In contrast to its behavior in visible light, the Sun is extremely variable at ultraviolet wavelengths, at radio wavelengths, and in its charged particle emissions.

This variability is controlled by the magnetic field of the Sun, which varies not only over the well-known 11-year solar cycle but on shorter and longer time scales as well.

The Sun's variable magnetic field is brought to the Earth by the supersonically expanding outer atmosphere of the Sun, the solar wind. When the magnetic field in the solar wind arriving at Earth is southward, in the direction opposite to that of the Earth's magnetic field, the solar wind and the magnetosphere couple most strongly.

The variability in the coupling leads to variability in the energy state of the Earth's magnetosphere, and to the periodic release of energy into the night magnetosphere known as the magnetospheric substorm.

Prolonged periods of strong solar wind/magnetosphere coupling lead to geomagnetic storms in which the entire system is disturbed.

Solar wind high-speed streams increase outer zone electron fluxes, while CME-driven interplanetary shocks inject and trap solar energetic protons as well as outer zone electrons in the inner magnetosphere.

As the disturbed magnetospheric system returns to its quiet-time state, much of the energy in the magnetosphere is translated into heating and ionization of the upper atmosphere.

In addition to coupling with the magnetosphere, the upper atmosphere couples to the lower atmosphere through gravity waves, tides, and the electric fields associated with weather systems.

The input from above and below both deposit net momentum and heat, altering the mean circulation and temperature structure.

Solar radiation is directly absorbed within the upper atmosphere, initiating an extensive photochemical chain that redistributes the energy. The thermal balance is controlled by a complex combination of dynamical, radiation and chemical pathways. Non- local thermodynamic equilibrium cooling is important here.

At wavelengths important for the upper atmosphere, the solar output is highly variable on both short and long time scales.