Higher frequency components in auroral light pulsations are shown to be considerably damped relative to similar components in associated X-ray pulsations. This effect can be explained in terms of velocity dispersion of electrons of different energies whose flux in the loss cone is modulated simultaneously in a localized region some 105 km out along auroral zone field lines. An alternative explanation is suggested which involves an energy-selective modulation mechanism which could be acting quite close to the ionosphere.