  In recent experiments of laser pulse interaction at relativistic intensities
with a low density plasma, the proton radiography technique showed evidence of
long--lived field structures generated after the self-channeling of the laser
pulse. We present 2D particle-in-cell simulations of this interaction regime,
where the dynamics of similar structures has been resolved with high temporal
and spatial resolution. An axially symmetrical field pattern, resembling both
soliton-like and vortex structures, has been observed. A study of the physics
of such structures and a comparison with experimental data is in progress.
