Statistical model of electron pitch angle diffusion in the outer radiation belt

Abstract

We calculated the bounce averaged electron pitch angle diffusion coefficients ? using the statistical characteristics of lower band chorus activity collected by the Cluster mission from 2001-2009. Nine years of Cluster observations provide the distributions of the θ angle between wave vectors and the background magnetic field; and the distributions of the wave total intensity B_w² for relatively wide ranges of the magnetic latitude λ, the magnetic local times, and the K_p indices. According to Cluster observations, the probability of observing a larger B_w² increases with λ and depends upon the magnetic local time and K_p. We compared the obtained results with the diffusion coefficients ? that were calculated under an assumption of parallel whistler wave propagation with a constant intensity B_w² = 10⁴ pT². The last calculations substantially underestimated pitch angle diffusion for the small equatorial pitch angles, α_eq, but likely overestimates ? for α_eq > 60°. An important increase in ? for α_eq < 30° can be explained by a large mean value for the θ distribution and by an increase of B_w² at λ > 15°. We took the probability density distribution of the wave mean amplitude into consideration instead of the averaged value. The obtained distribution of the diffusion coefficients indicated that approximately 20% of the most intense waves can provide the main portion of pitch angle diffusion for the dawn/day sector. For the dusk/night sector, wave intensity was significantly weaker and the relative importance of intense waves was not clearly pronounced.