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 w 2 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 w 2 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 w 2 = 10 4 pT 2 . 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 w 2 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.