Investigations of the Many Distinct Types of Auroras

In this chapter, the many distinct types of auroras are introduced and briefly explained. Background is given on auroral studies throughout history, and the broad categories of diffuse and discrete aurora are explained. The field-aligned current system, auroral acceleration regions, and auroral oval are introduced. The diffuse aurora, and the pitch angle diffusion resulting from wave-particle interactions that generate the diffuse aurora are then covered more extensively, including more recent investigations. The two primary acceleration mechanisms that generate the discrete aurora, inverted-V acceleration and Alfvénic acceleration, are examined in detail. Temporal changes in the aurora, particularly over the course of an auroral substorm, are the subject of the next section, followed by small-scale deformations of auroral arcs such as curls, folds, spirals, omega bands, and streamers. Finally, investigations of localized aurora that deviate from the classic auroral oval are presented, such as cusp aurora, detached dayside aurora, and evening corotating patches.