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Statistical study on the universal time\udvariations in the mean hourly auroral electrojet index (AE-index) has been\udundertaken for a 21 y period over two solar cycles (1957–1968 and 1978–1986).\udThe analysis, applied to isolated auroral substorm onsets (inferred from rapid\udvariations in the AE-index) and to the bulk of the AE data, indicates that the\udmaximum in auroral activity is largely confined to 09–18 UT, with a distinct\udminimum at 03–06 UT. The diurnal effect was clearly present throughout all\udseasons in the first cycle but was mainly limited to northern winter in the\udsecond cycle. Severe storms (AE > 1000 nT) tended to occur between 9–18 UT\udirrespective of the seasons whereas all larger magnetic disturbances (AE >\ud500 nT) tended to occur in this time interval mostly in winter. On the whole the\uddiurnal trend was strong in winter, intermediate at equinox and weak in summer.\udThe implication of this study is that Eastern Siberia, Japan and Australia are\udmostly at night, during the period of maximum auroral activity whereas Europe\udand Eastern America are then mostly at daytime. The minimum of auroral activity\udcoincides with near-midnight conditions in Eastern America. It appears that the\uddiurnal UT distribution in the AE-index reflects a diurnal change between\udinterplanetary magnetic field orientation and the Earth's magnetic dipole\udinclination.  <b>Key words.</b> Ionosphere (auroral ionosphere) ·\udMagnetospheric physics (auroral phenomena; storms and substorms)

Longitudinal (UT) effect in the onset of auroral disturbances over two solar cycles as deduced from the AE-index