The Distribution of Auroral Power Increases and Decreases

The auroral substorm was originally identified as a large increase in auroral power. There does not seem to be any investigation of just how large a power increase is needed to identify a substorm. It is not clear whether auroral brightenings and fadings form a continuous spectrum, or even whether large decreases in auroral power likewise occur over a short time scale (which could be considered a negative, or inverse substorm). We used Polar UVI images of global auroral power to investigate these and related questions. Specifically we considered the distribution of dP/dt and (dP/dt)/P, that is, the distribution of absolute and relative changes in auroral power. At small values of |dP/dt|, negative changes are much more frequent than positive changes. In fact, a small negative change in auroral power is by far the most common type of change. Hence the typical behavior of the aurora is a slow decline in power from one image to the next. Large magnitude changes are rare, but turn out to be almost exclusively positive (i.e., inverse substorms do not exist). Beyond about 0.2%/s relative change in auroral power (amount to a 20% change over 100 s), only positive events occur, within the measureable noise levels. However no clear boundary exists which divides substorms from other types of auroral boundaries: the spectrum of large positive changes in auroral power is continuous, without any apparent inflection point.