Chorus Wave Scattering Responsible for the Dayside Diffuse Auroral Precipitation

We perform a comprehensive theoretical and numerical analysis on the conjunction measurements of dayside diffuse aurora and whistler-mode chorus waves by the South Pole all-sky imager and THEMIS spacecraft at 16 -18 UT on August 13, 2009. A high correlation is identified between the intensities of the diffuse aurora at 557.7 nm near the THEMIS ionospheric footprints and chorus emissions. Using the simultaneous wave, plasma density and particle datasets of THEMIS observations, we compute the matrices of bounce-averaged diffusion coefficients due to chorus wave scattering in the realistic magnetosphere at a series of representative time stamps, which are subsequently utilized to quantitatively compare with the rate of strong diffusion for evaluating the energy dependent loss cone filling index associated with chorus-induced pitch angle scattering. Fits of Maxwellian-type energy spectrum to the modeled electron differential fluxes inside the loss cone produce a temporal variation of the total energy flux and characteristic energy of precipitating electrons. The obtained dominant precipitation energies are within 2 - 5 keV, which agrees well with the major electron population for the dayside green-line aurora excitation. The modeled change of the total precipitation energy flux is remarkably consistent with that of the observed green-line diffuse aurora intensity. The trend of decreases and increases in the aurora luminosity is also reasonably reproduced in a time consistent manner. Through a systematic combination of quasi-linear theory, realistic non-dipolar magnetic field mapping, and the concept of strong diffusion on the basis of conjugated space and ground observations, we have demonstrated that dayside chorus scattering can dominantly account for the dayside green-line diffuse aurora activity, while variations in electron differential flux also play a role. In addition, changes in the ambient density can affect the portion of diffuse auroral electrons that undergo efficient scattering into the loss cone for precipitation loss, and consequently modify the diffuse aurora activity.