Are repetitive particle injections during high‐speed solar wind streams classic substorms?

Abstract

The relation between substorm occurrences, energetic particle injections, and High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) events has been an issue. To understand their relationship, we use global auroral images from the Wideband Imaging Camera (WIC) on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft to investigate auroral activity near the times of repetitive particle injections that occur every ∼0.5 to 4 h during HILDCAA events. For the examined HILDCAA intervals during January–July 2003, we identified a total of 481 injection events. Good WIC imaging was available for 121 of these and showed substorm onset brightenings for a significant fraction of the 121 injections. Specifically, excluding 16 injections with ambiguous timings owing to the lack of a near‐midnight spacecraft, we find that 93 of 105 injections (∼88%) occurred within ±5 min of a substorm auroral onset. If we include cases with up to 10‐min timing difference, the percentage becomes 97%. These results indicate that a significant fraction of HILDCAA‐time repetitive particle injections are substorm‐associated and thus suggest that substorms are a major component of HILDCAA geomagnetic activity and a possible contributor to the sustained small negative Dst during these periods.