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