Series of ionospheric modification experiments have shown the possibility of artificially exciting short-scale field-aligned irregularities in the E region at high latitudes. During magnetically quiet conditions the heating facility situated close to Trom¿, Norway, produced 1-m irregularities. The observations were done by the two radars constituting the STARE system operating at 140 MHz and 144 MHz. A statistical analysis of the data shows a preference for overdense (fo,Heat<foE) O-mode heating. In 5% of the experiments, X-mode polarization of the heater wave was exciting irregularities. A smaller portion of the data confirmed underdense heating. The e-folding growth times varied from less than 2 s to 15 s with the shortest growth times as the typical values. The estimated average phase velocity of artificial irregularities amounted to 175 m s-1. Due to features in the backscatter pattern observed by STARE we can conclude that not only one type of instability was operating. Nonlinear saturation processes may be the cause of the observations. But also two-step processes, where an instability type with low threshold conditions modifies the plasma of the ionosphere for a more efficient instability mechanism, may explain the data.