Non-dipolar solar wind structure observed in the cycle 23/24 minimum

M. Tokumaru,1 M. Kojima,1 K. Fujiki,1 and K. Hayashi2

Received 14 January 2009; revised 25 March 2009; accepted 2 April 2009; published 5 May 2009.

[1] Interplanetary scintillation (IPS) observations made inthe Cycle 23/24 minimum using the Solar-TerrestrialEnvironment Laboratory (STEL) multi-station systemindicated that during intervals the solar wind had asignificantly non-dipolar structure that consisted of fast windcomponents at the poles and the equator and slower windcomponents in between. The solar wind structure revealedfrom the IPS observations was consistent with a markedincrease in the occurrence of fast winds observed in situ nearthe earth. The poleward boundary of the slow wind regionobserved during this minimum was ±30 north and south. Inaddition, our IPS observations revealed that the organizationof the 3-dimensional solar wind was highly variable during2007–2008. These features greatly differ from those observedduring the previous minima. This fact may be attributed to theweak magnetic field intensity at the poles during the Cycle 23/24minimum.Citation: Tokumaru, M., M. Kojima, K. Fujiki, and

K. Hayashi (2009), Non-dipolar solar wind structure observed in the

cycle 23/24 minimum,Geophys. Res. Lett., 36, L09101, doi:10.1029/

2009GL037461.

1. Introduction

[2] Systematic changes in the 3-dimensional structure ofthe solar wind, which are closely linked with the 11-yearsunspot cycle, have been disclosed from ground-based obser-vations using interplanetary scintillation (IPS) for more thanthree decades [e.g., Coles et al., 1980; Kojima and Kakinuma,1990]. The solar wind observed in the previous two minimumperiods revealed a stable structure which was composed of alow-speed stream in a narrow latitude range around theequator and a high-speed stream over the poles. Here wedemonstrate that the solar wind structure determined frominterplanetary scintillation (IPS) observations of the Solar-Terrestrial Environment Laboratory (STEL) of Nagoya Uni-versity for the Cycle 23/24 minimum differs significantlyfrom this simple dipole-like feature. In section 2, we brieflydescribe our IPS observations of the solar wind. In section 3,we present the solar wind speed data obtained from our IPSobservations in the cycle 23/24 minimum. In section 4, weshow in situ observations which support our finding. Insection 5, we discuss the implications of our findings and insection 6 we summarize the results of this study.

2. IPS Observations

[3] The IPS, which is a diffraction phenomenon causedby density irregularities in the interplanetary medium, has

been used as a useful ground-based method to observe thesolar wind out of the ecliptic [Dennison and Hewish, 1967].As the diffraction pattern drifts owing to the solar windmotion, the flow speed can be determined by measuring thetime delay between two signals at geographically separatedsites. We have carried out IPS measurements of the solarwind speed since the early 1980s, on a daily basis betweenApril and November/December, using the STEL 327-MHzmulti-station system [Kojima and Kakinuma, 1990]. OurIPS observations enable us to produce a synoptic map of thesolar wind speed in a Carrington longitude-versus-latitudeformat for a given solar rotation, since approximately 1000of lines-of-sight data are usually available during thecorresponding 27-day period. The IPS data have beendeconvolved to retrieve the intrinsic distribution of solarwind speed using the computer-assisted (CAT) tomographymethod [Kojima et al., 1998]. The high reliability of thesolar wind speed data deconvolved from our IPS observa-tions has been confirmed by earlier works [e.g., Kojima etal., 1998, 2007]. For example, the solar wind data showedexcellent agreement with Ulysses latitude scan observationsduring 2001 [Fujiki et al., 2003].

3. Solar Wind Speed Data

[4] Figures 1a–1d show the solar wind speed data obtainedfrom STEL IPS measurements for representative Carringtonrotations in the last three solar sunspot minima; i.e., CR 2070,CR 2075, CR 1910, and CR 1790, which correspond to theperiods between May 13 to June 9, September 26 to October10, 2008 (Cycle23/24), June 1 to 28, 1996 (Cycle 22/23), andJune 16 to July 13, 1987 (Cycle 21/22), respectively. The solidline in the synoptic map denotes the magnetic neutral line atthe source surface determined from WSO (Wilcox SolarObservatory) magnetogram observations (http://wso.stanford.edu/synsourcel.html) [Hoeksema et al., 1983]. Figures 1(left) and 1 (right) are the synoptic source surface velocitymap and the latitude profile of the solar wind speed, respec-tively. As shown here, the high-speed solar wind that ema-nates from the polar region is a common feature duringsunspot minimum periods. However, the solar wind observedin CR 2070 (Figure 1a) showed a clear difference in the flowspeed at the equator from those observed during the previoustwo minimum periods (Figures 1c and 1d). The solar windspeed in CR 2070 significantly increased at the equator, whileits peak value (�600 km/s) was somewhat lower than theflow speed of the fast polar streams. The mid-latitude regionsin CR 2070 were mostly dominated by the slow winds. Thus,the solar wind observed for CR 2070 exhibited a significantlynon-dipolar structure composed of polar and equatorial faststreams with slow streams in between. Another importantpoint to note is that the non-dipolar structure found for CR2070 was not discernible for CR 2075 (Figure 1b). This factsuggests that the solar wind structure evolved rapidly during

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1Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya,Japan.

2W. W. Hansen Experimental Physics Laboratory, Stanford University,Stanford, California, USA.

Copyright 2009 by the American Geophysical Union.0094-8276/09/2009GL037461$05.00

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2008. The latitude width of the slowwind region for CR 2075is considerably larger than that for CR 1910. This fact may beattributed to the complexity of the solar wind at each latitudeduring 2008.[5] A series of synoptic solar wind speed maps from

STEL IPS observations are displayed in Figures 2a–2c for

2008, 2007, and 1996, respectively. As revealed fromFigure 2, the solar wind speed structure evolved consider-ably during 2007–2008 despite quite a low level of solaractivity. The non-dipolar feature of the solar wind speedwas discerned for CRs 2070–2072 in 2008, and lessprominently for CR 2058 in 2007, and it appeared to be

Figure 1. (left) Synoptic source surface maps and (right) latitude profiles of the solar wind speed. Maps were derived fromSTEL IPS observations for Carrington Rotations (CRs) (a) 2070, (b) 2075, (c) 1910, and (d) 1790. CRs 2070 and 2075correspond to the Cycle 23/24 minimum, and CRs 1910 and 1790 correspond to 22/23 and 21/22 minima, respectively.Synoptic map is drawn in the Carrington longitude-versus-latitude format, and red (blue) colours in the map represents slow(fast) solar winds. Solid line in synoptic map denotes the magnetic neutral line at the source surface determined from WSOmagnetogram observations. Vertical bars in the latitude profile correspond to an rms value around the mean speed for agiven latitude.

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less distinct after CR 2074. Such a rapid evolution of the solarwind structure during 2007–2008 is in marked contrast tothat observed in the previous minima. The solar windobserved in 1996 (Figure 2c) showed a stable structure whichlasted over several solar rotations. In addition, Figure 2reveals a significant difference in the latitude extent of theslow wind region between Cycle 23/24 and 22/23 minimumperiods. The slow wind region observed during 2007–2008extended to �30� north and south, while the fast windoccasionally penetrated this latitude band. On the other hand,the latitude extent of the slow wind observed in 1996 wasapproximately ±15�.

4. In Situ Observations at 1 AU

[6] The significant growth of the equatorial fast windrevealed during 2007–2008 has been supported by in situ

observations conducted at earth orbit. Histograms of theoccurrence rate of the solar wind speed measured by space-craft near the earth are indicated in Figures 3a–3e for 2008,2007, 2003, 2001, and 1996, respectively. Here, we used thesolar wind speed data collected by the Advanced Composi-tion Explorer, ACE (http://www.srl.caltech.edu/ACE/ASC/)[McComas et al., 1998] or Wind (http://web.mit.edu/space/www/wind/wind_data.html) [Ogilvie et al., 1995] spacecraft.Note that the histogram for 2008 (Figure 3a) produced fromthe ACE data from the beginning to July 22 of the year (thelatest date at which the Level 2 (verified) data were availableas of this writing). We have confirmed that the distributionrevealed by the histogram remains basically unchanged evenif the realtime ACE data covering a fuller span for 2008 areused. The histogram of 1996 data (Figure 3e) showed a sharp,single peak profile with a maximum at �400 km/s, andthis fact is consistent with the solar wind speed distribution

Figure 2. A series of synoptic source surface maps of solar wind speed from STEL IPS observations for (a) 2008,(b) 2007, and (c) 1996. Color scale used here is the same as the one in Figure 1. White area in the map corresponds tothe region where no observational constraint is placed by IPS observations.

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revealed from our IPS observations during the Cycle 22/23minimum, since the earth’s orbit is located near the equatorwhere the slow speed component dominates (see Figure 2c).The wind speed data in 2001 (Cycle 23 maximum) werebasically the same as those in 1996, as shown in Figure 3d,since the slow speed stream became ubiquitous in theheliosphere during the sunspot maximum. However, thehistogram profile revealed in 2007 and 2008 data was ratherdifferent from the typical one. Although the primary peaks of2007 and 2008 data still occurred at �400 km/s, they were

much less noticeable, and a secondary peak significantlydeveloped at �600 km/s (see Figures 3a and 3b). Thischange was consistent with the emergence of the equatorialfast wind revealed by our IPS observations. We examined insitu data of the solar wind at earth orbit for other yearsbetween 1995 and 2006, and confirmed that the pronounceddominance of the slow-speed component was a commonfeature for almost all years. One exception was in situ datataken in 2003, which corresponds to the declining phase ofthe Cycle 23 (Figure 3c). The histogram for 2003 is

Figure 3. Histograms of the occurrence rate of the solar wind speed measured at earth orbit in (a) 2008, (b) 2007, (c) 2003,(d) 2001, and (e) 1996. Here, in situ observations made by the ACE spacecraft are used for Figures 3a–3d, and Windspacecraft data are used for Figure 3e. Ndat, Vave, and Vmode shown in each plot correspond to number of data, average andmodal values of solar wind speed, respectively.

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somewhat similar to those for 2007 and 2008. Furtherdiscussion on this similarity is beyond the scope of this paper.

5. Discussion

[7] The peculiarity of the solar wind structure observed inthe Cycle 23/24 minimum can be attributed to that of theglobal magnetic field structure of the corona. Figure 4illustrates the MHD model calculations of the coronalmagnetic field within 2.5 Rs (the source surface) [Hayashiet al., 2008] and the synoptic coronal hole maps for CR2070 (Figure 4a) and CR 1910 (Figure 4b). The photo-spheric magnetic field data from the Michelson DopplerImager, MDI [Scherrer et al., 1995] onboard the Solar andHeliospheric Observatory (SOHO) spacecraft have beenused in these model calculations. The white and red linesin Figure 4 represent the closed and open field lines, whilethe blue and red colours on the sphere indicate the positiveand negative polarities of the large-scale photosphericmagnetic field. As for CR 1910, the low-to-mid-latituderegion was entirely occupied by the large-scale closed fieldlines, and the open magnetic fluxes originated exclusivelyfrom the polar regions. In contrast, the open fluxes of CR

2070 emanated not only from the polar regions but alsofrom the low-latitude region, which is a source of theequatorial fast wind. Many mid- and low-latitude coronalholes were found for CR 2070, while the coronal holes wereconfined in the polar regions during CR 1910. The differ-ence in coronal hole distribution between CRs 2070 and1910 is consistent with what was revealed from the modelcalculations of the coronal magnetic field and also from ourIPS observations.[8] The formation of an enhanced low-latitude open flux

was considered to be a manifestation of the solar magneticdynamo activity during the Cycle 23/24 minimum. Thesolar magnetograph observations at the Wilcox Solar Ob-servatory indicated that the polar field strength of the Cycle23/24 minimum was significantly weaker than that of theprevious two minima (http://wso.stanford.edu/gifs/Polar.gif)[Hoeksema, 1995]. We consider that such a weak dipolecomponent is essential to account for the increase of thelow-latitude open flux and thus the non-dipolar solar windstructure in the Cycle 23/24 minimum. Furthermore, thesunspot counts in the Cycle 23/24 minimum were very low,and the number of spotless days in 2008 was the highest inthe last 50 years. This was also a sign of the solar dynamo

Figure 4. (left) Coronal magnetic fields within 2.5 solar radii and (right) synoptic coronal hole maps for CRs (a) 2070 and(b) 1910. The coronal magnetic fields were calculated from MDI/SOHO observations using the MHD code. The coronalhole map for CR 2070 was taken from the NSO/GONG magnetogram synoptic map archive (http://gong.nso.edu/data/magmap/archive.html). Green and red areas in this map denote positive and negative coronal holes, respectively. Coronalhole map for CR 1910 was from the NSO/Kit Peak synoptic coronal hole map archive (ftp://nsokp.nso.edu/kpvt/coronal_holes/synoptic/).

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activity in this minimum [e.g., Schatten, 2005; Svalgaard etal., 2005]. Observations during Ulysses’s third orbit space-craft showed that the polar fast wind was slightly slower,significantly less dense, cooler, and had less mass andmomentum flux than during the previous solar minimumorbit [McComas et al., 2008]. Thus, the Cycle 23/24minimum exhibits several peculiar aspects from the view-point of solar wind and magnetism. We consider that itprovided an excellent opportunity to gain a better insightinto the solar wind formation and dynamo process.

6. Summary

[9] Our IPS observations have disclosed that the solarwind structure in the Cycle 23/24 minimum greatly differsfrom what was observed during the previous two minima.Namely, the fast solar wind emanated not only from thepoles but also from the equatorial region, and the source ofthe slow solar wind split into two mid-latitude regions. Thepoleward boundary of the slow wind during this minimumextended to �30� north and south, and the solar windstructure evolved rapidly. These features significantly differfrom those observed during the previous minima. A distinctincrease of the occurrence rate of the high flow speedmeasured in situ at 1 AU is consistent with our IPSobservations. Our finding is regarded as another manifesta-tion to indicate the peculiarity of the solar magnetic activityin this minimum.

[10] Acknowledgments. The IPS observations were carried out underthe solar wind program of the Solar-Terrestrial Environment Laboratory(STEL) of Nagoya University. This work was partly supported by theGrant-in-Aid for Creative Scientific Research (17GS0208) from the Min-istry of Education, Culture, Sports, Science, and Technology of Japan. Wewish to thank the ACE Science Center and the MIT space plasma group forproviding the solar wind plasma data collected by the ACE and Windspacecraft.

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�����������������������K. Fujiki, M. Kojima, and M. Tokumaru, Solar-Terrestrial Environment

Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan. (fujiki@stelab.nagoya-u.ac.jp; kojima@stelab.nagoya-u.ac.jp;tokumaru@stelab.nagoya-u.ac.jp)K. Hayashi, W. W. Hansen Experimental Physics Laboratory, Stanford

University, 491 South Service Road, Stanford, CA 94305-4085, USA.(keiji@sun.stanford.edu)

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