Local acceleration driven by whistler mode chorus waves is fundamentally important for the acceleration of seed electrons in the outer radiation belt to relativistic energies. This mechanism depends strongly on substorm activity and on the source and seed electron populations injected by substorms into the inner magnetosphere. In this work, a selection of geospace disturbance events, emerging from single and isolated interplanetary drivers, is divided into two groups, one resulting in enhancement and one in depletion of the average relativistic electron Phase Space Density (PSD). Because substorm activity does not always coincide with or depend on magnetic storm occurrence, we have not limited our study to storms, but have included also non-storm events that are able to cause enhancements and depletions of the relativistic electrons in the outer radiation belt. We investigate solar wind and geomagnetic parameters, wave activity and the seed electron PSD in the outer Van Allen radiation belt, looking for the occurrence of characteristic patterns, by performing a Superposed Epoch Analysis (SEA). Our study indicates the importance of substorm-associated enhancements of seed electrons, along with prolonged, intense ULF and VLF wave activity and an earthward displaced plasmapause, as conditions leading to substantial enhancements of relativistic electrons in the outer Van Allen belt. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870437 for the SafeSpace project.

During the second half of 2019, a series of recurring, moderate geomagnetic storms (Dst ≈ - 70 nT) emerged after a sequence of high-speed solar wind streams (Vsw ≥ 600 km/s) impacted the magnetosphere. During one of these storms, intense substorm activity was also recorded (SML ≈ - 2000 nT on August 31 and September 1), as well as a longer-lasting solar wind pressure pulse. We investigate this series of events, using particle measurements from three missions that recorded significant enhancements of relativistic electron fluxes: the Van Allen Probes, Arase and Galileo 207 & 215 satellites. We use both the flux intensity and the phase space density (PSD) of electrons, along with interplanetary parameters and information on ultra-low frequency (ULF) and chorus wave activity for a detailed analysis of this event. Our study demonstrates the importance of substorm injections, even during moderate or weak geomagnetic storms. The presence of seed electrons at L* = 4-5, in addition to intense ULF and chorus wave activity, seems to result in very efficient electron acceleration to relativistic and ultra-relativistic energies. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870437 for the SafeSpace project.