Solar wind velocity structure around the solar maximum
observed by interplanetary scintillation
K. Fujiki , M. Kojima , M. Tokumaru , T. Ohmi , A. Yokobe , K. Hayashi, D. J.
McComas † and H. A. Elliott †
Solar-Terrestrial Environment Laboratory, Nagoya University, 3-13 Honohara, Toyokawa, Aichi Japan
†
Southwest Research Institute, San Antonio, TX, USA
Abstract. Ulysses observed a latitude structure of solar wind in its second fast latitude scan and found that the global structure
of solar wind near the solar maximum is significantly different from that in the solar minimum. Also soon after the solar
maximum, Ulysses measured that the fast solar wind which has magnetic polarity of the new solar cycle appeared at high
latitude in northern hemisphere. This fast wind appeared and disappeared a few times. We introduced a new tomographic
algorithm, time-series tomography, to reconstruct IPS velocity map using all data observed in the year from 1998 to 2001 and
analyzed the variation of the solar wind structure through these four year. Especially in 2001, we compared the Ulysses’ fast
latitude scan data. As results, it is found that disappearance and recovery of the fast solar wind around the north pole precedes
that around the south pole for several months. And also found that the IPS observation shows high level agreement to the
Ulysses observation especially for high latitudes.
INTRODUCTION
OBSERVATION
It is well known that the solar wind is consisted by two
velocity components (so called bimodal) in the solar
minimum. The slow solar wind is confined around the
Sun’s streamer belt and the fast solar wind is onserved in
other regions. When the solar activity increases the low
speed solar wind region extends to higher latitude.
Ulysses observed a latitude structure of the solar wind
in its second fast latitude scan (Nov., 2000 - Oct., 2001)
and show us a remarkably different solar wind structure
in the solar maximum from that in the solar minimum
(McComas et al.[1]). The fast solar wind observed by
Ulysses above N40Æ , soon after the solar maximum,
disappeared and appeared a few times.
We study the variation of the solar wind velocity structure during the years from 1998 (rising phase) to 2001
(after the solar maximum) using interplanetary scintillation (IPS) tomographic analysis. The IPS has an advantage for studies of the solar wind velocity structure
because it is possible to obtain the velocity structure for
each Carrington rotation. In this study, we introduce a
modified tomographic technique which is mentioned in
the next section.
In this paper, we will briefly report the results of
the solar wind velocity structure derived by the new
tomography technique and the comparison between the
IPS and Ulysses second fast latitude scan observations.
We employ the interplanetary scintillation (IPS) of natural radio sources at the Solar-Terrestrial Environment
Laboratory, Nagoya University, Japan. Two-dimensional
solar wind velocity distribution map (V-map) at 2.5 R s
is available for each carrington rotation. IPS signals of a
natural radio source is affected by a bias which is caused
by a weighted-integration along the line of sight, though
tomographic analysis is introduced by Kojima et al.[2] to
remove the bias and to improve spatial resolution. The
tomographic analysis requires stable solar wind velocity structure over one solar rotation because all IPS data
in a given Carrington rotation are used to reproduce a
V-map. Around the solar maximum, however, the solar wind structure changes rapidly, which disturbs tomographic V-map.
A new tomographic technique called time-series tomography (TST) here is introduced to improve the reliability of the V-map around the solar maximum, which
use all IPS data observed in a year and reproduces one
V-map for the year. This technique allow us to analyze
the continuously-changing velocity structure of the solar
wind such as that around the solar maximum. We derived the velocity structure using the TST technique in
the years from 1998 to 2001 which correspond to the period from rising phase to soon after the solar maximum.
Ulysses/SWOOPS daily averaged data during the second fast latitude scan in 2001 are compared to the TST
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
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and CR 1976 and appeared again in CR 1977, and finally the fast solar wind surrounds the north pole in 1980.
At the south pole, on the other hand, the fast solar wind
appeared in CR 1977 and disappeared in CR 1980. Especially for CR from 1977 to 1978 (also 1979), the fast
winds at the both poles expand into lower latitudes, about
40 degree.
Circle plots show the Ulysses observation with same
gray scale which are ballistically mapped back to 2.5
Rs using the observed solar wind speed. In this simple
mapping method, the estimated longitudes at the source
surface are slightly shifted with respect to the longitudes
of true stream sources because the mapped surface is
inside the Alfven surface. Gaps of the Ulysses plots,
seen in the boundary between CR 1977 and 1978 for
an example, attributes large difference of observed solar
wind speed because a traveling time of slow solar wind
observed by Ulysses has about two times longer than that
of fast solar wind. Thus the gaps are always seen when
Ulysses passes a boundary of fast and slow solar wind
regions.
Lower panel in figure 2 are the comparison between
IPS and Ulysses data. Error bar of the tomographic analysis is also plotted. Rapid increase of the solar wind velocity appeared in the low latitude around DOY 110 is
coronal mass ejection. Ulysses observed fast solar wind
around DOY 170, 200, 220 and then reaches fast solar
wind region near pole (CR 1980 and 1981). According to
the Ulysses trajectory in upper panel, it is clear that the
Ulysses passed near the boundary of fast and slow solar wind regions. On the other hand, the IPS and Ulysses
data in low latitude where slow solar wind are observed
do not correlate well each other.
FIGURE 1. Solar wind velocity structure around the solar
maximum. Top to bottom panels shows continuous variation
of solar wind structure in the years from 1998 to 2001. White
regions are data gap.
V-map. Although the second fast latitude scan started in
November, 2000, we did not compare the data observed
in 2000 because the data coverage in November and December in this year are not sufficient to reproduce the
solar wind structure at high latitudes in southern hemisphere in which Ulysses passed during this period.
RESULTS
SUMMARY AND DISCUSSION
Figure 1 shows results of TST analysis for the year from
1998 (top panel) to 2000 (bottom panel). The velocity
structure in 1998 is still bimodal, and the fast solar wind
around the north pole disappeared in CR 1951 (1999).
However the fast solar wind around the south pole still
exists in this year. Then the fast solar wind at high
latitudes disappeared completely in 2000. Note that there
are about four month gap between these maps because
IPS observations are stopped in winter season due to
heavy snow.
Upper panel of figure 2 shows a v-map in 2001 (Carrington rotations from 1974 (end-March) to 1981 (endOctober). In end of July, powerful lightening struck our
observation system at the foot of Mt. Fuji unfortunately.
The system was severely damaged and the observation
stopped about one month (white region in CR 1979).
According to this figure, the fast solar wind already appeared in CR 1974 at the north pole but longitudinally
localized. The fast solar wind disappeared in CR 1975
We employed time-series tomography for the analysis
of the solar wind velocity structure around the solar
maximum. As results, It is found that the fast solar wind
at the north pole disappeared in mid-1999 and that at
south pole disappeared in 2000. Slow solar wind region
occupied almost area on the source surface (2 5Rs) in this
year. Recovery of the fast solar wind at the north pole
precedes that at the south pole for a few solar rotations.
Ulysses also observed recovery of the fast solar wind
at from mid to high latitudes. The fast solar wind appeared and disappeared in a few times. According to IPS
observation, it is clear that the appearance and disappearance of the fast solar wind attributes to that the fast solar
wind region localized in limited longitudes and Ulysses
passed near the boundary of the fast and slow solar wind
regions. Although we have no data for CR 1979 the localized fast solar wind region existed for three Carring-
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FIGURE 2. Comparison between IPS and Ulysses observations. Velocity map in Carrington rotations from 1974 to 1981 are
estimated from IPS observation. Black and white contours are 700km/s and 400km/s, respectively. Circle plots show Ulysses data
which is traced back to 2 5R s . The gray scale of the circle plots are same as the background image. (upper figure). Velocity observed
by IPS along the Ulysses trajectory is compared in lower panel. Thick and thin lines shows IPS and Ulysses observations.
difference comes from insufficient data coverage of the
IPS observation around the north pole because error bars
of tomographic process in this region are also large.
Figure 3 shows correlation between IPS and Ulysses
observations. If we do not include the data observed in
CR 1981 when the IPS data contain large error, latitudes
higher than 45 Æ (circle) are concentrate around y x line.
On the other hand, the data at the latitude lower than
45Æ (triangle) are rather scattered. It is said that the slow
wind is closely related to the active region (e.g.Uchida et
al.[3] have found that the corona above an active region
expands almost continually). The photospheric magnetic
structure and shape of coronal streamer change rapidly
in comparison with the period which is required to be
stable structure by the TST analysis (about 2 weeks in
this case). Though the velocity structure at low latitude
reconstructed by the TST is temporary averaged and the
difference from Ulysses observations become large.
FIGURE 3. Correlation between IPS and Ulysses observation. Triangle and circle plots show the data at latitudes lower
and higher than 45 Æ, respectively.
ACKNOWLEDGMENTS
We would like to thank to the Japan Science Society
which supported financially for this presentation in Pisa,
Italy.
ton rotations (CR 1978-1979) because Ulysses observed
fast solar wind in almost same Carrington longitude in
CR 1979. Then the fast solar wind region extend around
the north pole in CR 1980. Difference between IPS and
Ulysses observations becomes large in CR 1981. This
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