Comment on “A statistical study of the ionospheric convection response to changing interplanetary magnetic field conditions using the assimilative mapping of ionospheric electrodynamics technique” by A.J. Ridley et al.

Introduction Ridley et al. [1998] have presented some very interesting maps of ionospheric potential and discussed how they change in response to changes in the orientation of the interplanetary magnetic field (IMF).Their results on propagation delays and the response of the transpolar voltage are very valuable, particularly as they have studied a relatively large number of cases.However, there is one important point of interpretation that we believe needs clarification.The authors employ a valuable new technique of subtracting the background potential distribution, prior to the IMF change, to highlight the convection changes.They term these difference patterns the "residual" convection pattern.In their discussion (p.4032), they state "In almost all the events that we examine the residual potential pattern shows very little motion.The lack of motion in the residual potential patterns implies that there is no propagation of the change from the dayside to the nightside".They later state (p.4037) "This finding is different from that of past studies, which show convection changes spreading out from the cusp [e.g.Saunders et al., 1992] but is consistent with the intensification of localized field aligned currents, as described by Banks et al. [1984] and Clauer and Banks [1986]."In this comment, we wish to point out that the examples they give do show an expansion of the convection change away from the cusp and that the propagation speeds are consistent with those observed in previous studies.Furthermore, the above statements imply that such an expansion of a convection change is inconsistent with an intensification of the field-aligned currents.This is not so.We also wish to point out that the incompressibility of the ionosphere has been incorrectly invoked as a reason why there cannot be an expansion of the convection pattern. Previous StudiesShort time scale responses of ionospheric convection to the reconnection rate changes induced by IMF orientation changes were first inferred from magnetometer data by Nishida [1968a, b].The propagation of the convection change was first reported