Solar Cycle Dependence of High-Latitude Solar Wind
K. Fujiki , M. Kojima , M. Tokumaru , T. Ohmi , A. Yokobe and K. Hayashi
Solar-Terrestrial Environment Laboratory, Nagoya University, 3-13 Honohara, Toyokawa, Aichi Japan
Abstract.
How has the high-latitude solar wind velocity changed over the solar activity cycle? We analyzed interplanetary scintillation data during the years 1985-2001 (excluding the few years around solar maximum) and obtained the following results: (1)
the solar wind in the polar region did not change its speed even during the phases of rising and declining solar activity, (2)
the N-S asymmetry of the high-latitude solar wind speed is a stable structure from 1987 to 1998, (3) the latitudinal velocity
gradient at high latitude becomes steeper with increasing solar activity.
INTRODUCTION
Carrington rotation. The IPS observation is always affected by line-of- sight bias. To reduce this effect, we apply the computer-assisted tomography (CAT) technique
(Kojima et al., 1998).
Each V-map is obtained using IPS data for three solar
rotations to improve image quality. We average over the
longitudinal structure in this study because of our focus
on the latitudinal variations.
It is well known that the velocity structure of the solar
wind changes drastically over the course of the solar cycle. In solar minimum, the solar wind is generally characterized by two velocity components (bimodal), fast and
slow solar wind (Woch et al., 1997). Ulysses found in its
first rapid latitude scan that the high-latitude solar wind
had a speed in range of 700-800 km/s and that there was
a small but noticeable gradual increase of the solar wind
toward higher latitudes. In the latitude scan the solar
wind velocity at the northern high latitude was faster than
that at the southern high latitude (Goldstein et.al.,1996).
As the solar activity increases the slow solar wind region
extends to higher latitudes. Then fast solar wind region
is greatly reduced in solar maximum (McComas et al.,
2000).
Ground-based interplanetary scintillation (IPS)
observations provide valuable estimates of the threedimensional velocity structure of the inner heliosphere
on a continuous basis. We use the interplanetary scintillation observations of natural radio sources obtained
with the Solar-Terrestrial Environment Laboratory system at Nagoya University in Japan. In this work, we
investigate the variation of the solar wind structure
through solar cycle using IPS data.
SOLAR WIND STRUCTURE THROUGH
THE SOLAR CYCLE
Figure 1 is stack map in years from 1985 to 2001. It is
clearly seen that solar wind structure is bimodal except
for a few years around solar maximum. In solar maximum, the equatorial slow wind region extends to higher
latitudes and the area of the fast solar wind is greatly reduced.
Velocity at the pole
Figure 2 shows the variation of velocity of the fast solar wind around the north pole. We calculated a mean velocity for the latitude range of 80 Æ -90Æ in the V-map derived using the CAT analysis. The CAT procedure sometimes underestimates the velocity around the poles because the data coverage (number of line-of-sight) around
the pole is insufficient. To remove this effect, we corrected the mean velocity around the pole by model calculation. The mean velocity is 78968 km/s which agrees
well with the velocity estimated by extrapolating the
Ulysses observations to the polar region. Almost all data
points scatter in the one σ (68 km/s) belt and there is
no systematic velocity change with change in the area of
IPS OBSERVATION
The IPS observations at a frequency of 327 MHz were
obtained using four remote stations. Observations are obtained 8 hours a day from each station except during winter (mid-December to March) when heavy snow lies in
the antenna reflector. Velocity maps are obtained in each
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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FIGURE 1. Stack map from 1985 to 2001. White regions are data gaps.
FIGURE 2. Solar wind speed at the northern high latitudes.
The latitude range of N80 Æ-N90Æ in each V-map are averaged.
Shaded area is the solar maximum when the fast solar wind
disappeared around the poles.
high-speed flow.
FIGURE 3. N-S asymmetry of the solar wind. The latitude
range of 70 Æ-80Æ in each V-map are averaged. Circle and square
show north and south hemisphere, respectively.Shaded area
A is a period when the data coverage of high latitudes is
insufficient. Shaded area B is the solar maximum.
N-S asymmetry
We compared the solar wind velocities between the
northern and southern high latitudes (Fig. 3). For this
comparison we used mean velocity over the latitudes
70Æ -80Æ where the data coverage is better than the latitude range of 80 Æ -90Æ. Then the mean velocity is averaged over each year. As a result, we found that the
solar wind velocity at northern high latitudes is usually
higher than that at southern high latitudes. Ulysses detected hemispherical differences of 13 km/s at a latitude
of 8 0Æ (Goldstein et al., 1996). Ulysses sampled a velocity along its trajectory in latitude and longitude, while
our analyses were made by averaging velocities over all
longitudes and in the latitude range 70 Æ -80Æ .
Figure 3 and Figure 1 (bin of 1991) also show that the
recovery of the fast wind around the north pole precedes
that around the south pole by several months after the
22nd solar maximum. A similar trend is observed in the
23rd solar maximum (Fujiki et al.in this book).
Velocity gradient in the fast wind region
We analyzed the latitudinal velocity gradient of the
fast solar wind (Fig. 4). At high latitudes, there are several fine structures in the velocity map derived from the
CAT analysis, such as a lower speed island and abnormally high-speed regions. In order to remove the lower
speed island from the data set, we first checked the velocity distribution at each latitude along a meridian, and
then search the mode of the velocity of high-speed wind.
As a result, the velocity gradient is low at the solar mini-
142
3. Kojima, M., M. Tokumaru, H. Watanabe, A. Yokobe,
K. Asai, B. V. Jackson, and P. L. Hick, Heliospheric
tomography using interplanetary scintillation observations,
2, Latitude and heliospheric distance dependence of
solar wind structure at 0.1-1 AU, J. Geophys Res., 103,
1981-1989, 1998
4. McComas D. J., B. L. Barraclough, H. O. Funsen, J.
T. Gosling, E. Santiago-Muñoz, R. M. Skoug, B. E.
Goldstein, M. Neugebauer, P. Reley, and A. Balogh, Solar
wind observation over Ulusses’ first full poler orbit, J.
Grophys. Res., 105, 10419-10422, 2002.
5. Woch, J., W. I. Axford, U. Mall, B. Wilken, S. Livi, J.
Geiss, G. Gloecker, and R. J. Forsyth, SWICS/Ulysses
observation: The three-dimensional structure of the
heliosphere in the declining/minimum phase of the solar
cycle, Geophys. Res. Lett., 24, 2885-2888, 1997
FIGURE 4. Velocity gradient in the high speed region. Thick
line shows Ulysses result measured in the first fast latitude scan.
mum and increases with solar activity.
SUMMARY
We analyzed the variation of the high-latitude solar wind
structure through the solar cycle (1985-2001). The solar
wind structure derived from IPS observations, when augmented with the successful CAT technique, agrees well
with Ulysses measurements.
Analysis method used in this study is not applicable
during solar maximum because the solar wind structure
becomes quite complex in this period. First results of a
study relating to solar maximum are reported in Fujiki et
al. (2002).
First results of a study relating to solar maximum are
reported in Fujiki et al.(2002).
ACKNOWLEDGMENTS
We would like to thank to the Japan Science Society
which supported financially for this presentation in Pisa,
Italy.
REFERENCES
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