Four subauroral HF radio paths are investigated during substorms of various intensities. The principal substorm effects are both an increase of the signal strength 1--2 hours before the substorm expansion phase and a change in the azimuth of the received signal. The latter is explained by increasing the intensities of irregularities caused by particle precipitation on the poleward edge of the main ionospheric trough. Model calculation of the signal amplitude variations during the substorm growth phase has been carried out. Excellent agreement between the model and observations is achieved when the critical frequency of the F2 layer is increased by several megacycles and the height of the F2 layer maximum is reduced by 20--40 km at the reflection point the nearest to the receiving center. Similar investigations may be useful for quantitative description of any ionospheric model within disturbed periods. ¿ 2000 American Geophysical Union