Ionospheric convection during nonsteady interplanetary magnetic field conditions

We investigate six cases in which the interplanetary magnetic field (IMF) orientation is changing. The assimilative mapping of ionospheric electrodynamics (AMIE) technique is used to compute the ionospheric electric potential in a 5‐min time step over the nonsteady periods. For each period a steady convection pattern prior to the convection change is determined, and this pattern is then subtracted from each of the nonsteady convection patterns. From these residual potential patterns a number of parameters can be determined, such as the start and end time of the convection change, the amount of change and the location of the strongest changes. By further examining the residual patterns, we reach the following conclusions: (1) convection changes occur in a fixed region of space, while the magnitude of the convection change grows linearly with time; (2) the strength of the convection change and the rate of the change are linearly proportional to the change observed in the IMF B y and B z components; and (3) the merging region may not react to long duration changes in the IMF until the ratio of B z /| B y | passes through some threshold value. This can cause the ionospheric reconfiguration time to be less than the IMF reorientation time.