17:15 〜 19:15
[PEM10-P16] UT dependence of severe space weather event defined by geomagnetic indices: another unusual aspect of the May 2024 event
キーワード：severe geomagnetic strom、afternoon peak of AU、Kp indices、dipole tilt effect、ionospheric conductivity、11 May 2024 G5 event
The dipole tilt makes the stations in the same geomagnetic (GM) latitude range locate at different geographic (GG) longitudes. Resultant ionospheric dayside conductivity for the same local time (LT) is higher at stations in the tilted direction (North American and Australian sectors), causing higher geomagnetic disturbances at these stations, than the other stations in the same GM latitude ranges. This applies to stations that are used in calculating the geomagnetic indices such as AE and Kp, resulting UT variations of these indices.
Since high AU tends to be given by dayside AE-stations, we expect more frequent occurrence of very high AU (e.g., > 1200 nT) when the North America is near noon (15-21 UT) than the other UT. Actually, these high AU events are detected by the dayside stations, predominantly in the afternoon sector (most often at 13-15 LT in both GG and GM coordinates). For Kp, some adjustment is made to compensate this effect for local K values of 1-8, but local K =9 or 8 at dayside Kp stations are still required to give highest Kp value (=9 or 9-) even for stations away from the dipole tile. As the result, we again expect more frequent occurrence of the highest Kp when the East coast of North America or Australia is near noon (18-21 UT and 0-03 UT) than the other UT.
We confirmed the expected UT dependence using all available data (AU minute values for 1978-2019, Kp values for 1932-2024), while minimum occurrence is also found at the 09-15 UT range for both indices. On the other hand, equinox peaks of Kp still applies to the occurrence of Kp=9 or 9-, while majority of very high AU took place during summer such that equinox would be minimum if the AE station cover even the southern hemisphere. Such a different seasonal dependence is actually consistent with the ionospheric conductivity effect that is more obvious at higher latitudes.
The consistency in the UT distribution between AU and Kp indicates that some G4 geomagnetic storms taking place at 09-15 UT might be as severe as G5 storm. This problem might remain even for the new Hp30 index unless the longitudinal difference is further compensated. In this respect, the 11 May 2024 G5 event (Kp=9 at 09-12 UT on 11 May) is actually a very severe storm, with the largest ever recorded AU value of 1431 nT during 08-15 UT (registered at Abisko at 12:11 UT). Finally, there is one more peculiar feature of this large AU: unlike the other large AU events that takes place during the recovery phase of AL < -1000 substorms, this event was preceded only by normal substorm (AL ≈ -600 nT) and followed by a strong negative excursion in the Alaska-Pacific sector instead.
Since high AU tends to be given by dayside AE-stations, we expect more frequent occurrence of very high AU (e.g., > 1200 nT) when the North America is near noon (15-21 UT) than the other UT. Actually, these high AU events are detected by the dayside stations, predominantly in the afternoon sector (most often at 13-15 LT in both GG and GM coordinates). For Kp, some adjustment is made to compensate this effect for local K values of 1-8, but local K =9 or 8 at dayside Kp stations are still required to give highest Kp value (=9 or 9-) even for stations away from the dipole tile. As the result, we again expect more frequent occurrence of the highest Kp when the East coast of North America or Australia is near noon (18-21 UT and 0-03 UT) than the other UT.
We confirmed the expected UT dependence using all available data (AU minute values for 1978-2019, Kp values for 1932-2024), while minimum occurrence is also found at the 09-15 UT range for both indices. On the other hand, equinox peaks of Kp still applies to the occurrence of Kp=9 or 9-, while majority of very high AU took place during summer such that equinox would be minimum if the AE station cover even the southern hemisphere. Such a different seasonal dependence is actually consistent with the ionospheric conductivity effect that is more obvious at higher latitudes.
The consistency in the UT distribution between AU and Kp indicates that some G4 geomagnetic storms taking place at 09-15 UT might be as severe as G5 storm. This problem might remain even for the new Hp30 index unless the longitudinal difference is further compensated. In this respect, the 11 May 2024 G5 event (Kp=9 at 09-12 UT on 11 May) is actually a very severe storm, with the largest ever recorded AU value of 1431 nT during 08-15 UT (registered at Abisko at 12:11 UT). Finally, there is one more peculiar feature of this large AU: unlike the other large AU events that takes place during the recovery phase of AL < -1000 substorms, this event was preceded only by normal substorm (AL ≈ -600 nT) and followed by a strong negative excursion in the Alaska-Pacific sector instead.