Role of ULF waves in relativistic electron loss by whistler-mode wave scattering

In this study we describe observations of whistler-mode emission modulated by compressional ultra-low-frequency (ULF) waves around the flank magnetosphere. We show that ULF waves modulate whistler-mode wave generation (via modulation of energetic electron fluxes) and may affect wave propagation (via modulation of small-scale palsma density gradients). In the presence of density gradients, whistler-mode ray propagation can be ducted along field-aligned density peaks (ducts) which suppresses ray divergence, reduces wave damping and prevents significant wave intensity decrease. Such ducting can therefore result in scattering of relativistic electrons resonating with whistler-mode waves on high geomagnetic latitudes (far from the equator). We employ simultaneous near-equatorial and ground-based measurements of whistler-mode waves and low-altitude relativistic electron precipitation measurements by the ELFIN CubeSat. We show that when waves are ducted (as evidenced by their appearance on the ground) they scatter relativistic electrons into the loss cone. Our results suggest that whistler-mode wave modulation and ducting by ULF-induced density gradients may indeed be significant for relativistic electron losses. Thus, such population of ducted whistler-mode waves should be further studied statistically and possibly incorporated in radiation belt models.