First observations of the ELF emission generated by the modulated ionospheric HF-heating using the HAARP facility have been presented. The prime focus is on the dependence of the polarization and of the amplitude of ELF wave upon the frequency and polarization of the heating radio wave. A simple 1D computational model of the ELF generation based on the earlier developed model of a horizontal magnetic dipole caused by the HF ionospheric heating [1] is presented. The model develops a qualitative understanding of the changes in the ELF amplitude and polarization due to variations of the heating frequency. The obtained results imply that polarization of the ELF emission generated by the ionospheric HF-heating can be controlled by changing the frequency or polarization of the radio wave.