Scale‐free statistics of spatiotemporal auroral emissions as depicted by POLAR UVI images: Dynamic magnetosphere is an avalanching system

Abstract

We report first results from a spatiotemporal statistical analysis of ionospheric emissions as observed by the Ultraviolet Imager (UVI) onboard the POLAR spacecraft during 4 months of 1997 and 1998. Approximately 12,300 individual emission events near local midnight with durations exceeding the sampling time of the image sequences are investigated. The probability distributions of these events over the lifetime T, maximum area A, integrated area S, maximum power W, and integrated energy output are shown to obey distinct power law relations p ∼ T−2.2, p ∼ A−1.8, p ∼ S−1.6, p ∼ W−1.7, p ∼ E−1.5 over a wide range of scales. The observed behavior is consistent with the behavior of statistical–physical avalanche models near a stationary critical state. These results support the hypothesis of self‐organized critical dynamics of the magnetosphere and suggest an important role for cross‐scale coupling effects in the development of geomagnetic disturbances.