HF radio propagation through the auroral oval during substorms

Dynamical changes of the polar ionosphere are most pronounced during geomagnetic disturbances like the substorms and storms . The similar behavior of the polar and auroral ionosphere leads to the crucial, as a rule, unfavorable changes in the HF propagation at high latitudes. In the present article the features of radio wave propagation on the St. Petersburg-Bely Nos sub-polar path, having a length of 1800 km, were analyzed by the use of the oblique ionospheric sounding data. Observations were carried out during the substorm activity within the summer months of 1997. The main goal of the study is to reveal the typical features in HF radio propagation during substorms which could be useful for both the scientific investigation and radio communications at high latitudes. The results obtained demonstrate the following typical features in the HF propagation on this radio path: (i) the range of the operational frequencies is significantly narrowed during the substorm period; (ii) a change of the propagation mechanism takes place; (iii) a significant growth of auroral absorption occurs during the expansion phase; (iv) the multi-ray index for the received signals grows sharply. Evidence for the changes of the radio wave propagation parameters was found, which can be used for possible forecasting of the substorm onset. Typical features related to the HF propagation along the auroral zone are interpreted from the geophysical point of view. Those are significant for the tasks of the communication improvement and problems investigated by the Space Weather program.