Activity Cycles in StarsActivity Cycles in Stars

Dr. David H. HathawayDr. David H. Hathaway

NASA Marshall Space Flight CenterNASA Marshall Space Flight Center

National Space Science and Technology CenterNational Space Science and Technology Center

OutlineOutline

• The Sun’s Activity CycleThe Sun’s Activity Cycle

• Dynamo Processes for Activity CyclesDynamo Processes for Activity Cycles

• Detecting Activity Cycles in StarsDetecting Activity Cycles in Stars

• Making Sense of Activity Cycles in StarsMaking Sense of Activity Cycles in Stars

• ConclusionsConclusions

The Sun’s Activity Cycle

The sunspot cycle has had an average period of about 11 years since Cycle 1 which began in 1755. Amplitudes ( in terms of the average daily sunspot number) range from ~50 to ~200.

The “11-year” Sunspot Cycle

The Maunder MinimumThe Maunder MinimumThe Sun went through a period of 70 years (the Maunder Minimum - The Sun went through a period of 70 years (the Maunder Minimum - 1645 to 1715) with virtually no spots. The existence of the Maunder 1645 to 1715) with virtually no spots. The existence of the Maunder Minimum is now well established by the efforts of Hoyt and Minimum is now well established by the efforts of Hoyt and Schatten. They have tabulated daily observations with nearly Schatten. They have tabulated daily observations with nearly complete coverage over the period of the Maunder Minimum . complete coverage over the period of the Maunder Minimum .

Sunspot Latitudes Sunspots appear in two bands on either side of the equator. These bands spread in latitude and migrate toward the equator as each cycle progresses. Cycles overlap at the time of minimum. The hemispheres can be asymmetric but are basically synchronized. Sunspots are rarely seen poleward of about 40°.

Magnetic Fields are the KeyMagnetic Fields are the Key

Sunspot number is well correlated with other measures of solar Sunspot number is well correlated with other measures of solar activity. The 400-year length of the sunspot number record helps activity. The 400-year length of the sunspot number record helps to characterize the solar cycle. The connection with cosmic rays to characterize the solar cycle. The connection with cosmic rays leaves even longer records of solar activity in tree rings (leaves even longer records of solar activity in tree rings (1414C) and C) and ice cores (ice cores (1010Be).Be).

Sunspots and Solar ActivitySunspots and Solar Activity

Sunspot AreaSunspot Area 10.7cm Radio Flux10.7cm Radio Flux GOES X-Ray FlaresGOES X-Ray Flares

Climax Cosmic-Ray FluxClimax Cosmic-Ray FluxGeomagnetic Geomagnetic aaaa index indexTotal IrradianceTotal Irradiance

EUV Luminosity VariationsEUV Luminosity Variations

Dynamo Processes for Activity Cycles

The 11-year time-scale for the solar cycle is too short to be attributed to relic fields. Flows within the convection zone are thought to be the source of the solar cycle. Several aspects of convection zone and magnetic flux tube dynamics indicate that the interface layer or “tachocline” is the seat of the solar cycle.

The Solar Interior

Internal Rotation RateHelioseismic determinations of the internal rotation rate show that the latitudinal differential rotation seen at the surface extends through the convection zone. Layers of strong radial shear are found near the surface and at the base of the convection zone (the tachocline).

The Basic Dynamo ProcessesDifferential rotation amplifies the poloidal field by wrapping it around the Sun to produce a strong toroidal field.

Lifting and twisting the toroidal field can produce a poloidal field with the opposite orientation.

The Ω-effect The α-effect

In/CCWIn/CCW

Out/CWOut/CW

Cycle PeriodsCycle PeriodsMany models of the Sun’s magnetic dynamo make use of the Many models of the Sun’s magnetic dynamo make use of the meridional circulation. In these models the period of the cycle is meridional circulation. In these models the period of the cycle is inversely related to the flow velocity at the base of the convection inversely related to the flow velocity at the base of the convection zone. In other models the period is inversely related to zone. In other models the period is inversely related to √∆Ω√∆Ωαα..

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

Detecting Activity Cycles in Stars

Wilson’s Studies at Mt. WilsonWilson’s Studies at Mt. WilsonStarting in 1966 Olin Wilson used the 100-inch telescope on Mt. Wilson to Starting in 1966 Olin Wilson used the 100-inch telescope on Mt. Wilson to examine the emission in the cores of the Ca II H and K lines in 91 main examine the emission in the cores of the Ca II H and K lines in 91 main sequence stars from M2 to F5. This emission is well correlated with the sequence stars from M2 to F5. This emission is well correlated with the presence of magnetic fields on the Sun.presence of magnetic fields on the Sun.

The Sun in Ca II KThe Sun in Ca II K The Sun’s Magnetic FieldThe Sun’s Magnetic Field

Spectral bands inSpectral bands inCa II H & KCa II H & K

Ca II and the Sunspot CycleCa II and the Sunspot CycleViewing the Sun as a star in Ca II K over a sunspot cycle shows a Viewing the Sun as a star in Ca II K over a sunspot cycle shows a strong correlation between Ca II K emission and sunspot number.strong correlation between Ca II K emission and sunspot number.

Baliunas & Soon (1995)Baliunas & Soon (1995)

Ca II Variations in StarsCa II Variations in Stars

Dobson et al. (1990)Dobson et al. (1990)

These observation yield information on the level of activity and its These observation yield information on the level of activity and its variability – both short term (rotation) and long term (stellar cycles).variability – both short term (rotation) and long term (stellar cycles).

Activity Cycles on Other StarsActivity Cycles on Other StarsWilson’s work has been extended by others (e.g. Vaughn, Preston, Wilson’s work has been extended by others (e.g. Vaughn, Preston, Baliunas and Radick) and we now have information about activity Baliunas and Radick) and we now have information about activity cycles in many stars. Cycle amplitudes and periods show cycles in many stars. Cycle amplitudes and periods show considerable variation from star-to-star.considerable variation from star-to-star.

Radick (2000)

Cycle CharacteristicsCycle CharacteristicsBaliunas et al. (1995) found cycle periods from 2 years to more than Baliunas et al. (1995) found cycle periods from 2 years to more than 20. The more active stars tended to be faster rotating and have more 20. The more active stars tended to be faster rotating and have more chaotic activity. The least active stars tended to be rotating slowly chaotic activity. The least active stars tended to be rotating slowly an often didn’t show cyclic behavior. Note: these are stars of an often didn’t show cyclic behavior. Note: these are stars of different ages and spectral types.different ages and spectral types.

V – VariableC – CyclicL – Long termF - Flat

Making Sense of Activity Cycles in Stars

Convection Zones

CNO Cyclep-p Chain

The lowest mass stars are full convective. Stars like the Sun have outer convection zone. More massive stars have inner convection zones. The transition between envelope and core convection is at M ~1.3 M - at least theoretically.

Cycle Period vs. Rossby NumberCycle Period vs. Rossby Number

Noyes, Weiss, & Vaughn (1984) Noyes, Weiss, & Vaughn (1984) showed that the showed that the Rossby Number Rossby Number - - the ratio of a convective the ratio of a convective turnover time to the rotation turnover time to the rotation period – was a good predictor of period – was a good predictor of cycle periods. Shorter rotation cycle periods. Shorter rotation rates relative to the convective rates relative to the convective turnover time corresponded with turnover time corresponded with shorter cycles.shorter cycles.

Activity vs. Rossby NumberActivity vs. Rossby NumberThe scatter plot of activity level vs spectral color falls into place when The scatter plot of activity level vs spectral color falls into place when activity level is plotted against Rossby Number. Young stars (filled activity level is plotted against Rossby Number. Young stars (filled dots) are more active than old stars (open circles). Fast rotating stars dots) are more active than old stars (open circles). Fast rotating stars are more active than slow ones.are more active than slow ones.

Convection in Solar Type StarsConvection in Solar Type Stars

Brown et al. 2007

Numerical models for convection in solar type stars show how Numerical models for convection in solar type stars show how differential rotation is produced by the effects of rotation on the differential rotation is produced by the effects of rotation on the convective flows. Cells stretched north to south are key.convective flows. Cells stretched north to south are key.

Dynamic Variations in Solar Type StarsDynamic Variations in Solar Type StarsAs rotation increases from the Sun’s rate differential rotation first increases and then decreases with further increase in rotation rate. Faster rotation rate gives slower meridional flow.

Brown et al. 2007

If the cycle period is set by the meridional flow speed then faster If the cycle period is set by the meridional flow speed then faster rotating stars should have longer, not shorter periods according to rotating stars should have longer, not shorter periods according to this model. this model.

Doppler ImagingDoppler ImagingSpectral lines from rotating stars are broadened by the area & intensity Spectral lines from rotating stars are broadened by the area & intensity weighted fraction of the stellar surface at different Doppler velocities. weighted fraction of the stellar surface at different Doppler velocities. The presence of a starspot leaves a bump in the spectral line that The presence of a starspot leaves a bump in the spectral line that moves from the blue to the red as the spot rotates across the disk.moves from the blue to the red as the spot rotates across the disk.

Doppler ImagesDoppler ImagesDoppler imaging gives information on sizes and latitudes of starspots. Doppler imaging gives information on sizes and latitudes of starspots. Some observations indicate large polar spots - which raise more Some observations indicate large polar spots - which raise more questions. Are they produced by strong meridional flows? Are they questions. Are they produced by strong meridional flows? Are they produced by poleward moving dynamo waves?produced by poleward moving dynamo waves?

AG DorAG Dor HU VirHU Vir

ConclusionsConclusions• Young stars (< 1 Gyr) have high levels of Young stars (< 1 Gyr) have high levels of activity, rapid rotation, and chaotic variationsactivity, rapid rotation, and chaotic variations

• Intermediate age stars have moderate levels of Intermediate age stars have moderate levels of activity and rotation rate and have smooth cyclesactivity and rotation rate and have smooth cycles

• Stars as old as the Sun have low levels of Stars as old as the Sun have low levels of activity, slower rotation, some periods of activity, slower rotation, some periods of inactivity (Maunder Minima)inactivity (Maunder Minima)

•Changes in differential rotation and meridional Changes in differential rotation and meridional flow speed with rotation rate in numerical models flow speed with rotation rate in numerical models favors activity cycle periods set by favors activity cycle periods set by αα and and ∆Ω∆Ω..