Abstract. 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.
Abstract. The temporal and spatial behaviour of the ionospheric parameters foF2 and h F during isolated substorms are examined using data from ionospheric stations distributed across Europe and western Asia. The main purpose is finding the forerunners of the substorm disturbances and a possible prediction of these disturbances. During the period from March 1998 to March 1999, 41 isolated substorms with intensities I = 60 − 400 nT were identified and studied. The study separated occasions when the local magnetometers were affected by the eastward electrojet (positive substorms) from those influenced by the westward electrojet (negative substorms). The deviations of the ionospheric parameters from their monthly medians ( foF2 and h F) have been used to determine the variations through the substorm. Substorm effects occurred simultaneously (< 1 h) across the entire observatory network. For negative substorms, foF2-values increase > 6 h before substorm onset, T o , reaching a maximum 2-3 h before T o . A second maximum occurs 1-2 h after the end of the substorm. The h F values 3-4 h before T o have a small minimum but then increase to a maximum at T o . There is a second maximum at the end of the expansion phase before δh F drops to a minimum 2-3 h after ending the expansion phase. For positive substorms, the timing of the first maximum of the δfoF2 and δh F values depends on the substorm length -if it is longer, the position is closer to T o . The effects on the ionosphere are significant: foF2 and h F reach 2-3 MHz (δfoF2 = 50-70% from median value) and 50-70 km (δ h F = 20-30% from median value), respectively. Regular patterns of occurrence ahead of the first substorm signature on the magnetometer offer an excellent possibility to improve short-term forecasting of radio wave propagation conditions.
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