Preliminary results from coordinated SOHO-Ulysses
observations
S. Parent!*, G. Poletto1", B.J.I. Bromage*, S.T. Suess**, J.C. Raymond*, G. Noci§ and
G.E. Bromage*
* Centre for Astrophysics, University of Central Lancashire, Preston, UK
^ Osservatorio diArcetri, Firenze, Italy
**Marshall Space Flight Center, Huntsville, AL, USA
^Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
§
Universita di Firenze, Firenze, Italy
Abstract. SOHO-Ulysses quadratures occur at times when the SOHO-Sun-Ulysses angle is 90° and offer a
unique possibility to compare properties of plasma parcels observed in the low corona with properties of the
same parcels measured, in due time, in situ. The June 2000 quadrature occurred at a time Ulysses was at 3.35 AU
and at a latitude of 58.2 degrees in the south-east quadrant. Here we focus on the UVCS observations made on
June 11, 12, 13, 16. UVCS data were acquired at heliocentric altitudes ranging from 1.6 to 2.2 solar radii, using
different grating positions, in order to get a wide wavelength range. The radial direction to Ulysses, throughout the
4 days of observation, traversed a region where high latitude streamers were present. Analysis of the spectra taken
by UVCS along this direction shows a variation of the element abundances in the streamers over our observing
interval: however, because the radial to Ulysses crosses through different parts of streamers in different days, the
variation could be ascribed either to a temporal or to a spatial effect. The oxygen abundance, however, seems to
increase at the edge of streamers, as indicated by previous analyses. This suggests the variation may be a function
of position within the streamer, rather than a temporal effect. Physical conditions in streamers, as derived from
UVCS observations, are also discussed.
INTRODUCTION
4" x 4" rasters within the same field of view, out to 1.2
solar radii. SUMER data were acquired in the range between 1.04 and 1.6 solar radii. UVCS will be discussed
SOHO-Ulysses quadratures occur when the SOHOextensively in the following. LASCO data were providSun-Ulysses angle is 90°. At such times, we may obing the overall coronal configuration at the time of the
serve the plasma parcels that leave the corona in the
quadrature.
direction of Ulysses first, remotely, with SOHO, and
It is hard to overestimate the role that studies of ellater on, in situ, with Ulysses. This geometric configuement abundances have played in our understanding of
ration occurs twice per year and a number of coordinated
the coronal-solar wind relationship: here it suffices to reSOHO Ulysses campaigns have been run at those times.
mind the reader of those analyses [e. g. 1], which, from
The June 2000 quadrature occurred when Ulysses was
the depletion of the He/H ratio, at magnetic sector boundat 3.35 AU and at a latitude of 58.2 degrees in the
south-east quadrant. A JOP - JOP 112 - was running aries, lead the authors to suggest streamers as plausible
source regions of the wind measured at those positions.
at that time, with the participation of CDS, SUMER,
Where, within streamers, does slow wind emerge from,
UVCS, LASCO on SOHO and SWOOPS and SWICS
is still a matter of debate: Raymond et al. [2], from the
experiments on Ulysses. The JOP aimed at deriving eleoxygen abundance measured in the streamer legs, proment abundances along the radial to Ulysses, at different
pose streamers lateral branches as the site where slow
heliocentric distances, with the purpose of establishing
wind originates, Noci et al. [3], on the other hand, point
whether and how the element abundance varies with alto a more complicated configuration where the open retitude and compare coronal and in situ abundances. CDS
gions between substreamers are possible sources of the
data are composed by normal incidence telescope (NIS)
slow wind. Because JOP 112 occurred at a phase of maxrasters of 120" x 120" centred at altitudes that reach up
imum activity in the solar cycle, we may expect to obto 1.18 solar radii, and grazing incidence telescope (GIS)
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
83
FIGURE1.1. LASCO
LASCOC2
C2images
imagesofofthe
thewhite
whitelight
light corona
corona taken
taken on
on 11,
11, 12,
12, 13,
13, 16
16 June
June 2000.
FIGURE
2000. The
The inner
inner and
and outer
outer circles
circles indicate,
indicate,
respectively,the
thesolar
solardisk
disk and
and the
the lower
lower edge
edge of
of the
the C2
C2 coronagraph.
coronagraph. The
respectively,
The radial
radial to
toUlysses,
Ulysses,atat-58.2°
-58.2Æ in
inthe
the south-west
south-westquadrant,
quadrant,
hasbeen
been superposed
superposed on
on the
the images.
images. The
The UVCS
UVCS slit
slit was
was set
set normal
normal to
has
to the
the radial
radial to
to Ulysses;
Ulysses; the
theCDS
CDSFOV
FOVisisshown
shownininfigure
figure2;2;
theSUMER
SUMERslit,
slit,which
whichcan
canbe
bemoved
movedonly
only along
along the
the North-South
North-South direction,
direction, was
the
was set
set at
at altitudes
altitudes allowing
allowing SUMER
SUMER to
to observe
observe areas
areas
alongthe
theSun-Ulysses
Sun-Ulyssesdirection.
direction.
along
servestreamers,
streamers,ininthe
thedirection
directionpointing
pointingtotoUlysses
Ulysses and
and
serve
relateabundances
abundancesatatcoronal
coronallevels
levelstotoininsitu
situabundances
abundances
relate
andwind
windspeed.
speed.
and
In
this
contribution we
we present
present preliminary
preliminary results
results
In this contribution
from
JOP
112,
focussing
on
an
analysis
of
UVCS
data.
from JOP 112, focussing on an analysis of UVCS data.
After
a
description
of
the
observations
we
made
and
of
After a description of the observations we made and of
the
morphology
of
the
regions
traversed
by
the
radial
to
the morphology of the regions traversed by the radial to
Ulysses,we
wedescribe
describethe
thetechniques
techniquesused
used to
to derive
derive eleeleUlysses,
ment
abundances,
and
discuss
the
results
we
obtained.
ment abundances, and discuss the results we obtained.
We conclude by outlining future work.
We conclude by outlining future work.
observations. As
observations.
As we
we might
might expect
expect at
at this
this phase
phase of
of the
the
solar cycle,
cycle, streamers
solar
streamers extend
extend to
to high
high latitudes,
latitudes, and
and the
the
radial to
radial
to Ulysses
Ulysses either
either runs
runs at
at the
the edge
edge of
of streamers
streamers or
or
crosses
the
streamer
core.
crosses the streamer core.
UVCS acquired
1.9 R
UVCS
acquired data
data at
at 1.6
1.6 and
and 1.9
RQ.. The
The slit
slit (100
(100
jum
wide)
was
set
normal
to
the
radius
of
the
Sun,
µm wide) was set normal to the radius of the Sun, with
with
its center
center along
its
along the
the direction
direction to
to Ulysses.
Ulysses. Spectra
Spectra have
have
been
obtained
using
different
grating
positions
been obtained using different grating positions (usually
(usually
5), with a spatial resolution of w 70" and a 2 pixel spec5), with a spatial resolution of 70 00 and a 2 pixel spectral binning (1 pixel = 9.25 mA). Observing times at each
tral binning (1 pixel = 9.25 mÅ). Observing times at each
grating positions were on the order of 90 to 120 mingrating positions were on the order of 90 to 120 minutes. Spectra span the 951 A - 1117 A interval, in the
utes. Spectra span the 951 Å - 1117 Å interval, in the
O VI primary channel and the 1180 A to 1250 A interO VI primary channel and the 1180 Å to 1250 Å interval in the redundant channel and include the HI Lya,
val in the redundant channel and include the H I Ly α ,
Lyp, Ly lines, the O VI 1032 and 1037 A lines, several
Lyβ , LyYγ lines, the O VI 1032 and 1037 Å lines, several
lines from minor ions in different ionization stages, e.g.
lines from minor ions in different ionization stages, e.g.
Fex through Fe XIII, S X and , Ar XII andAr XIV, in
Fe X through Fe XIII, S X and , Ar XII and Ar XIV, in
addition to Si XII, Mg X, Ca X and N V lines.
XII , Mg X , Ca X and N V lines.
addition
to Sihow
To show
coordinated observations have been
To
show
how
have
been
made during JOP coordinated
112, we giveobservations
in Fig. 2 the
position
made during JOP 112, we give in Fig. 2 the position
THE OBSERVATIONS
OBSERVATIONS
THE
The June 2000 quadrature campaign extended for two
The June 2000 quadrature campaign extended for two
weeks, around the quadrature date, which occurred on
weeks, around the quadrature date, which occurred on
June 13. JOP 112 has been run during the second week
June 13. JOP 112 has been run during the second week
of the campaign. Here we focus mainly on observations
of the campaign. Here we focus mainly on observations
taken on 11, 12, 13 and 16 June. Fig. 1 shows the mortaken
on 11,
12, corona
13 and on
16 those
June. Fig.
shows
the morphology
of the
days1from
LASCO
C2
phology of the corona on those days from LASCO C2
84
contribution to
to the
the line
line intensity
intensity is
is given
given by
by plasma
plasma in
in
contribution
the
plane
of
the
sky.
In
this
case
we
can
write
[6]
the plane of the sky. In this case we can write [6]
f Il 12
2 =
I
13
/13
C12 N1 + 4π hνj1212
C13 N1 + 4π hνj1313
(1)
(1)
where 1,2,3,
1, 2, 3, indicate,
indicate, respectively,
respectively,the
the ground
ground level
level and
and
where
the lower
lower and
and upper
upper level
level of
of the
the transitions
transitions from
from which
which
the
the O
O VI
VI doublet
doublet lines
lines originate
originate and
and 712,713
j 12 ; j13 are
are the
the line
line
the
emissivities.
Eq.
(1)
can
be
rewritten
as
emissivities. Eq. (1) can be rewritten as
I12
In
I13
=
g2
g 2 1 + g3 θ
g3 1 + θ
i+e
(2)
(2)
withgg statistical
statistical weights
weightsof
of the
the levels
levels and
and the
the ratio
ratio 6θ ==
with
I13 rad
between
the
radiative
and
collisional
component
of
between
the
radiative
and
collisional
component
of
I13 coll
the O
OVI
VI 1032
1032AÅline
linegiven
givenby
by
the
;
;
from an HIT
EIT image in 195 A
Å taken
taken
FIGURE 2. An excerpt from
the direction
direction to
to Ulysses
Ulysses and
and the
the positions
positions
on 13 June, showing the
and GIS
GIS spectromspectromwhere CDS made observations with the NIS and
eters. The CDS SUMER slits are along the north-south
north-south direcdirection. NIS
NIS the
the full
full spectra
spectra are
are taken
taken in
in 120"
12000 x 150"
15000 rasters;
rasters;GIS
GIS
spectra are acquired at several positions within
within the
the NIS
NISrasters
rasters
in
in a 4"
400 by 4"
400 pixel.
λ2 exp EkT13e
2 13
p
R
13 Iex (λ)dλ
Rsun 2
h(r)
1
r
gNe (∆λ2cor + ∆λ2ex) 2
(3)
(3)
theexciting
excitingchromospheric
chromosphericradiation,
radiation, other
other
whereIIexex isis the
where
symbolshave
havetheir
theirusual
usualmeaning
meaningand
andh(r)
h(r) isis aa geometgeometsymbols
rical
ricalfactor.
factor.
θ = 5:75 10
Te
b)
b)Electron
Electrontemperatures
temperatures
As we
we mentioned,
mentioned,our
our spectra
spectracontain
contain lines
lines from
fromFe
Fe inin
As
different
different ionization
ionization stages,
stages, as
as well
well as
as HHII Lyman
Lyman lines.
lines.
Then,
Then,ififwe
weidentify
identifythe
thecollisional
collisionalcomponent
componentof
ofthe
theLyp
Ly β
line
linel 1, ,calculate
calculatethe
theratio
ratioRR
betweenthe
theobserved
observedFeFe
0bobs
s between
ion
ion intensities
intensities toto the
the observed
observed Lyp
Ly β component
componentand
andthe
the
ratio
ratioRRththbetween
betweenthe
thepredicted
predictedFe
Feion
ionintensities
intensitiesand
andthe
the
predicted
componentatat different
different temperatures,
temperatures,we
we
predicted Lyp
Ly β component
may
vs.log
log T,
T , for
forall
all
maybuild
buildaaplot
plotof
of log
logRR == log(^-)
log ( RRth )vs.
obs
the
Fe
ions
of
the
spectra
[7].
The
common
intersection
the Fe ions of the spectra [7]. The common intersection
of
of the
the curves
curvesbuilt
built from
fromdifferent
differentions
ions gives
gives aa good
good inindication
dication of
of the
theplasma
plasmaelectron
electron temperature.
temperature. Clearly,
Clearly, ifif
the
theplasma
plasmaisisnot
notisothermal,
isothermal,curves
curvesintersect
intersectatatdifferent
different
temperatures.
temperatures.
Temperatures
Temperaturescan
canbe
bederived
derivedalso
alsovia
viathe
theDEM
DEM(Differ(Differential
entialEmission
EmissionMeasure)
Measure)technique:
technique:aaplot
plotof
ofthe
theDEM
DEM
of
ofdifferent
differentfrom
fromdifferent
differentlines
linesvs.
vs.TT indicate
indicatethe
thetempertemperature
atureatatwhich
whichthe
thepeak(s)
peak(s)ininemission
emissionoccur(s).
occur(s).We
Werefer
refer
the
reader
to
Raymond
et
al.,
this
volume,
for
a
definition
the reader to Raymond et al., this volume, for a definition
of
ofthe
theDEM.
DEM.
of the CDS field
field of view: as we mentioned, CDS
CDS -– and
SUMER -– made spectra along the radial to Ulysses (or at
nearby positions) at altitudes 11.02
< r/R®
1.6.
:02 r=R <
1:6. These
These
observations will allow us to find
find the profile
profile of
of abunabundances vs. height, as well as the density
density and
and electron
electron
temperature profile
profile vs. height [see e.g.
e. g. 4], over several
days at the different
different positions traversed by the radial
radial to
to
Ulysses. In this preliminary analysis, however, we report
report
only on results from
from the analysis of 4 days of UVCS observations. In the next section diagnostic techniques
techniques to
to
derive
derive density,
density, temperature
temperature and
and oxygen
oxygen abundance
abundance will
will
be
be illustrated.
illustrated.
DIAGNOSTIC TECHNIQUES
TECHNIQUES
We
We give
give now
now aa short
short description
description of
of the
the diagnostic
diagnostic techtechniques
niques used
used to
to derive
derive density,
density, electron
electron temperature
temperature and
and
oxygen
oxygen abundance.
abundance. We
We refer
refer the
the reader
reader to
to Raymond
Raymond [5]
[5]
for
for aa more
more detailed
detailed discussion
discussion of
of these
these methods.
methods.
c)c)Element
Elementabundances
abundances
Element
Element abundances
abundances can
can be
be derived,
derived, atat the
the same
same time
time
as
T
is
derived,
from
the
techniques
briefly
e
as Te is derived, from the techniques briefly described
described
a)
a) Electron
Electron densities
densities
In
In the
the hypothesis
hypothesis that
that the
the plasma
plasma speed
speed is
is negligible
negligible
at
the
positions
where
UVCS
took
data
(heliocentric
at the positions where UVCS took data (heliocentric
altitudes:
1.6 and
1.9 R
R®,
in streamer
streamer areas)
areas) we
altitudes: 1.6
and 1.9
we can
can
, in
make
a
crude
evaluation
of
the
electron
density
in
make a crude evaluation of the electron density NNee in
the
the streamer
streamer from
from Ovi
O VI lines,
lines, assuming
assuming that
that the
the major
major
1
1 this
thiscan
canbe
beevaluated
evaluatedfrom
fromthe
themeasured
measuredintensities
intensitiesofofthe
theHI
H Ilines,
lines,
taking
takinginto
intoaccount
accountthe
thepredicted
predictedratios
ratiosbetween
betweentheir
theircollisional
collisionaland
and
radiative
components,
see
[2].
radiative components, see [2].
85
above. For instance, iron abundance is derived from the
log R vs. log T curves: at the temperature where the curve
intersect, log R — 0, if the abundance used in the theoretical calculation were corresponding to the abundance in
the regions where observations were taken. The shift in
the log R vs. log T profiles, required to make log R — 0,
gives the correct value of the iron abundance.
The above method cannot be applied to oxygen ions,
though, as we do not have oxygen ions from different
ionization stages. Hence, we used an alternative technique, which requires first the identification of the radiative and collisional components of the O VI doublet
lines.
From the radiative components we have
A
l/ra</
=
an anomalous behaviour also as far as densities are concerned: while its density at 1.6 RQ is lower, its density
gradient is less steep than in any other day. We notice,
however, that an error of 20% will raise the density at
1.6 RQ to values found on other days.
16 JUN, R=1.6
7^(1032)____________
J«w/(Lyp) Qr/#0F//10327<//5£(1032) 8v///
6.00
(4)
6.10
6.20
6.30
6.40
6.50
log T
while from the collisional components we have
'col
—
4o/(1032) C/// #Lyp #LyB
—
——— -—— ———
7co/(Lyp) toy/BQVI qm2
,-,
(5)
FIGURE 3. Plot of log fy vs. logT for June 16, 2000, at the
heliocentric altitude of 1.6/?0. Data refer to the UVCS pixel
lying along the radial to Ulysses. Lines from three ionization
stages of Ulysses - FeX, Fexn, Fexm- have been used to
build the plot, theoretical line emissivities have been taken from
[8].The streamer appears to be approximately isothermal.
where (Afo/MOco/ and (No/Nu)raei indicate the oxygen
abundance value derived from the collisional and radiative components of the line intensity; 7CO/ and7ra</ are the
collisional and radiative components of the intensity in
photon cm~2 s"1 sr"1; Covi/Qii is the ratio of ion concentration (which, in the log T interval 6.1-6.3 changes
by < 16%); / is the oscillator strength; B is the branching ratio; I^isk is the disk intensity in O Vl/Lyp lines; 5v
is the line width and q the excitation rate.
16 JUN, R=1.6 R 0
RESULTS
Figure 3 gives an example of the application of the technique described in b) of the previous Section to observations acquired at 1.67?0 on June 16,2000. The plot shows
that plasma is isothermal, at the position to which data refer, as the log-K-^vs. log T curves for different ions interobs
sect at practically the same temperature. Figure 4 gives
an example of the DEM technique, mentioned in b) of
the previous Section, applied to data taken on the same
day as in Figure 3. Both methods yields approximately
the same temperature. Results for the four days we are
analyzing appear in Table 1. Temperature and densities
are given along the radial to Ulysses at 1.6 and 1.9 R®.
On June 11 we made observations only at the lower altitude, hence physical parameters are given only at the
lower height. The error in temperature values is on the
order of 50 103 K, in densities is on the order of 20%.
Temperatures at 1.6 RQ are w 1.2 106 K, for all days but
June 16. It also looks like temperature decreases with altitude, with the exception of June 12. This day may have
K
log T [K]
FIGURE 4. Plot of logDEM vs. logT for June 16, 2000, at
the heliocentric altitude of 1.6R0. Data refer to the UVCS pixel
lying along the radial to Ulysses. Lines from different ions have
been used (see symbols); the temperature at which the DEM
peaks is approximately the same as given by the plot of log ^
vs. logT" (Figure 3).
In order to interpret our results we need to understand
which feature we are observing. To this end we cannot rely on LASCO images, as they refer to higher altitudes than our data. Hence we made plots of the total
intensity of the O VI 1032 A line vs. distance along the
UVCS slit, at 1.6 and 1.9 RQ. Because the UVCS slit
86
TABLE 1. Electron temperature, Te, and density, Ne, along the radial to Ulysses
\\June
1 2 June
1 3 June
1 6 Jt/fle
1.6*0
.6*©
.9*©
.6*0
.9*0
.6*0
.9*0
Tel MK
Nel 106 cm~3
1.17
1.20
1.48
1.23
1.03
1.38
1.26
5.00
4.62
1.73
5.60
1.52
5.58
2.50
technique described in c) of the previous Section we
need to know the disk intensities in the H Lyp and O VI
1032 A lines and to identify the collisional and radiative components of the O VI and hydrogen lines. As disk
intensities vary with time, a precise knowledge of these
parameters is essential to a good determination of the
oxygen abundance. Also, to identify correctly the collisional and radiative components of the Lyp line, we need
to know the ratio between the radiative and collisional
components of the Lya and Lyp lines, which depend as
well from the chromospheric illumination.
There is no room here for a detailed description of how
we evaluated these factors. Briefly, we can say that, as we
do not have disk intensities measured at the time our data
have been acquired, we estimated disk intensities starting
from measured values (SOLSTICE values for Lya and
UVCS Lya and O VI 1032 disk measurements in 1996
and 1997) and extrapolated these to the time of our observations on the basis of the temporal increase predicted
by [ 10] for the Lyman continuum and the NeVIII and NV
lines, assumed to be representative of the Lya, and of the
Lyp and O VI lines, respectively. Extrapolated intensities
agree with indications given by [11] for the variation between maximum and minimum of the solar cycle for Lya
and other lines.
As for the separation of the collisional and radiative
components of the lines, Raymond et al. [2] give values
for these ratios. But, as we mentioned, these as well depend on disk intensities: as a consequence, we estimated
new ratios, from the disk intensities we predicted. However, in evaluating abundances from Eq. 4 and Eq. 5, we
let the ratios vary in between Raymond et al. values and
the newer estimates, until (^a)rad = (^ a )coii- In this way,
we sort of took care of uncertainties in the determination of disk intensities, as well as of the unknown short
temporal variation they may go through over the time interval when data were acquired. Alternatively, as more
usually done, we might have considered average values
from the collisional and radiative abundance determination. Practically, the two technique are equivalent, as
values from the two determinations are pretty close: the
maximum discrepancy we had, using the old vs. the new
factors, is on the order of 35%.
Oxygen abundance values are given in Table 2. As
with other parameters, the behavior of June 12 is anomalous, as abundances increase with altitude. In the other
days, we find an higher abundance value, when observing the streamer edges (June 11 and 13), than when observing the streamer body (June 16). This result confirms
previous determination by [2, 12], at altitudes comparable to those of this work, and by Zangrilli et al. [9] at
higher heliocentric distances.
center lies along the radial to Ulysses, those plots show
that the direction to Ulysses traverses a region with an
approximately flat intensity distribution, i.e. through the
streamer body, on June 16, while it is adjacent to a region
of very steep intensity gradient, i.e. intersects a streamer
edge, on June 11 and 13. On June 12, however, there is
an ambiguous situation, because we have contrasting information at the two altitudes: it looks like we observe
the streamer edge, at the low heliocentric distance, and
the streamer body at high heliocentric distance. Possibly, projection and/or temporal effects do not allow us
to identify clearly where the radial to Ulysses is lying.
Hence, from now on, we consider data from June 11 and
13 as representative of conditions of the streamer's legs,
data from June 16 as representative of conditions of the
streamer's "body" and leave the June 12 case apart. In a
later study, we plan to check whether coronagraph observations at lower heights (e. g., from Mauna Loa) provide
any useful indications to understand the morphology of
the June 12 case.
In this scenario, we may conclude that the electron
temperature is lower at the streamer's edges, than in
streamer's cores. This conclusion should be further tested
calculating the electron temperature distribution across a
streamer, because we cannot rule out, so far, the possibility that the lower Te of June 11 and 13, is lower throughout the streamer than the Te derived for the streamer's
body on June 16. However, should the present indication
be confirmed by a more detailed analysis, the abundance
obtained in streamer's legs by different authors should
be rediscussed in terms of a core-leg variation of the
streamer electron temperature.
Densities, on the other hand, appear to have, at 1.6 *0,
the same value independent of whether they refer to the
streamer edge or to the streamer body. In the streamer
body the density gradient is possibly less steep than in the
legs, but once more we need to confirm these indications
by a more extended analysis. We notice, however, that
densities inferred by LASCO at higher altitudes seem
to give analogous indications (see Zangrilli et al., this
volume [9]).
In order to derive the oxygen abundance with the
87
TABLE 2. Oxygen abundances along the
radial to Ulysses
Oxygen Abundance
(Phot. = 8.93)
1 1 June
\2June
1.6*0
1.6*©
1 3 June
1.6*0
1.9*0
1.6*0
1.9*0
1 6 June
8.61
8.38
8.53
8.67
8.55
8.43
8.49
SUMMARY AND CONCLUSIONS
In this paper we have presented results from a preliminary analysis of UVCS data taken in June 2000, at the
time of an Ulysses-SOHO-Sun quadrature. Our analysis
provides further evidence in favor of an enhanced oxygen
abundance in the legs of streamers, with respect to the
streamer's core. This enhancement favors the hypothesis that streamer legs are sources of the slow solar wind:
because abundances in the core are too low to be compatible with abundances in the wind, as measured by in
situ experiments. It may be worth pointing out that the §
ratio in the slow wind, takes the value § = 1890 ± 600
while in the fast wind g = 1590 ± 500, [13]. The oxygen
abundances we derived from June 11 and 13 are consistent with the slow wind estimates, while both the June 12
and 16 values are inconsistent with reported values.
One of the aims of quadrature campaigns is the comparison between coronal and in situ data. Hopefully, we
might be able to check values of § from data measured
by Ulysses versus our coronal values. However, the two
values quoted above for slow and fast wind are only
marginally different and it may be difficult to draw definite conclusions from this kind of comparison.
Obviously, we need to extend the analysis of quadrature observations to the other days when we acquired
data. Also, we are planning to extend the analysis of element abundances to include other elements as well. Estimates of the Fe abundance lead to values higher than
found by [2]. For instance, Figure 3 shows that the iron
abundance is photospheric on May 16, at 1.6 RQ. Fe is a
low FIP element: it will be interesting to check whether
this overabundance is common to other low FIP elements. Also, analysis of CDS and SUMER data will allow us to compare the values we find from UVCS with
values derived from other data sets, at different altitudes.
Another interesting result we got is related to the
electron temperatures in streamers. There have been a
few estimates of Te in streamers, but no attempt has been
made, yet, to derive the profile of Te across a streamer.
We plan to extend our analysis to different location than
so far examined, to check whether the indications from
this work, which favor an increase of Te towards the
streamer edge is confirmed. This would allow a better
evaluation of the abundance profile across streamers.
ACKNOWLEDGMENTS
The work of GP has been partially funded by MURST
- the Italian Ministry for University and Scientific Research. SP acknowledges support from ASI, the Italian
Space Agency. SOHO is a mission of international cooperation between ESA and NASA.
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