1. 1 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet The Unusual Heliospheric Current Sheet at the End of Cycle 23 A Comparison of Cycles 21,22,& 23 J. Todd Hoeksema Stanford University

2. 2 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Comparison of Solar Cycles 21 – 22 – 23 – Consistent observations from WSO since 1976 – the beginning of Cycle 21 Typical evolution of model heliospheric current sheet follows pattern through cycle Minimum HCS resembles dipole – until now

3. 3 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Potential Field – Source Surface Simple Model of Coronal Field Carrington Rotation 1912 – August 1996 Whole Sun Month – Elephant’s Trunk

4. 4 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Typical Solar Cycle – 22 Minimum 1986 Rising 1988 Max & Polar Reversal 1989 Early Decline 1991 Later Decline 1993 Pre-Min 1995

5. 5 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Solar Cycle 21 1976 - 1986 Every 8 th Rotation X2 CR 1645-1781

6. 6 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Solar Cycle 22 1986 - 1996 Every 8 th Rotation X2 CR 1780 - 1916

7. 7 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Solar Cycle 23 1996 - 2007 Every 8 th Rotation X2 CR 1920 - 2056

8. 8 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Comparison of Cycles 21 & 22 Every 9 th Rotation X2 1977 – 1986 CR 1650 – 1776 1987 – 1996 CR 1790 – 1916

9. 9 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Comparison of Cycles 22 & 23 Every 9 th Rotation X2 1987 – 1996 CR 1790 – 1916 1997 – 2006 CR 1920 - 2046

10. 10 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Comparison of Cycles 21 & 23 Every 9 th Rotation X2 1977 – 1986 CR 1645 – 1776 1997 – 2006 CR 1920 – 2046

11. 11 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Comparison of Minima at Start of Cycles 21-22-23-24 1976 – Cycle 21.01996 – Cycle 23.0 1986 – Cycle 22.02007 – Cycle 23.95(?)

12. 12 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 1976 1981 1986 1991 1996 2001 2006

13. 13 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Polar Field Measured at WSO from 1976 - 2007

14. 14 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet Summary Each solar cycle follows a similar pattern of evolution Each cycle shows different features depending on AR emergence Polar field dominates HCS@solar minimum Polar field at end of Cycle 23 is very weak HCS reaches higher latitude and has more structure in 2007 than expected

15. 15 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet North Polar Field Strength