SUN COURSE - SLIDE SHOW 2 Show 1 -- photosphere & sunspots --> 1. Introduction, 2. Magnetic fields, 3. Corona
1. INTRODUCTION: PS Age of Sun ? • If Sun consists of coal, it could generate heat/light for … a few thousand yrs • Kelvin (1862) - if Sun slowly contracts (grav. Energy -> heat), it could have illuminated Earth for … 100 m yrs ! • Radioactivity discovered -> Earth rocks older than this !! • Paradox resolved - new source heat discovered - nuclear fusion Radioactive dating method (eg uranium --> lead): - measure ratio masses + rate of decay --> time since rocks have formed • Oldest rocks in s. system (Moon/meteorites) formed 4.6b yrs ago -- when Sun thought to have been born
PS Diffusion Start with field B0(x) and watch diffuse. Current density
MOST of UNIVERSE is PLASMA: Ionosphere --> Sun (8 light mins) • stars (4 light years) [At 186,000 miles per sec, 1 light year = 6 trillion miles] Learn basic behaviour of plasma from Sun
Eqns of Magnetohydrodynamics PDE's for v and B as functions of x,y,z,t As beautiful as the Sun !! -- Nonlinear Impossible to solve completely So approximate in imaginative ways -- ODE's or PDE's and solve (standard techs) --> Construct models for many dynamic phenomena on Sun
ATMOSPHERE Photosphere --> Sunspots Dark because cool Galileo discovered (1610) w. telescope Surprise (1908): -- Greeks, Chinese site of strong magnetic field ? Planets, clouds, smoke, slag
2. MAGNETIC MAP ("Magnetogram") White -- towards you; Black -- away from you SURPRISES: -- patterns of sunspots Regions around sunspots -- "Active Regions" -- intense magnetic fields over whole Sun
Movie But why the active region patterns ?
Magnetic Field (B) and Plasma Coupled B exerts force on plasma -- move together In interior, plasma dominates --> moves B Rotation shears B --> 2 tubes B --> Emerge:
Magnetic Tube Emerges Creates pair of sunspots +/- Opposite pattern in s. hemisphere
Global Magnetic Field of Sun At photosphere <|B|> ~ 3 - 20 G, but most is strong (1 kG) & vertical (i) ACTIVE REGION FLUX Spreads out --> remnant active region Merges --> unipolar region -- one dominant polarity Migrates to the poles --> polar field • Emerges as • sunspots (B = 3 kG, • flux = 1022 Mx) • * active regions --> faculae (bright)
Solar Cycle Sunspots vary with period 8-15 years (average 11.1 yr) Two zones between -35o and +35o latitude In opposite hemispheres, leading spots have opposite polarities (Hale) Leading polarity is closer to equator (Joy)
New spots emerge at start of cycle at high latitudes w. opposite polarity As cycle progresses, spots form at lower latitudes (Sporer) Polar Fields: * maximum extent at s. minimum * change polarity 1-2 years after s. maximum
No sunspots varies - 11-year cycle Also cycle in X-rays And in shape of corona:
(ii) Quiet Sun - Network mixed polarity -- at edge of supergranule cells -- covers Sun at s. minimum ephemeral active regions (1019 Mx) -- emerge & migrate to boundary --> network fragments (F = 2x1018 Mx, B = 1 kG) -- at edges of cells internetwork fragments (F = 6x1016 Mx, B = 10-100 G ?) -- inside cells
("crown") -- See at ECLIPSE of Sun 3. CORONA SURPRISE (1940) -- Temperature is million degrees !!!! SURPRISE (1868) -- Light from new element "Helium" Faint open structures -- Bright closed structures
Another ECLIPSE Corona created by B (heat, structure) -- open/closed (iron filings round bar magnet) B here -- unaware -- no effect B in plasma -- coupled (intimate, subtle) CORONA is in 4th state ("PLASMA") PLASMA behaves differently from normal gas: T = Million --> Corona is NOT a normal gas !!! 3 states of matter ...
How Bright is Corona/Full Moon ? Lang: Moon = 0.1 x Sunspot ?? Base corona (C0) = 10-5 x Photosphere Corona at 1 Ro = 10-8 x Photosphere Full Moon = 2 x 10-6 x Photosphere = 0.2 Co Sunspot: Umbra = 0.2 Photosphere Penumbra = 0.75 Photosphere So Moon = 10-5 Sunspot
CORONA: Normally need eclipse to see -- Glare of surface Observe direct with X-ray telescope As T increases (furnace), object becomes bluer
Picture with X-ray telescope: Coronal holes -- loops -- bright points Bright --> Denser
Yohkoh A magnetic world
Original Skylab Images Latest images from new satellite:
of Magnetic Loops in Corona (TRACE) Close-up B strong Dominates plasma
along Magnetic Loops Plasma Motions Conjuring trick -- plasma appearing by magic ?