  We have studied the element abundances and energy spectra of the small
"He-poor" impulsive solar energetic-particle (SEP) events, comparing them with
other impulsive SEP events with more-normal abundances of He. He-poor events
can have abundances as low as He/O ~ 2, while both impulsive and gradual SEP
events usually have source abundances of 30 < He/O < 100 with mean values of 50
- 60. He/C ratios are not only low, but often decrease with energy in He-poor
events. Abundance enhancement patterns of other elements with atomic numbers 6
< Z < 56, and likely values of their mass-to-charge ratios A/Q, are generally
unaltered in He-poor events, as are the probable source-plasma temperatures of
2.5 - 3.2 MK for all impulsive SEP events. One He-poor event is also an example
of a rarer C-poor event with C/O = 0.08 +- 0.04, sup-pressed by a factor over 5
from the mean. We discuss suggestions of a possible A/Q threshold during
acceleration and of the sluggish ionization of He entering the corona, because
of its uniquely high first ionization potential (FIP), but the suppression of
He and the decline of He/C with energy is difficult to explain if both He and C
are fully ionized with A/Q = 2 as expected at 2.5 - 3.2 MK. Although less
dramatic, a possible excess enhancement of Ne in some impulsive SEP events is
also considered. Possible causes of the large ~30% spectral and abundance
variations in impulsive events are also discussed. However, the physics of the
He-poor events remains a mystery.
