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Solar Magnetism

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Solar Magnetism 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 • 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/im ages/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 Photo Credit (T.Rimmele (NSO), penumbra 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 http://csep10.phys.utk.edu/astr162/lect/light/zeeman-split.html • Sun spot field is 1000- 4000 gauss – Earth at surface is 0.3 gauss 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.
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