Dawn-Dusk Morphology of Auroral Arcs in Substorm Growth Phase

The substorm growth phase plays a critical role in magnetospheric energy storage through magnetotail plasma sheet thinning and magnetic flux loading, and the study of auroral morphology and structure helps us understand the process of magnetotail energy accumulation and release. This study focuses on quiescent auroral arcs, which characterizes the growth phase of substorms and precedes the subsequent expansion thereon; we utilize the observational data from Time History of Events and Macroscale Interactions (THEMIS) all-sky imagers and obtain 47 substorm growth phase arcs from 2014 to 2022. We find that the auroral arcs typically maintain structural stability, and most of them (∼72.3%) move equatorward in the growth phase during which a latitudinal minimum is recorded on the duskside. Statistical examination of the 47 distinct substorm events confirms this dawn-dusk asymmetry in growth phase arc distribution. Such morphology of the growth phase arcs may originate from the magnetotail current sheet that is thinner on the duskside, we propose that this morphological characteristic likely reflects the dusk-favored magnetotail current sheet thinning process, constituting a systematic duskside preference in magnetospheric dynamics that may originate from the interplay between solar wind-magnetosphere coupling and the Hall current system in the magnetotail.