The Unusual Heliospheric Current Sheet at the End

Comparison of Solar Cycles 21 – 22 – 23 – • Consistent observations from

Potential Field – Source Surface Simple Model of Coronal Field Carrington Rotation 1912 –

Typical Solar Cycle – 22 Minimum Early 1986 Decline 1991 Rising 1988 Max &

Solar Cycle 21 1976 - 1986 Every 8 th Rotation X 2 CR 1645

Solar Cycle 22 1986 - 1996 Every 8 th Rotation X 2 CR 1780

Solar Cycle 23 1996 - 2007 Every 8 th Rotation X 2 CR 1920

Comparison of Cycles 21 & 22 Every 9 th Rotation X 2 1977 –

Comparison of Cycles 22 & 23 Every 9 th Rotation X 2 1987 –

Comparison of Cycles 21 & 23 Every 9 th Rotation X 2 1977 –

Comparison of Minima at Start of Cycles 21 -22 -23 -24 July 31, 2007

1976 July 31, 2007 1981 1986 1991 1996 SHINE 2007 – Heliospheric Plasma Sheet

Polar Field Measured at WSO from 1976 - 2007 July 31, 2007 SHINE 2007

Summary • Each solar cycle follows a similar pattern of evolution • Each cycle

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

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

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 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 2

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

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

Solar Cycle 21 1976 - 1986 Every 8 th Rotation X 2 CR 1645 -1781 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 5

Solar Cycle 22 1986 - 1996 Every 8 th Rotation X 2 CR 1780 - 1916 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 6

Solar Cycle 23 1996 - 2007 Every 8 th Rotation X 2 CR 1920 - 2056 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 7

Comparison of Cycles 21 & 22 Every 9 th Rotation X 2 1977 – 1986 CR 1650 – 1776 1987 – 1996 CR 1790 – 1916 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 8

Comparison of Cycles 22 & 23 Every 9 th Rotation X 2 1987 – 1996 CR 1790 – 1916 1997 – 2006 CR 1920 - 2046 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 9

Comparison of Cycles 21 & 23 Every 9 th Rotation X 2 1977 – 1986 CR 1645 – 1776 1997 – 2006 CR 1920 – 2046 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 10

Comparison of Minima at Start of Cycles 21 -22 -23 -24 July 31, 2007 1976 – Cycle 21. 0 1996 – Cycle 23. 0 1986 – Cycle 22. 0 2007 – Cycle 23. 95(? ) SHINE 2007 – Heliospheric Plasma Sheet 11

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

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

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 July 31, 2007 SHINE 2007 – Heliospheric Plasma Sheet 14

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