Spatially Quasi-periodic Finger-like Auroras during Substorms

North–south-aligned discrete auroras are common phenomena during substorms. A quasi-periodic form of such auroras distributed azimuthally in the nightside resembles a finger-like morphology. They were noticed in sawtooth-time substorms, but have not yet been paid much attention during other geomagnetic times. In this study, we investigate the longitudinal periodicity of those auroras for different types of substorms using IMAGE data from 2001 to 2003. Using an automated detection algorithm, we identified multiple north–south-aligned thin auroral structures in auroral images and found that their average center and length in latitude, and longitudinal wavelength (∼25°) are essentially the same. However, the occurrent rates for finger-like auroras are substantially different, 83%, 64%, and 18% for sawtooth-time substorms, storm-time substorms and isolated substorms, respectively. Therefore, statistical results indicate that finger-like auroras occur in favor of geomagnetically more active times. Combining our data analysis and previous simulations, we suggest that interchange instability is a possible mechanism of spatially quasi-periodic finger-like auroras when a substorm injection happens in a very wide local time. It is particularly important to point out that the sawtooth events not only have a temporal periodicity as defined in particle injections, but also have a spatial periodicity as observed in auroral morphology for the majority of them.