The Internal Structure and Dynamics of the Sun

• The Sun's energy is generated in the core by nuclear fusion.
• The internal structure of the Sun consists of a large central radiative zone surrounded by a convection zone.
• The Sun does not rotate as a solid body.

Variation of Solar Radiation

• The Sun is a variable star at all wavelengths.
• The overall solar irradiance changes in phase with the solar cycle.
• The Sun varies least at visible wavelengths, changing by about 0.2% from solar maximum to solar minimum.
• The solar irradiance has short-term (< 2 weeks) variations due to sunspots (decreased irradiance) and faculae (increased irradiance).
• At UV, EUV, X-ray, and radio wavelengths the Sun can vary by orders of magnitude on timescales of seconds to years.

The Generation and Transport of the Sun's Magnetic Fields

• The Sun's magnetic field is generated by dynamo actions.
• The Sun has a magnetic activity cycle lasting about 11 years.
• The polar field reverses during the maximum of solar activity.
• Surface fields are concentrated into small flux tubes.
• Active regions (sunspot groups) emerge in a limited range of latitudes centered on the solar equator.
• The rate of magnetic field emergence from the quiet regions of the Sun exceeds that of active regions.
• Emerging active regions contribute to the large-scale magnetic structure.
• At the surface of the Sun the convective flows dominate the magnetic fields, but higher in the solar atmosphere the magnetic fields dominate the plasma.
• Eruptions of magnetic structures from the Sun are transported through the solar wind and can profoundly affect the Earth's space environment, producing geomagnetic storms.

The Formation and Dynamics of the Solar Atmosphere

• The Sun's outer atmosphere is heated to millions of degrees by non-thermal processes.
• There are two types of solar wind: high- and low-speed. The high-speed wind originates from predominantly open field areas in coronal holes. The low-speed wind originates from the predominantly closed field areas near the equator.
• The solar wind has a variable temperature and composition.
• Ultra-relativistic particles are injected sporadically into the heliosphere.
• Shocks from coronal mass ejections can accelerate particles to high energies.
• Magnetic fields are continuously restructuring.
• The chemical composition of the upper solar atmosphere is variable and different from that of the surface layers of the Sun

The Generation of Solar Activity by Variations in the Sun's Magnetic Field

• The free energy stored in magnetic fields can be released rapidly in flares.
• High-energy particles can be accelerated in seconds by flares.
• Magnetic fields erupt into the heliosphere.