Observations of oblique propagation of whistler mode chorus

Whistler-mode chorus is known as an intense wave emission, naturally occurring in the inner magnetosphere. It was shown that these waves can play a role in the process of local acceleration of electrons in the outer Van Allen radiation belt. Numerous theoretical studies have been recently published concerning generation of chorus and its effects, using both quasi-linear and nonlinear approaches to the description of wave interactions with energetic electrons. Most of these studies assume that the waves propagate parallel to the terrestrial magnetic field. Here we show that this is not always the case. We examine properties of whistler-mode chorus using data from the WBD and STAFF-SA instruments on board the Cluster spacecraft to characterize propagation and spectral properties of chorus. We identify the source region of chorus by the Poynting flux analysis using multicomponent wave measurements of the STAFF-SA instrument using three magnetic and two electric antennas. The high-resolution waveforms from the WBD instrument show that chorus in the source region can be formed by a succession of discrete wave packets with increasing or decreasing frequency that sometimes changes into shapeless hiss with the same propagation characteristics. Multi-component measurements show that in both these regimes the wave vectors can be found at large angles to the terrestrial magnetic field. This has consequences on wave-particle interactions in the chorus source region.