The response of high‐latitude convection to a sudden southward IMF turning

We describe the response of the high‐latitude convection to a sudden southward turning of the IMF. In this event B Z recorded at the WIND satellite switched from +6 nT to −6 nT in 2½ minutes and effects in the ionosphere were observed by the SuperDARN HF radars. In the vicinity of noon MLT, the convection switched suddenly (∼2 min) from sunward to antisunward with the velocity change reaching magnitudes as great as 1500 m/s in about 6 minutes. Surprisingly, the responses at the other MLTs distributed from noon to midnight began at virtually the same time. Meridional flows near 22 MLT were observed to reverse direction in about 4 minutes with the velocity change reaching magnitudes as great as 750 m/s in about 8 minutes. This example shows that the convection can respond to a sudden IMF change practically simultaneously (∼2 min) over the entire high‐latitude zone and reconfigure from a pattern characteristic of B Z + to one characteristic of B Z − in a very short (2–4 min) time.