Remote, long-term auroral kilometric radiation observations as a geomagnetic indicator of substorm onset

Abstract

Auroral kilometric radiation (AKR) is a cyclotron maser instability generated radio emission that occurs in the region above the auroral oval; the observed intensity increases with the growth rate of the instability, indicating an active source region and the presence of accelerated electrons, while the emission frequency is inversely proportional to the altitude of a source along a field line. Remote observations can thus provide a direct insight into the spatial development of the primary coupling region between the magnetosphere and ionosphere during energetic phenomena. During substorms in particular, the auroral acceleration region has been shown to increase in altitude and increase the low-frequency power of the AKR spectrum, particularly towards dusk (Morioka et al. 2007, Waters et al. 2022). However, AKR is beamed anisotropically, which makes it difficult to observe global variability of the emission when a spacecraft is not in an ideal position, namely at dayside local times. With an automatic extraction of AKR observations from the Wind spacecraft, we have access to nearly 30 years of data from a variety of viewing positions. The latest 20 years of observations are made from the  dayside, near L1. To evaluate the efficacy of the AKR observations as an indicator of substorm dynamics and further constrain the visibility effects, we compare AKR bursts from Wind (Fogg, Jackman, Waters et al. 2022) to a published list of substorm events derived from the SuperMAG magnetometer network. We calculate the binary classification statistics in four local time sectors with more than 10 years of AKR observation. When evaluated over a 2 hour window, AKR bursts observed from the nightside and duskside have a good (> 0.6) recall of substorm events, while the duskside observations have a more favourable false alarm probability (< 0.4). Dayside observations have a high miss rate (~0.8), but a high specificity (> 0.9), thus exhibiting a reliable proxy for substorm activity. Observations from all local time sectors except the nightside have positive forecast skill as determined by the Heidke skill score. Occurrence distributions of AKR burst frequencies from each local time sector and event group highlight the components present in each local time sector. This work lays the foundations for further parameterisation of the visibility of AKR sources at different locations within the inner magnetosphere by Wind when observing from L1, where the effects of the frequency, magnetic local time and magnetic latitude of the source can be examined more finely. Such work is useful for providing context for past AKR observations, as well as for the interpretation of future radio observations (with the JUICE flyby of Earth, for example), observation scheduling or planning of future missions.