Introduction

The tenuous, partially ionized plasma in planetary upper atmospheres is vulnerable to explosive and dynamic events from both the Sun and the lower atmosphere. The power of the Sun is continuously bombarding the atmospheres of planets with photons, energetic particles, and plasma. Some of the most dramatic solar events are the sudden release of electromagnetic energy during solar flares, and plasma from interplanetary coronal mass ejections (ICME). The intense solar radiation from a flare is the first to impact a planetary system, shortly followed by the arrival of relativistic energetic particles. Some time later, hours to days depending on the planet's distance from the Sun, the bulk of the plasma arrives to interact with, in some cases, the planetary magnetosphere; energy is then channeled into the upper atmospheres and ionospheres. The upper atmospheres are subjected to dramatic changes in external forcing by these types of events, by as much as a factor of two in total energy deposited, by an order of magnitude for individual processes, and by several orders of magnitude in some wavelength bands.

The upper atmospheres of planets are also being pushed and jostled by energy and momentum propagating upward from the dynamic chaotic lower atmospheres. The total solar irradiance driving the lower atmospheres is invariant except for the fraction of one percent changes observed over a solar cycle. Estimates have been made of the impact of longer-term changes in solar radiative output on Earth's climate, an area that is explored further in Vol. III.