  The large and rapidly varying electric and magnetic fields induced by the
spectator systems moving at ultrarelativistic velocities induce a charge
splitting of directed flow, $v_1$, of positive and negative pions in the final
state of the heavy ion collision. The same effect results in a very sizeable
distortion of charged pion spectra as well as ratios of charged pions
($\pi^+/\pi^-$) emitted at high values of rapidity. Both phenomena are
sensitive to the actual distance between the pion emission site and the
spectator system. This distance $d_E$ appears to decrease with increasing
rapidity of the pion, and comes below $\sim$1~fm for pions emitted close to
beam rapidity. In this paper we discuss how these findings can shed new light
on the space-time evolution of pion production as a function of rapidity, and
on the longitudinal evolution of the system created in heavy ion collisions.
