  We describe the statistical properties of narrowband drifting auroral
kilometric radiation ('striated' AKR) based on observations from the Cluster
wideband receiver during 2002-2005. We show that the observed characteristics,
including frequency drift rate and direction, narrow bandwidth, observed
intensity, and beaming angular sizes are all consistent with triggering by
upward traveling ion solitary structures (`ion holes'). We calculate the
expected perturbation of a horseshoe electron distribution function by an ion
hole by integrating the resonance condition for a cyclotron maser instability
(CMI) using the perturbed velocity distribution. We find that the CMI growth
rate can be strongly enhanced as the horseshoe velocity distribution contracts
inside the passing ion hole, resulting in a power gain increase greater than
100 dB. The gain curve is sharply peaked just above the R-mode cut-off
frequency, with an effective bandwidth ~50 Hz, consistent with the observed
bandwidth of striated AKR emission. Ion holes are observed in situ in the
acceleration region moving upward with spatial scales and speeds consistent
with the observed bandwidth and slopes of SAKR bursts. Hence, we suggest that
SAKR bursts are a remote sensor of ion holes and can be used to determine the
frequency of occurrence, locations in the acceleration region, and lifetimes of
these structures.
