Ground magnetic response to an extraordinary IMF BY flip during the May 2024 storm: Travel time from the magnetosheath to dayside high latitudes

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.