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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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Solar Magnetism
Differential Rotation, Sunspots, Solar Cycle
Guest lecture: Dr. Jeffrey MorgenthalerJan 30, 2006
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
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
Solar Oscillations
Global Oscillations Network Group (GONG)
• Allows continuous viewing of Sun
Helioseismology probes solar interior
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
Differential rotation—pre GONG
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?
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
SOHO Michelson Doppler Imager (MDI)
• Measures rotation speeds (red faster, blue slower than average)
• Shows plasma flow under equator
• Hint of flow towards poles
SOHO GOLF (Global Oscillations at Low Frequencies)
• Measured sound speed compared to theory• Something funny going on at base of convection
zone
Brief Review
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
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)
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
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
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”
Big Bear Solar Observatory H-alpha images
http://www.bbso.njit.edu/images.html
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
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
Arch Prominences
• H-alpha emission traces out magnetic field lines
• Out one sunspot, in another
NSO/Sac Peak/NOAO
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