveeffects
effects will
willdecrease
decreaseour
ouruncertainty
uncertainty
in
the
iron
isotope
ratios
by
perhaps
a
factor
in the iron isotope ratios by perhaps a factor of
oftwo.
two.ItItisis
also
planned
to
examine
the
Fe
isotope
ratios
in
also planned to examine the Fe isotope ratios in higher
higher
speed
speed solar
solar wind
wind flows.
flows.
ACKNOWLEDGMENTS
ACKNOWLEDGMENTS
We
We are
are grateful
grateful to
to all
all the
the individuals
individuals who
who participated
participated
in
the
design,
the
construction,
and
the
in the design, the construction, and the calibration
calibration of
of
CELIAS/MTOF.
CELIAS/MTOF. This
This research
researchwas
wassupported
supportedby
byNASA
NASA
grants
grants NAG5-7678
NAG5-7678and
andNAG5-9282.
NAG5-9282.
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126