  Photoionization by a femtosecond, terawatt laser pulse generates a plasma
column in a neutral ambient gas. Velocity of electrons, pushed by the laser
ponderomotive force along the column surface, couples to the the radial density
gradient at the column boundary, generating an azimuthally polarized THz
rotational current (RC). The same mechanism produces the low-frequency RC in a
leaky plasma channel. Applying external voltage to the channel induces a
radially non-uniform electron flow (direct current) and a constant, azimuthally
polarized magnetic field. Coupling them to the electron density perturbations
adds two more terms to the RC. The surface RC in the plasma column supports a
broadband, evanescent THz signal accompanying the wake. A few millimeters away
from the column, rapid evanescence of high-frequency components turns this THz
signal into a radially polarized, single-cycle pulse.
