Line widths and empirical most-probable speeds for neutral hydrogen
(H^0) and ionized oxygen (O^5+) derived from emission line profiles
measured with UVCS above polar coronal holes in late 1996 and early
1997.  The symbols with error bars denote the observed half-widths of
the lines (in Doppler velocity units) as a function of heliocentric
distance, and the thin black lines are a fit to these values.  The
derived ranges of most-probable speeds (w) are plotted as filled
regions bounded by thick lines.  For hydrogen, the uncertainty range
of the speeds parallel to the radial magnetic field (yellow) extend
up to the speeds in the perpendicular direction (green).  However,
self-consistent models involving the conservation of mass flux
indicate that the parallel speeds are likely to be smaller than
the perpendicular speeds.  For oxygen, above about 2.2
solar radii, the parallel speeds (blue) are constrained by
Doppler-dimmed intensity ratios to be decisively smaller than the
perpendicular speeds (red).  Note also that above about 1.9 solar
radii, the perpendicular speed of oxygen is larger than that of
hydrogen; this implies that the ratio of oxygen's to hydrogen's
perpendicular temperature is greater than proportional to their
masses.

So far, the best theoretical explanation for the faster oxygen
perpendicular speeds (and for the faster oxygen outflow) is that these
particles absorb energy from high-frequency (short period) waves in
the solar wind.  These waves must have periods of about a millisecond
for the particles to be able to sap their energy.  In contrast, most
of the observed waves in the solar wind have periods of more than a
few seconds, up to minutes and hours.  The UVCS observations are thus
suggestive evidence for the production of these shorter period waves
in the solar corona.

(Kohl et al. 1998, Astrophys. J., 501, L127; Cranmer et al. 1999,
Astrophys. J., 511, 481)