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Ulysses Observations of Weaker Solar Wind in Cycle 23

Ulysses Observations of Weaker Solar Wind in Cycle 23. Heather A. Elliott (1), David J. McComas(1), R. W. Ebert (1), B. E. Goldstein (2), J. T. Gosling (3), N. A. Schwadron (4), and R. M. Skoug (5) 1) Southwest Research Institute, 2) Jet Propulsion Lab, 3) University of Colorado ,

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Ulysses Observations of Weaker Solar Wind in Cycle 23

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  1. Ulysses Observations of Weaker Solar Wind in Cycle 23 Heather A. Elliott (1), David J. McComas(1), R. W. Ebert (1), B. E. Goldstein (2), J. T. Gosling (3), N. A. Schwadron (4), and R. M. Skoug (5) 1) Southwest Research Institute, 2) Jet Propulsion Lab, 3) University of Colorado, 4) Boston University, and 5) Los Alamos National Lab

  2. Overview of the Solar Cycle and Ulysses Solar Wind Observations • The current sheet tilt decreased more slowly than in the previous minimum. • The fast wind regions are smaller in the third orbit (cycle 23 minimum) than in the 1st orbit (cycle 22 minimum). • The last of the Ulysses observations show the speed was dropping off at the edge of the polar coronal hole.

  3. Long Term Trends In the Solar Wind • Tilt lower in the previous two minima. • Speed of holes similar to before, but the density is lower. • Pressure lower this minimum than in the previous three. Ulysses ACE OMNI

  4. Time Evolution: Ulysses-ACE Comparison • The density and speed at ACE and Ulysses is similar when they are closer in latitude. • However, the dynamic pressure at both ACE and Ulysses is quite similar also indicating that the pressure is lower at all latitudes. • The pressure has been decreasing since about 1992.

  5. Solar Wind Dynamic Pressure • The solar wind dynamic pressure is significantly lower than the previous minimum. • The whole Sun (all latitudes) is emitting a lower pressure wind.

  6. Solar Wind Power and Magnetic Flux Coronal Hole Fast Wind • Both the field strength and the dynamic pressure have been decreasing. • The power in the fast wind correlates well with the Sun’s total open magnetic flux. V>500 km/s and Lat >40 V<400 km/s and Lat <20 Schwadron and McComas 2008

  7. Even the Declining Phase of Cycle 23 Was Unusual:Fast Wind At Low Latitudes 1996 2003 • Most years the data can be fit well with one fit. • In 2003 much of the fast wind is shifter to higher speeds. • The wind was faster than typically observed at low latitudes. • This explains the higher Ap and Kp in 2003.

  8. Ulysses Speed-Latitude Relationship For Coronal Holes

  9. Low Latitude Polar Coronal Extension:Emitted Faster Than Normal Wind • During the declining phase of cycle 23 (2003) a large outward polar coronal hole extension emitted wind much faster than typical for low latitudes. • Blue curve is the speed predicted from the Ulysses speed-latitude relationship.

  10. Differences in Cycle 23 • The current sheet tilt stayed higher longer. • Faster than normal wind at low latitudes in 2003. • Ulysses crossed the edges of the polar holes at higher latitudes. • The solar wind dynamic pressure for the whole Sun was lower. • The field strength was lower. • The overall distribution and sizes of the coronal holes is different. The polar coronal holes seem smaller, but there were more holes at low latitudes in the declining phase. • A long lived low latitude hole emitted faster wind than is typically observed at low latitudes causing Ap and Kp to be high in the declining phase. • The large low latitude coronal holes keep the current sheet tilt high. • CME rate is low for a long time, and what transients do occur are slow and small and difficult to distinguish from the slow solar wind (Kilpua et al.,2009).

  11. Conclusions • The overall configuration and properties of the coronal holes are different this cycle • the current sheet tilt stays high • the solar wind at low latitudes is faster than what was observed at low in the previous two cycles • the higher speeds at low latitudes causes the Ap and Kp indices to be high. • The long-term drop in solar wind dynamic pressure occurs over the whole Sun. • The implications of a long term drop in pressure for the entire Sun is that the overall size of the heliosphere has been shrinking • Ulysses observations indicate a global reduction in the Sun's energy and mass below the sonic point since the speed of the polar coronal holes is similar to that in the previous minimum, but the mass flux is significantly reduced. • The solar wind power and magnetic flux are correlated.

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