Central polar cap convection response to short duration southward Interplanetary Magnetic Field

P T Jayachandran,J W Macdougall

doi:10.1007/s00585-000-0887-z

Abstract

Abstract. Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (~1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary.Key words: Ionosphere (plasma convection; polar ionosphere) - Magnetospheric physics (solar wind - magnetosphere interaction)

Highlights

A number of observations over the past two decades have shown that the plasmaow in the Earth's polar ionosphere is driven by the solar wind-magnetosphericionospheric coupling
We ®nd that the central polar cap convection changes associated with short duration southward turning of Interplanetary Magnetic Field (IMF) has 3 components: (1) an initial response to a southward transition that is nearly simultaneous for the entire polar cap, (2) a peak of convection speed that propagates poleward from the ionospheric footprint of the merging region and, if the change in IMF is rapid enough, and (3) a step in convection that appears to start at the cusp and propagates antisunward over the polar cap with velocity of the maximum convection
In the previous section we have described the changes in convection at a central polar cap station in response to a southward turning of the IMF

Summary

Introduction

A number of observations over the past two decades have shown that the plasmaow in the Earth's polar ionosphere is driven by the solar wind-magnetosphericionospheric coupling. Most of the previous studies (Todd et al, 1998; Etamadai et al, 1988; Saunders et al, 1992; Ridley et al, 1998; Ruohoniemi and Greenwald, 1998) were either con®ned to auroral regions (closed ®eld lines) or regions near to the polar cap boundary Some of these have shown that the high-latitude convection responds with a delay after southward transition of IMF (Etamadi et al, 1988; Todd et al, 1988), there are signi®cant dierences in the measured delays. The exact nature of theow response is a subject of debate (Lockwood and Cowley, 1999; Ridley et al, 1999) Based on these previous studies, there are two schools of thought pertaining to the ionospheric convection response to the southward transition of IMF. We ®nd that the central polar cap convection changes associated with short duration southward turning of IMF has 3 components: (1) an initial response to a southward transition that is nearly simultaneous for the entire polar cap, (2) a peak of convection speed (attributed to the maximum merging electric ®eld) that propagates poleward from the ionospheric footprint of the merging region and, if the change in IMF is rapid enough, and (3) a step in convection that appears to start at the cusp and propagates antisunward over the polar cap with velocity of the maximum convection

Observations

Discussion

Conclusions