We presented experimental results of strong plasma modifications induced by X-mode powerful HF radio waves injected towards the magnetic zenith into the high latitude F region of the ionosphere. The experiments were conducted in 2009–2011 using the EISCAT Heating facility, UHF incoherent scatter radar and the EISCAT ionosonde at Tromsø, Norway; and the CUTLASS SuperDARN HF coherent radar at Hankasalmi, Finland. The results showed that the X-mode HF pump wave can generate strong smallscale artificial field aligned irregularities (AFAIs) in the F region of the high-latitude ionosphere. These irregularities, with spatial scales across the geomagnetic field of the order of 9–15 m, were excited when the heater frequency (fH) was above the ordinary-mode critical frequency (foF2) by 0.1–1.2 MHz. It was found that the X-mode AFAIs appeared between 10 s and 4 min after the heater is turned on. Their decay time varied over a wide range between 3 min and 30 min. The excitation of X-mode AFAIs was accompanied by electron temperature (Te) enhancements and an increase in the electron density (Ne) depending on the effective radiated power (ERP). Under ERPs of about 75–180 MW the Te enhances up to 50% above the background level and an increase in Ne of up to 30% were observed. Dramatic changes in the Te and Ne behavior occurred at effective radiated powers of about 370– 840 MW, when the Ne and Te values increased up to 100% above the background ones. It was found that AFAIs, Ne and Te enhancements occurred, when the extraordinary-mode critical frequency (fxF2) lied in the frequency range fH–fce/2rfxF2rfHþfce/2, where fce is the electron gyrofrequency. The strong Ne enhancements were observed only in the magnetic field-aligned direction in a wide altitude range up to the upper limit of the UHF radar measurements. In addition, the maximum value of Ne is about 50 km higher than the Te enhancement peak. Such electron density enhancements (artificial ducts) cannot be explained by temperature-dependent reaction rates. They can be attributed to HF-induced ionization production by accelerated electrons. The possible mechanisms for plasma modifications induced by powerful X-mode HF radio waves were discussed. ; We would like to thank the EISCAT Scientific Association for providing the Tromsø heating experiments in conjunction with the EISCAT UHF radar measurements. CUTLASS is supported by the UK Science and Technology Facilities Council Grant PP/E007929/1, the Finnish Meteorological Institute, and the Swedish Institute of Space Physics. TKY is supported by the Science and Technology Facilities Council Grant ST/H002480/1. ; Peer-reviewed ; Publisher Version