  Because conical segments of quasispherical ultrarelativistic blastwaves are
causally disconnected on angular scales larger than the blastwave inverse
Lorentz factor, astrophysical blastwaves can sustain initial anisotropy,
imprinted by the process that drives the explosion, while they remain
relativistic. We show that initial angular energy fluctuations in
ultrarelativistic blastwaves imply a production of vorticity in the blastwave,
and calculate the vortical energy production rate. In gamma-ray burst (GRB)
afterglows, the number of vortical eddy turnovers as the shocked fluid crosses
the blastwave shell is about unity for marginally nonlinear anisotropy. Thus
the anisotropy must be nonlinear to explain the magnetic energy density
inferred in measured GRB spectra.
