Association of substorm chorus events with drift echoes

Over recent years, substorm chorus events (SCEs) have been proposed as a useful indicator of substorm onset. The events are regularly seen in the data from the VELOX (VLF/ELF Logger Experiment) instrument at Halley, Antarctica, which has provided over a decade of near continuous observations. SCEs are generally thought to be excited by the injection of electrons near midnight as they gradient-curvature drift toward dawn. On close one-to-one inspection of SCEs seen at Halley and energetic electron signatures seen with the LANL geostationary spacecraft, we have found that many events are associated with the drift echo of the injected electrons rather than the initial injection. In this paper we present some example events as well as the relative statistics. We find that approximately 1/4 of SCEs where a clear signature can be seen in the LANL data are associated with drift echoes rather than the initial substorm injection. We argue that rather than being a direct signature of substorm onset, SCEs are a signature of enhanced electron fluxes in the chorus generation region, which are often, but not exclusively, associated with substorm injections.