Contour plots of the oxygen velocity distribution function in a
theoretical model of ion heating by cyclotron resonance with
high-frequency (10 to 10,000 Hz) Alfven waves.  The velocity
distribution function is plotted at five temporal "snapshots" in a
kinetic simulation, where the oxygen ions are assumed to be in
a homogeneous plasma that contains a power-law wavenumber spectrum
of Alfven waves propagating parallel to the background magnetic
field.  The ions begin (at t=0) in an isotropic distribution
characterized by a temperature of approximately 2 million K.
As time evolves in the simulation, the ions diffuse to large
perpendicular speeds, and the temperature in the perpendicular
direction grows to values exceeding 300 million K.

The distribution functions are plotted versus the components of
the ion velocity parallel to the magnetic field (horizontal axis)
and perpendicular to the magnetic field (vertical axis).
The white contours in the plots enclose the central "core" of
the distribution (i.e., where most of the particles reside in
velocity space), and the colors represent decreasing levels of
the distribution function away from the core, from purple to
blue to green to yellow to red.

(Cranmer 2001, J. Geophys. Res., 106, in press)