The Sun

The Sun imaged in white light by the SOHO spacecraft

Six views of the Sun from SOHOWhite light

Magnetogram

EUV 171 A (1 million K)

White light coronagraphEUV 284 A (2 million K)

EUV 304 A (70,000 K)

EUV 195 A1.5 million K

Sun Facts

Solar radius = 695,990 km = 109 Earth radiiSolar mass = 1.989 x1030 kg = 333,000 Earth massesSolar luminosity (energy output of the Sun) = 3.8 x1026 watts

Surface temperature = 5770 ºK Surface density = 2.07 x10-7 g/cm3 = 1.6 x10-4 density of airSurface composition = 70% H, 28% He, 2% (O,C,N,..) by mass

Central temperature = 15,600,000 ºK Central density = 150 g/cm3

Central composition = 35% H, 63% He, 2% (O,C,N,..) by mass

Solar age = 4.57 x109 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 SOHOMichelson 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, 1991-1995, Yohkoh images every 4 months

11 year sunspot cycle

1980

1991

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