Pulsating aurora and cosmic noise absorption associated with growth-phase arcs

Derek Mckay,Juha Vierinen,Noora Partamies

doi:10.5194/angeo-36-59-2018

Derek Mckay, Juha Vierinen + Show 1 more

Open Access

https://doi.org/10.5194/angeo-36-59-2018

Copy DOI

Abstract

Abstract. The initial stage of a magnetospheric substorm is the growth phase, which typically lasts 1–2 h. During the growth phase, an equatorward moving, east–west extended, optical auroral arc is observed. This is called a growth-phase arc. This work aims to characterize the optical emission and riometer absorption signatures associated with growth-phase arcs of isolated substorms. This is done using simultaneous all-sky camera and imaging riometer observations. The optical and riometric observations allow determination of the location of the precipitation within growth-phase arcs of low- (<10 keV) and high- (> 10 keV) energy electrons, respectively. The observations indicate that growth-phase arcs have the following characteristics: The peak of the cosmic noise absorption (CNA) arc is equatorward of the optical emission arc. This CNA is contained within the region of diffuse aurora on the equatorward side. Optical pulsating aurora are seen in the border region between the diffuse emission region on the equatorward side and the bright growth-phase arc on the poleward side. CNA is detected in the same region. There is no evidence of pulsations in the CNA. Once the equatorward drift starts, it proceeds at constant speed, with uniform separation between the growth-phase arc and CNA of 40±10 km. Optical pulsating aurora are known to be prominent in the post-onset phase of a substorm. The fact that pulsations are also seen in a fairly localized region during the growth phase shows that the substorm expansion-phase dynamics are not required to closely precede the pulsating aurora. Keywords. Ionosphere (auroral ionosphere)

Highlights

Polar magnetospheric substorms follow a sequence starting with a latitudinally narrow auroral arc, called the growthphase arc, which extends hundreds of kilometres east–west
Cosmic radio noise absorption (CNA) is detected in the same region
We report that when there is an isolated substorm, an arc of CNA is observed on the equatorward edge of the discrete growth-phase arc

Summary

Introduction

Polar magnetospheric substorms follow a sequence starting with a latitudinally narrow auroral arc, called the growthphase arc, which extends hundreds of kilometres east–west. This arc evolves until the sudden commencement of complex and conspicuous activity, referred to as the expansion phase. The start of the expansion phase is accompanied by a deviation from the mean ground-level magnetic field (the onset) and an increase in high-energy precipitation (Wing et al, 2013). The onset can be identified by an abrupt decrease of the north–south component of the ground-level magnetic field (e.g. local electrojet index IL of more than 80 nT in 15 min) (Tanskanen, 2009). The substorm recovery process usually lasts 1–3 h (Tanskanen, 2009)

Objectives

Findings

Discussion

Conclusion