Unusually large positive geomagnetic variation (AU) near noon on 11 May, 2024

During the May 2024 space weather event, Kiruna magnetometer (KIR) registered historically large positive deviation of the northward geomagnetic disturbance (dX = +1300 nT) at around 12 UT (14 MLT, i.e., postnoon).  The large dX is observed entire Scandinavia, giving AU = 1431 nT at 12:11 UT, but not in the Atlantic or North American sectors (although we do not know the disturbance at 15-23 ML because no data at > 55° Mlat is available).  

Such large positive dX of dayside stations is not very rare, most of them are observed in the North American continent.  Out of total 21 AU peaks of > +1300 nT separated by more than 1 hour (12 magnetic storms) during 1978-2019, 2 events are peaked at 09-15 UT, 8 events at 15-21 UT, 6 events at 21-03 UT, and 5 events at 03-09 UT.

For the European sector, dX value in the May 2024 event is the second largest after the 24 November 2001 event in both AU statistics (1978-2019) and Kiruna magnetometer (1962-2024).  The same uncommon nature is even seen in Kp=9 that was registered at 09-12 UT.  During 1932-2024, Kp=9 was observed only during 4 events at 09-15 UT, whereas Kp=9 was observed during 10 events at 15-21 UT, 8 events at 21-03 UT, and 5 events at 03-09 UT.

Although these UT anomaly is within the statistical fluctuation, we attribute this to the geomagnetic tilt toward the North American sector.  This makes stations at the same geomagnetic latitudes (e.g., AE stations and Kp stations) located at lower geographic latitudes (i.e., under higher ionospheric conductivity) in the North American sector than the other longitudes when the stations are located near noon (09-15 MLT).  Accordingly, the dayside dX and local K tends to register higher in the North American sector than the other longitudes.  Since extremely large AU (> 1300 nT) tends to occur near noon (this is the case with the 12 storms mentioned above), we expect more frequent large dX when the North America is near noon (15-24 UT).  For Kp, large Kp requires K=9 at Kp station even in the dayside where the disturbance is normally smaller than the nightside.  Then the North America may easier to register large K even during daytime due to higher conductivity.  If the rareness of high AU and Kp during 09-15 UT has such solid reason, the May 2024 space weather event was actually very unusual. 

Finally, there is one more peculiar feature of the large dayside AU for the May 2024 event is that it is preceded only by normal substorm (AL ≈ -600 nT) and followed by a strong negative excursion in the Alaska-Pacific sector instead.  This is quite different from ordinary dayside positive dX that is normally preceded by substorm of large AL (which is the case for the 24 November 2001 event with AL < -1300 nT).

Acknowledgment: We used provisional AE, SuperMAG, INTERMAGNET and Kp.  We thank all contributing observatories and institutions for these datasets.