Periodic Excitation of Chorus and ECH Waves Modulated by Ultralow Frequency Compressions

Abstract

Spacecraft observations often reveal the presence of marginally stable chorus and electron cyclotron harmonic (ECH) waves across a spatial scale about several Earth radii (RE), which is much larger than the spatial scale of the localized injections that are thought to generate them. Given the importance of these waves for electron acceleration, scattering, and loss to the atmosphere, here we investigate how such broad, periodic wave excitation might occur. We show that it can be attributed to modulation of hot (resonant) electron distributions by ultralow frequency (ULF) waves, which transport electrons radially inward/outward by as much as ∼1.5 RE. Unstable electrons (and chorus waves generated by them) from higher L-shells (presumably from the azimuthally extended flow-braking region) are thus brought to lower L-shells. Half a ULF cycle later, outward electron transport increases the loss cone anisotropy (the perpendicular velocity gradient near the loss cone), resulting in periodic generation of ECH waves with intensities that are anticorrelated with chorus wave intensity. Such wave generation and intermittent electron precipitation (modulated on the spatial/temporal scale of a ULF wavelength/wave period) across large spatial scales could explain the pulsating aurora.