  In this paper, we analyze hard X-ray spikes observed by RHESSI to understand
their temporal, spectral, and spatial properties. A recently developed
demodulation code was applied to hard X-ray light curves in several energy
bands observed by RHESSI. Hard X-ray spikes were selected from the demodulated
flare light curves. We measured the spike duration, the energy-dependent time
delay, and count spectral index of these spikes. We also located the hard X-ray
source emitting these spikes from RHESSI mapping that was coordinated with
imaging observations in visible and UV wavelengths. We identify quickly varying
structures of <1 s during the rise of hard X-rays in five flares. These hard
X-ray spikes can be observed at photon energies over 100 keV. They exhibit
sharp rise and decay with a duration (FWHM) of less than 1 s. Energy-dependent
time lags are present in some spikes. It is seen that the spikes exhibit harder
spectra than underlying components, typically by 0.5 in the spectral index when
they are fitted to power-law distributions. RHESSI clean maps at 25-100 keV
with an integration of 2 s centered on the peak of the spikes suggest that hard
X-ray spikes are primarily emitted by double foot-point sources in magnetic
fields of opposite polarities. With the RHESSI mapping resolution of ~ 4 arsec,
the hard X-ray spike maps do not exhibit detectable difference in the spatial
structure from sources emitting underlying components. Coordinated
high-resolution imaging UV and infrared observations confirm that hard X-ray
spikes are produced in magnetic structures embedded in the same magnetic
environment of the underlying components. The coordinated high-cadence TRACE UV
observations of one event possibly reveal new structures on spatial scales <1-2
arsec at the time of the spike superposed on the underlying component. They are
probably sources of hard X-ray spikes.
