Abstract
Most of the quasi-linear and nonlinear theoretical studies assume that the waves propagate parallel to the terrestrial magnetic field. We show a case where this assumption is invalid.
We use data from the Cluster spacecraft to characterize propagation and spectral properties of chorus. Multicomponent measurements show that waves in both regimes (discrete chorus wave packets ro shapeless hiss) can be found at large angles to the terrestrial magnetic field.
The hiss intervals contain waves propagating less than one tenth of a degree from the resonance cone. In the regime of discrete wave packets the peak of the wave energy density is found at a few degrees from the resonance cone in a broad interval of azimuth angles.
The wave intensity increases with the distance from the magnetic field minimum along a given field line, indicating a gradual amplification of chorus in the source region.