  Transition region (TR) spectra typically show the Si IV 1402.8 A line to be
enhanced by a factor of 5 or more compared to the neighboring O IV 1401.2 A,
contrary to predictions of ionization equilibrium models and the Maxwellian
distribution of particle energies. Non-equilibrium effects in TR spectra are
therefore expected. To investigate the combination of non-equilibrium
ionization and high-energy particles, we apply the model of the periodic
electron beam, represented by a kappa-distribution that recurs at periods of
several seconds, to plasma at chromospheric temperatures of 10^4 K. This simple
model can approximate a burst of energy release involving accelerated
particles. Instantaneous time-dependent charge states of silicon and oxygen
were calculated and used to synthesize the instantaneous and period-averaged
spectra of Si IV and O IV. The electron beam drives the plasma out of
equilibrium. At electron densities of N_e = 10^10 cm^-3, the plasma is out of
ionization equilibrium at all times in all cases we considered, while for a
higher density of N_e = 10^11 cm^-3, ionization equilibrium can be reached
toward the end of each period, depending on the conditions. In turn, the
character of the period-averaged synthetic spectra also depends on the
properties of the beam. While the case of kappa = 2 results in spectra with
strong or even dominant O IV, higher values of kappa can approximate a range of
observed TR spectra. Spectra similar to typically observed spectra, with the Si
IV 1402.8 A line about a factor 5 higher than O IV 1401.2 A, are obtained for
kappa = 3. An even higher value of kappa = 5 results in spectra that are
exclusively dominated by Si IV, with negligible O IV emission. This is a
possible interpretation of the TR spectra of UV bursts, although an
interpretation that requires a density that is 1-3 orders of magnitude lower
than for equilibrium estimates.
