The aurora changes in brightness, shape, color, dynamics, and location in response to changes in the state of the magnetosphere. This variability is dramatically exemplified by the auroral substorm, which begins when the explosive release of the energy stored in the magnetotail during the substorm growth phase causes an abrupt brightening of the most equatorward discrete arc in the midnight sector of the auroral oval (top panel) and the disruption of the quiet-time nightside arc system (the "auroral break-up"). As more energy is released during the substorm expansion phase, in a series of pulse-like intensifications, the emissions in the midnight and pre-midnight sectors become very bright (a few hundred kilorayleighs) and form a luminous bulge that expands into the polar cap (bottom panel). This "auroral bulge" is made up of a variety of highly dynamic forms--arc fragments, spirals, curls, pulsating patches, etc. At the end of the expansion phase, the aurora begins to dim and becomes less active; the nightside portion of the oval thins and returns to its quiet-time configuration and location.
The aurora's electromagnetic emissions register the dynamic response of the magnetosphere to the transfer of energy, mass, and momentum from the solar wind. However, interpreting the information that these emissions provide requires a knowledge of the mapping of specific precipitation regions to particular magnetospheric plasma domains as well as an understanding of the processes involved in the acceleration and precipitation of the auroral particles. Both of these issues, which are central to the discipline of auroral physics, present researchers with a number of unresolved questions and uncertainties and thus remain the subject of ongoing research.
The images above show the development of an auroral substorm over
northern Canada, Scandinavia, and northern Russia. They were acquired
with the Far-Ultraviolet Wideband Imaging Camera (WIC) on the IMAGE
spacecraft and show auroral emissions produced by energetic electrons
striking the molecular nitrogen in the upper atmosphere.