The time scales on which the Sun varies range from seconds to billions of years. Explosive events on the Sun involve the changing of magnetic configurations and subsequent liberation of energy, and processes associated with this energy transformation occur on the shortest of these time scales. However, the nature of the explosive events can change over the course of the Sun's evolutionary lifetime. This chapter covers the history of explosive events throughout the evolution of the solar system, with a focus on the observed manifestations of explosive events and what we can learn by studying them. Chapters in other volumes of this series (see Table 1.2) address parts of this topic. Chapter 2 of Vol. III describes the long-term evolution of magnetic activity of Sun-like stars, and Ch. 5 of Vol. II discusses observations of solar and stellar eruptions, flares, and jets. A description of radiative signatures of accelerated particles (primarily with application to solar flares) can be found in Ch. 4 of Vol. II, while Ch. 6 of that volume tackles models of coronal mass ejections and flares.
While the main theme of this chapter is the changing nature of these explosive events over evolutionary time scales, the influence of other important stellar parameters – such as rotation rate, convection zone depth, and the influence of binarity on rotation – are also discussed where appropriate. The focus of the chapter is to use an event-driven discussion of explosive events, keeping in mind how the parameters change with some key stellar parameters. This treatment is meant to illustrate key properties of explosive events that are known in stars of these age ranges.
An explosive event is made up of several distinct components, from energetic particles to coronal mass ejections to flares. While on the Sun these three components can be studied separately, on stars other than the Sun the identification and study of explosive events is limited to the radiative manifestation of the event, namely stellar flares. As diagnosed from the Sun, flares involve particle acceleration and plasma heating. They occur as a consequence of magnetic reconnection somewhere in the outer atmosphere, yet involve all stellar atmospheric layers. Owing to a diversity of physical processes involved, they produce emissions across the electromagnetic spectrum, that for the Sun have been recorded from km-wavelength radio waves to the highest-energy gamma-rays.