UT dependence of severe space weather events defined by geomagnetic indices: an unusual aspect of the May 2024 event

Abstract Due to geomagnetic dipole tilt, the solar zenith angles and the resultant ionospheric conductivities at the same geomagnetic (GM) latitude and local time are highest in the tilt direction, which are the North American sector in the northern hemisphere and the Australian-New Zealand sector in the southern hemisphere. As a result, the geomagnetic disturbances (raw nT or its 3 h-variation scaled by K = 0–9) at specific observatories used for calculating a global geomagnetic index should be higher in the tilt direction than those at others if the index is affected by dayside activities, such as Kp (global average of K) and AU (upper envelope of AE). Such an index potentially shows small UT dependence for the same level of ionospheric and magnetospheric activities. We examine this for very high values of AU > 1200 nT and Kp ≥ 9–. AU peak values of > 1000 nT were almost always recorded at dayside stations, most frequently in the postnoon sector, whereas a local K value of 9 or 8 occur much less frequently in the dayside than in the nightside. Applying the above unevenness, AU peak values of > 1200 nT are expected to occur most frequently when the AE stations in North America are located in the postnoon sector (15–21 UT). For Kp, Kp = 9 is most easily recorded when the East coast of North America, Australia, or New Zealand is near local noon (15–21 UT and 00–03 UT). These expected UT dependences are consistent with both the Kp statistics over 93 years and the AU statistics over 31 years. The data also show the lowest occurrence rate of AU > 1200 nT and Kp ≥ 9– at the same UT sector (09–15 UT). The statistics suggest that some G4 geomagnetic storms occurring at 09–15 UT may be as severe as G5 storms. In this respect, the May 2024 G5 event (Kp = 9 at 09–12 UT on 11 May) was a very severe storm, with the highest AU value recorded during 08–15 UT and only the second recorded case of Kp = 9 at 09–12 UT. Graphical Abstract