This plot shows the relative ability of heavy ions (in the extended
solar corona) to damp high-frequency Alfven waves that propagate
along the solar magnetic field in coronal holes.  The ions are sorted
in order of their charge-to-mass ratio Z/A, where Z is the number of
electrons missing from the atom and A is the number of protons and
neutrons in its nucleus.

The vertical axis shows the damping ability, expressed as the
logarithm of a dimensionless "Sobolev optical depth" tau. When tau
is much less than one (i.e., for negative values on the vertical
axis) the plasma is "transparent" to the waves, which are not
damped by the presence of the ion.  When tau is larger than one
(i.e., for positive values on the vertical axis), the medium is
"opaque," and waves are easily damped by the ions.  Many more ion
species are opaque than had been previously assumed, so models of
Alfven wave propagation and damping must take them into account.

(Cranmer 2000, Astrophys. J., 532, 1197)