  The relativistic Vlasov-Maxwell system of plasma physics is considered with
initial data on a past light cone. This characteristic initial value problem
arises in a natural way as a mathematical framework to study the existence of
solutions isolated from incoming radiation. Various consequences of the
mass-energy conservation and of the absence of incoming radiation condition are
first derived assuming the existence of global smooth solutions. In the
spherically symmetric case, the existence of a unique classical solution in the
future of the initial cone follows by arguments similar to the case of initial
data at time $t=0$. The total mass-energy of spherically symmetric solutions
equals the (properly defined) mass-energy on backward and forward light cones.
