The storm-time ring current: a statistical analysis at two widely separated low-latitude stations

P Francia,N Adorante,U Villante,W D Gonzalez

doi:10.5194/angeo-22-3699-2004

Abstract

Abstract. We conducted a statistical analysis of the geomagnetic field variations during the storm main phase at two low-latitude stations, separated by several hours in magnetic local time, in order to investigate the asymmetry and longitudinal extent of the storm-time ring current. The results show evidence for an asymmetric current which typically extends from evening to noon and, during moderate solar wind electric field conditions, up to the early morning, confirming the important role of the magnetospheric convection in the ring current energization. We also analyzed a possible relationship between the local current intensity during the storm main phase and the substorm activity observed at different time delays τ with respect to the storm onset. The results show a significant anticorrelation for τ =-1h, indicating that if the substorm activity is high just before the storm, a weaker ring current develops.

Highlights

A growing interest about the relative importance of the symmetric and asymmetric component of the storm-time ring current has been recently documented in the study of the development and decay of geomagnetic storms (e.g. Grafe, 1999)
It is generally accepted that the enhanced magnetospheric convection caused by a long lasting, strong southward interplanetary magnetic field (IMF) component has a central role in the intensification of the ring current (Gonzalez et al, 1994; Tsurutani and Gonzalez, 1997); the cross-tail convection electric field, which is proportional to the eastward solar wind (SW) electric field, drives ions, mainly from the plasma sheet, into the inner magnetosphere, causing the westward ring current to grow significantly
We examined time intervals characterized by a SW eastward electric field Ey>5 mV/m for at least 3 h and selected 20 events; they were associated to geomagnetic storms with Dst minimum value ≤−100 nT (Tsurutani and Gonzalez, 1997)

Summary

Introduction

A growing interest about the relative importance of the symmetric and asymmetric component of the storm-time ring current has been recently documented in the study of the development and decay of geomagnetic storms (e.g. Grafe, 1999). It is generally accepted that the enhanced magnetospheric convection caused by a long lasting, strong southward interplanetary magnetic field (IMF) component has a central role in the intensification of the ring current (Gonzalez et al, 1994; Tsurutani and Gonzalez, 1997); the cross-tail convection electric field, which is proportional to the eastward solar wind (SW) electric field, drives ions, mainly from the plasma sheet, into the inner magnetosphere, causing the westward ring current to grow significantly In this context, Liemohn et al (2001) studied the contributions from the symmetric and asymmetric component of the ring current to the storm-time corrected Dst * index (i.e. removing the effects of the magnetopause currents and induced currents in the conducting Earth). On the basis of the Kozyra (2001) results, we analyzed the possible relationship between the local storm-time ring current intensity and the substorm activity at different time delays τ with respect to the storm onset and found a significant anticorrelation for τ =−1 h

Data and analysis

Summary and discussion