Investigation of the sources of the slow solar wind
Lucia Abbo , Ester Antonucci , Maria Adele Doderoy and Carlo Benna
Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, Pino Torinese, Italy
yUniversità degli Studi di Torino, Torino, Italy
Abstract. Aim of this analysis is to study the variation of the physical conditions of the coronal plasma across the streamer
boundary in order to identify the coronal sources of the slow solar wind during the minimum of solar activity. The analysis
is based on the observations of equatorial streamers, obtained in the outer corona during the years 1996 and 1997 with the
Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO. The outflow velocity, the electron density and the oxygen
abundance relative to hydrogen of the coronal plasma have been determined, in the range between 1.6 and 3.5 solar radii
(R ), by means of a spectroscopic analysis of the OVI 1032, 1037 Å and the HI Ly 1216 Å lines. Coronal expansion at low
velocity, in the range 80–100 km/s, is observed along regions 15 o –20o wide, surrounding the streamer boundary. Evidence
for coronal plasma outflows at low velocity is also found further out in the region along the streamer axis. In this case the
outflows become significant beyond 2.7 R . Hence, the slow solar wind during solar minimum flows just outside the denser
and brighter zone of a streamer, characterized by closed magnetic field lines and in a lane around the heliospheric current
sheet, forming just above the closed field line region.
INTRODUCTION
ponent of the coronal plasma uniquely on the basis of the
OVI 1032, 1037 doublet, and to compute the abundance
of oxygen relative to hydrogen by including also the HI
Ly 1216 line data. The technique fully accounts for the
effect of Doppler dimming in an expanding corona.
The ultraviolet lines emitted in the extended corona are
formed both via collisional and radiative excitation. The
radiative contribution becomes predominant in low density conditions. The collisional and radiative components
are separated by considering a pair of lines emitted from
the same ion, such as the OVI 1032 and 1037 Å lines.
The electron density is then derived from the ratio of the
collisional to radiative component of a spectral line, being proportional to this ratio multiplied by a factor depending on the outflow velocity. Therefore in order to
derive electron density and outflow velocity, at the same
time, we need a further constraint given by the mass flux
conservation along the flow tube connecting the corona
and the heliosphere: ne wA=const, where ne is the electron density, w is the outflow speed, A = F (r) r 2 is the
cross section of the flux tube and the quantity F (r) takes
into account the deviation from radial expansion.
The abundance of oxygen relative to hydrogen is computed from the ratio of the radiative components of the
OVI 1032 line and the HI Ly 1216 line, both functions
of the outflow velocity (Antonucci and Giordano, 2001),
by assuming conditions close to ionization equilibrium.
According to the Ulysses observations, during solar minimum the slow solar wind tends to be confined in the
equatorial streamer belt, whereas the fast wind fills most
of the heliosphere. This study is an attempt to identify the
coronal regions where the heliospheric slow wind originates, by analyzing the observations obtained with the
Ultraviolet Coronagraph Spectrometer (UVCS) onboard
SOHO. The most probable sources of the slow wind are
the regions of open magnetic field lines at the border
of the large quiescent equatorial streamers observed during solar minimum. Therefore we focus on the comparison of the dynamical conditions observed in the central
brightest streamer region, corresponding to plasma predominantly confined by closed field lines, and in the dim
lanes running along the borders of these bright regions.
The UVCS observations of the outer corona, extending
from 1.5 R to a few solar radii, provide a set of spectroscopic data apt to identify plasma outflows by means of
the Doppler dimming analysis of the resonantly scattered
coronal emission.
DIAGNOSIS OF THE CORONAL
ULTRAVIOLET EMISSION
The spectroscopic diagnostic technique used in this study
(Antonucci et al., 2002) allows us to determine the electron density and the outflow velocity of the OVI ion com-
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
238
Date
19
22
30
1
30
5
Aug
Aug
Aug
Sep
Apr
May
1996
1996
1996
1996
1997
1997
Streamer axis
(PA, o )
Altitude range
(R )
120
60
240
280
280
80
1.6 – 3.5
1.6 – 3.5
1.6 – 3.5
1.6 – 3.5
1.7 – 3.3
1.8 – 3.3
Kinetic Temperature OVI (K)
TABLE 1. Coronal streamers used in the analysis.
108
107
106
1
2
3
4
Heliocentric Distance
5
OBSERVATIONS AND DATA ANALYSIS
FIGURE 1. Kinetic temperature of OVI ions as a function
of heliocentric distance (in solar radii) relative to both the
streamers (full dots) and the regions adjacent to the streamer
boundary (full triangles).
The present analysis is based on a set of six streamers,
observed with UVCS at high spectral resolution. They
are detected at mid and low latitudes during the solar
minimum period 1996–1997. Table I reports the following quantities: date of observation, position angle (PA,
in degrees, counterclockwise from the North Pole) of the
streamer axis, range of the UVCS fields of view, in solar
radii (R ).
For each streamer we define the streamer boundary
as the contour level corresponding to the 1/e value of
the OVI line intensity maximum (different definitions
of streamer boundary are discussed by Abbo and Antonucci, 2001). The intensities of the OVI 1032, 1037 and
HI Ly 1216 lines are then computed by integrating the
line emission in two regions: within the streamer boundaries and in the external region, approximately 15o -20o
wide, adjacent to the boundary. The integrated emissions
are then fitted with a gaussian curve to obtain the line
intensity and width, that can be expressed in terms of
the kinetic temperature, Tk , quantity that measures the
velocity distribution of the emitting particles along the
line-of-sight (l.o.s.).
The kinetic temperatures derived from the OVI 1032 line
profiles are plotted in Figure 1 as a function of heliocentric distance (the values found for streamers are reported
as full dots and those relative to the regions surrounding streamers as full triangles). This quantity is clearly
increasing with height and the values found outside are
significantly higher than inside streamers (Antonucci et
al., 1997).
The diagnostic technique to be applied to the UVCS
data is based on the computation of resonance absorption along the direction of the incident radiation coming
from the disk, within a solid angle subtended by the solar disk itself. Therefore the analysis has to include the
distribution of the oxygen ion velocity in three dimensions, whereas only the distribution along the l.o.s. is
observed. Within the streamer, that is, in higher density
conditions, we assume an isotropic maxwellian velocity
distribution with the width defined by the observed T k .
Outside the streamer, in the thinner regions adjacent to
the streamer boundary, we assume a bi–maxwellian velocity distribution of the ions, with a kinetic temperature
equal to the observed one in the plane perpendicular to
the radial direction, and equal to the electron temperature along the radial direction. This assumption is dictated by the fact that line broadening in the regions close
to streamers is about twice larger than in the streamer
itself, thus representing an intermediate condition between closed field regions and the core region of coronal
holes, where the ion velocity distributions are found to
be highly anisotropic (e.g., Kohl et al. 1998, Cranmer at
al. 1999, Antonucci et al., 2000).
The coronal electron temperature, Te , assumed in the
analysis of streamers is that derived by Gibson, et al.
1999. In the open field line regions it is inferred from
the coronal hole measurements by David, et al. 1998 (see
Antonucci et al. 2000).
In order to define the constraint imposed by mass flux
conservation in the hypothesis of coronal expansion, the
flux tube geometry has been modeled by assuming the
expansion factor of the magnetic field lines as derived by
Wang and Sheeley (1990) for the regions immediately
adjacent to a streamer. In this model the magnetic field
is taken to be potential everywhere except in an equatorial current sheet (with the inner boundary of the current
sheet located at 2.5 R ), where the magnetic field radial
component changes sign discontinuously and the latitudinal component vanishes. In this model the open magnetic field lines adjacent to the streamer are characterized
by a maximum expansion factor, F(r)max = 13:5, much
larger than that expected in the core of coronal holes,
where F(r)max = 4. In a previous analysis (Abbo and
239
108
Electron density (cm-3)
Electron density (cm-3)
108
107
106
105
104
1
2
3
4
Heliocentric Distance
107
106
105
104
1
5
FIGURE 2. Electron density as a function of heliocentric
distance (in solar radii) derived for streamers (full dots) and for
the regions surrounding the streamer boundary (full triangles).
2
3
4
Heliocentric Distance
5
FIGURE 3. Streamer electron density as a function of heliocentric distance (in solar radii) calculated by assuming static
conditions (full dots) and dynamic conditions (asterisks), compared with that derived by Gibson et al, 1999 (dashed line) from
visible light observations.
Antonucci, 2002) we presented preliminary results of the
same data set, obtained with a factor equal to the total expansion factor of a coronal hole, as measured by Munro
and Jackson, 1977.
corresponding to a portion of the spectrometer slit, set
to 1.6 R , far from the center) and around the streamer
axis above 2.7 R , the oxygen ions kinetic temperatures
are higher than within the inner part of the streamer
(Figure 1). If this large increase in kinetic temperature
is interpreted as energy deposition as suggested by the
coronal hole observations (Antonucci et al. 1997b, Kohl
et al. 1997, Noci et al. 1997), these regions where the
plasma expands are regions of energy dissipation. Such
an effect, however, occurs only beyond 2.7 R along
the streamer axis, presumably in coincidence with the
transition from closed to open magnetic field lines, that
is, close to the inner boundary of the heliopsheric current
sheet.
In the regions where coronal expansion is found to
be compatible with the OVI emission data, we find an
outflow velocity ranging from 90 km/s to 100 km/s
along the streamer border and a value about 80–90 km/s
above 2.7 R along the streamer axis. These values
are compatible with a regime of slow wind. In fact in
coronal holes between 1.8 R and 3.5 R the fast wind
expands much more rapidly with a speed that varies
typically from about 100 km/s to 400 km/s.
A distinctive feature of the heliospheric slow and fast
wind, besides the wind speed, is the plasma composition.
Therefore we also derive the oxygen abundance relative
to hydrogen, although, being a high FIP element, oxygen
does not show a marked variation between slow and fast
wind. The oxygen abundance is plotted in Figures 4,
inside (full dots) and outside streamers (full triangles),
as a function of heliocentric distance. The abundance
decreases with increasing heliodistance, as found in quiescent streamers (Marocchi et al. 2001). In streamers it
ranges from values close to the photospheric abundance,
RESULTS
The electron densities averaged over an individual
streamer and over its surrounding regions have been
derived in both cases in the assumption of a dynamically
expanding atmosphere and of a static plasma. The values
have been further averaged over the set of streamers
considered in the analysis. The results are plotted versus
heliocentric distance in Figure 2.
A comparison of the streamer electron density with
that derived from the visible light coronal observations
by Gibson et al. 1999 is shown in Figure 3. It is clear
that only the values obtained for a static atmosphere
(full dots) are compatible with the white light results
(dashed line), except for the density at 3.5 R , where
both values, obtained in static and dynamic conditions
(asterisks), are compatible with the visible light result.
Hence, we conclude that the plasma in the bright emission region, delimited by the 1/e intensity contour line,
is predominantly static out to 2.7 R . At higher altitudes
data are also consistent with outward expansion.
In the regions adjacent to streamers, on the other hand,
the results obtained for a static plasma are unrealistic,
whereas those obtained for a dynamic expanding corona
(triangles in Figure 2) are acceptable, implying that
the narrow region, 15o –200 wide, along the streamer
boundary, is dominated by open magnetic field lines.
It is interesting to note that all along the streamer border
(that in the present set of data starts at 1.8 R , height
240
A(O/H)
10-3
10-4
10-5
1
visible and ultraviolet emission of an equatorial streamer
observed with UVCS. At last, considerations on the
oxygen abundance in the regions of plasma outflows do
support the interpretation of these regions as sources
of the slow wind. The resulting scenario is compatible
with the model proposed by Wang et al., 2000, of a
two–component slow wind: one component flowing
along the rapidly diverging open magnetic field lines
adjacent to the streamer boundary, and the second one
confined to the region of the denser equatorial plasma
sheet.
photospheric value
streamer core
2
3
4
Heliocentric Distance
5
REFERENCES
FIGURE 4. Abundance of oxygen relative to hydrogen as
a function of heliocentric distance (in solar radii) derived
for streamers (full dots) and for the regions surrounding
the streamer boundary (full triangles). The abundance value
for the streamer core, 110;4, and the photospheric value,
6.710;4, are plotted as dashed-dotted, dashed lines, respectively.
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6.7 10;4, to 1 10;4, abundance found in the core of
a streamer (e.g. Raymond et al., 1997). This supports
the interpretation given by Marocchi et al. that the
abundance of the wind close to the heliospheric current
sheet might derive by open field lines existing within
the streamer, and separating substreamers not always
resolved in the outer corona (following the hypothesis
of slow wind formed between substreamers formulated
by Noci et al. 1997). In the regions surrounding the
streamer border the decrease of abundance tends to be
less marked. Since also the slow wind oxygen abundance
tends to be lower than the fast wind abundance, the trend
found in Figure 4 for the regions adjacent to the streamer
border (full triangles) is supporting the conclusion that
these are sources of slow wind.
This study, based on the OVI coronal emission, leads
to the conclusion that the regions running along the
streamer boundaries are site of coronal outflows at low
velocities and therefore these are the likely sources of
the slow wind. In addition, coronal expansion at low
velocities is also likely to occur beyond 2.7 R along the
streamer axis, presumably where the transition between
closed and open magnetic field lines takes place and the
interplanetary current sheet forms. The values of the
outflow velocity found, in the range between 80 km/s
and 100 km/s, are consistent with the results obtained by
Sheeley et al., 1997 and Wang et al., 1998 for the outward motions of the density inhomogeneities detected
with LASCO (SOHO), which originate at the cusp of
streamers. They are also consistent with the results found
by Strachan et al., 2002, obtained by analyzing, with
the traditional Doppler dimming technique, the coronal
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