Photosphere:

The photosphere is the effective ``surface'' of the Sun since it is the point where the photons break free of scattering and zip into outer space. However, the photosphere is not a thin surface, but rather has a thickness of about 100 km. Within that 100 km, the temperature drops from 6000 K at the bottom to 4000 K at the top. Lower temperature means less luminosity from Planck's curve, so the edge of the Sun's disk is darker than the center, this is called limb darkening.

There are several features seen within the photosphere:

  1. faculae - large, bright regions
  2. granules - small (1000 km wide), bright regions that change brightest on the order of several minutes = tops of convection cells
  3. sunspots - dark regions that travel in pairs (north and south magnetic poles)

Sunspots pairs are due to magnetic flux tubes on the surface of the Sun. The flux tubes carry energy away causing the surface to be cooler (1800 degrees cooler) than the surrounding material, thus their darker appearance. The sunspots always travel with a north and south pole, oriented along the equator. The number of sunspots per year varys with an 11 year cycle and the peaks are associated with times of high solar activity (many flares and solar storms).

During the 11 year cycle, sunspot pairs are created at high solar latitudes and move towards the equator during the cycle. This effect is seen in the Maunder butterfly diagram. The typical lifetime of a sunspot is one or two solar rotations.

The origin of sunspots and the 11 year cycle are related. The solar magnetic field is unlike the magnetic fields of planets in that it is a surface magnetic field, instead of extending into space it is confined to the photosphere. Magnetic flux tubes can only be created when the surface field lines distort and overlap until a loop pops off the photosphere. The endpoints of the loop become the north and south poles of the sunspot. The process begins with a quiet Sun (low activity) and magnetic field lines that are smooth and lined up north/south on the Sun's surface. The differential rotation of the Sun causes the magnetic lines to wrap up.

After the solar cycle peaks, the energy is released from the magnetic fields and the field lines relax to their original, smooth north/south orientation. Then, cycle begins again. Total time for the whole process is 11 years.

Chromosphere:

The chromosphere is a pinkish atmosphere above the Sun's photosphere. It emits an emission spectrum to indicate it is a very hot gas (20,000 K). The most complex and transient solar phenomena occur in the chromosphere including:

  1. solar flares
  2. solar arcs
  3. prominences
Solar flares, arcs and prominences are linked to sunspot activity. Gas is trapped in the flux lines created by the sunspot pairs and lifted off the photosphere into the chromosphere. Over a period of a few hours, the magnetic fields collapse hurling the hot gas outward (much like a breaking rubber band). These event also create a large flux of high energy particles which reach the Earth as magnetic storms and cause a sharp increase aurora activity.

Corona:

The corona of the Sun is a large, white halo of glowing gas visible during a total eclipse. The corona gas is extremely hot (temperatures on order of a million degrees) and is the source of the solar wind.

The solar wind is a constant stream of solar particles moving at faster than the escape velocity of the Sun's gravitational field. They escape through windows in the solar corona called coronal holes, regions where the magnetic fields are weak and the charged solar wind particles are not trapped in magnetic bottles.

X-ray and the above UV picture of the corona show that the hot gas is connected to the magnetic features in the photosphere. Those low level structures extending into long streamers in the outer corona and heat the corona to its million degree temperatures.