Multi‐Platform Observations of the Radial Penetration of Substorm Injected Electrons and Subsequent Slot‐Filling Event

On 15 February 2018 a co-rotating interaction region (CIR) from an equatorial coronal hole reached the Earth. The CIR initiated a moderate and slowly intensifying geomagnetic storm, which began with a large and strong substorm injection. The substorm injection was exceptionally well-observed by an array of spacecraft including LANL-GEO satellites, Van Allen Probes (RBSP), Arase (ERG), and MetOp/POES, as well as ground-based instruments. These observations enable the unambiguous identification of several important features that have been impossible to measure directly in other events. The substorm injection extended well inside the geosynchronous orbit. A fortuitous conjunction of RBSP-A (moving inbound) and Arase (simultaneously moving outbound at the same magnetic local time) allows us to establish, very precisely, the location of the inner edge of the injection region at L = 3.8−3.9. In supporting observations, North American riometers saw precipitation extending down to L ≈ 4 but not lower. Arase and RBSP-A also observed whistler-mode hiss waves inside the plasmasphere. Analysis of the resonance conditions shows, conclusively, and for the first time, that they were produced by the drifting injected electrons. RBSP-A observations also show the injection (or transport) of electrons into or through the slot region within hours of the substorm injection onset. Previous studies were not able to clearly connect or separate substorm injections and slot-filling processes. These new observations clearly identify slot-filling as a spatially and temporally separate process that is not a direct result of substorm injection.