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/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 rotationpre 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 Suns and Earths 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 rotations effect on magnetic field most

likely reason for sunspot activity

Solar MagnetismNeutrino SummaryReview HelioseismologySolar OscillationsGlobal Oscillations Network Group (GONG)Helioseismology probes solar interiorGONG resultsDifferential rotationpre GONGDifferential rotation seen by GONGSolar and Heliospheric ObservatorySOHO Michelson Doppler Imager (MDI)SOHO GOLF (Global Oscillations at Low Frequencies)Brief ReviewSunspotsSunspot structureSunspot behaviorSunspot number variationSunspots are related to magnetic fieldBig Bear Solar Observatory H-alpha imagesZeeman EffectBabcock Model for Sunspot formationArch ProminencesSunspot, Differential Rotation, Magnetic field Summary