The images shows neutral hydrogen Lyman alpha emission along the
UVCS slit during the passage of a polar jet on August 5, 1997.  The
vertical axis is the spatial coordinate along the UVCS entrance
slit (in arc seconds) centered on the north heliographic pole, at
a heliocentric distance of 1.65 solar radii.

We have observed several polar jets with UVCS/SOHO.  They usually
correlate with the EIT and LASCO jet events.  We analyzed
spectroscopic observations of these jets and found that they typically
undergo two phases: during the first phase the O VI lines show a
brief intensity enhancement and narrowing, while the H I Lyman alpha
line is not enhanced; during the second phase, about 25 minutes later,
the H I Lyman alpha line shows an intensity enhancement and
narrowing, while the O VI line is relatively unchanged.  According
to our empirical jet model, the first phase is the fast (more
than 280 km/s), dense centroid of the jet passing by the UVCS slit.
During the second phase, the model required a further decrease in
the electron temperature (with a jet temperature of only 150,000 K),
along with a weaker electron density and an outflow velocity of
205 km/s.  Possible scenarios of the electron temperature variations
needed to account for observed conditions on August 5, 1997 indicate
that some heating is required. We computed models of the temperature
and nonequilibrium ionization state of an expanding plasma using
various forms for the heating rates. We found that the jet had to
leave the Sun at an electron temperature below 2.5 million K and
that a heating rate of the same order as the average coronal hole
heating is required. Such low initial temperatures are consistent
with the idea that the jets observed by LASCO, EIT, and UVCS are
different than previously observed coronal X-ray jets.

(Dobrzycka et al. 2000, Astrophys. J., 538, 922)