Oxygen Abundance in the Extended Corona at Solar
Minimum
Ester Antonucci* and Silvio Giordano*
*Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
Abstract.
We present a study on the abundance of oxygen relative to hydrogen in the solar minimum corona and for
the first time we measure this quantity in polar coronal holes. The results are derived from the observations of
the extended corona obtained with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The diagnostic
method used to obtain the oxygen abundance is based on the resonant components of the O VI 1032 A and
HI 1216 A emission lines. This method fully accounts for the effects of the outflow velocity of the solar wind,
which can be determined through Doppler dimming, and of the width of the absorbing profiles of the coronal
ions or neutral atoms involved in resonant scattering. The oxygen abundance is higher in the polar coronal hole
regions, where the fast wind is accelerated, than in the streamer belt. In the polar regions the observed oxygen
abundance is consistent with the photospheric value and with the composition results obtained with Ulysses for
the fast wind. The oxygen abundance values derived with UVCS suggest that the plasma remains substantially
contained in quiescent streamers, that therefore do not contribute significantly to the solar wind.
INTRODUCTION
gestion that gravitational settling of heavier ions is occurring in the closed field line region present in the center
of streamers. The magnetic configuration, in this case, is
roughly that of Figure 1. The OVI core dimming, however, can also be related to the origin of a slow wind flowing outward along open magnetic field lines that separate
the OVI bright structures existing within a streamer, that
in this case are interpreted as evidence for substreamers
(Figure 2). Therefore in this case the depletion of oxygen
is related to the outflows in the magnetically complex
structure of the equatorial belt at solar minimum [16].
A statistical study of the quiescent streamer belt performed by Marocchi, Antonucci and Giordano [15] has,
on the one hand, confirmed the deficiency of oxygen in
the core of the streamer belt, with an abundance value
relative to hydrogen close to 1.0 x 10~4 (8.0); on the other
hand, it has shown that the oxygen depletion is dependent on heliodistance, and the dependence is more pronounced in the lateral branches where the abundance decreases from 3.9xlO~ 4 (8.59) at 1.5 R0 to 2.2xlO~ 4
(8.34) at 2.2 R0 .
In the solar wind the oxygen abundance remains close
to photospheric values. Although the SWICS/Ulysses instrument has detected a tendency to a systematic variation of the oxygen abundance between 5.3 x 10~4 (8.7)
and 6.3 x 10~4 (8.8), related to the variation of wind
speed between 400 km s"1 and 800 km s"1, during
the period of declining activity between mid-1992 and
The capability of determining elemental abundances in
the extended corona, the region marking the transition between the inner solar atmosphere and the heliosphere, has been achieved only recently with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard
SOHO. This instrument has opened the possibility of determining the ultraviolet emissions of neutral hydrogen
and minor ions beyond 1.5 RQ, and, on the basis of these
quantities, the abundance of minor elements relative to
hydrogen.
Most of the attention has been focussed up to now
on the abundance of oxygen, since the OVI 1032 and
1037 are, with the HI Ly a at 1216, the strongest lines
in the extended corona [e. g. 17]. Moreover, during solar
minimum the oxygen abundance has been measured first
in streamers where emission lines are brighter.
A new phenomenon was immediately evident when
imaging the extended corona with UVCS. Solar minimum quiescent streamers are characterized by a depletion of oxygen which is particularly strong in their
core (of roughly an order of magnitude with respect to
the photosphere) and less severe in the lateral branches
[16, 17]. This result, coupled with the fact that Feldman
et al. [7] and Feldman [8] found an oxygen abundance
value close to the limb, at 1.03 R0, consistent with the
photospheric value, led Raymond et al. [17] to the sug-
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
77
mid-1993, this difference might not be fully significant
[19]. The large discrepancy in the oxygen abundance
of streamers and wind suggests that it is unlikely that
the streamer plasma does significantly contribute to the
wind, and in particular to the slow wind confined in the
low-latitude/equatorial heliospheric regions during solar
minimum. Hence, a possible hypothesis is that the slow
solar wind would not originate primarily from quiescent
streamers but from the layers surrounding the streamers, where the plasma is flowing outward channeled in
flux tubes with large expansion factors [15]; that is, in
a region of transition between streamers and the core of
coronal holes. The above hypothesis is reasonable since
quiescent streamers are mainly characterized by closed
field lines. If instead the magnetic topology is such that a
streamer is structured in substreamers, as suggested by
the OVI images, there is the possibility of leakage of
coronal material flowing along the open magnetic field
lines separating adjacent substreamers (Figure 2).
Active streamers might certainly behave differently
by significantly contributing to the slow wind; but the
alternating slow and fast wind streams observed with
SWICS/Ulysses persists substantially unmodified independently of the level of activity of a streamer. Therefore the degree of change in the magnetic topology of
the equatorial streamer belt, in principle, should not be
determinant in the formation of the slow wind.
If the interpretation of the data analyzed up to now is
correct we then have to measure the oxygen abundance
in the coronal regions outside the streamer belt, predominantly characterized by open field lines, in order to identify the abundance signature of the solar wind. Furthermore we might attempt to identify possible oxygen variations in the slow and fast wind streams. This task might
be not so simple when studying oxygen since this is a
high FIP element and therefore its concentration is not
so variable from slow to fast wind streams as expected
for the low FIP element concentrations [e.g. 21]. Here
we address the first problem, that of determining the oxygen abundance close to the Sun in a polar coronal hole,
with a new diagnostic method that allows to derive such
quantity in a region of significant outflows related to the
acceleration of the fast wind.
FIGURE 1. Dipolar magnetic configuration of a streamer.
lines used in the analysis. This is because the main mechanism of line formation in the tenuous extended corona
is resonant scattering of photons originating in the lower
atmosphere, since the importance of the radiative relative
to the collisional contribution to the intensity of a coronal
line is increasing with heliodistance (the radiative component decreases with density, while the collisional one
decreases as density squared), and the resonant emission,
in presence of a radial outward flow, undergoes Doppler
dimming. This effect is due to the red shift of the incident radiation in the frame of reference of the expanding
plasma. The oxygen coronal abundance relative to hydrogen is derived from the ratio of the bright oxygen line, the
OVI 1032 line and the dominant coronal hydrogen line,
H I Lyoc 1216. Since in the corona the latter is formed
almost exclusively by resonant scattering, the abundance
is derived by computing the ratio of the radiative component of O VI1032, and the H I Lyoc 1216 line, according
to the formula derived by Antonucci and Giordano [2]:
NH
(1)
where Ir,ovi and /r?/// are the intensities of the radiative
components of O VI and the HI line, bni and bovi are
the branching ratios of the considered transitions, B\2,m
and B\2,ovi are the Einstein coefficients for stimulated
emission, AO,#/ and ho^ovi are the reference wavelengths
of the transitions, and 2Q&HQ &UHH are the concentrations of
the OVI ions and HI atoms, respectively.
The quantities FD (w) are the functions which take into
account the line intensity dimming due to the outflow
velocity of coronal plasma, w. They also account for
the width of the coronal absorbing profiles along the
direction of the incident radiation. Coronal absorbing
profiles are usually much wider than thermal profiles in
regions of open magnetic field lines, and their width may
OXYGEN ABUNDANCE DIAGNOSTICS
IN AN EXPANDING CORONA
In order to determine the oxygen abundance of the extended corona outside the streamer belt, we need to adopt
a diagnostic method that accounts for the presence of
outward plasma flows related to the expansion of the solar corona along open field lines. The radial flows induce
a dimming in the emission of the oxygen and hydrogen
78
hydrogen of 6.0 ± 1.1 10 4. The error is due to the low
statistics found in the faint coronal hole regions. Possible
systematic errors (< 30%) can be due to uncertainties in
the atomic parameters.
The electron density and temperature values used correspond to the model of tenuous, cool corona considered
by Antonucci, Dodero and Giordano [1], to derive the
outflow velocity in coronal holes. As discussed in that
paper, the still existing discrepancies in the observational
results on the physical conditions in coronal holes allow
also a model of relatively denser and hotter plasma, if
we use the temperature derived by Ko et al. [13], on the
basis of the in situ composition measurements of the solar wind performed with SWICS/Ulysses, and the density derived from the visible light data obtained with
UVCS/SOHO by Kohl et al. [14]. In this case, at 1.6
RQ the outflow velocity is negligible and the corresponding oxygen abundance is a factor 2.4 larger than that derived for a tenuous and cool corona (first model). Since
this value is clearly unsound, the cool, tenuous coronal
model is preferred, in agreement with other indications
in favor of this model (for instance, the electron density
value as derived for the extended corona with the spectroscopic method discussed by Antonucci and Giordano
[3] is consistent with the Guhathakurta et al. [12] density
curve derived from visible light data). For what concerns
the electron temperature, although the results obtained by
David et al. [5] do trace this quantity only out to roughly
1.3 RQ , its values remain below 1 x 106 K and there is a
tendency to decrease outward when approaching 1.3 R©
. Therefore, considering this tendency and that the interplanetary value is of the order of 105 K, we do not expect
significant temperature variation from the inner corona
out to 1.6 RQ , where we assume as a value 8 x 105 K.
The slow variation of temperature and relatively low expansion velocity ensure that, even in presence of an expanding corona, we can determine the concentrations of
neutral hydrogen and oxygen ions which do not vary significantly out to 1.6 R0 .
The oxygen abundance derived for a polar coronal
hole in this analysis is consistent with the fast wind values obtained with SWICS/Ulysses, 6.3 x 10~4, and the
photospheric value as shown in Table I, where we report both the photospheric value, 6.7 x 10~4 obtained
by Grevesse et al [10] and the value recently re-evaluated
by Grevesse [11], which is somewhat lower, 5.9 x 10~4.
Table I summarizes the oxygen concentration values resulting from the UVCS observations of the extended
corona, compared with the heliospheric data measured
with SWIGS and the photospheric data.
FIGURE 2. Magnetic topology of a complex streamer consisting of substreamers separated by open field lines. The
dashed lines with arrows indicate slow plasma flows along the
open magnetic field regions [16].
vary depending on the direction, since in polar coronal
holes the velocity distribution of the oxygen ions are biMaxwellian above 1.8 R© [14].
The separation of the radiative and collisional component of the OVI1032 is performed by taking into account
the OVI1037 line [3].
RESULTS ON THE OXYGEN
ABUNDANCE IN A POLAR CORONAL
HOLE DURING SOLAR MINIMUM
The oxygen abundance is determined in a polar coronal
hole observed at North on May 21, 1996. The intensities of the OVI 1032 and 1037 and HI 1216 lines are
averaged over an instantaneous field of view (30 arcmin
x 84 arcsec) perpendicular to, and centered on, the radial direction at 1.6 R© . Therefore the average height of
the measurement is 1.64 R0 . The local outflow velocity, w = 80 km s"1, is derived from the Doppler dimming of the OVI doublet by means of the the OVI 1037,
1032 line ratio diagnostics. The code used in the analysis is discussed in the paper by Dodero et al. [6] and
Antonucci, Dodero and Giordano [1]. The ion velocity
distribution is considered to be Maxwellian since at the
coronal level of 1.6 R there is no evidence for anisotropy
in the ion velocity distribution. In order to derive Doppler
dimming we have assumed a local electron temperature,
8 x 105 K, compatible with the low temperature values
determined in the same coronal hole by David et al. [5]
and the electron density as derived by Guhathakurta et al.
[12]. The same electron temperature is assumed to compute the concentrations of the HI atoms and OVI ions. In
these conditions we find an oxygen abundance relative to
79
TABLE 1. Oxygen Abundance.
Extended Corona
Heliosphere
streamer core
streamer lateral structures
coronal holes
slow wind
current sheet
fast wind
Photosphere
DISCUSSION
1 x 1(T4
3.5-2.2 x 1(T4
6.0 x 1(T4
5.3 x 1(T4
< 4.0 x 10~4
6.3 x 10~4
6.7- 5.9 x 10~4
In the case of this magnetic topology, the clear heliodistance dependence observed in the streamer lateral structures could be interpreted in terms of gravitational settling in the closed field lines outlining substreamers.
In this scenario the most probable source of the slow
wind is likely to be found in a region of transition between streamers and the core of coronal holes. However, whether the oxygen abundance signature can contribute to the identification of the source of the slow wind
remains an open question. In fact, slow and fast wind
compositions certainly differ for what concerns the 'first
ionization potential' (FIP) effect, which is enhanced by
a factor of 1.5-2 in the slow wind relative to the fast
wind, with a quite sharp transition of the low FIP element abundance at the fast/slow wind boundary [21]. Although oxygen is a high FIP element, and therefore it is
probably not the best candidate to identify the transition
between slow and fast wind in the corona, future studies
might, however, provide information on possible variations in composition in the open field line regions outside
streamers, that might also exist in the heliosphere but be
masked by the statistical uncertainty.
Our analysis indicates that, on the basis of the oxygen
abundance, the plasma of coronal holes is the same as
that observed in the fast wind streams. In addition, this
plasma, accelerated in the extended corona to form the
fast wind, retains the photospheric abundance of a highFIP element such as oxygen. Therefore the present result, that provides the first determination of oxygen abundance in coronal holes, shows that this quantity is preserved from the convection zone to the heliosphere along
the open field lines that are channeling the fast wind.
Regarding the low-latitude/equatorial regions, in the
introduction it was pointed out that the oxygen abundance of quiescent streamers derived in previous studies
is significantly lower than the slow wind abundance. In
fact in the bright lateral structures of streamers oxygen
is depleted from 3.5 x 10~4 at 1.5 R0 to 2.2 x 10~4 at
2.2 R0 and in the core of a streamer from 1.3 x 10~4 to
0.8 x 10~4, in the same range of heliodistance [15].
In order to reconcile the abundance values in the
outer corona and heliosphere to the observational evidence pointing to a general association of slow wind and
streamer equatorial belt, Raymond et al. [18] suggested
the following interpretation. Whereas the core plasma
is magnetically confined for long intervals of time, and
thus the confinement is causing the observed severe oxygen depletion due to gravitational settling, the slow wind
might originate in the lateral branches where the plasma
might be confined for shorter time and therefore partially
escape outward (Figure 1). However even considering
possible systematic errors in the spectroscopic analysis
of the oxygen abundance (< 30%), the oxygen depletion
in the streamer lateral structures is significant; whereas
the wind abundance, even in the case of the slow wind
which is showing a tendency for a decrease relative to
fast wind, does not differ significantly from the photospheric abundance on the basis of the statistical errors
of the available data. The only channels for plasma leakage in the equatorial belt, therefore, remain the open field
lines between substreamers present in the magnetic configuration proposed by Noci et al. [16], as shown in (Figure 2). This plasma might contribute, but only to the heliospheric region close to the interplanetary sector bounbary, that indeed is noticeably poor in oxygen (Table I).
ACKNOWLEDGMENTS
This work was supported by ASI (Italian Space Agency)
and MURST (Ministero dell'Universita e della Ricerca
Scientifica e Tecnologica) contracts.
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