2,5 minute values of auroral AL indices are, used as indicators of magnetospheric substorms. By means of appropriate threshold values of their variation we define the commencement of the events and the enhancement of their expansion phase, according to the pre-perturbation levels. The events thus selected (here termed V), are then compared with bays declared by middle and low latitude observatories (termed B). In this way, in the interval August 1967 through July 1968 some 25 to 50% of B events could be identified as isolated bays, according to the different months, considering as isolated those events where the auroral substorm is preceded by an undisturbed interval. O The principal causes of failures are discussed as well as the characteristic manifestations of the enhancements in disturbed periods.


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