The ionospheric response to interplanetary magnetic field variations: Evidence for rapid global change and the role of preconditioning in the magnetosphere

We have found observational evidence for a rapid communication of interplanetary magnetic field (IMF) changes to the global ionosphere and evidence for the state of the magnetosphere in the previous hour conditioning this response. These conclusions are drawn from a case study of sunward flow bursts on the nightside polar cap boundary observed by geomagnetically conjugate HF radars. The flow burst excitation consists of two factors: (1) At the time of the flow burst, the magnetosphere still held a memory of the stable and northward IMF period that had persisted up until 1 hour before the flow burst (internal condition). During the northward IMF period a theta aurora associated with a sunward flow channel was formed in the polar cap. After that the IMF turned southward, and the transpolar arc decayed antisunward. However, by the time of the flow burst (i.e., 1 hour after the IMF southward turning), the Sun‐aligned arc had not yet completely vanished, and in the poleward expanded portion of the northern plasma sheet, there was still a remnant of the sunward flow channel susceptive to an external forcing. (2) One hour after the southward turning of the IMF a sharp IMF transition from southward to northward B z impinged on the dayside magnetopause (external condition). On arriving at the dayside cusp ionosphere the B z transition signal pervaded the entire polar cap ionosphere instantaneously (<1 min) and reached the nightside plasma sheet. There, the remnant of the sunward flow channel was reactivated by the B z transition, and a sunward flow burst was observed first in the northern ionosphere and then in the southern ionosphere with a 7‐min time delay. Thus the sunward flow burst represents a rapid global response of the ionosphere starting 2–3 min after the IMF change at the subsolar magnetopause.