  We present a theoretical investigation of terahertz (THz) generation in
laser-induced gas plasmas. The work is strongly motivated by recent
experimental results on micro-plasmas, but our general findings are not limited
to such a configuration. The electrons and ions are created by
tunnel-ionization of neutral atoms, and the resulting plasma is heated by
collisions. Electrons are driven by electromagnetic, convective and diffusive
sources and produce a macroscopic current which is responsible for THz
emission. The model naturally includes both, ionization current and
transition-Cherenkov mechanisms for THz emission, which are usually
investigated separately in the literature. The latter mechanism is shown to
dominate for single-color multi-cycle lasers pulses, where the observed THz
radiation originates from longitudinal electron currents. However, we find that
the often discussed oscillations at the plasma frequency do not contribute to
the THz emission spectrum. In order to predict the scaling of the conversion
efficiency with pulse energy and focusing conditions, we propose a simplified
description that is in excellent agreement with rigorous particle-in-cell
simulations.
