We develop a one-dimensional model of THz emissions induced by laser-driven, time-asymmetric ionization and current oscillations in a hydrogen gas. Our model highlights complex scalings of the THz fields with respect to the laser and gas parameters, in particular, a non-monotonic behavior against the laser parameters. Analytical expressions of the transmitted and reflected fields are presented, explaining the THz spectra observed in particle-in-cell and forward-pulse propagation codes. The backward-propagating THz wave is mainly driven by the electron current oscillations at the plasma frequency, and its resulting spectrum operates below the plasma frequency. The transmitted THz wave is emitted from both plasma current oscillations and photo-ionization. Their respective signal presents a contribution below and around the plasma frequency, plus a contribution at higher frequencies associated to the photo-induced current. The interplay between these two mechanisms relies on the ratio between the propagation length and the plasma skin depth.
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