Diffuse auroral scattering by whistler mode chorus waves: Dependence on wave normal angle distribution

Abstract

[1] Using the statistical CRRES measurements of the electric field intensities of lower band chorus (LBC) and upper band chorus (UBC) around L = 6 under geomagnetically moderate conditions, we evaluate the variations in modeled magnetic field spectral intensity and the resultant changes in resonant scattering rates of plasma sheet electrons caused by different choices of the wave normal distribution. UBC scattering rates inferred from electric field measurements show a common trend of decreasing scattering with increasing peak wave normal angle, θm, for the plasma sheet electrons at all resonant pitch angles. This trend is mainly due to the lower power of magnetic field as derived from the electric field measurements for oblique waves. The LBC resonant diffusion inferred from electric field measurements shows a considerable increase in scattering rates with increasing θm for ∼1 keV electrons at all resonant pitch angles and for 3–30 keV electrons over certain ranges of pitch angles, which is contrary to the decrease in wave magnetic field amplitude and results mainly from the decrease in resonant energy and redistribution of the majority of wave power at large wave normal angles for increased peak wave normal angle. LBC-induced scattering rates of 3–10 keV electrons decrease with increasing θm at low pitch angles, consistent with the decrease in wave magnetic field amplitude when θm increases. Our investigation demonstrates that the knowledge of the wave normal distribution of LBC and UBC is essential for an accurate quantification of the net resonant scattering rates and loss timescales of the plasma sheet electrons for an improved global simulation of diffuse auroral precipitation and the evolution of plasma sheet electron pitch angle distribution if only measurements of wave electric field intensity are available. In contrast, the diffuse auroral scattering rates calculated from magnetic field measurements are much less sensitive to the assumption on wave normal angle distribution. While UBC scattering with constant magnetic field power is roughly insensitive to the assumed wave normal distribution, LBC scattering with constant magnetic field power becomes more dependent on the assumed wave normal angle distribution, especially for ∼1 keV electrons.