Propagation of lower‐band whistler‐mode waves in the outer Van Allen belt: Systematic analysis of 11 years of multi‐component data from the Cluster spacecraft

Lower‐band whistler‐mode emissions can influence the dynamics of the outer Van Allen radiation belts. We use 11 years of measurements of the STAFF‐SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position. We determine probability distributions of wave vector angle weighted by the wave intensity. The results show that wave vector directions of intense waves are close to a Gaussian‐shaped peak centered on the local magnetic field line. The width of this peak is between 10 and 20 degrees. The cumulative percentage of oblique waves is below 10–15%. This result is especially significant for an important class of whistler‐mode emissions of lower‐band chorus at higher latitudes, well outside their source region, where a simple ray tracing model fails and another mechanism is necessary to keep the wave vectors close to the field‐aligned direction.