Comparing the Large-Scale Magnetic Field During the Last

Three Solar Cycles

Todd Hoeksema

Evolution of the Large-Scale Magnetic Field OverThree Solar Cycles

Global Properties

Corona & Heliosphere

Zonal Magnetic Field

Toroidal Magnetic Field

Daily Solar Mean Magnetic Field – Sun as a Star3 Cycles – 1975 - 2009

WSO Sensitivity Issues in ~2001

RotationModulationObservedAt < 10 uT

4 Solar Minima, 3 Polar Reversals1976 - 2009

WSO Large-scale Polar Aperture 55 to Pole

Surge Arrival

South strongerIn Cycles 21 + 22

MDI MeasuredPolar Field during Cycle 231996-2008

Movie shows observed andInterpolated polar field above 62

Smaller 75+ Polar Field IsMore Symmetric & ReversesLater

Difference in Strength is Significant, but Smaller

MDI MeasuredPolar Field during Cycle 231996-2008

Movie shows observed andInterpolated polar field above 62

Smaller 75+ Polar Field IsMore Symmetric & ReversesLater

Difference in Strength is Significant, but Smaller

Axial, Equatorial, and Total Solar Dipole – 3 Cycles1976 - 2009

Total

Axial

Equatorial

Total Solar Dipole, Quadrupole, Octupole – 3 Cycles1976 - 2009

Dipole

Quadrupole

l = 3

Magnitude @ 2.5Rs

Computed Source Surface Magnetic Field

The ComputedCoronal Field & HCS

During Cycle 231996-2009

Every 10th RotationEach panel 2 Rots wide

Reversal in 1999-2000

Stable Structures

Dipole and QuadrupoleDominate

3 Cycles of the Computed HCS Inclination

Best SS Radius may be 1.8 Rs this minimum – Sun & Hoeksema SH11A-1487

From McComas et al., GRL 2008

Ulysses Observed Different Solar Wind StructureDuring Solar Minimum in 1996 compared with 2008Low-latitude is more complex and wider due to Decreased importance of Polar Coronal Holes

Thanks to J. Luhmann

Note: Coronal snapshot useful, but may mislead since structure reflects rotation

Ulysses also saw low IMF at high heliolatitudes in 2007 (Smith et al., GRL, 2008) –OMNI data from NSSDC

Thanks to J. Luhmann

IMF Measured at L1 is Significantly Weaker thisMinimum than the Previous Three

IMF observed to be uniform at all latitudes away from HCS

Thanks to J. Luhmann

Computed open flux foot points – Coronal Hole ProxyEach Panel shows ~10 years starting 1972.

Low latitude holes are relatively more important during this solar minimum

1973

1983

1993

2003

(GONG websitemovie, MSFC sunspotillustrations)

The long decline of Solar Cycle 23 has been marked by very few, well separated Active Regions

Comparison ofLarge-ScaleCorona in Cycles 21 & 23

Panels spacedBy 12 rotations.

CYCLE 23:

•Longer•Fewer Contours•Gradual Flattening•Structures Persist•More Dipole in Declining Phase

Cycle 21 Cycle 23

Comparison ofLarge-ScaleCorona in Cycles 22 & 23

Panels spacedBy 12 rotations.

Cycle 23:

•Fewer Contours•Slower Flattening•Structures Persist Thanks to Weak Poles•Both Dipolar During Reversal

Cycle 22 Cycle 23

Back to the Photospheric Field

Zonal Average of Total Flux and Net Flux – 3 Cycles1976-2009 from WSO

~2001 WSO Sensitivity

Antisymmetric Zonal

Symmetric Zonal

OldPolaritySurge2003/4

Toroidal (East-West) Field – 3 Cycles from WSO

Cycles overlap several years and are extendedWo

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From wattsupwiththat website

Comparison of SSN for last 5 Cycles

From Wikipedia website

Perspective Comes from Taking the Long View

Summary - 1

•Cycle 23 was longer and weaker (relative to 21 and 22):

•MF, pole, meridional flow, corona, solar wind, dipole

•Polar Reversal on time, but no subsequent growth – no organized poleward migration

•Corona/Heliosphere not as constrained by dominant Polar Coronal Holes

Summary - 2

•Corona/Heliosphere not constrained by dominant Polar Coronal Holes in declining phase of Cycle 23

•HCS continued to reach high latitude•Mid latitude structure in solar wind and low latitude coronal holes

•Very few spots in last few years.

Summary - 3

•Zonal Flux Reduced•Poleward flows small•Antisymmetric flows at low latitude only

•2002 not like 1991, 1981

•Old polarity anti-symmetric flow in 2003•Interesting to learn what was ‘absent’ in 2003-2004•Toroidal flux pattern about the same•New flux emerges just after previous max at high latitude and expands equatorward