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Keywords:

  • solar cycle variation;
  • solar wind acceleration;
  • solar wind variations

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[1] Observations of solar wind from both large polar coronal holes (PCHs) during Ulysses' third orbit showed that the fast solar wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum (first) orbit. In addition, while much more variable, measurements in the slower, in-ecliptic wind match quantitatively with Ulysses and show essentially identical trends. Thus, these combined observations indicate significant, long-term variations in solar wind output from the entire Sun. The significant, long-term trend to lower dynamic pressures means that the heliosphere has been shrinking and the heliopause must be moving inward toward the Voyager spacecraft. In addition, our observations suggest a significant and global reduction in the mass and energy fed in below the sonic point in the corona. The lower supply of mass and energy may result naturally from a reduction of open magnetic flux during this period.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[2] The NASA/ESA Ulysses mission has been a remarkable mission of international collaboration and scientific discovery. Launched in October 1990, Ulysses performed a Jupiter gravity assist in February 1992 that diverted it into a nearly polar orbit around the Sun (>80° latitude). All other spacecraft to date have trajectories relatively close to the ecliptic plane, so Ulysses has single handedly explored the previously unsampled mid- and high-latitude heliosphere. Over the past 16 years, Ulysses completed orbits over both poles roughly each six years. Unfortunately, owing to a declining power source and transmitter failures, the Ulysses spacecraft has reached the end of its journey and will cease operations sometime in the summer of 2008.

[3] Ulysses' first orbit revealed a simple bimodal structure of the three dimensional (3-D) solar wind around solar minimum in solar cycle (SC) 22. The wind is comprised of essentially two types: 1) fast, tenuous and relatively homogeneous solar wind at high heliolatitudes emanating from large PCHs that persisted throughout most of the SC and 2) slower, denser, and highly variable wind at lower latitudes [e.g., McComas et al., 1998a, 2000]. At mid-latitudes Ulysses repeatedly traversed a large, stable co-rotating interaction region (CIR) [e.g., Gosling, 1996, and references therein] formed by the interaction between slow wind and faster PCH wind overtaking it. Around solar minimum, the band of solar wind variability was narrow, and confined to within a few tens of degrees of the heliographic equator [Gosling et al., 1995, 1997; McComas et al., 1998a] indicative of a small magnetic dipole tilt angle at these times.

[4] In contrast to the simple structure seen in Ulysses' first orbit, its second orbit, which occurred over the rise to and through solar maximum in SC 23, showed a complex global structure of the 3-D solar wind at all heliolatitudes. This complex structure was driven by a complicated mixture of flows arising from multiple sources at all heliolatitudes [McComas et al., 2002], including streamers, coronal mass ejections (CMEs), small coronal holes, and active regions [Neugebauer et al., 2002]. The two magnetic sector structure largely persisted throughout this interval, but the apparent magnetic dipole axis rotated as the Sun reversed magnetic polarity with its orientation being nearly perpendicular to the Sun's rotation axis around maximum [Smith et al., 2001].

[5] Observations from the end of Ulysses' second orbit through the beginning of its third polar orbit (∼37°S - the end of 2005) [McComas et al., 2006] showed that while the heliosphere was generally returning to a more ordered state, the solar wind stream structure was more variable and contained more interplanetary coronal mass ejections (ICMEs) than observed during Ulysses' first orbit at the same distances and latitudes. This greater variability actually occurred later in the SC when the sunspot number was lower than during its first orbit. McComas et al. [2006] suggested that the differences were likely due to a more complicated current sheet structure, including both a larger average heliospheric current sheet tilt relative to the solar equator and a significant non-planarity to the belt of low-speed flow. They also suggested that such differences might be a regular feature of the alternating magnetic polarity of the Sun's ∼22-year Hale cycle.

[6] In this study we examine new observations from the Ulysses Solar Wind Observations Over the Poles of the Sun (SWOOPS) experiment [Bame et al., 1992]. Observations shown here for the first time span from 2006 when the spacecraft was at mid-latitudes and heading southward, through its third southern polar pass, fast latitude scan, and northern polar pass to essentially the end of the Ulysses mission (and end of SC 23). This study examines the quite different solar wind conditions observed in the PCH flows in SC 23, over Ulysses' third orbit, compared to those taken during its first orbit at a very similar phase of cycle 22. We also examine the properties of the in-ecliptic solar wind, comparing long term trends in the low latitude Ulysses data with data from the Solar Wind Electron Proton Alpha Monitor (SWEPAM) [McComas et al., 1998b] on the Advanced Composition Explorer (ACE).

2. Observations

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[7] Figures 1a1c show polar plots of the solar wind speed over all three of Ulysses' orbits. Underlying the SWOOPS data are composite images of the Sun and corona, which illustrate the solar conditions for each orbit: minimum in SC 22, maximum in SC 23, and minimum in SC 23, respectively. Figures 1a and 1b are essentially replots of figures by McComas et al. [1998a, 2003]. Figure 1d displays the smoothed sunspot number (black) and averaged current sheet tilt relative to the solar equator (red), taken from the Wilcox Solar Observatory (WSO).

image

Figure 1. (a–c) Polar plots of the solar wind speed, colored by IMF polarity for Ulysses' three polar orbits colored to indicate measured magnetic polarity. In each, the earliest times are on the left (nine o'clock position) and progress around counterclockwise. (d) Contemporaneous values for the smoothed sunspot number (black) and heliospheric current sheet tilt (red), lined up to match Figures 1a–1c. In Figures 1a–1c, the solar wind speed is plotted over characteristic solar images for solar minimum for cycle 22 (8/17/96), solar maximum for cycle 23 (12/07/00), and solar minimum for cycle 23 (03/28/06). From the center out, we blend images from the Solar and Heliospheric Observatory (SOHO) Extreme ultraviolet Imaging Telescope (Fe XII at 1950 nm), the Mauna Loa K coronameter (700–950 nm), and the SOHO C2 white light coronagraph.

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[8] Around minimum in SC 22, the band of solar wind variability was narrow, and confined to low latitudes (∼30° to ∼20° north) [Gosling et al., 1995, 1997; McComas et al., 1998a]. This configuration was consistent with the small dipole tilt angles seen at the time and confinement of the helmet streamers to low latitudes. In contrast, the tilt of the heliospheric current sheet has remained substantially higher thus far through the minimum of SC 23, even though the sunspot number declined to very low values. Figure 1c shows the comparable plot for Ulysses' third orbit. Generally, Figures 1a and 1c look very similar except for the reversed solar magnetic field. Also note that the band of solar wind variability extends to somewhat higher latitudes in the third orbit observations. The brief low speed interval (∼5:30 position) in the otherwise fast PCH wind was caused by significant mass loading of the flow by comet McNaught [Neugebauer et al., 2007].

[9] Figure 2 shows a comparison of various plasma properties taken as a function of heliolatitude for the PCH flows observed over Ulysses' first and third orbits. From top to bottom, the plots show proton speed, proton density normalized by R2, proton temperature normalized by R [McComas et al., 2000], the alpha particle to proton ratio, and the full normalized momentum flux, or dynamic pressure, mp(npvp2 + 4nαvα2)(R/Ro)2. We separated the one-hour averaged SWOOPS data into 4° bins in heliolatitude from 40° to 80° and calculated mean values (symbols) and ±1σ variations (bars) for each bin.

image

Figure 2. Solar wind parameters for different segments of Ulysses first orbit (red) and third orbit (blue), when the spacecraft was imbedded in fast solar wind from large PCHs. While the alpha to proton density ratio is statistically identical, in the third orbit, the speed is slightly slower and the density, temperature, and dynamic pressure are all significantly lower compared to values measured during Ulysses' first orbit.

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[10] While there were small variations between the fast and slow latitude scans (small vs. large symbols) and northern and southern PCH observations (circles vs. squares), the most significant differences in Figure 2 are clearly between first (red) and third (blue) orbits. The PCH solar wind observed in Ulysses' third orbit is significantly slower, less dense and cooler than that observed in Ulysses' first orbit. Of these, the speed shows the least difference, particularly at the highest latitudes, although in combination, these four-degree binned samples show a consistently lower speed in the third orbit. In addition, the speed also continued to show its characteristic, but still unexplained, increase of ∼1 km s−1 per degree of heliographic latitude [McComas et al., 2000, 2002, 2003]. Because the wind was slower and less dense, the dynamic pressure was also lower in the third orbit. In contrast to these bulk properties, however, the alpha to proton ratio, which is a measure of the plasma composition, was essentially identical.

[11] Table 1 provides the mean values for selected plasma parameters. All values were calculated by averaging all one-hour averaged data samples obtained above 40° heliolatitude. The columns show the first orbit mean value, the third orbit mean value, and the percentage change of the third orbit value compared to the first. The short interval around the comet McNaught encounter was removed so as not to bias the sample. Clearly, the PCH solar wind was consistently weaker in Ulysses' third orbit compared to its first. In a related study, Issautier et al. [2008] use radio wave observations from Ulysses to independently confirm the significantly lower density (∼20% in their measurements) and show that the core electrons are also significantly cooler (∼15%) in the third orbit observations. Also, Smith and Balogh [2008] examined the magnetic field properties and showed that the radial magnetic field observed at high latitudes in Ulysses' third orbit was around one third weaker than in its first orbit.

Table 1. Solar Wind Parameters From Ulysses First and Third Orbits for Latitudes >40°
Parameterequation image1stequation imageequation image3rdequation imageequation image
vp (km s−1)761739−3%
vα (km s−1)772746−3%
npR2 (cm−3)2.652.19−17%
nαR2 (cm−3)0.120.10−17%
TpR × 105 (K)2.662.30−14%
TαR × 106 (K)1.121.00−11%
Mass Flux (kg m−2 s−1) ρiviR2 × 10−153.963.17−20%
Dynamic Pressure (nPa) ρivi2R23.012.34−22%
Proton Thermal Pressure (pPa) npkTpR39.897.43−25%
Alpha Particle Thermal Pressure (pPa) nαkTαR31.801.39−23%
Alpha to proton ratio, nα/np0.04370.0444not significant

[12] The substantial differences between the PCH observations in Ulysses' first and third orbits led us to consider if these differences were limited to high latitudes or represented global changes in the solar wind and Sun itself. Figure 3 shows the SWOOPS observations overlaid with the same parameters from the nearly identical SWEPAM experiment on the nearly ecliptic ACE spacecraft. Intervals when Ulysses was at low latitudes (<±30°) are indicated by the light shading. The ACE and Ulysses observations agree extremely well for essentially all of the intervals when Ulysses was at low latitudes. Furthermore, the ACE measurements also reveal a steady decline in solar wind density and dynamic pressure.

image

Figure 3. Solar wind bulk parameters from Ulysses (black) compared to ACE (red), in the ecliptic plane, averaged over 25.4 day solar rotations. Intervals when Ulysses was within ±30° of the ecliptic plane (shaded) match very closely in speed, density and dynamic pressure. Dynamic pressure continues to quantitatively match, even over Ulysses' high latitude excursions.

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3. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[13] The observations presented here demonstrate significant differences between the fast high-latitude solar wind from the large PCHs observed at high latitudes in Ulysses' first (SC 22) and third (SC 23) orbits. In the third orbit the wind was slightly slower (∼3%), less dense (∼17%), and cooler (∼14%) than in the first and had significantly lower mass flux (∼20%), dynamic pressure (∼22%), and ∼25% less thermal pressure. The observations were taken at very similar phases of the SC, leading us initially to consider if one of these were the “usual” PCH conditions and the other one “unusual.” In fact, the SC 23 activity minimum has been somewhat less usual in that polar magnetic fields remained weak and well formed PCHs didn't begin to emerge early in the solar minimum phase of this cycle [Schatten, 2005]. Since then, the PCHs have remained somewhat less well formed and the current sheet has retained a significantly higher inclination than typically seen around most solar minima.

[14] Lazarus and McNutt [1990] used near ecliptic observations from Voyager to show that the dynamic pressure of the solar wind varied over a SC by almost a factor of two, with the smallest pressure near solar maximum and the peak pressure a couple of years later. Subsequently, another large (nearly factor of 2) variation from 1991 through 2001 was noted by Richardson et al. [2001], with the near ecliptic dynamic pressure again rising steeply just after solar maximum and then slowly decreasing over the rest of the SC. The Ulysses observations through 2002, however, did not show such a large increase during or just after the latest solar maximum, but rather revealed a more modest ∼50% increase from ∼2 to ∼3 nPa [McComas et al., 2003] and that increase was very short-lived.

[15] While this study is the first to show long term variations of multiple solar wind properties simultaneously at both high and low latitudes, Richardson and Wang [1999] previously compared the trends in dynamic pressure from Ulysses' first polar orbit with measurements from IMP-8 and Voyager 2. Those authors did not compare absolute values of quantities and did not include alpha particles. The alphas provide tens of percent of the total pressure, which is highly variable in the slower solar wind, especially around solar maximum [McComas et al., 2003, 2006]. Nonetheless, Richardson and Wang [1999] did show that the large-scale trends in the dynamic pressure were similar at high and low latitudes, suggesting that the solar wind might vary globally over the SC.

[16] The solar wind's dynamic pressure is particularly interesting because it inflates the heliosphere, and balances the external pressure from the surrounding local interstellar medium. The termination shock crossing by Voyager 1 at a heliocentric distance of 94 AU on 16 December 2004 [Stone et al., 2005, and references therein] occurred during a significant inward motion of the shock. This was likely caused by the substantial drop in the pressure in early 2004 [McComas et al., 2006] (also see Figure 3). While the pressure initially rebounded to ∼2.6 nPa (at 1 AU) around the end of 2004, this was a temporary increase and the pressure has continued to drop off since. The overall decrease in dynamic pressure to ∼2 nPa has surely made the heliosphere smaller than it was a decade ago, and is likely at least a contributing factor in Voyager 2 crossing the termination shock at just 84 AU in September 2007 [Richardson et al., 2008]. More importantly, such low dynamic pressure also reduces the distance to the heliopause, meaning that if it stays low, Voyager 1 will reach this critical boundary earlier than previously anticipated.

[17] Another reason that the dynamic pressure (or momentum flux) is an important parameter is because nearly all of the energy of the solar wind is carried off in the form of bulk flow energy. Thus, this quantity is an excellent measure of the average energy being introduced in the corona, which ultimately accelerates the solar wind. Leer and Holzer [1980] developed a simple model to examine the effects of solar wind energization by heating and momentum addition. These authors showed that addition of energy above the sonic point primarily increases the wind speed whereas the addition of energy by heating below the sonic point increases the mass and momentum flux roughly comparably, leading to little change in the wind speed. In contrast, they showed that addition of momentum in the subsonic region increases the mass flux while keeping the solar wind speed roughly constant. While numerous subsequent theories and models have addressed the issues of coronal heating and solar wind acceleration (e.g., see review by McComas et al. [2007, and references therein]), the basic connection of solar wind properties and the coronal energization processes described by Leer and Holzer [1980] remain intact.

[18] The addition of mass and energy below the sonic point is consistent with models that power the solar wind through emerging magnetic flux [e.g., Schwadron and McComas, 2003]. On the basis of such models, Schwadron et al. [2006] predicted that the solar wind power, which is about a quarter lower in the third orbit compared to the first, should be proportional to the radial magnetic flux of the open field. Remarkably, the average radial field strength in the PCHs during Ulysses third orbit was significantly lower (about one third) [Smith and Balogh, 2008] compared to the measured radial field in the fast wind during the first orbit [Balogh et al., 1995].

[19] The Ulysses observations of relatively similar (slightly reduced) solar wind speed with significant and comparable drops in mass and momentum fluxes suggest cooler conditions with substantially less heating below the sonic point in the PCH flows observed in Ulysses' third orbit compared to its first. Because of the quantitative agreement in the trends and values with ACE when Ulysses was at low latitudes, this result likely applies globally and represents a real change of the whole Sun. This change likely caused the Sun's polar fields not to strengthen as much or as early in this solar minimum as they did in the previous one [Schatten, 2005, and references therein]. In fact, models based on solar magnetic field observations indicate significantly lower open magnetic flux and suggest that the radial field could be as much as 40% lower at the start of 2007 compared to previous cycle [see Svalgaard and Cliver, 2007, and references therein].

[20] By correlating the Ulysses observations with independent measurements from ACE in the ecliptic plane, this study is the first to show a quantitative match between these properties. Thus we are able to tie the differences in the much less variable solar wind properties in the PCHs, especially density and dynamic pressure, to nearly identical variations in the ecliptic measurements. This combination indicates that the differences seen at high latitudes were largely driven by a global change in the Sun and its solar wind output and not just differences in the PCHs. It seems likely that the globally weaker solar wind is directly related to the lower average strength of the Sun's open field. An almost constant rate of open field reconfiguration through flux emergence would naturally cause the power in the solar wind to scale with the amount of open magnetic flux [Schwadron et al., 2006], and, in turn, explain why the solar wind power is lower in the third orbit. It is interesting to consider if such whole Sun variations are typical of the 22-year Hale Cycle.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[21] We thank all the wonderful men and women who have made the Ulysses program such an outstanding success. For Figure 1, we also specifically thank the Ulysses magnetometer team for data used to identify the magnetic polarities and H. Morgan for help in assembling the images in the top right plot. Work at LANL was performed under the auspices of the U. S. DOE. This work was funded by NASA under the Ulysses program.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information
FilenameFormatSizeDescription
grl24924-sup-0001-t01.txtplain text document1KTab-delimited Table 1.

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