In search of multi-scale plasma instabilities at the heart of substorm onset: implications for the Plasma Observatory mission

The physical trigger of substorm onset remains one of the key unresolved problems in magnetospheric physics. Understanding how, when, and why stored energy in Earth’s magnetotail is explosively released is central to space-weather science. To identify the instability responsible for detonation, recent studies have focused on the earliest auroral signatures of onset—small-scale, quasi-periodic structures known as auroral beads. Previous work has linked these beads to plasma instabilities and to magnetotail dynamics through kinetic Alfvén waves.

To further understand the substorm onset mechanism, we use new measurements from a narrow-field, high-cadence auroral imager. By extending the Kalmoni et al. (2018) methodology, we track the temporal evolution and dispersion characteristics of “mini beads”, in effect beads-within-beads. Our analysis shows that all types of beads move in the same eastward direction but that mini beads precede the larger beads by at least one minute. However, in contrast to larger-scale beads, mini beads obey different dispersion relations, suggesting that mini beads arise from a distinct physical process and represent an earlier or new stage of the instability development leading to substorm onset.  This means that we need to understand the near-Earth transition region on multiple scales far earlier than currently thought, challenging all current substorm onset paradigms. 

We discuss the implications of this analysis for determining the role of multi-scale physical processes in substorm onset for multi-spacecraft missions such as Plasma Observatory.