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

Abstract : [1] We calculated the bounce averaged electron pitch angle diffusion coefficients 〈D a eq a eq 〉 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 q 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 l, the magnetic local times, and the K p indices. According to Cluster observations, the probability of observing a larger B w 2 increases with l and depends upon the magnetic local time and K p. We compared the obtained results with the diffusion coefficients 〈D a eq a eq 〉 const 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, a eq , but likely overestimates 〈D a eq a eq 〉 const for a eq > 60. An important increase in 〈D a eq a eq 〉 for a eq < 30 can be explained by a large mean value for the q distribution and by an increase of B w 2 at l > 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.