Statistical analysis of ionospheric potential patterns for isolated substorms and sawtooth events

X Cai,C R Clauer,A J Ridley

doi:10.5194/angeo-24-1977-2006

Abstract

Abstract. We present here results which contrast isolated substorms with individual sawtooth events. Sawtooth events are defined as quasi-periodic, large-amplitude oscillations in the energetic particle flux with a periodicity of 2–4 h observed at the geosynchronous orbit. Sawtooth events have several similarities to isolated substorms leading therefore, to different opinions about whether sawtooth events are just an intense periodic form of substorms or if they deserve a category of their own. To help resolve this, we examine the ionospheric potential patterns in the northern polar region for isolated substorms and sawtooth events using the assimilative mapping of ionospheric electrodynamics (AMIE) technique. First we show a statistical analysis of isolated substorm potential patterns. In order to examine the seasonal variation, isolated substorms are identified by mid-latitude positive bay in the north-south geomagnetic perturbation in each season, respectively. Superposed epoch analysis (SEA) is applied to obtain the typical polar potential patterns for each season. By examining the time evolution of the potential patterns and cross polar cap potential (CPCP) for isolated substorms during each season, we find only subtle seasonal variations in the results obtained using the AMIE analysis. This provides a basis for comparison with sawtooth events in the next step. From the averaged potential patterns of 213 isolated substorms and those of 184 individual sawtooth events, we find the sawtooth events show signatures similar to subtorms: the DP 1 potential pattern develops and dominates the polar region after the onset. However, the DP 1 potential cell of sawtooth events encompasses a larger area than that of isolated substorms. Moreover, the CPCP of sawtooth is stronger than that of isolated substorms. It is also shown that the sawtooth events displays greater variability between individual events than isolated substorms. We conclude that in terms of ionospheric electrodynamics, the sawtoothe events have features that are similar to those of isolates substorms, though larger in spatial extent and in magnitude.

Highlights

The purpose of this paper is to investigate the similarities and differences between isolated substorms and individual sawtooth events by examining the ionospheric electric potential patterns in the northern polar region
We examine following characteristics for each season: 1) time evolution of residual potential patterns which show the spatial similarities between substorms; and 2) difference between maximum residual potential and minimum residual potential, defined as cross polar cap potential (CPCP) ∗, which tells the strength of DP 1 potential pattern
Utilizing Superposed epoch analysis (SEA) methodology on the residual potential patterns determined from assimilative mapping of ionospheric electrodynamics (AMIE), we find there are only subtle seasonal variations in the DP 1 potential pattern developing during the expansion phase of isolated substorms

Summary

Introduction

The purpose of this paper is to investigate the similarities and differences between isolated substorms and individual sawtooth events by examining the ionospheric electric potential patterns in the northern polar region. Sawtooth events are global storm-time phenomena, so named because of the shape of the energetic particle flux measurements at geosynchronous orbit. These particle flux data show repetition of slow decreases followed by rapid large amplitude increases (especially clear in the proton flux). The dispersionless particle injection is observed with a wider local time range than that of a typical isolated substorm which is confined to near local midnight only. The individual sawtooth event shows several similarities to substorm signatures: auroral observation in the polar region, magnetic dipolarization at geosynchronous orbit and magnetic perturbation measured by ground magnetometers (e.g., Henderson, 2004; Henderson et al, 2005; Huang, 2002; Huang et al, 2003b; Clauer et al, 2006). An important research issue concerns whether a individual sawtooth event is just an intense form of a magnetospheric substorm or whether it is a new global mode of magnetosphere activity

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