The IMF dependence of the local time of transpolar arcs: Implications for formation mechanism

R C Fear,S E Milan

doi:10.1029/2011ja017209

Abstract

Transpolar arcs are auroral features that extend from the nightside auroral oval into the polar cap. It is well established that they occur predominantly when the interplanetary magnetic field (IMF) has a northward component (Bz > 0). Results concerning how the magnetic local time at which transpolar arcs form might depend upon the IMF dawn‐dusk component (BY) are more mixed. Some studies have found a correlation between these two variables, with Northern Hemisphere arcs forming predominantly premidnight when BY > 0 and postmidnight when BY < 0 and vice versa in the Southern Hemisphere. However, a more recent statistical study found that there was no significant correlation, and other studies find that the formation of moving arcs is triggered by a change in the sign of the IMF BY component. In this paper, we investigate the relationship between the magnetic local time at which transpolar arcs form and the IMF BY component. It is found that there is indeed a correlation between the magnetic local time at which transpolar arcs form and the IMF BY component, which acts in opposite senses in the Northern and Southern hemispheres. However, this correlation is weak if the IMF is only averaged over the hour before the first emergence of the arc and becomes stronger if the IMF is averaged 3–4 h beforehand. This is consistent with a mechanism where the magnetic local time at which the arc first forms depends on the BY component in the magnetotail adjacent to the plasma sheet, which is determined by the IMF BY component during intervals of dayside reconnection in the hours preceding the first emergence of the arc. We do not find evidence for the triggering of arcs by an IMF BY sign change.

Highlights

A03213 observations of polar cap arcs are observations of part of a transpolar arc or whether they are driven by different processes
[2] This paper studies the formation of transpolar arcs (TPA), which can potentially shed light on the complex magnetic topology that the magnetosphere can attain during prolonged periods of northward interplanetary magnetic field (IMF)
Valladares et al [1994] did not find a clear BY dependency on the side of the polar cap at which the arcs were located. (They attributed this in part to selection criteria; their observations were mainly of weak polar cap arcs, which may not make up a significant proportion of events in satellite-based studies.) Kullen et al [2002] presented the first, and so far only statistical study of globally imaged transpolar arcs

Summary

Introduction

[2] This paper studies the formation of transpolar arcs (TPA), which can potentially shed light on the complex magnetic topology that the magnetosphere can attain during prolonged periods of northward IMF. We find that the correlation is strongest if the IMF is averaged not over the preceding hour or two (as per Gussenhoven [1982]) or during the lifetime of the arc [Kullen et al, 2002], but if the IMF is considered 3–4 h before the first emergence of the arc This time scale is consistent with the time expected for newly opened magnetospheric flux at the dayside to reach the inside edge of the lobe, adjacent to the magnetotail plasma sheet, as described by the Dungey cycle of magnetospheric convection [Dungey, 1961]. [6] In section 2, we will provide an overview of various models which have been proposed for the formation of polar cap arcs (including transpolar arcs), and summarize the predictions they make for a BY dependence upon the local time at which the arcs form. The cross-tail component described by Cowley [1981] is initially added to the outside edge of the lobe, but as dayside and nightside reconnection continue,

Discussion

Findings

Conclusions