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Solar Magnetism Differential Rotation, Sunspots, Solar Cycle Guest lecture: Dr. Jeffrey Morgenthaler Jan 30, 2006

Solar Magnetism

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Solar Magnetism. Differential Rotation, Sunspots, Solar Cycle Guest lecture: Dr. Jeffrey Morgenthaler Jan 30, 2006. Neutrino Summary. Principle of CONSERVATION OF ENERGY led to proposal of neutrino by Wolfgang Pauli - PowerPoint PPT Presentation

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Page 1: Solar Magnetism

Solar Magnetism

Differential Rotation, Sunspots, Solar Cycle

Guest lecture: Dr. Jeffrey MorgenthalerJan 30, 2006

Page 2: Solar Magnetism

Neutrino Summary

• Principle of CONSERVATION OF ENERGY led to proposal of neutrino by Wolfgang Pauli

• Neutrino flux from sun measured by several experiments (in units of SNU) fell short from solar model expectations (solar neutrino problem)

• Solar model proves reliable for many thing• Led to proposal of MSW (neutrino oscillations)

• Solar astronomy helped particle physics

Page 3: Solar Magnetism

Review Helioseismology

• Review effect of sound speed on waves– Vacuum cleaner analogy– Car hitting puddle on freeway

analogy– Waves propagate faster

towards center of Sun• Refraction (bending) depends

on wavelength– Bigger wheels on vacuum

cleaner– Better treads on tires

• Particular wavelengths reinforce constructively– http://gong.nso.edu/gallery/

images/harmonics/pmode.mpg

Page 4: Solar Magnetism

Solar Oscillations

Page 5: Solar Magnetism

Global Oscillations Network Group (GONG)

• Allows continuous viewing of Sun

Page 6: Solar Magnetism

Helioseismology probes solar interior

Page 7: Solar Magnetism

GONG results

• Blow-up of GONG results (axes reversed) shows fantastic detail

• Gives accurate depth of convective zone– 0.713 of solar radius

• Measures differential rotation– Rotation speed as a

function of depth

Page 8: Solar Magnetism

Differential rotation—pre GONG

Page 9: Solar Magnetism

Differential rotation seen by GONG

• GONG has been operating for more than 10 years

• Pattern of rotation vs. depth in 1995 was almost the same now as it was in 2005

• What is going on?

Page 10: Solar Magnetism

Solar and Heliospheric Observatory

• Orbits Sun every 365 days

• Pulled equally by Sun’s and Earth’s gravity– Lagrange point L1

• Has 12 scientific instruments, including 3 for Helioseismology

Page 11: Solar Magnetism

SOHO Michelson Doppler Imager (MDI)

• Measures rotation speeds (red faster, blue slower than average)

• Shows plasma flow under equator

• Hint of flow towards poles

Page 12: Solar Magnetism

SOHO GOLF (Global Oscillations at Low Frequencies)

• Measured sound speed compared to theory• Something funny going on at base of convection

zone

Page 13: Solar Magnetism

Brief Review

Page 14: Solar Magnetism

Sunspots• First recorded observations by Chinese

astronomers around 28 BC• First detailed study by Galileo and other

astronomers around 1610• Profound implications for Aristotelian cosmology

Page 15: Solar Magnetism

Sunspot structure

• Recall granules• Recall black body

radiation– Sunspot darker because it

is cooler• Umbra darkest part

– Size of 1-10 earths• Penumbra is transition

zone• Fibrils are penumbral

intrusions into umbra• Plages (beaches) light

regions beyond penumbra

Photo Credit (T.Rimmele (NSO), M.Hanna (NOAO)/AURA/NSF)

Page 16: Solar Magnetism

Sunspot behavior

• Tend to come in pairs• Come and go in a few days

to a month• Were first evidence of

differential rotation– But move around themselves

(mostly north/south)

• Movie from SOHO SOI instrument

Page 17: Solar Magnetism

Sunspot number variation• Sunspot number varies with an 11-year period

– SOLAR CYCLE

• Period discovered by amateur astronomer– Heinrich Schwabe of Dessau, Germany, ~1840– 17 years of observations

• Almost certainly related to period observed helioseimologically

• Sunspots start each solar cycle appearing 30 deg above and below equator

• As cycle progresses, spots appear more toward equator

Page 18: Solar Magnetism
Page 19: Solar Magnetism

Sunspots are related to magnetic field

• First proposed by George Ellery Hale around 1900– Looked at sun a different way (narrow-band filter)

• Discovered chromosphere, above photosphere– Saw light only emitted by Hydrogen atoms

• Sunspots looked like “cyclonic whirls”• Hypothesized electrons in whirls would generate a magnetic

field– Devised an experiment to measure magnetic field

using the Zeeman effect• Found magnetic field• Does that mean electrons generated magnetic field?

• Magnetic field is buried in Sun and “bubbles up”

Page 20: Solar Magnetism

Big Bear Solar Observatory H-alpha images

http://www.bbso.njit.edu/images.html

Page 21: Solar Magnetism

Zeeman Effect

• Quantum mechanical effect

• Allows measurement of magnetic field– Strength is distance

between spectral lines– Polarization is direction of

magnetic field

• Sun spot field is 1000-4000 gauss– Earth at surface is 0.3

gauss

http://csep10.phys.utk.edu/astr162/lect/light/zeeman-split.html

Page 22: Solar Magnetism

Babcock Model for Sunspot formation

• See fig 5-13 in text• Differential rotation distorts N/S field so it starts to run E/W• Explains why sunspots come in pairs

– Magnetic field pokes out of sun and back in– Like a thread stitching through fabric

• Explains generally why sunspots start at 30 degrees N and S latitude

• Explains why sunspots get “squeezed” into lower latitudes• Generally explains why sunspot activity dies down after spots reach

equator– Magnetic field cancels itself out

• Explains why sunspots show up with reverse polarity the next time around

• Details of where magnetic field is formed might be wrong in Babcock model – field seems to be deeper than he thought

Page 23: Solar Magnetism

Arch Prominences

• H-alpha emission traces out magnetic field lines

• Out one sunspot, in another

NSO/Sac Peak/NOAO

Page 24: Solar Magnetism

Sunspot, Differential Rotation, Magnetic field Summary

• Sunspots observed on an 11-year cycle– SOLAR CYCLE

• Sunspots linked to magnetic field– Observed using Zeeman effect

• Differential rotation “winds up” magnetic field– Differential rotation best observed with

helioseismology• 25 days at equator, 35 days at pole

– Sunspots also show differential rotation– Differential rotation’s effect on magnetic field most

likely reason for sunspot activity