this figure is
actually taken from the interactive ``xspace'' program used to teach space
plasma physics, see http://www-ssc.igpp.ucla.edu/ssc/software/xspace.html
and http://www-ssc.igpp.ucla.edu/pictures/edu_particle.gif
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.