Geophysical Research Letters

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

Authors


Abstract

[1] Interplanetary scintillation (IPS) observations made in the Cycle 23/24 minimum using the Solar-Terrestrial Environment Laboratory (STEL) multi-station system indicated that during intervals the solar wind had a significantly non-dipolar structure that consisted of fast wind components at the poles and the equator and slower wind components in between. The solar wind structure revealed from the IPS observations was consistent with a marked increase in the occurrence of fast winds observed in situ near the earth. The poleward boundary of the slow wind region observed during this minimum was ±30 north and south. In addition, our IPS observations revealed that the organization of the 3-dimensional solar wind was highly variable during 2007–2008. These features greatly differ from those observed during the previous minima. This fact may be attributed to the weak magnetic field intensity at the poles during the Cycle 23/24 minimum.

1. Introduction

[2] Systematic changes in the 3-dimensional structure of the solar wind, which are closely linked with the 11-year sunspot cycle, have been disclosed from ground-based observations using interplanetary scintillation (IPS) for more than three decades [e.g., Coles et al., 1980; Kojima and Kakinuma, 1990]. The solar wind observed in the previous two minimum periods revealed a stable structure which was composed of a low-speed stream in a narrow latitude range around the equator and a high-speed stream over the poles. Here we demonstrate that the solar wind structure determined from interplanetary scintillation (IPS) observations of the Solar-Terrestrial Environment Laboratory (STEL) of Nagoya University for the Cycle 23/24 minimum differs significantly from this simple dipole-like feature. In section 2, we briefly describe our IPS observations of the solar wind. In section 3, we present the solar wind speed data obtained from our IPS observations in the cycle 23/24 minimum. In section 4, we show in situ observations which support our finding. In section 5, we discuss the implications of our findings and in section 6 we summarize the results of this study.

2. IPS Observations

[3] The IPS, which is a diffraction phenomenon caused by density irregularities in the interplanetary medium, has been used as a useful ground-based method to observe the solar wind out of the ecliptic [Dennison and Hewish, 1967]. As the diffraction pattern drifts owing to the solar wind motion, the flow speed can be determined by measuring the time delay between two signals at geographically separated sites. We have carried out IPS measurements of the solar wind speed since the early 1980s, on a daily basis between April and November/December, using the STEL 327-MHz multi-station system [Kojima and Kakinuma, 1990]. Our IPS observations enable us to produce a synoptic map of the solar wind speed in a Carrington longitude-versus-latitude format for a given solar rotation, since approximately 1000 of lines-of-sight data are usually available during the corresponding 27-day period. The IPS data have been deconvolved to retrieve the intrinsic distribution of solar wind speed using the computer-assisted (CAT) tomography method [Kojima et al., 1998]. The high reliability of the solar wind speed data deconvolved from our IPS observations has been confirmed by earlier works [e.g., Kojima et al., 1998, 2007]. For example, the solar wind data showed excellent agreement with Ulysses latitude scan observations during 2001 [Fujiki et al., 2003].

3. Solar Wind Speed Data

[4] Figures 1a1d show the solar wind speed data obtained from STEL IPS measurements for representative Carrington rotations in the last three solar sunspot minima; i.e., CR 2070, CR 2075, CR 1910, and CR 1790, which correspond to the periods between May 13 to June 9, September 26 to October 10, 2008 (Cycle23/24), June 1 to 28, 1996 (Cycle 22/23), and June 16 to July 13, 1987 (Cycle 21/22), respectively. The solid line in the synoptic map denotes the magnetic neutral line at the source surface determined from WSO (Wilcox Solar Observatory) magnetogram observations (http://wso.stanford.edu/synsourcel.html) [Hoeksema et al., 1983]. Figures 1 (left) and 1 (right) are the synoptic source surface velocity map and the latitude profile of the solar wind speed, respectively. As shown here, the high-speed solar wind that emanates from the polar region is a common feature during sunspot minimum periods. However, the solar wind observed in CR 2070 (Figure 1a) showed a clear difference in the flow speed at the equator from those observed during the previous two minimum periods (Figures 1c and 1d). The solar wind speed in CR 2070 significantly increased at the equator, while its peak value (∼600 km/s) was somewhat lower than the flow speed of the fast polar streams. The mid-latitude regions in CR 2070 were mostly dominated by the slow winds. Thus, the solar wind observed for CR 2070 exhibited a significantly non-dipolar structure composed of polar and equatorial fast streams with slow streams in between. Another important point to note is that the non-dipolar structure found for CR 2070 was not discernible for CR 2075 (Figure 1b). This fact suggests that the solar wind structure evolved rapidly during 2008. The latitude width of the slow wind region for CR 2075 is considerably larger than that for CR 1910. This fact may be attributed to the complexity of the solar wind at each latitude during 2008.

Figure 1.

(left) Synoptic source surface maps and (right) latitude profiles of the solar wind speed. Maps were derived from STEL IPS observations for Carrington Rotations (CRs) (a) 2070, (b) 2075, (c) 1910, and (d) 1790. CRs 2070 and 2075 correspond 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 WSO magnetogram observations. Vertical bars in the latitude profile correspond to an rms value around the mean speed for a given latitude.

[5] A series of synoptic solar wind speed maps from STEL IPS observations are displayed in Figures 2a2c for 2008, 2007, and 1996, respectively. As revealed from Figure 2, the solar wind speed structure evolved considerably during 2007–2008 despite quite a low level of solar activity. The non-dipolar feature of the solar wind speed was discerned for CRs 2070–2072 in 2008, and less prominently for CR 2058 in 2007, and it appeared to be less distinct after CR 2074. Such a rapid evolution of the solar wind structure during 2007–2008 is in marked contrast to that observed in the previous minima. The solar wind observed in 1996 (Figure 2c) showed a stable structure which lasted over several solar rotations. In addition, Figure 2 reveals a significant difference in the latitude extent of the slow wind region between Cycle 23/24 and 22/23 minimum periods. The slow wind region observed during 2007–2008 extended to ∼30° north and south, while the fast wind occasionally penetrated this latitude band. On the other hand, the latitude extent of the slow wind observed in 1996 was approximately ±15°.

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 to the region where no observational constraint is placed by IPS observations.

4. In Situ Observations at 1 AU

[6] The significant growth of the equatorial fast wind revealed during 2007–2008 has been supported by in situ observations conducted at earth orbit. Histograms of the occurrence rate of the solar wind speed measured by spacecraft near the earth are indicated in Figures 3a3e for 2008, 2007, 2003, 2001, and 1996, respectively. Here, we used the solar wind speed data collected by the Advanced Composition 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 from the ACE data from the beginning to July 22 of the year (the latest date at which the Level 2 (verified) data were available as of this writing). We have confirmed that the distribution revealed by the histogram remains basically unchanged even if the realtime ACE data covering a fuller span for 2008 are used. The histogram of 1996 data (Figure 3e) showed a sharp, single peak profile with a maximum at ∼400 km/s, and this fact is consistent with the solar wind speed distribution revealed from our IPS observations during the Cycle 22/23 minimum, since the earth's orbit is located near the equator where the slow speed component dominates (see Figure 2c). The wind speed data in 2001 (Cycle 23 maximum) were basically the same as those in 1996, as shown in Figure 3d, since the slow speed stream became ubiquitous in the heliosphere during the sunspot maximum. However, the histogram profile revealed in 2007 and 2008 data was rather different from the typical one. Although the primary peaks of 2007 and 2008 data still occurred at ∼400 km/s, they were much less noticeable, and a secondary peak significantly developed at ∼600 km/s (see Figures 3a and 3b). This change was consistent with the emergence of the equatorial fast wind revealed by our IPS observations. We examined in situ data of the solar wind at earth orbit for other years between 1995 and 2006, and confirmed that the pronounced dominance of the slow-speed component was a common feature for almost all years. One exception was in situ data taken in 2003, which corresponds to the declining phase of the Cycle 23 (Figure 3c). The histogram for 2003 is somewhat similar to those for 2007 and 2008. Further discussion on this similarity is beyond the scope of this paper.

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 Wind spacecraft data are used for Figure 3e. Ndat, Vave, and Vmode shown in each plot correspond to number of data, average and modal values of solar wind speed, respectively.

5. Discussion

[7] The peculiarity of the solar wind structure observed in the Cycle 23/24 minimum can be attributed to that of the global magnetic field structure of the corona. Figure 4 illustrates the MHD model calculations of the coronal magnetic field within 2.5 Rs (the source surface) [Hayashi et al., 2008] and the synoptic coronal hole maps for CR 2070 (Figure 4a) and CR 1910 (Figure 4b). The photospheric magnetic field data from the Michelson Doppler Imager, MDI [Scherrer et al., 1995] onboard the Solar and Heliospheric Observatory (SOHO) spacecraft have been used in these model calculations. The white and red lines in Figure 4 represent the closed and open field lines, while the blue and red colours on the sphere indicate the positive and negative polarities of the large-scale photospheric magnetic field. As for CR 1910, the low-to-mid-latitude region was entirely occupied by the large-scale closed field lines, and the open magnetic fluxes originated exclusively from the polar regions. In contrast, the open fluxes of CR 2070 emanated not only from the polar regions but also from the low-latitude region, which is a source of the equatorial fast wind. Many mid- and low-latitude coronal holes were found for CR 2070, while the coronal holes were confined in the polar regions during CR 1910. The difference in coronal hole distribution between CRs 2070 and 1910 is consistent with what was revealed from the model calculations of the coronal magnetic field and also from our IPS observations.

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 coronal hole 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. Coronal hole map for CR 1910 was from the NSO/Kit Peak synoptic coronal hole map archive (ftp://nsokp.nso.edu/kpvt/coronal_holes/synoptic/).

[8] The formation of an enhanced low-latitude open flux was considered to be a manifestation of the solar magnetic dynamo activity during the Cycle 23/24 minimum. The solar magnetograph observations at the Wilcox Solar Observatory indicated that the polar field strength of the Cycle 23/24 minimum was significantly weaker than that of the previous two minima (http://wso.stanford.edu/gifs/Polar.gif) [Hoeksema, 1995]. We consider that such a weak dipole component is essential to account for the increase of the low-latitude open flux and thus the non-dipolar solar wind structure in the Cycle 23/24 minimum. Furthermore, the sunspot counts in the Cycle 23/24 minimum were very low, and the number of spotless days in 2008 was the highest in the last 50 years. This was also a sign of the solar dynamo activity in this minimum [e.g., Schatten, 2005; Svalgaard et al., 2005]. Observations during Ulysses's third orbit spacecraft showed that the polar fast wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum orbit [McComas et al., 2008]. Thus, the Cycle 23/24 minimum exhibits several peculiar aspects from the viewpoint of solar wind and magnetism. We consider that it provided an excellent opportunity to gain a better insight into the solar wind formation and dynamo process.

6. Summary

[9] Our IPS observations have disclosed that the solar wind structure in the Cycle 23/24 minimum greatly differs from what was observed during the previous two minima. Namely, the fast solar wind emanated not only from the poles but also from the equatorial region, and the source of the slow solar wind split into two mid-latitude regions. The poleward boundary of the slow wind during this minimum extended to ∼30° north and south, and the solar wind structure evolved rapidly. These features significantly differ from those observed during the previous minima. A distinct increase of the occurrence rate of the high flow speed measured in situ at 1 AU is consistent with our IPS observations. Our finding is regarded as another manifestation to indicate the peculiarity of the solar magnetic activity in this minimum.

Acknowledgments

[10] The IPS observations were carried out under the solar wind program of the Solar-Terrestrial Environment Laboratory (STEL) of Nagoya University. This work was partly supported by the Grant-in-Aid for Creative Scientific Research (17GS0208) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We wish to thank the ACE Science Center and the MIT space plasma group for providing the solar wind plasma data collected by the ACE and Wind spacecraft.

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