5. Conclusions

[18]  The following results are achieved.

[19]  1. Parameters of radio signals, the MOF and the LOF on the path, can be considered as data of the oblique ionosphere sounding (OIS). The OIS method is rather reliable to estimate the state of the high-latitude ionosphere as a medium of propagation during strong magnetic storms. In contrast data of vertical ionosphere sounding concerning ionosphere dynamics during disturbed periods are very poor (~50%).

[20]  2. For quiet geomagnetic conditions, the frequency range MOF-LOF on the transauroral radio path is practically constant, whereas for the disturbed conditions there are sudden leaps and changes of the MOF and LOF values. (1) On the falling off of a storm or a substorm the frequency range MOF-LOF is narrow due to diminishing the MOF and increasing the LOF values; (2) for an intensive disturbance with sharp onset there is a growth of the MOF values several hours before the moment T₀ (the onset of sharp increase of the AE index), as well as during several hours after the end of the disturbance; and (3) mighty precipitation during the substorm period changes the ionosphere structure. Intensive sporadic layers develop in the E region of the ionosphere. The MOF values for signals reflected from these layers can exceed the MOF values due to the F2 layer of the ionosphere.

[21]  3. Geophysical data ( AE and Kp indexes, satellite, riometer and ionospheric data) help essentially to interpret the received regularities of variations of the signal parameters MOF and LOF on the high-latitude radio path. So, an appreciable riometer absorption influences the LOF value but a perceptible level of AE index influences mainly the MOF value.

[22]  4. The analysis enables it to formulate the following recommendations to be put into practice of radio communications: (1) It is possible to use the most favorable conditions of the signal transmission during a period of 2-3 hours before the storm or substorm onset when the MOF is great and the LOF is low; and (2) It is necessary to organize the work of the HF channel after the ionospheric disturbance with the proper operational frequencies from a narrow range MOF-LOF.