  Models of the solar transition region made from lines other than those of
helium cannot account for the strength of the helium lines. However, the
collisional excitation rates of the helium resonance lines are unusually
sensitive to the energy of the exciting electrons. Non-thermal motions in the
transition region could drive slowly-ionizing helium ions rapidly through the
steep temperature gradient, exposing them to excitation by electrons
characteristic of higher temperatures than those describing their ionization
state. We present the results of calculations which use a more physical
representation of the lifetimes of the ground states of He I and He II than was
adopted in earlier work on this process. New emission measure distributions are
used to calculate the temperature variation with height. The results show that
non-thermal motions can lead to enhancements of the He I and He II resonance
line intensities by factors that are comparable with those required. Excitation
by non-Maxwellian electron distributions would reduce the effects of
non-thermal transport. The effects of non-thermal motions are more consistent
with the observed spatial distribution of helium emission than are those of
excitation by non-Maxwellian electron distributions alone. In particular, they
account better for the observed line intensity ratio I(537.0 A)/I(584.3 A), and
its variation with location.
