Echoes from the D region have been obtained during disturbed periods with sweep-frequency ionosondes at stations in the northern part of Sweden. Typical D-layer situations are described, and the time and height distribution of D layers is shown for one of the stations in the subauroral zone. Arguments are advanced in support of the hypothesis that these D layers are caused by precipitation of energetic electrons from the outer Van Allen belt.
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Arne Pedersen (1962) studied this question.
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