The Sun The Sun imaged in white light by the SOHO spacecraft

Six views of the Sun from SOHO White light Magnetogram EUV 171 A (1 million K) White light coronagraphEUV 284 A (2 million K) EUV 304 A (70,000 K)

EUV 195 A 1.5 million K

Sun Facts Solar radius = 695,990 km = 109 Earth radii Solar mass = x10 30 kg = 333,000 Earth masses Solar luminosity (energy output of the Sun) = 3.8 x10 26 watts Surface temperature = 5770 ºK Surface density = 2.07 x10 -7 g/cm 3 = 1.6 x10 -4 density of air Surface composition = 70% H, 28% He, 2% (O,C,N,..) by mass Central temperature = 15,600,000 ºK Central density = 150 g/cm 3 Central composition = 35% H, 63% He, 2% (O,C,N,..) by mass Solar age = 4.57 x10 9 yr

White light image of photosphere showing sunspot and granulation

Swedish 1-m Solar telescope, La Palma, Canary Islands

What is a sunspot? High plasma  (kilogauss field) stops convective flow locally, limiting heat transport to surface; so that part of photosphere is cooler and hence darker.

H  image of chromosphere showing prominences (filaments) and plages

In high  photosphere, convection carries B field to edges of cells where it concentrates. H  image of chromosphere showing supergranules with magnetic field spicules around edges.

EUV image of lower corona showing coronal hole and active regions

White light coronagraph showing prominances and streamers

The Magnetic Sun Solar Rotation and Magnetic Field Dynamo Solar magnetic field is generated by a dynamo powered by the flow kinetic energy in solar convection and differential rotation. A small “seed” magnetic field that is frozen into the highly conducting fluid is amplified as it is twisted up into tighter knots. Intense magnetic fields become buoyant and rise to the surface erupting as sunspots

Solar rotation is not uniform. blue = slow rotation red = fast rotation Angular rotation rate Derived from SOHO Michelson Doppler Imager

Formation of Sunspots Regions of strong magnetic field generated by the solar dynamo are less dense than there suroundings, so they rise to the surface where they emerge as pairs of sunspots. Sunspots slowly dissipate as their magnetic fields are convected away.

The Magnetic Sun The 11-year Sunspot Cycle The solar magnetic field reverses every 11 years (approximately) At the time of the reversal the sun’s magnetic field is very disordered and the sun has many sunspots (magnetic active regions) -- this is solar maximum. Between reversals the number of sunspots drops to almost none – this is solar minimum.

X-ray Sun, , Yohkoh images every 4 months 11 year sunspot cycle

The Active Sun (preview of what’s to come) Erupting Prominences Solar Flares Coronal Mass Ejections Three different but often related phenomena:

A prominence is free to erupt when the magnetic field lines tying it down reconnect Hot plasma accelerated at the reconnection site flows down the field lines and heats the photosphere Erupting Prominence

An Example of SXI Solar Imaging: Filament Eruption and Coronal Mass Ejection LASCO Data Courtesy of SOHO SXI 01:53UT LASCO C2 02:42 UT NOAA Space Environment Center

The End