  Since the early 1980s, decimetric spike bursts have been attributed to
electron cyclotron maser emission (ECME) by the electrons that produce hard
X-ray bursts as they precipitate into the chromosphere in the impulsive phase
of a solar flare. Spike bursts are regarded as analogous to the auroral
kilometric radiation (AKR), which is associated with the precipitation of
auroral electrons in a geomagnetic substorm. Originally, a loss-cone-driven
version of ECME, developed for AKR, was applied to spike bursts, but it is now
widely accepted that a different, horseshoe-driven, version of EMCE applies to
AKR. We explore the implications of the assumption that horseshoe-driven ECME
also applies to spike bursts. We develop a 1D model for the acceleration of the
electrons by a parallel electric field, and show that under plausible
assumptions it leads to a horseshoe distribution of electrons in a solar flare.
A second requirement for horseshoe-driven ECME is an extremely low plasma
density, referred to as a density cavity. We argue that a coronal density
cavity should develop in association with a hard X-ray burst, and that such a
density cavity can overcome a long-standing problem with the escape of ECME
through the second-harmonic absorption layer. Both the horseshoe distribution
and the associated coronal density cavity are highly localized, and could not
be resolved in the statistically large number of local precipitation regions
needed to explain a hard X-ray burst. The model highlights the "number problem"
in the supply of the electrons needed to explain a hard X-ray burst.
