Equatorial Coronal Holes and Their Relation to High-Speed
Solar Wind Streams
Lidong Xia and Eckart Marsch
Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany
Abstract. Using together SUMER, EIT and MDI onboard SOHO, we examine plasma properties and magnetic fields at the
base of three equatorial coronal holes (ECHs) occurring during August and October 1996 near solar minimum. We estimate
the electron density, flow speed deduced from UV/EUV lines as well as the average magnetic field of the photosphere. These
ECHs produced distinct high-speed streams in the ecliptic plane with average flow speeds of larger than 500 kms 1 . With
SWE and MFI both onboard the WIND satellite, we also determine the parameter values of the plasma and magnetic field
of these high-speed streams at 1 AU in the Earth’s orbit. We discuss the relationships between observations of high-speed
streams at 1 AU and coronal holes at the coronal base.
TABLE 2. Instruments
Instruments Measurements
INTRODUCTION
The nascent fast solar wind may first be accelerated in
magnetic funnels [1]. This idea is supported by measurements of Doppler shifts of the Ne VIII line with SUMER
(Solar Ultraviolet Measurements of Emitted Radiation),
which show that large blue shifts occur mainly along the
chromospheric network [2]. Another interesting result is
that regions of large blue shifts spatially coincide with
very dark regions, if we compare the Dopplergram of the
hole with its intensity image in the same line [3]. However, the cited studies were carried out in polar coronal
holes (PCHs). In that case there are several observational
disadvantages due to the line-of-sight geometry. In this
contribution, we study three ECHs observed during the
solar minimum in the second half of 1996 and investigate their relation to interplanetary high-speed streams.
SUMER
EIT
MDI
SWE
MFI
The three ECHs and their related high-speed streams
are shown in Figs. 1 and 2. ECHs appear as dark regions in the EIT (Extreme ultraviolet Imaging Telescope)
meridian maps obtained in the Fe XII line (195Å).
The ECH1 and ECH2 including the positions of the
SUMER slit are shown in Figs. 3 and 4. Because the
coronal lines are quite weak in these ECHs, we have
binned the data along the slit in the Y direction selecting areas obtained from homogeneous dark parts of the
holes, from 190" to 335" for ECH1 (Fig. 3) and from -60"
to 50" for ECH2 (Fig. 4).
We deduce the outflow speed by measuring Doppler
shifts of the Mg X line (624.968Å, log T e =6.04) observed
in 2nd order. For wavelength calibration we used cool
lines of C I. Chae et al. [5] have estimated that C I lines
have only a small average red shift of about 1.5 kms 1 .
This value has been subtracted from the average Doppler
shift of the Mg X line. The Ne VIII line (770.428Å, log
Te =5.8) observed in ECH3 is strong enough, so that line
center positions can be determined for every pixel.
After getting the intensity ratio of two Mg IX lines
694/706 (log T e =5.98), the CHIANTI atomic database
[6] has been used to calculate the electron density in
OBSERVATIONS AND DATA ANALYSIS
We selected three ECHs that have been observed by
SUMER (Table 1). The instrument has been described
in detail elsewhere [4]. The detector B and slit 2 with the
size of 1"300" were used. In this study, simultaneous
data obtained by five instruments onboard SOHO and
WIND were analysed (Table 2).
TABLE 1.
ECHs observed by SUMER
Item
Date
Wavelengths
ECH1
ECH2
ECH3
August 27, 1996
October 12/13, 1996
October 19/20, 1996
Ref. Spec.: 660-1500Å
Ref. Spec.: 660-1500Å
Raster: 750-790Å
intensity, Doppler shift, line width and
electron density (Mg IX 694/706)
coronal hole boundaries (Fe XII 195Å)
photospheric magnetic field
solar wind speed and density at 1AU
magnetic field at 1AU
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
319
FIGURE 1. Left: EIT meridian map during August 22 and September 3, 1996, showing the recurrent ECH “Elephant’s Trunk”.
Right: Solar wind speed observed by SWE at 1AU during the same period showing a related high-speed stream. Note the
displacement in time, which is due to the 3-days travel time of the stream from the corona to 1AU.
FIGURE 2. Top: EIT meridian map during October 8 and November 1, 1996, showing three ECHs. Bottom: Solar wind speed
observed by SWE at 1AU during the same period showing three related high-speed streams.
FIGURE 3. Left: EIT map on August 27, 1996, showing part of the ECH “Elephant’s Trunk”. Middle: MDI magnetoheliogram.
Right: Intensity of the Mg IX 706Å line along the slit of SUMER. Note the coronal hole boundary indicated by a dotted
line and the SUMER slit position indicated by a solid line in the EIT and MDI images.
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FIGURE 4.
The same as Fig. 3 but for the coronal hole observed on October 12, 1996.
FIGURE 5. Left: EIT map on October 20, 1996, showing part of the ECH3. Middle: MDI magnetoheliogram (white: positive
polarity; black: negative polarity). Right: Contour plots of Doppler-shifts of the Ne VIII line overlaid on the MDI magnetoheli-
ogram. We take the average speed outside this ECH as 0 kms 1 , since we have no absolute wavelength calibration in
this spectral window (black dotted: -6, black solid: -2, white solid: 2, and white dotted: 6 kms 1 ; negative speed for
blue shifts and positive for red shifts). Note the coronal hole boundary indicated by a dotted line in the EIT and MDI
images.
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TABLE 3. Coronal parameters deduced from
SUMER and MDI
Ne
V
fe =Ne *V
B0
(cm 3 )
(kms 1 ) (cm 2 s 1 ) (G)
ECH1 and ECH2. The average electron density along the
slit below 335" in ECH1 has been measured to be about
2108cm 3 , which is consistent with the result obtained
by Del Zanna and Bromage [7] in the same hole.
MDI (Michelson Doppler Imager) full-disk magnetograms (sampled at a rate of 15/day) are used to estimate
the average photospheric magnetic field across areas that
spatially coincide with the ones selected to deduce the
outflow speed and electron density by using SUMER
data (Figs. 1 and 2).
A simple computation is performed to determine the
parameter values of the plasma and magnetic field of the
high-speed streams at 1AU, by averaging the measured
values across the least variable part of the high-speed
streams between the leading and trailing edges.
ECH1
ECH2
0.8108
1.8108
7.9
1.4
6.31013
2.51013
3.1
1.0
TABLE 4. In-situ parameters deduced from SWE
and MFI
Np
Vp
f p =N p *V p Br
(cm 3 ) (kms 1 ) (cm 2 s 1 ) (nT)
ECH1
ECH2
4.3
3.6
606
594
2.6108
2.2108
2.8
2.7
SUMMARY AND CONCLUSION
RESULTS
We have correlated coronal with in-situ measurements.
We found that the larger blue shifts, as deduced from
coronal lines, are associated mainly with those photospheric regions where large magnetic flux with a single
polarity is concentrated. This observation agrees with the
model prediction that the fast solar wind is initially accelerated in the coronal funnels rooted there. Coronal holes
with a larger photospheric magnetic flux may result in a
larger expansion factor of the solar wind stream tube, and
by mass and magnetic flux conservation, have a larger
initial flow speed at the coronal base.
By inspection of Table 3, containing the data obtained
by SUMER and MDI, and Table 4, giving the parameters obtained from in-situ observations of SWE (Solar
Wind Experiment) and MFI (Magnetic Fields Investigation), we can infer that the two high-speed streams
produced by ECH1 and ECH2 have similar properties. If we trace back the proton flux and magnetic
field under the assumption of a radial expansion of the
streams, they should have an average proton flux of about
1013 cm 2 s 1 and a magnetic field of about 1 G at the
Sun’s surface. Comparison with the photospheric magnetic field measured by MDI indicates that ECH1 has
an expansion factor of about 3, and ECH2 of about 1.
This may be interpreted as ECH1 having a stronger photospheric magnetic field. If we determine the expansion
factor by means of the mass flux measured by SUMER
in the holes and by SWE at 1AU, this expansion factor
should be about 5.5 for ECH1 and about 2.5 for ECH2.
The lower expansion factors determined by the magnetic
field in both holes may mainly result from our underestimation of the net field strength caused by the noise level
of the field data measured by MDI. In addition, Measurements of the line widths show that both the Mg IX and
Mg X lines have larger Doppler widths for ECH1 than
for ECH2.
Finally, we give 2-D images of ECH3 in Fig. 5 to show
the close relationship between the Doppler shift and the
magnetic network. Except for the area occupied by a
bright point, where a mixed-polarity magnetic structure
is present and the velocity field becomes very complicated, the Ne VIII line is more blue shifted inside than
outside the hole. Moreover, the contour plots show that
the larger blue shifts with speeds above 6 kms 1 are associated mainly with those photospheric regions where
large magnetic flux with a single polarity is concentrated.
ACKNOWLEDGMENTS
The SUMER project is financially supported by DLR,
CNES, NASA and the ESA PRODEX program (Swiss
contribution). We thank the MDI, EIT, SWE and MFI
teams for use of their data.
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