Gaitskell

PH0021Astronomy

Lecture 19 (The Sun)021203v7

The Sun, in all its glory!

Prof Rick Gaitskell

Department of PhysicsBrown University

See course pages for source

http://gaitskell.brown.edu

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Sun Data

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

The Sun - Photosphere

• Sizeo Scaleo Composition

• Compare terrestrial planets• Compare Jovian planets• Sun

—GAS/PLASMA - No solids, even at core, despite pressure…

• Why does it appear as a well defined “ball”?o (i) Gas distribution is nearly sphericalo (ii) Only seeing gas from thin layer of gas

• PHOTOSPHERE o Thin layer of gas 400 km o 1/2000th radius (Rsun~0.7x106 km)o “Limb darkening”

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Absorption Lines - Photosphere

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

The Sun - Structure of Photosphere

Typical grain ~1000 km (hires solar images)[DEMO Silicon Oil & Al flakes]

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Chromosphere

• Above Photosphere (Photo~0.01% of Earth atmosphere)

• Is even less dense Chromosphere (Chromo~0.01% of Photo )o Difficult to see, except during real or artificial eclipseo Pink in colour! Why pink??

• Emission Spectrum [BOARD]

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Emission Lines

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Corona

• Outermost layer of Sun is the Corona (“Crown”)

o Requires eclipse (or coronagraph to observe)o Not sphereical

• Streamers/projectionso Emission lines [see next] indicate hotter than Chromosph.

• Not just Fe+ but Fe13+ • => T~2x106

o Intensity? (Flux ~ T4 )

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Corona in x-rays

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Coronal Mass Ejection

• SOHO Satellite observationo C3 coronagraph

• (obs range 3.5-30 solar corona radii)

• Coronal Mass Ejectiono 18 Feb, 2000 (Medium Sized)o Follows onset of solar flare previous day (but not necessarily linked)

o Takes ~2 days to arrive (this one is heading direct for Eath, hence “halo” like appearance

o Up to 109 tonnes plasma @ 0.01 c

http://www.southpole.com/headlines/y2000/ast18feb_1.htm

Like solar flares, they occur whenever there's a rapid, large-scale change in the sun's magnetic field. Solar flares and CMEs often occur together, but not necessarily because the flare triggers the CME or vice versa. One can happen without the other and frequently during solar maximum we see CMEs without an associated flare

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Sun Spots

• And the Brightest and Hottest parts of Corona are…o …directly above another conspicuous feature: SUN SPOTS

4300K

5700K

Photosphere6300K

[DEMO]

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Differential Rotation of Sun

• Sun Spot Observationo Clear that different horizontal bands (latitudes) rotate at different rates

• Gallileo made first rot obs—(Live ~2 months)

• Richard Carrington, 1859—Differential rotation

• Helioseismologyo 1980’s able to determine how bulk is behaving

26 days

36 days

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Sun Spot Cycle (# of Sun Spots & Position)

• 11 year cycleo i.e. ~11 years between max #o max -> min (no sun spots) ->

max

• Position migrates over cycle

o Min -> Starts ~30 deg lato Max as moves toward

equatoro Next Min as Spots annihilate

at equator

[BOARD]

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Zeeman Spliting of Abs. Lines in Sun Spots

• George Hale 1908• Permit determination of magnetic field strength and polarity

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Magnetic-Dynamo Model (explain 22-year cycle)

• Horace Babcock 1960o Proposes Magnetic-Dynamo Model to explain 22-year cycle (with polarity flips)o Makes use of

• Differential Rotation• Convection

See http://science.msfc.nasa.gov/ssl/pad/solar/quests.htm for dynamo discussion and figs

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Sun Spot Movie

• First parto Differential Rotation of Sun Spotso Sphere is then mapped onto sheet

• Second Parto Again see differential rotation

• Makes higher latitudes move slower than equator

o Evolution of Sun Spots over 22 year cycle• 1980 max

—Top Hemisph: Yellow leads Blue—Bot Hemisph: Blue leads Yellow

• 1986 minima• 1991 max (Poles Reversed)

—Top Hemisph: Blue leads Yellow—Bot Hemisph: Yellow leads Blue

• 1997 minima

QuickTime™ and aVideo decompressorare needed to see this picture.

http://science.msfc.nasa.gov/ssl/pad/solar/images/MagMovie.mov

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Solar Flares

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Hydrostatic Equilibrium

• [DEMO - Balloons]

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Regions of Sun

• What happens at radius~0.7?

o Inside - radiationo Outside - convection

• T is low enough that neutral hydrogen forms

o H absorbs visible light much better

o Opacity increaseso ->Convection

PH0008 Gaitskell Class Spring2002 Rick Gaitskell

Summary

• Starting at the top…

• Corona (Flares/CME)o Very high temperature gas/plasmao Flares (hot ionized gas from sun spot) and CME (larger amounts of coronal gas)

• Chromasphere• Photosphere (Sun Spots)

o 400 km thick, 5800 K (Blackbody appears Yellow)o Sun spots show 22-year cycle (magnetic behaviour of Sun)

• Convection Zone [cell structure]• Radiation Zone [r<0.7]• Core (Fusion) [r<0.25]

o Has it gone out, no! We see neutrinos!o But not as many as we expected…particle physics!