Ground Magnetic Response to an Extraordinary IMF B_Y Flip During the May 2024 Storm: Travel Time From the Magnetosheath to Dayside High Latitudes

In the present study we investigate the response of the dayside ground magnetic field to the sequence of interplanetary magnetic field (IMF) B_Y changes during the May 2024 geomagnetic storm. We pay particular attention to its extraordinarily large (>120 nT) and abrupt flip, and use GOES-18 (G18) magnetic field measurements in the dayside magnetosheath as a time reference. In the dayside auroral zone, the northward magnetic component changed by as much as 4,300 nT from negative to positive indicating that the direction of the auroral electrojet changed from westward to eastward. The overall sequence was consistent with the conventional understanding of the IMF B_Y driving of zonal ionospheric flows and Hall currents, which is also confirmed by a global simulation conducted for this storm. Surprisingly, however, the time delay from G18 to the ground increased significantly in time. The delay was 2–3 min for a sharp B_Y reduction ∼30 min prior to the B_Y flip, but it became as long as 10 min for the zero-crossing of the B_Y flip. It is suggested that the prolonged time delay reflected the travel time from G18 to the reconnection site, which sensitively depends on the final velocity at the magnetopause, that is, the inflow velocity of the magnetic reconnection. Around the B_Y flip, the solar wind number density transiently exceeded 100 cm⁻³, and should have increased further through the bow shock crossing. It is suggested that this unusually dense plasma reduced the reconnection rate, and therefore, the solar wind-magnetosphere energy coupling due to the extraordinary IMF.