Abstract
Previously published statistics based on Cluster spacecraft measurements surprisingly show that in the outer radiation belt, lower band whistler mode waves predominantly propagate unattenuated parallel to the magnetic field lines up to midlatitudes, where ray tracing simulations indicated highly attenuated waves with oblique wave vectors. We explain this behavior by considering a large fraction of ducted waves.
We argue that these ducts can be weak and thin enough to be difficult to detect by spacecraft instrumentation while being strong enough to guide whistler mode waves in a cold plasma ray tracing simulation. After adding a tenuous hot electron population, we obtain a strong effect of Landau damping on unducted waves, while the ducted waves experience less damping or even growth.
Consequently, the weighted average of amplitudes and wave normal angles of a mixture of ducted and unducted waves provides us with strong quasi-parallel waves, consistent with the observations.