The Solar Dynamo Saga: Chapter 11 Dr. David Hathaway NASA Marshall Space Flight Center 2009 August 15 Huntsville Hamfest.

The Solar Dynamo Saga:The Solar Dynamo Saga:Chapter 11Chapter 11

Dr. David HathawayDr. David Hathaway

NASA Marshall Space Flight CenterNASA Marshall Space Flight Center

2009 August 15 Huntsville Hamfest2009 August 15 Huntsville Hamfest

OutlineOutline

• Sunspots, space weather, and climateSunspots, space weather, and climate

• Solar cycle characteristicsSolar cycle characteristics

• Solar dynamo basics (40s and 50s)Solar dynamo basics (40s and 50s)

• Solar dynamo models (60s to Present)Solar dynamo models (60s to Present)

• Dynamo dilemmas (Starting in the 70s)Dynamo dilemmas (Starting in the 70s)

• The role of the meridional circulationThe role of the meridional circulation

• Conclusions Conclusions

Sunspots, Space Weather, Sunspots, Space Weather, and Climateand Climate

(why should YOU care)(why should YOU care)

SunspotsSunspotsSunspots are dark (and cooler) regions Sunspots are dark (and cooler) regions on the surface of the Sun. They have a on the surface of the Sun. They have a darker inner region (the Umbra) darker inner region (the Umbra) surrounded by a lighter ring (the surrounded by a lighter ring (the Penumbra).Penumbra).

Sunspots usually appear in groups that Sunspots usually appear in groups that form over hours or days and last for days form over hours or days and last for days or weeks.or weeks.

These early sunspot observations These early sunspot observations indicated that the Sun rotates once in indicated that the Sun rotates once in about 27 days.about 27 days.

Solar ActivitySolar ActivitySolar Flares, Prominence Eruptions, and Coronal Mass Ejections are all Solar Flares, Prominence Eruptions, and Coronal Mass Ejections are all forms of solar activity. Each can occur on its own but they often occur forms of solar activity. Each can occur on its own but they often occur together as in “The Bastille Day Event.”together as in “The Bastille Day Event.”

Space WeatherSpace WeatherSpace weather refers to conditions on the Space weather refers to conditions on the Sun and in the space environment that Sun and in the space environment that can influence the performance and can influence the performance and reliability of space-borne and ground-reliability of space-borne and ground-based technological systems, and can based technological systems, and can endanger human life or health. endanger human life or health.

Effects of Solar Activity:Effects of Solar Activity:On Radio Wave PropagationOn Radio Wave PropagationVariations in ionizing radiation (UV, EUV, X-rays) from the Sun, as well Variations in ionizing radiation (UV, EUV, X-rays) from the Sun, as well as solar induced changes to the Earth’s magnetosphere, alter the as solar induced changes to the Earth’s magnetosphere, alter the ionosphere – changing the Maximum Usable Frequency for high ionosphere – changing the Maximum Usable Frequency for high frequency radio communications.frequency radio communications.

HF Communication only

• Polar flights departing from North America use VHF (30-300 MHz) comm or Polar flights departing from North America use VHF (30-300 MHz) comm or Satcom with Canadian ATCs and Arctic Radio.Satcom with Canadian ATCs and Arctic Radio.

• Flights rely on HF (3 – 30 MHz) communication inside the 82 degree circle.Flights rely on HF (3 – 30 MHz) communication inside the 82 degree circle.

• Growth: Airlines operating China-US routes goes from 4 to 6 and then Growth: Airlines operating China-US routes goes from 4 to 6 and then number of weekly flights goes from 54 to 249 over the next 6-years.number of weekly flights goes from 54 to 249 over the next 6-years.

Effects of Solar Activity:Effects of Solar Activity:On Airline OperationsOn Airline Operations

Total Irradiance and ClimateTotal Irradiance and ClimateThe 0.1% change in the Total Solar Irradiance seen over the last three The 0.1% change in the Total Solar Irradiance seen over the last three solar cycles only produces a 0.1solar cycles only produces a 0.1° C temperature change in climate ° C temperature change in climate models.models. However, the Sun seems to have a bigger impact. However, the Sun seems to have a bigger impact.

Two other mechanisms (besides direct forcing by the Total Solar Two other mechanisms (besides direct forcing by the Total Solar Irradiance variations) are under study: 1) solar ultraviolet and extreme Irradiance variations) are under study: 1) solar ultraviolet and extreme ultraviolet variability and 2) Cosmic Ray modulation on cloud cover. ultraviolet variability and 2) Cosmic Ray modulation on cloud cover.

Solar Cycle CharacteristicsSolar Cycle Characteristics(that dynamo theories should reproduce)(that dynamo theories should reproduce)

Sunspots!Sunspots!

Sunspots are cooler and darker regions on the surface of the Sunspots are cooler and darker regions on the surface of the Sun. Typical sunspots are about the size of Earth.Sun. Typical sunspots are about the size of Earth.

The Sunspot CycleThe Sunspot Cycle

The average cycle lasts 131±14 months and has a smoothed The average cycle lasts 131±14 months and has a smoothed sunspot number maximum of 114±40.sunspot number maximum of 114±40.

Equatorward Drift & Cycle OverlapEquatorward Drift & Cycle Overlap

Sunspots appear in two bands on either side of the equator. These Sunspots appear in two bands on either side of the equator. These bands drift toward the equator as the cycle progresses and cycles bands drift toward the equator as the cycle progresses and cycles overlap by 2-3 years at minimum.overlap by 2-3 years at minimum.

Active Latitude PositionActive Latitude Position

The equatorward drift rate is not constant. The drift rate slows as The equatorward drift rate is not constant. The drift rate slows as the active latitude approaches the equator. Bigger cycles have the active latitude approaches the equator. Bigger cycles have faster drift rates.faster drift rates.

Active Latitude WidthActive Latitude Width

The width of the active latitude band increases from cycle The width of the active latitude band increases from cycle minimum to cycle maximum and then decreases after the minimum to cycle maximum and then decreases after the equatorward edge approaches the equator. Bigger cycles have equatorward edge approaches the equator. Bigger cycles have wider active latitude zones.wider active latitude zones.

It’s MagneticIt’s Magnetic

Hale’s Magnetic Polarity LawHale’s Magnetic Polarity Law

The magnetic polarity of the sunspots in active regions switches The magnetic polarity of the sunspots in active regions switches from one hemisphere to the other and from one cycle to the next.from one hemisphere to the other and from one cycle to the next.

Sunspot Group Tilt- Joy’s LawSunspot Group Tilt- Joy’s Law

Sunspot groups are tilted with the leading spots closer to the Sunspot groups are tilted with the leading spots closer to the equator than the following spots. This tilt increases with latitude.equator than the following spots. This tilt increases with latitude.

Polar Field ReversalsPolar Field Reversals

The magnetic polarities of the Sun’s poles reverse from one The magnetic polarities of the Sun’s poles reverse from one cycle to the next at about the time of sunspot cycle maximum.cycle to the next at about the time of sunspot cycle maximum.

Magnetic Cycle in MotionMagnetic Cycle in Motion

Notice the Differential Rotation – flow to the right (faster than average Notice the Differential Rotation – flow to the right (faster than average rotation) near the equator and flow to the left (slower than average rotation) near the equator and flow to the left (slower than average rotation) near the poles – and Meridional Flow from the equator rotation) near the poles – and Meridional Flow from the equator towards the poles.towards the poles.

The Surface EvidenceThe Surface Evidence

The evolution of the Sun’s surface magnetic field pattern must be The evolution of the Sun’s surface magnetic field pattern must be matched with any viable dynamo model.matched with any viable dynamo model.

What about the Interior?What about the Interior?

We will return to this later!We will return to this later!

Solar DynamosSolar Dynamos

Dynamo?Dynamo?• Cowling (1934) “Anti-Dynamo Theorem” – no dynamo action from Cowling (1934) “Anti-Dynamo Theorem” – no dynamo action from

axisymmetric flows.axisymmetric flows.

• Cowling (1945) Decay time for “fossil” field (10Cowling (1945) Decay time for “fossil” field (101010 years) incompatible years) incompatible with 11-year cycle – must have dynamo activity.with 11-year cycle – must have dynamo activity.

• Elsassar (1946) & Bullard (1949) – dynamo action from non-Elsassar (1946) & Bullard (1949) – dynamo action from non-axisymmetric flows.axisymmetric flows.

• Parker (1955) Dual effects of differential rotation and cyclonic motions Parker (1955) Dual effects of differential rotation and cyclonic motions give oscillatory mean field with traveling dynamo waves.give oscillatory mean field with traveling dynamo waves.

Babcock (1961)Babcock (1961)

a) Dipolar field at cycle minimum a) Dipolar field at cycle minimum threads through a shallow layer threads through a shallow layer below the surface.below the surface.

b) Differential rotation shears out b) Differential rotation shears out this poloidal field to produce a this poloidal field to produce a strong toroidal field (first at the strong toroidal field (first at the mid-latitudes then progressively mid-latitudes then progressively lower latitudes).lower latitudes).

c) Buoyant fields erupt through c) Buoyant fields erupt through the photosphere giving Hale’s the photosphere giving Hale’s polarity law and Joy’s Tilt.polarity law and Joy’s Tilt.

d) d) Meridional flow away from the Meridional flow away from the mid-latitudes gives reconnection mid-latitudes gives reconnection at the poles and equator.at the poles and equator.

Leighton (1969)Leighton (1969)• Supergranules (which were Supergranules (which were “discovered” by Leighton, Noyes, “discovered” by Leighton, Noyes, & Simon in 1962) provide diffusive & Simon in 1962) provide diffusive transport of 10transport of 1044 km km22/s in the /s in the photosphere (photosphere (no meridional flow no meridional flow requiredrequired).).

• A rotation rate increasing inward A rotation rate increasing inward is required for equatorward drift of is required for equatorward drift of the active latitudes.the active latitudes.

• A numerical model for the depth A numerical model for the depth and longitude averaged magnetic and longitude averaged magnetic field can reproduce many observed field can reproduce many observed aspects – provided several aspects – provided several adjustable parameters are set.adjustable parameters are set.

αΩαΩ Dynamos Dynamos• Toroidal field, Toroidal field, BBɸɸ, is , is

generated from poloidal generated from poloidal field by the Omega effect field by the Omega effect – shearing by differential – shearing by differential rotation - rotation - ΩΩ..

• Poloidal field, Poloidal field, AAɸɸ, is , is

generated from toroidal generated from toroidal field by the alpha effect – field by the alpha effect – cyclonic flows – cyclonic flows – αα~v·~v·v.v.

• A dynamo wave travels A dynamo wave travels along isorotation surfaces along isorotation surfaces with frequency ~ with frequency ~ ααΩΩ

3D-MHD Dynamos3D-MHD Dynamos

• Gilman & Miller (1981) “Dynamically consistent nonlinear Gilman & Miller (1981) “Dynamically consistent nonlinear dynamos driven by convection in a rotation spherical shell”dynamos driven by convection in a rotation spherical shell”

• Glatzmeier (1985) “Numerical simulations of stellar convective Glatzmeier (1985) “Numerical simulations of stellar convective dynamos. II. Field propagation in the convection zone”dynamos. II. Field propagation in the convection zone”

• Oscillatory dynamos were producedOscillatory dynamos were produced

• The dynamo waves moved polewardThe dynamo waves moved poleward

• The periods were too short by 10xThe periods were too short by 10x

Dynamo Dilemma #1Dynamo Dilemma #1

Equatorward propagation could Equatorward propagation could be achieved if the rotation rate be achieved if the rotation rate increased inward across the increased inward across the convection zone.convection zone.

However, the However, the αα-effect in the bulk -effect in the bulk of the convection zone is orders of the convection zone is orders of magnitude too large and gives of magnitude too large and gives short period (2-year instead of short period (2-year instead of 22-year) dynamos no matter how 22-year) dynamos no matter how the internal rotation rate varies.the internal rotation rate varies.

Rapid twisting motions produced very short solar cycles.Rapid twisting motions produced very short solar cycles.

Dynamo Dilemma #2Dynamo Dilemma #2The magnetic field produced in the convection zone should be The magnetic field produced in the convection zone should be buoyant and rise rapidly to the surface (Parker, 1975).buoyant and rise rapidly to the surface (Parker, 1975).

Buoyant magnetic fields can’t stay down long enough to get amplified.Buoyant magnetic fields can’t stay down long enough to get amplified.

Dynamo Dilemma #3Dynamo Dilemma #3

Observed (Helioseismology) Hydrodynamic Model Kinematic ModelObserved (Helioseismology) Hydrodynamic Model Kinematic Model

The internal rotation profile determined with helioseismic methods The internal rotation profile determined with helioseismic methods shows shear layers at the top and bottom of the convection zone with shows shear layers at the top and bottom of the convection zone with nearly constant rotation rate in between – unlike the rotation profiles nearly constant rotation rate in between – unlike the rotation profiles produced in the hydrodynamic models or assumed in the kinematic produced in the hydrodynamic models or assumed in the kinematic dynamo models.dynamo models.

The Sun’s internal rotation didn’t match ANY of the models.The Sun’s internal rotation didn’t match ANY of the models.

Interface DynamosInterface Dynamos

All three dilemmas could be circumvented if the dynamo action took All three dilemmas could be circumvented if the dynamo action took place at the base of the convection zone.place at the base of the convection zone.

Parker (1975) made this suggestion early on to solve the magnetic Parker (1975) made this suggestion early on to solve the magnetic buoyancy dilemma.buoyancy dilemma.

DeLuca & Gilman (1986) produced dynamo models in which the DeLuca & Gilman (1986) produced dynamo models in which the overshooting convective motions act on the magnetic field and overshooting convective motions act on the magnetic field and could produce longer period dynamo oscillations.could produce longer period dynamo oscillations.

The dynamics of flux tubes rising rapidly through a rotating The dynamics of flux tubes rising rapidly through a rotating convection zone dramatically reduces the twisting otherwise convection zone dramatically reduces the twisting otherwise produced by the convection itself.produced by the convection itself.

Interface Dynamo WavesInterface Dynamo Waves

The shear layer at the base The shear layer at the base of the convection zone has of the convection zone has rotation rate decreasing rotation rate decreasing inward at latitudes below inward at latitudes below about 30º and increasing about 30º and increasing inward at higher latitudes.inward at higher latitudes.

This gives two dynamo This gives two dynamo waves – one moving waves – one moving toward the equator below toward the equator below 30º and another moving 30º and another moving poleward at higher poleward at higher latitudes.latitudes.

Dynamo Dilemma # 4 – there isn’t any evidence for a poleward wave.Dynamo Dilemma # 4 – there isn’t any evidence for a poleward wave.

Flux Transport DynamosFlux Transport DynamosDikpati & Choudhuri (1994, 1995) proposed flux transport dynamos like Dikpati & Choudhuri (1994, 1995) proposed flux transport dynamos like those of Babcock and Leighton but with key differences. those of Babcock and Leighton but with key differences.

1) The 1) The ΩΩ-effect is in the shear layer at the base of the convection zone. -effect is in the shear layer at the base of the convection zone.

2) The 2) The αα-effect is produced by the Coriolis force on rising flux tubes. -effect is produced by the Coriolis force on rising flux tubes.

3) A poleward meridional flow at the surface facilitates polar field 3) A poleward meridional flow at the surface facilitates polar field reversals. The return flow at the base of the convection zone gives the reversals. The return flow at the base of the convection zone gives the equatorward drift of the active latitudes and the 22-year period of the equatorward drift of the active latitudes and the 22-year period of the magnetic cycle.magnetic cycle.

The deep Meridional Flow gives only The deep Meridional Flow gives only equatorward motions and is slow enough to equatorward motions and is slow enough to give 11-year sunspot cycle.give 11-year sunspot cycle.

The Meridional FlowThe Meridional Flow

A poleward meridional flow of A poleward meridional flow of about 12 m/s is observed at the about 12 m/s is observed at the surface. This poleward flow is also surface. This poleward flow is also observed below the surface with observed below the surface with helioseismology.helioseismology.

The Deep Return FlowThe Deep Return Flow

Nandy & Choudhuri (2002) Braun & Fan (1998)Nandy & Choudhuri (2002) Braun & Fan (1998)

The poleward surface flow must sink inward in the polar regions and The poleward surface flow must sink inward in the polar regions and return to the equator at some depth. If the flow reverses at a point return to the equator at some depth. If the flow reverses at a point half way through the convection zone then a 12 m/s flow at the half way through the convection zone then a 12 m/s flow at the surface gives a 1 m/s flow at the base of the convection zone.surface gives a 1 m/s flow at the base of the convection zone.

A Flux Transport DynamoA Flux Transport Dynamo

In/CCWIn/CCW

Out/CWOut/CW

Dikpati & Charbonneau (1999) investigated the characteristics of Dikpati & Charbonneau (1999) investigated the characteristics of these dynamos and their sensitivity to changes in flow parameters. these dynamos and their sensitivity to changes in flow parameters. The meridional flow speed controls both the dynamo period and its The meridional flow speed controls both the dynamo period and its strength.strength.

Dikpati & Charbonneau (1999)Dikpati & Charbonneau (1999)

The Dikpati & Charbonneau The Dikpati & Charbonneau (1999) model produced 20 (1999) model produced 20 year cycles with a latitude year cycles with a latitude drift that slowed as it drift that slowed as it approached the equator and approached the equator and polar fields that reversed at polar fields that reversed at cycle maximum.cycle maximum.

The cycle period was The cycle period was inversely proportional to the inversely proportional to the meridional flow speed.meridional flow speed.

Fast meridional flow gives Fast meridional flow gives strong polar fields.strong polar fields.

The model has a “memory” The model has a “memory” of about 2 sunspot cycles.of about 2 sunspot cycles.

Corroborating EvidenceCorroborating Evidence

Hathaway, Nandy, Wilson, & Reichmann (2003, 2004) studied the Hathaway, Nandy, Wilson, & Reichmann (2003, 2004) studied the equatorward drift of the active latitudes from 128 years of sunspot equatorward drift of the active latitudes from 128 years of sunspot data. 1) There was no evidence of a poleward moving dynamo wave. data. 1) There was no evidence of a poleward moving dynamo wave. 2) The drift rate slows as the activity band approaches the equator. 3) 2) The drift rate slows as the activity band approaches the equator. 3) There is a positive correlation between the speed of the drift in cycle There is a positive correlation between the speed of the drift in cycle N and the amplitude of the N+2 cycle. N and the amplitude of the N+2 cycle.

Dikpati Dynamo PredictionDikpati Dynamo Prediction

Dikpati, de Toma & Gilman (2006) fed sunspot areas and Dikpati, de Toma & Gilman (2006) fed sunspot areas and positions into their numerical model for the Sun’s dynamo and positions into their numerical model for the Sun’s dynamo and reproduced the amplitudes of the last eight cycles with reproduced the amplitudes of the last eight cycles with unprecedented accuracy (RMS error < 10).unprecedented accuracy (RMS error < 10).

Cycle 24 Prediction ~ 180 Cycle 24 Prediction ~ 180 ± 15± 15(with slow flow: late and ~ (with slow flow: late and ~ 165 165 ± 15± 15

Meridional Flow VariationsMeridional Flow Variations

Hathaway (1996) Basu & Antia (2003)Hathaway (1996) Basu & Antia (2003)

The strength of the meridional flow shows variability. The delayed The strength of the meridional flow shows variability. The delayed start of Cycle 24 in the Dikpati prediction was based on the start of Cycle 24 in the Dikpati prediction was based on the slowdown reported by Basu & Antia (2003). In Dikpati’s model this slowdown reported by Basu & Antia (2003). In Dikpati’s model this gives a long cycle and weak polar fields at the end of Cycle 23.gives a long cycle and weak polar fields at the end of Cycle 23.

Flow Speed and Polar FieldsFlow Speed and Polar Fields

Flux Transport Dynamos produce weak polar fields and long cycles Flux Transport Dynamos produce weak polar fields and long cycles when the meridional flow is weak. when the meridional flow is weak.

From SURYA code of Choudhuri et al.From SURYA code of Choudhuri et al.

Cycle 24 MinimumCycle 24 Minimum

We see weak polar fields (about half as strong as in previous We see weak polar fields (about half as strong as in previous cycles) and a long Cycle 23 (12 years instead of 11).cycles) and a long Cycle 23 (12 years instead of 11).

Meridional Flow VariabilityMeridional Flow VariabilityWith Summer Intern Lisa Rightmire (Univ. Memphis), I have measured With Summer Intern Lisa Rightmire (Univ. Memphis), I have measured the meridional motion of the weak field magnetic elements by cross-the meridional motion of the weak field magnetic elements by cross-correlation full disk magnetograms. The results for 1996-2002 are correlation full disk magnetograms. The results for 1996-2002 are consistent with the strength and variations seen by Basu & Antia consistent with the strength and variations seen by Basu & Antia (2003) but they show a return to even faster flow from 2003-2009.(2003) but they show a return to even faster flow from 2003-2009.

Dynamo Dilemma # 5 – the faster flow after 2002 SHOULD have made Dynamo Dilemma # 5 – the faster flow after 2002 SHOULD have made stronger polar fields and a shorter cycle.stronger polar fields and a shorter cycle.

Previous Cycle VariabilityPrevious Cycle Variability

Komm,Howard, & Harvey Komm,Howard, & Harvey (1993) made similar (1993) made similar measurements from Kitt Peak measurements from Kitt Peak daily magnetograms and daily magnetograms and found fast flow at minimum found fast flow at minimum that slows down at maximum.that slows down at maximum.

The slow down in the The slow down in the meridional flow from 1996 to meridional flow from 1996 to 2002 represented the normal 2002 represented the normal solar cycle variation in the solar cycle variation in the meridional flow and cannot meridional flow and cannot explain the weak polar fields!explain the weak polar fields!

Cycle 23 Latitude DriftCycle 23 Latitude DriftThe equatorward drift of the active latitudes during cycle 23 looks The equatorward drift of the active latitudes during cycle 23 looks very average – neither slow nor fast and its variations are not in very average – neither slow nor fast and its variations are not in sync with the surface flows.sync with the surface flows.

Where is Cycle 24?Where is Cycle 24?

Cycle 24 is now underway. Almost all the sunspots since Cycle 24 is now underway. Almost all the sunspots since September of 2008 have been Cycle 24 sunspots. The smoothed September of 2008 have been Cycle 24 sunspots. The smoothed sunspot number went through a minimum in November 2008.sunspot number went through a minimum in November 2008.

Where is Cycle 24 Going?Where is Cycle 24 Going?With the exception of the Flux Transport Dynamo predictions, all With the exception of the Flux Transport Dynamo predictions, all indications are that Cycle 24 will be a weak sunspot cycle.indications are that Cycle 24 will be a weak sunspot cycle.

ConclusionsConclusions A magnetic dynamo is required to explain the 11-year A magnetic dynamo is required to explain the 11-year solar activity cycle.solar activity cycle.

The basic processes of the The basic processes of the ΩΩ-effect, the -effect, the αα-effect, and -effect, and transport by meridional flow and cellular convection must transport by meridional flow and cellular convection must be at play.be at play.

Models relying on deep meridional flow seemed to be the Models relying on deep meridional flow seemed to be the most appropriate.most appropriate.

But … Measurements of meridional flow variations do not But … Measurements of meridional flow variations do not support these modelssupport these models..

Solar Cycle 24 has begun but is expected to be very Solar Cycle 24 has begun but is expected to be very weak.weak.