Acceleration of radiation belt electrons driven by a foreshock bubble

Foreshock transients are mesoscale structures upstream of the Earth’s bow shock and they evolve from solar wind discontinuities. Foreshock transients are commonly observed, but their impact on the radiation belt dynamics has not been studied before. These structures have a global impact on the magnetosphere and, in particular, can launch ultra-low frequency (ULF) waves which can energize radiation belt electrons through resonant interactions. We present a case study of a foreshock transient event that is associated with prompt acceleration in electron fluxes, drift echoes and localized ULF wave activity using multi-satellite observations.

The transient is characterized by hot and tenuous plasma, strong flow deflection and is bounded by a compressed edge on its sunward side indicating that it is a foreshock bubble. Using multiple satellite missions, Van Allen Probes, Arase and GOES, we can assess the global view of the transient’s effects on the dayside inner magnetosphere. The electron fluxes from these satellites show signatures of an initial energization and subsequent drift echoes. This injection exhibits energy dispersion and boomerang stripes in the pitch angle distributions. Analysis of energy and pitch angle dependent drift speeds shows that the acceleration is consistent with the timing and geometry of the impact of the foreshock bubble. Wave measurements from these spacecraft show enhanced ULF wave activity at the time of the electron injection. This study shows, for the first time, that foreshock transients may play an important role in radiation belt dynamics.