  We present experimental evidence supported by simulations of a relativistic
ionization wave launched into surrounding gas by the sheath field of a plasma
filament with high energy electrons. Such filament is created by irradiating a
clustering gas jet with a short pulse laser ($\sim$115 fs) at a peak intensity
of $5 \times 10^{17}$ W/cm$^2$. We observe an ionization wave propagating
radially through the gas for about 2 ps at 0.2-0.5 $c$ after the laser has
passed, doubling the initial radius of the filament. The gas is ionized by the
sheath field, while the longevity of the wave is explained by a moving field
structure that traps the high energy electrons near the boundary, maintaining a
strong sheath field despite the significant expansion of the plasma.
