Evidences for Low-speed Streams from Small Coronal Hole
Tomoaki Ohmi∗ , Masayoshi Kojima∗ , Keiji Hayashi∗ , Atushi Yokobe∗ , Munetoshi
Tokumaru∗ , Ken’ichi Fujiki∗ and Kazuyuki Hakamada†
†
∗
Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442-8507, Japan
Department of Natural Science and Mathematics, Chubu University, Kasugai 487-8501, Japan
Abstract. Using the WIND spacecraft data, we have studied properties of the locally bunched low-speed stream which was
found in association with active regions by tomographic analysis of interplanetary scintillation observations. The source region
of this low-speed stream was inferred to be a small coronal hole at vicinity of active regions by tracing potential magnetic
field lines. The following WIND spacecraft observations support this inference of coronal hole origin. (1) Observed magnetic
fields have properties of coronal hole origin: IMF polarity is the same as that of the coronal hole, and a magnetic neutral
sheet was not observed in the stream. (2) Variations of velocity and density in the stream are as steady and uniform as those
in typical high-speed wind. In addition, we have found that the relative He abundance Nα /NP in this low-speed stream has
0.032, which is more than two times higher than that in low-speed wind in the heliospheric plasma sheet (0.013) and very near
to that of high-speed wind from the large coronal hole (0.040). However, proton mass flux density and freeze-in temperature
from the ratio of O7+ /O6+ are about 1.5 times higher than those in the coronal hole high-speed streams. These results imply
that the low-speed steam is originated from a small coronal hole with high mass flux density and is strongly heated in the
lower corona.
INTRODUCTION
[15] is a small coronal hole or not. This can be verified by
in situ measurements when a spacecraft traverses these
streams: If the compact low-speed streams are originated
from a small coronal hole, a magnetic polarity change
will not be observed when the spacecraft traverses this
low-speed region. In addition, the measured flow parameters within these streams will be steadier and more uniform than those within other low-speed streams. In order
to study in this manner, plasma and magnetic field data
measured by the WIND spacecraft are investigated. In
this paper, we report these results bliefly, and details of
this study and discussions will be submitted to the Journal of Geophysical Research as a full paper.
It has been reported that a small coronal hole is a source
of a low-speed stream [1, 2, 3, 4]. An inverse correlation
between the solar wind speed and the expansion factor of
a magnetic flux tube indicates that the lower speed wind
flows out from a more largely diverging region [5, 6, 7].
According to the potential-field model, these large diverging regions are at the boundaries of the large coronal
holes and in the small coronal holes [8, 9]. In the solar activity mimimum phase, comparison study between
the spacecraft measurements and several coronal magnetic field models shows that the low-speed streams originate not only from coronal hole boundaries but also from
small coronal holes at low latitudes [10].
Low-speed regions (≤ 350 km s −1 ) are often observed
bunched into a compact area associated with active regions [11]. In recent years, the tomographic analysis
for the interplanetary scintillation (IPS) observations has
been developed to obtain the solar wind velocity accurately with high spatial resolution [12, 13, 14]. By using
this analysis, the origin of low-speed streams observed in
association with active regions at solar activity minimum
phase was investigated, and it was found that these lowspeed streams originated from open field regions located
at vicinity of one polarity side of active regions [15].
Our focus in this study is to confirm whether the origin of the the locally bunched low-speed stream found by
LOW-SPEED WIND FROM SMALL
OPEN FIELD REGION
Figure 1 shows the synoptic maps for the solar wind velocity and magnetic structure at the Sun for Carrington
rotation number (CR) 1896 (May 16 to June 12 in 1995).
The solar wind velocity distribution at 2.5 solar radii (R S )
(Figure 1a) is obtained from IPS tomographic analysis.
The distribution of solar wind velocity is constructed on
a projection surface with assumptions of radial and constant velocity. For comparing the coronal structures, the
projection surface is set at 2.5 R S . Figure 1b and 1c show
the synoptic maps of photospheric magnetic fields ob-
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
137
FIGURE 2. Magnetic potential-field lines from the photosphere to the source surface at 2.5 RS for CR 1896. To avoid
plotting too many magnetic field lines, the plotted open field
lines are restricted to those which originate from the lower latitude boundaries of the open field regions. Closed loops in the
corona are shown for photospheric magnetic field lines stronger
than 15 G.
FIGURE 1. Synoptic maps for (a) solar wind velocity projected at 2.5 RS , (b) photospheric magnetic field, (c) Yohkoh
soft X-ray, and (d) open field footpoints at ths Sun for Carrington rotation number (CR) 1896 (May 16 to June 12 in
1995). The contour lines in the velocity map are for 500 and
350 km s−1 . The heavy solid line shown in (a) and (d) represents a magnetic neutral line derived from the potential-field
model. The black and gray sections of the WIND trajectory
are negative and positive magnetic polarities measured with the
WIND, respectively.
largely from the narrow open field regions into the interplanetary space where the low speed streams are observed.
PROPERTIES OF SOLAR WIND FROM
SMALL CORONAL HOLE
served at the National Solar Observatory at Kitt Peak
(NSO/Kitt Peak) and soft X-ray images from the Yohkoh
SXT observations. Figure 1d shows the magnetic field
regions on the photosphere from which magnetic fields
are open to interplanetary space. Open field regions and
the magnetic neutral lines (MNL) are estimated with a
potential-field analysis developed by [6] from the synoptic data of photospheric magnetic field observed at the
NSO/Kitt Peak. Black and gray points represent the open
field regions with negative and positive polarities, respectively. To verify this estimations, the polarities of interplanetary magnetic field (IMF) measured with the WIND
were compared. A measured magnetic polarities were
mapped back to the surface of 2.5 R S by using a constant
speed method with measured solar wind speed.
At longitudes of 10 ◦ –60◦ in Figure 1, there are (a)
low-speed regions (≤ 350 km s −1 ), (b) strong complex
magnetic structures, and (c) narrow low-intensity soft
X-ray region. This narrow low-intensty structure agree
with the computed open field regions. Figure 1d shows
good agreement between the observed and calculated
magnetic polarities, and magnetic polarity change was
not observed in this regions.
In order to investigate the magnetic field structure in
the regions where the low-speed winds were observed,
we calculated the potential-field lines in the corona (Figure 2), and found that the open field lines fanned out
By tracing the potential magnetic field lines, the source
region of the compact low-speed stream is inferred to
be a small open field region. (The term of “compact”
means “bunched into a compact area” hereafter.) In general the coronal hole and solar wind from there have following typical properties: Coronal hole is the regions of
low temperature, low density and unipolar open magnetic field [16]. Variations of velocity and density in the
streams from the coronal hole are steady and uniform
[17]. The helium to proton density ratio N α /NP in the
coronal hole stream is relatively higher than that in the
heliospheric plasma sheet with high-density low-speed
streamer [17, 18]. To verify weather the compact lowspeed streams are originated from the small coronal hole,
we have investigated the plasma data measured with the
WIND spacecraft.
Figure 3 shows the IPS velocity map and the plasma
parameters from the WIND observations. To compare
with the spacecraft measurements, the projection surface
of IPS tomography is set at 1 AU. The compact lowspeed region at 2.5 R S which is located around the 45 ◦
longitude in CR 1896 (Figure 1) is moved around the
300◦–330◦ longitude of CR 1897 (June 12 - July 10,
1995) at 1 AU. The IPS velocities extracted along the
138
TABLE 1. Averaged properties of solar wind for CR 1897.
Here the velocities are in km s−1 , the densities in cm−3 , the
temperatures in 106 K, and the fluxes in 108 cm2 s−1 .
Vp
Np
TO
Nα /NP
NPVP
Slow (HPS)
Slow (seCH)
Fast (leCH)
343±22
11.8±3.9
1.92±0.30
0.013±0.013
4.41±1.62
323±9
10.2±0.7
1.99±0.19
0.031±0.008
3.30±0.24
665±13
3.8±0.6
1.38±0.07
0.040±0.004
2.49±0.34
coronal hole (leCH) streams (N α /NP =0.040) but higher
than that in the HPS (N α /NP =0.013). The freeze-in temperature from the charge states of O 7+ /O6+ is about 2.0
MK, and the proton mass flux density is 1.5 times as large
as that in the leCH streams.
SUMMARY AND CONCLUSIONS
In order to study whether the compact low-speed streams
(≤ 350 km s−1 ) were originated from small equatorial
coronal hole, the solar wind plasma and magnetic field
data obtained from the WIND measurements were investigated. As a result, we have found the evidences that
they have properties of coronal hole origin: The IMF polarity is the same as that of the coronal hole, and the variations of velocity and density in the stream are as steady
and uniform as those in typical coronal hole streams. The
averaged properties of solar wind found in this study
are summarized in Tabale 1. In this table, the “Slow”
wind is defined as the streams with velocities less equal
400 km s−1 , while the velocities of “Fast” wind is greater
equal 600 km s −1 , and the data at the compressive interacting regions are excluded.
The helium abundance in the seCH streams was steady
and lower than that in the leCH streams. The similar
results to ours have been obtained by Neugebauer [19]
that the averaged value of N α /NP correlates with the
average speed of the flow. The temperature in the seCH
estimated from the charge states of O 7+ /O6+ was about
2.0 MK, which is higher than that in the typical coronal
holes (see also [20]), and the proton flux density was
higher than that in the fast solar wind. It is well known
that heat addition in the subsonic region in the lower
corona increases the mass flux and works against the
solar wind acceleration [21, 9]. Our results therefore
suggest that the solar wind originated from the small
coronal hole is strongly heated in the lower corona and
emanates the low-speed streams.
FIGURE 3. Comparison the IPS velocity with the solar wind
parameters measured by the WIND spacecraft for CR 1897
(June 12 - July 10, 1995). IPS velocity map is made at 1 AU.
The IPS velocities extracted along the WIND trajectory are
shown as a heavy solid line in the velocity plot of WIND data.
WIND trajectory are shown as a heavy solid line in the
velocity plot of WIND data.
In the gray shaded period between straight solid lines
(7 UT on DOY 165 – 1 UT on DOY 168) in the WIND
data plot, the compact low-speed streams are observed
with the unipolar magnetic field. The velocities of compact low-speed streams obtained from the IPS observations are in good agreement with measured bulk velocities by the WIND spacecraft. It is very interesting of this
compact low-speed streams that the density variation is
steadier and more uniform than that of other low-speed
streams around the heliospheric plasma sheet (HPS). In
the the dark gray shaded period, the steady and uniform
streams were observed from the near center of the small
equatorial coronal hole (seCH). Another interesting is
that the He abundance of N α /NP was as high as 0.032.
This is slightly lower than that in the large equatorial
139
ACKNOWLEDGMENTS
17. Bame, S. J., Asbridge, J. R., Feldman, W. C., and Gosling,
J. T., J. Geophys. Res., 82, 1487 (1977).
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19. Neugebauer, M., in Solar Wind Seven, edited by E. Marsh
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We would like to thank the National Solar Observatory at Kitt Peak for use of their synoptic magnetic
field data. The NSO/Kitt Peak data used here are produced cooperatively by NSF/NOAO, NASA/GSFC, and
NOAA/SEL. We would like to thank the use of WIND
plasma and magnetic field data. Hourly averages of the
WIND data were obtained from the World Wide Web
through NDADS (NSSDC (National Space Science Data
Center) Data Archives and Distribution System) and
WIND-SWE Data Page at MIT. We would like to thank
Yohkoh team for the soft X-ray solar images. We wish
to acknowledge engineering support of our IPS observations from Y. Ishida, K. Maruyama and N. Yoshimi. This
work was partially supported by the Scientific Research
Fund of the Japan Society for the Promotion of Sience
(grant 12440130).
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